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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 01/14] trace2: Documentation/technical/api-trace2.txt
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From: Jeff Hostetler <jeffhost@microsoft.com>

Created design document for Trace2 feature.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 Documentation/technical/api-trace2.txt | 1158 ++++++++++++++++++++++++
 1 file changed, 1158 insertions(+)
 create mode 100644 Documentation/technical/api-trace2.txt

diff --git a/Documentation/technical/api-trace2.txt b/Documentation/technical/api-trace2.txt
new file mode 100644
index 0000000000..501fd770f2
--- /dev/null
+++ b/Documentation/technical/api-trace2.txt
@@ -0,0 +1,1158 @@
+Trace2 API
+==========
+
+The Trace2 API can be used to print debug, performance, and telemetry
+information to stderr or a file.  The Trace2 feature is inactive unless
+explicitly enabled by setting one or more of the `GIT_TR2`, `GIT_TR2_PERF`,
+or `GIT_TR2_EVENT` environment variables.
+
+The Trace2 API is intended to replace the existing (Trace1)
+printf-style tracing provided by the existing `GIT_TRACE` and
+`GIT_TRACE_PERFORMANCE` facilities.  During initial implementation,
+Trace2 and Trace1 may operate in parallel.
+
+The Trace2 API defines a set of high-level events with known fields,
+such as (`start`: `argv`) and (`exit`: {`exit-code`, `elapsed-time`}).
+
+These high-level events are written to one or more Trace2 Targets
+in a target-specific format.  Each Trace2 Target defines a different
+purpose-specific view onto the event data stream.  In this mannor,
+a single set of Trace2 API event calls in the Git source can drive
+different types of analysis.
+
+Trace2 Targets
+--------------
+
+Trace2 defines the following set of Trace2 Targets.  These can be
+independently enabled and disabled.  Trace2 API events are sent to
+each enabled target.  Each enabled target writes an event message
+in a target-specific format (described later).
+
+`GIT_TR2` (NORMAL)::
+
+	a simple printf format like GIT_TRACE.
++
+------------
+$ export GIT_TR2=~/log.normal
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.normal
+12:28:42.620009 common-main.c:38                  version 2.20.1.155.g426c96fcdb
+12:28:42.620989 common-main.c:39                  start git version
+12:28:42.621101 git.c:432                         cmd_verb version (version)
+12:28:42.621215 git.c:662                         exit elapsed:0.001227 code:0
+12:28:42.621250 trace2/tr2_tgt_normal.c:124       atexit elapsed:0.001265 code:0
+------------
+
+`GIT_TR2_PERF` (PERF)::
+
+	a column-based format to replace GIT_TRACE_PERFORMANCE suitable for
+	development and testing, possibly to complement tools like gprof.
++
+------------
+$ export GIT_TR2_PERF=~/log.perf
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.perf
+12:28:42.620675 common-main.c:38                  | d0 | main                     | version      |     |           |           |            | 2.20.1.155.g426c96fcdb
+12:28:42.621001 common-main.c:39                  | d0 | main                     | start        |     |           |           |            | git version
+12:28:42.621111 git.c:432                         | d0 | main                     | cmd_verb     |     |           |           |            | version (version)
+12:28:42.621225 git.c:662                         | d0 | main                     | exit         |     |  0.001227 |           |            | code:0
+12:28:42.621259 trace2/tr2_tgt_perf.c:211         | d0 | main                     | atexit       |     |  0.001265 |           |            | code:0
+------------
+
+`GIT_TR2_EVENT` (EVENT)::
+
+	a JSON-based format of event data suitable for telemetry analysis.
++
+------------
+$ export GIT_TR2_EVENT=~/log.event
+$ git version
+git version 2.20.1.155.g426c96fcdb
+------------
++
+------------
+$ cat ~/log.event
+{"event":"version","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.620713","file":"common-main.c","line":38,"evt":"1","exe":"2.20.1.155.g426c96fcdb"}
+{"event":"start","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621027","file":"common-main.c","line":39,"argv":["git","version"]}
+{"event":"cmd_verb","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621122","file":"git.c","line":432,"name":"version","hierarchy":"version"}
+{"event":"exit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621236","file":"git.c","line":662,"t_abs":0.001227,"code":0}
+{"event":"atexit","sid":"1547659722619736-11614","thread":"main","time":"2019-01-16 17:28:42.621268","file":"trace2/tr2_tgt_event.c","line":163,"t_abs":0.001265,"code":0}
+------------
+
+Trace2 API Definitions
+----------------------
+
+All public Trace2 functions and macros are defined in `trace2.h` and
+`trace2.c`.  These are documented in detail in `trace2.h`.  All
+public symbols are prefixed with `trace2_`.
+
+There are no public Trace2 data structures.
+
+The Trace2 code also defines a set of private functions and data types
+in the `trace2/` directory.  These symbols are prefixed with `tr2_`
+and should only be used by functions in `trace2.c`.
+
+The functions defined by the Trace2 API can be grouped into the
+following groups.
+
+Basic Command Events::
+
+	These are concerned with the lifetime of the overall git process.
++
+----------------
+trace2_initialize(...)
+
+trace2_cmd_start(...)
+trace2_cmd_exit(...)
+trace2_cmd_error(...)
+trace2_cmd_path(...)
+----------------
+
+Git Command Detail Events::
+
+	These are concerned with describing the specific Git command
+	after the command line, config, and environment are inspected.
++
+----------------
+trace2_cmd_verb(...)
+trace2_cmd_subverb(...)
+trace2_cmd_alias(...)
+
+trace2_cmd_list_config(...)
+trace2_cmd_set_config(...)
+
+trace2_def_param(...)
+trace2_def_repo(...)
+----------------
+
+Git Child Process Events::
+
+	These are concerned with the various spawned child processes,
+	including sub-git processes and hooks,
++
+----------------
+trace2_child_start(...)
+trace2_child_exit(...)
+
+trace2_exec(...)
+trace2_exec_result(...)
+----------------
+
+Git Thread Events::
+
+	These are concerned with Git thread usage.
++
+----------------
+trace2_thread_start(...)
+trace2_thread_exit(...)
+----------------
+
+Region and Data Events::
+
+	These are concerned with recording performance data
+	over regions or spans of code.
++
+----------------
+trace2_region_enter(...)
+trace2_region_enter_printf(...)
+trace2_region_leave(...)
+trace2_region_leave_printf(...)
+
+trace2_data_string(...)
+trace2_data_intmax(...)
+trace2_data_json(...)
+
+trace2_printf(...)
+----------------
+
+Trace2 API Usage
+----------------
+
+Here is a hypothetical usage of the Trace2 API showing the intended
+usage (without worrying about the actual Git details).
+
+Initialization::
+
+	Initialization happens in `main()`.  The initialization code
+	determines which, if any, Trace2 Targets should be enabled and
+	emits the `version`, `start`, and `exit` events.  It causes an
+	`atexit` function and `signal` handler to be registered that
+	will emit `atexit` and `signal` events.
++
+----------------
+int main(int argc, const char **argv)
+{
+	int exit_code;
+
+	trace2_initialize();
+	trace2_cmd_start(argv);
+
+	exit_code = cmd_main(argc, argv);
+
+	trace2_cmd_exit(exit_code);
+
+	return exit_code;
+}
+----------------
+
+Command Details::
+
+	After the basics are established, additional process
+	information can be sent to Trace2 as it is discovered, such as
+	the command verb, alias expansion, interesting config
+	settings, the repository worktree, error messages, and etc.
++
+----------------
+int cmd_checkout(int argc, const char **argv)
+{
+	// emit a single "def_param" event
+	trace2_def_param("core.editor", "emacs");
+
+	// emit def_params for any config setting matching a pattern
+	// in GIT_TR2_CONFIG_PARAMS.
+	trace2_cmd_list_config();
+
+	trace2_cmd_verb("checkout");
+	trace2_cmd_subverb("branch");
+	trace2_def_repo(the_repository);
+
+	if (do_something(...))
+	    trace2_cmd_error("Path '%s': cannot do something", path);
+
+	return 0;
+}
+----------------
+
+Child Processes::
+
+	Git typically spawns many sub-processes, such as sub-git
+	commands, shell scripts, editors, pagers, and hooks.
+	Trace2 child events record time spent waiting for child
+	processes and help track performance problems across
+	multiple layers of child processes.
++
+----------------
+void run_child(...)
+{
+	int child_exit_code;
+	struct child_process cmd = CHILD_PROCESS_INIT;
+	...
+
+	trace2_child_start(&cmd);
+	child_exit_code = ...();
+	trace2_child_exit(&cmd, child_exit_code);
+}
+----------------
++
+For example, the following fetch command spawned ssh, index-pack,
+rev-list, and gc.  This example also shows that fetch took
+5.199 seconds and of that 4.932 was in ssh.
++
+----------------
+$ export GIT_TR2_BRIEF=1
+$ export GIT_TR2=~/log.normal
+$ git fetch origin
+...
+
+$ cat ~/log.normal
+version 2.20.1.vfs.1.1.47.g534dbe1ad1
+start git fetch origin
+worktree /Users/jeffhost/work/gfw
+cmd_verb fetch (fetch)
+child_start[0] ssh git@github.com ...
+child_start[1] git index-pack ...
+... (Trace2 events from child processes omitted)
+child_exit[1] pid:14707 code:0 elapsed:0.076353
+child_exit[0] pid:14706 code:0 elapsed:4.931869
+child_start[2] git rev-list ...
+... (Trace2 events from child process omitted)
+child_exit[2] pid:14708 code:0 elapsed:0.110605
+child_start[3] git gc --auto
+... (Trace2 events from child process omitted)
+child_exit[3] pid:14709 code:0 elapsed:0.006240
+exit elapsed:5.198503 code:0
+atexit elapsed:5.198541 code:0
+----------------
++
+When a git process is a (direct or indirect) child of another
+git process, it inherits Trace2 context information.  This
+allows the child to print a verb hierarchy.  This example
+shows gc as child[3] of fetch.  When the gc process reports
+its verb as "gc", it also reports the hierarchy as "fetch/gc".
+(In this example, trace2 messages from the child process is
+indented for clarity.)
++
+----------------
+$ export GIT_TR2_BRIEF=1
+$ export GIT_TR2=~/log.normal
+$ git fetch origin
+...
+
+$ cat ~/log.normal
+version 2.20.1.160.g5676107ecd.dirty
+start git fetch official
+worktree /Users/jeffhost/work/gfw
+cmd_verb fetch (fetch)
+...
+child_start[3] git gc --auto
+    version 2.20.1.160.g5676107ecd.dirty
+    start /Users/jeffhost/work/gfw/git gc --auto
+    worktree /Users/jeffhost/work/gfw
+    cmd_verb gc (fetch/gc)
+    exit elapsed:0.001959 code:0
+    atexit elapsed:0.001997 code:0
+child_exit[3] pid:20303 code:0 elapsed:0.007564
+exit elapsed:3.868938 code:0
+atexit elapsed:3.868970 code:0
+----------------
+
+Region Events::
+
+	Trace2 defines a region as a timed span of code.
++
+----------------
+void wt_status_collect(struct wt_status *s)
+{
+	trace2_region_enter("status", "worktrees", s->repo);
+	wt_status_collect_changes_worktree(s);
+	trace2_region_leave("status", "worktrees", s->repo);
+
+	trace2_region_enter("status", "index", s->repo);
+	wt_status_collect_changes_index(s);
+	trace2_region_leave("status", "index", s->repo);
+
+	trace2_region_enter("status", "untracked", s->repo);
+	wt_status_collect_untracked(s);
+	trace2_region_leave("status", "untracked", s->repo);
+}
+
+void wt_status_print(struct wt_status *s)
+{
+	trace2_region_enter("status", "print", s->repo);
+	switch (s->status_format) {
+	    ...
+	}
+	trace2_region_leave("status", "print", s->repo);
+}
+----------------
++
+In this example, scanning for untracked files ran from +0.012568 to
++0.027149 (since the process started) and took 0.014581 seconds.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |            | 2.20.1.160.g5676107ecd.dirty
+d0 | main                     | start        |     |           |           |            | git status
+d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
+d0 | main                     | cmd_verb     |     |           |           |            | status (status)
+...
+d0 | main                     | region_enter | r1  |  0.010988 |           | status     | label:worktrees
+d0 | main                     | region_leave | r1  |  0.011236 |  0.000248 | status     | label:worktrees
+d0 | main                     | region_enter | r1  |  0.011260 |           | status     | label:index
+d0 | main                     | region_leave | r1  |  0.012542 |  0.001282 | status     | label:index
+d0 | main                     | region_enter | r1  |  0.012568 |           | status     | label:untracked
+d0 | main                     | region_leave | r1  |  0.027149 |  0.014581 | status     | label:untracked
+d0 | main                     | region_enter | r1  |  0.027411 |           | status     | label:print
+d0 | main                     | region_leave | r1  |  0.028741 |  0.001330 | status     | label:print
+d0 | main                     | exit         |     |  0.028778 |           |            | code:0
+d0 | main                     | atexit       |     |  0.028809 |           |            | code:0
+----------------
++
+Regions may be nested.  This causes event messages to be indented.
+Elapsed times are relative to the start of the correpsonding nesting
+level as expected.  For example, if we add region events to:
++
+----------------
+static enum path_treatment read_directory_recursive(struct dir_struct *dir,
+	struct index_state *istate, const char *base, int baselen,
+	struct untracked_cache_dir *untracked, int check_only,
+	int stop_at_first_file, const struct pathspec *pathspec)
+{
+	enum path_treatment state, subdir_state, dir_state = path_none;
+
+	trace2_region_enter_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
+	...
+	trace2_region_leave_printf("dir", "read_recursive", NULL, "%.*s", baselen, base);
+	return dir_state;
+}
+----------------
++
+We can further investigate the time spent scanning for untracked files.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |            | 2.20.1.162.gb4ccea44db.dirty
+d0 | main                     | start        |     |           |           |            | git status
+d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
+d0 | main                     | cmd_verb     |     |           |           |            | status (status)
+...
+d0 | main                     | region_enter | r1  |  0.015047 |           | status     | label:untracked
+d0 | main                     | region_enter |     |  0.015132 |           | dir        | ..label:read_recursive
+d0 | main                     | region_enter |     |  0.016341 |           | dir        | ....label:read_recursive vcs-svn/
+d0 | main                     | region_leave |     |  0.016422 |  0.000081 | dir        | ....label:read_recursive vcs-svn/
+d0 | main                     | region_enter |     |  0.016446 |           | dir        | ....label:read_recursive xdiff/
+d0 | main                     | region_leave |     |  0.016522 |  0.000076 | dir        | ....label:read_recursive xdiff/
+d0 | main                     | region_enter |     |  0.016612 |           | dir        | ....label:read_recursive git-gui/
+d0 | main                     | region_enter |     |  0.016698 |           | dir        | ......label:read_recursive git-gui/po/
+d0 | main                     | region_enter |     |  0.016810 |           | dir        | ........label:read_recursive git-gui/po/glossary/
+d0 | main                     | region_leave |     |  0.016863 |  0.000053 | dir        | ........label:read_recursive git-gui/po/glossary/
+...
+d0 | main                     | region_enter |     |  0.031876 |           | dir        | ....label:read_recursive builtin/
+d0 | main                     | region_leave |     |  0.032270 |  0.000394 | dir        | ....label:read_recursive builtin/
+d0 | main                     | region_leave |     |  0.032414 |  0.017282 | dir        | ..label:read_recursive
+d0 | main                     | region_leave | r1  |  0.032454 |  0.017407 | status     | label:untracked
+...
+d0 | main                     | exit         |     |  0.034279 |           |            | code:0
+d0 | main                     | atexit       |     |  0.034322 |           |            | code:0
+----------------
++
+Trace2 regions are similar to the existing trace_performance_enter()
+and trace_performance_leave() routines, but are thread safe and
+maintain per-thread stacks of timers.
+
+Data Events::
+
+	Trace2 defines a data event to report important variables.
+	These events contain the active thread and are indented
+	relative to the current thread's open region stack.
++
+----------------
+int read_index_from(struct index_state *istate, const char *path,
+	const char *gitdir)
+{
+	trace2_region_enter_printf("index", "do_read_index", the_repository, "%s", path);
+
+	...
+
+	trace2_data_intmax("index", the_repository, "read/version", istate->version);
+	trace2_data_intmax("index", the_repository, "read/cache_nr", istate->cache_nr);
+
+	trace2_region_leave_printf("index", "do_read_index", the_repository, "%s", path);
+}
+----------------
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |            | 2.20.1.156.gf9916ae094.dirty
+d0 | main                     | start        |     |           |           |            | git status
+d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
+d0 | main                     | cmd_verb     |     |           |           |            | status (status)
+d0 | main                     | region_enter | r1  |  0.001791 |           | index      | label:do_read_index .git/index
+d0 | main                     | data         | r1  |  0.002494 |  0.000703 | index      | ..read/version:2
+d0 | main                     | data         | r1  |  0.002520 |  0.000729 | index      | ..read/cache_nr:3552
+d0 | main                     | region_leave | r1  |  0.002539 |  0.000748 | index      | label:do_read_index .git/index
+...
+----------------
+
+Thread Events::
+
+	Trace2 defines events to mark the start and end of a thread.
+	A thread name constructed during the start event. This name is
+	added to subsequent thread-specific events.  The thread exit
+	event reports the elapsed time in the thread.
++
+For example, the multithreaded preload-index code can be
+instrumented with a region around the thread pool and then
+per-thread start and exit events within the threadproc.
++
+----------------
+static void *preload_thread(void *_data)
+{
+	trace2_thread_start("preload_thread");
+
+	trace2_data_intmax("index", the_repository, "offset", p->offset);
+	trace2_data_intmax("index", the_repository, "count", nr);
+
+	do {
+	    ...
+	} while (--nr > 0);
+	...
+
+	trace2_thread_exit();
+	return NULL;
+}
+
+void preload_index(struct index_state *index,
+	const struct pathspec *pathspec,
+	unsigned int refresh_flags)
+{
+	trace2_region_enter("index", "preload", the_repository);
+
+	for (i = 0; i < threads; i++) {
+	    ... /* create thread */
+	}
+
+	for (i = 0; i < threads; i++) {
+	    ... /* join thread */
+	}
+
+	trace2_region_leave("index", "preload", the_repository);
+}
+----------------
++
+In this example preload_index() was executed by the `main` thread
+and started the `preload` region.  Seven threads, named
+`th01:preload_thread` through `th07:preload_thread`, were started.
+Events from each thread are atomically appended to the shared target
+stream as they occur so they may appear in random order with respect
+other threads. Finally, the main thread waits for the threads to
+finish and leaves the region.
++
+Data events are tagged with the active thread name.  They are used
+to report the per-thread parameters.
++
+----------------
+$ export GIT_TR2_PERF_BRIEF=1
+$ export GIT_TR2_PERF=~/log.perf
+$ git status
+...
+$ cat ~/log.perf
+d0 | main                     | version      |     |           |           |            | 2.20.1.156.gf9916ae094.dirty
+d0 | main                     | start        |     |           |           |            | git status
+d0 | main                     | def_repo     | r1  |           |           |            | worktree:/Users/jeffhost/work/gfw
+d0 | main                     | cmd_verb     |     |           |           |            | status (status)
+...
+d0 | main                     | region_enter | r1  |  0.002595 |           | index      | label:preload
+d0 | th01:preload_thread      | thread_start |     |  0.002699 |           |            |
+d0 | th02:preload_thread      | thread_start |     |  0.002721 |           |            |
+d0 | th01:preload_thread      | data         | r1  |  0.002736 |  0.000037 | index      | offset:0
+d0 | th02:preload_thread      | data         | r1  |  0.002751 |  0.000030 | index      | offset:2032
+d0 | th03:preload_thread      | thread_start |     |  0.002711 |           |            |
+d0 | th06:preload_thread      | thread_start |     |  0.002739 |           |            |
+d0 | th01:preload_thread      | data         | r1  |  0.002766 |  0.000067 | index      | count:508
+d0 | th06:preload_thread      | data         | r1  |  0.002856 |  0.000117 | index      | offset:2540
+d0 | th03:preload_thread      | data         | r1  |  0.002824 |  0.000113 | index      | offset:1016
+d0 | th04:preload_thread      | thread_start |     |  0.002710 |           |            |
+d0 | th02:preload_thread      | data         | r1  |  0.002779 |  0.000058 | index      | count:508
+d0 | th06:preload_thread      | data         | r1  |  0.002966 |  0.000227 | index      | count:508
+d0 | th07:preload_thread      | thread_start |     |  0.002741 |           |            |
+d0 | th07:preload_thread      | data         | r1  |  0.003017 |  0.000276 | index      | offset:3048
+d0 | th05:preload_thread      | thread_start |     |  0.002712 |           |            |
+d0 | th05:preload_thread      | data         | r1  |  0.003067 |  0.000355 | index      | offset:1524
+d0 | th05:preload_thread      | data         | r1  |  0.003090 |  0.000378 | index      | count:508
+d0 | th07:preload_thread      | data         | r1  |  0.003037 |  0.000296 | index      | count:504
+d0 | th03:preload_thread      | data         | r1  |  0.002971 |  0.000260 | index      | count:508
+d0 | th04:preload_thread      | data         | r1  |  0.002983 |  0.000273 | index      | offset:508
+d0 | th04:preload_thread      | data         | r1  |  0.007311 |  0.004601 | index      | count:508
+d0 | th05:preload_thread      | thread_exit  |     |  0.008781 |  0.006069 |            |
+d0 | th01:preload_thread      | thread_exit  |     |  0.009561 |  0.006862 |            |
+d0 | th03:preload_thread      | thread_exit  |     |  0.009742 |  0.007031 |            |
+d0 | th06:preload_thread      | thread_exit  |     |  0.009820 |  0.007081 |            |
+d0 | th02:preload_thread      | thread_exit  |     |  0.010274 |  0.007553 |            |
+d0 | th07:preload_thread      | thread_exit  |     |  0.010477 |  0.007736 |            |
+d0 | th04:preload_thread      | thread_exit  |     |  0.011657 |  0.008947 |            |
+d0 | main                     | region_leave | r1  |  0.011717 |  0.009122 | index      | label:preload
+...
+d0 | main                     | exit         |     |  0.029996 |           |            | code:0
+d0 | main                     | atexit       |     |  0.030027 |           |            | code:0
+----------------
++
+In this example, the preload region took 0.009122 seconds.  The 7 threads
+took between 0.006069 and 0.008947 seconds to work on their portion of
+the index.  Thread "th01" worked on 508 items at offset 0.  Thread "th02"
+worked on 508 items at offset 2032.  Thread "th04" worked on 508 itemts
+at offset 508.
++
+This example also shows that thread names are assigned in a racy manner
+as each thread starts and allocates TLS storage.
+
+Printf Events::
+
+	Trace2 defines a printf interface to write arbitrary messages
+	to the targets.  These are convenient for development, but
+	may more difficult to post-process.
+
+Trace2 Target Formats
+---------------------
+
+NORMAL Target Format
+~~~~~~~~~~~~~~~~~~~~
+
+NORMAL format is enabled when the `GIT_TR2` environment variable
+is set to "1" or an absolute pathname.
+
+Events are written as lines of the form:
+
+------------
+[<time> SP <filename>:<line> SP+] <event-name> [[SP] <event-message>] LF
+------------
+
+`<event-name>`::
+
+	is the event name.
+`<event-message>`::
+	is a free-form printf message intended for human consumption.
++
+Note that this may contain embedded LF or CRLF characters that are
+not escaped, so the event may spill across multiple lines.
+
+If `GIT_TR2_BRIEF` is true, the time, filename, and line number fields
+are omitted.
+
+This target is intended to be more of a summary (like GIT_TRACE) and
+less detailed than the other targets, and therefore does not emit
+messages for some of the Trace2 API verbs.  For example, thread,
+region, and data events are omitted.
+
+PERF Target Format
+~~~~~~~~~~~~~~~~~~
+
+PERF format is enabled when the `GIT_TR2_PERF` environment variable
+is set to "1" or an absolute pathname.
+
+Events are written as lines of the form:
+
+------------
+[<time> SP <filename>:<line> SP+
+    BAR SP] d<depth> SP
+    BAR SP <thread-name> SP+
+    BAR SP <event-name> SP+
+    BAR SP [r<repo-id>] SP+
+    BAR SP [<t_abs>] SP+
+    BAR SP [<t_rel>] SP+
+    BAR SP [<category>] SP+
+    BAR SP DOTS* <perf-event-message>
+    LF
+------------
+
+`<depth>`::
+	is the git process depth.  This is the number of parent
+	git processes.  A top-level git command has depth value 0.
+	A child of it has depth value 1.
+
+`<thread-name>`::
+	is a unique name for the thread.  The primary thread
+	is called "main".  Other thread names are of the form "th%d:%s"
+	and include a unique number and the name of the thread-proc.
+
+`<event-name>`::
+	is the event name.
+
+`<repo-id>`::
+	when present, is a number indicating the repository
+	in use.  A `def_repo` event is emitted when a repository is
+	opened.  This defines the repo-id and associated worktree.
+	Subsequent repo-specific events will reference this repo-id.
++
+Currently, this is always "r1" for the main repository.
+This field is in anticipation of in-proc submodules in the future.
+
+`<t_abs>`::
+	when present, is the absolute time in seconds since the
+	program started.
+
+`<t_rel>`::
+	when present, is time in seconds relative to the start of
+	the current region.  For a thread-exit event, it is the elapsed
+	time of the thread.
+
+`<category>`::
+	is present on region and data events and is used to
+	indicate a broad category, such as "index" or "status".
++
+Currently, this is advisory.  This field is in anticipation
+of category-based filtering, similar to the `GIT_TRACE_<key>`
+facility in Trace1.
+
+`<perf-event-message>`::
+	is a free-form printf message intended for human consumption.
+
+If `GIT_TR2_PERF_BRIEF` is true, the time, filename, and line number
+fields are omitted.
+
+This target is intended for interactive performance analysis
+during development and is quite noisy.
+
+EVENT Target Format
+~~~~~~~~~~~~~~~~~~~
+
+EVENT format is enabled when the `GIT_TR2_EVENT` environment
+variable is set to "1" or an absolute pathname.
+
+Each event is a JSON-object containing multiple key/value pairs
+written as a single line and followed by a LF.
+
+------------
+'{' <key> ':' <value> [',' <key> ':' <value>]* '}' LF
+------------
+
+Some key/value pairs are common to all events and some are
+event-specific.
+
+Common Key/Value Pairs
+^^^^^^^^^^^^^^^^^^^^^^
+
+The following key/value pairs are common to all events:
+
+------------
+{
+	"event":"version",
+	"sid":"1547659722619736-11614",
+	"thread":"main",
+	"time":"2019-01-16 17:28:42.620713",
+	"file":"common-main.c",
+	"line":38,
+	...
+}
+------------
+
+`"event":<event>`::
+	is the event name.
+
+`"sid":<sid>`::
+	is the session-id.  This is a unique string to identify the
+	process instance to allow all events emitted by a process to
+	be identified.  A session-id is used instead of a PID because
+	PIDs are recycled by the OS.  For child git processes, the
+	session-id is prepended with the session-id of the parent git
+	process to allow parent-child relationships to be identified
+	during post-processing.
+
+`"thread":<thread>`::
+	is the thread name.
+
+`"time":<time>`::
+	is the UTC time of the event.
+
+`"file":<filename>`::
+	is source file generating the event.
+
+`"line":<line-number>`::
+	is the integer source line number generating the event.
+
+`"repo":<repo-id>`::
+	when present, is the integer repo-id as described previously.
+
+If `GIT_TR2_EVENT_BRIEF` is true, the file and line fields are omitted
+from all events and the time field is only present on the `start` and
+`atexit` events.
+
+Event-Specific Key/Value Pairs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+`version`::
+	This event gives the version of the executable and the EVENT format.
++
+------------
+{
+	"event":"version",
+	...
+	"evt":"1",		       # EVENT format version
+	"exe":"2.20.1.155.g426c96fcdb" # git version
+}
+------------
+
+`start`::
+	This event contains the complete argv received by main().
++
+------------
+{
+	"event":"start",
+	...
+	"argv":["git","version"]
+}
+------------
+
+`exit`::
+	This event is emitted when git calls `exit()`.
++
+------------
+{
+	"event":"exit",
+	...
+	"t_abs":0.001227, # elapsed time in seconds
+	"code":0	  # exit code
+}
+------------
+
+`atexit`::
+	This event is emitted by the Trace2 `atexit` routine during
+	final shutdown.  It should be the last event emitted by the
+	process.  (The elapsed time reported here is greater than the
+	time reported in the `exit` event because it runs after all
+	other atexit tasks have completed.)
++
+------------
+{
+	"event":"atexit",
+	...
+	"t_abs":0.001227,
+	"code":0
+}
+------------
+
+`signal`::
+	This event is emitted when the program is terminated by a user
+	signal.  Depending on the platform, the signal event may
+	prevent the `atexit` event from being generated.
++
+------------
+{
+	"event":"signal",
+	...
+	"t_abs":0.001227,
+	"signal":13
+}
+------------
+
+`error`::
+	This event is emitted when one of the `error()`, `die()`,
+	or `usage()` functions are called.
++
+------------
+{
+	"event":"error",
+	...
+	"msg":"invalid option: --cahced", # formatted error message
+	"fmt":"invalid option: %s"	  # error format string
+}
+------------
++
+The error event may be emitted more than once.  The format string
+allows post-processors to group errors by type without worrying
+about specific error arguments.
+
+`cmd_path`::
+	This event contains the discovered full path of the git
+	executable (on platforms that are configured to resolve it).
++
+------------
+{
+	"event":"cmd_path",
+	...
+	"path":"C:/work/gfw/git.exe"
+}
+------------
+
+`cmd_verb`::
+	This event contains the primary verb for this command
+	and the hierarchy of command verbs.
++
+------------
+{
+	"event":"cmd_verb",
+	...
+	"name":"pack-objects",
+	"hierarchy":"push/pack-objects"
+}
+------------
++
+Normally, the `name` field contains the canonical name of the
+command.  When a canonical command verb is not available, one of
+these special values are used:
++
+------------
+"_query_"            # "git --html-path"
+"_run_dashed_"       # when "git foo" tries to run "git-foo"
+"_run_shell_alias_"  # alias expansion to a shell command
+"_run_git_alias_"    # alias expansion to a git command
+"_usage_"            # usage error
+------------
+
+`cmd_subverb`::
+	This event, when present, describes the variant of the main
+	command verb.  This event may be emitted more than once.
++
+------------
+{
+	"event":"cmd_subverb",
+	...
+	"name":"branch"
+}
+------------
++
+The `name` field is an arbitrary string that makes sense in the
+context of the primary verb. For example, checkout can checkout a
+branch or an individual file.  And these variations typically have
+different performance characteristics that are not comparable.
+
+`alias`::
+	This event is present when an alias is expanded.
++
+------------
+{
+	"event":"alias",
+	...
+	"alias":"l",		 # registered alias
+	"argv":["log","--graph"] # alias expansion
+}
+------------
+
+`child_start`::
+	This event describes a child process that is about to be
+	spawned.
++
+------------
+{
+	"event":"child_start",
+	...
+	"child_id":2,
+	"child_class":"?",
+	"use_shell":false,
+	"argv":["git","rev-list","--objects","--stdin","--not","--all","--quiet"]
+
+	"hook_name":"<hook_name>"  # present when child_class is "hook"
+	"cd":"<path>"		   # present when cd is required
+}
+------------
++
+The `child_id` field can be used to match this child_start with the
+corresponding child_exit event.
++
+The `child_class` field is a rough classification, such as "editor",
+"pager", "transport/*", and "hook".  Unclassified children are classified
+with "?".
+
+`child_exit`::
+	This event is generated after the git process has returned
+	from the waitpid() and collected the exit information from the
+	child.
++
+------------
+{
+	"event":"child_exit",
+	...
+	"child_id":2,
+	"pid":14708,	 # child PID
+	"code":0,	 # child exit-code
+	"t_rel":0.110605 # observed run-time of child process
+}
+------------
++
+Note that the session-id of the child process is not available to
+the current/spawning process, so the child's PID is reported here as
+a hint for post-processing.  (But it is only a hint because the child
+proces may be a shell script which doesn't have a session-id.)
++
+Note that the `t_rel` field contains the observed run time in seconds
+for the child process (starting before the fork/exec/spawn and
+stopping after the waitpid() and includes OS process creation overhead).
+So this time will be slightly larger than the atexit time reported by
+the child process itself.
+
+`exec`::
+	This event is generated before git attempts to `exec()`
+	another command rather than starting a child process.
++
+------------
+{
+	"event":"exec",
+	...
+	"exec_id":0,
+	"exe":"git",
+	"argv":["foo", "bar"]
+}
+------------
++
+Since `exec()` does not return if successful, this will be the last
+event generated for the current git command before the new git commit
+starts.
++
+The `exec_id` field is a command-unique id and is only useful if the
+`exec()` fails and a corresponding exec_result event is generated.
+
+`exec_result`::
+	This event is generated if the `exec()` fails and control
+	returns to the current git command.
++
+------------
+{
+	"event":"exec_result",
+	...
+	"exec_id":0,
+	"code":1      # error code (errno) from exec()
+}
+------------
+
+`thread_start`::
+	This event is generated when a thread is started.  It is
+	generated from *within* the new thread's thread-proc (for TLS
+	reasons).
++
+------------
+{
+	"event":"thread_start",
+	...
+	"thread":"th02:preload_thread"
+}
+------------
+
+`thread_exit`::
+	This event is generated when a thread exits.  It is generated
+	from *within* the thread's thread-proc (for TLS reasons).
++
+------------
+{
+	"event":"thread_exit",
+	...
+	"thread":"th02:preload_thread",
+	"t_rel":0.007328                # thread elapsed time
+}
+------------
+
+`def_param`::
+	This event is generated to log a global key/value pair for the
+	current command.
++
+------------
+{
+	"event":"def_param",
+	...
+	"param":"core.editor",
+	"value":"emacs"
+}
+------------
++
+This is intended to log "values of interest" for post-processing.  For
+example, this might be configuration settings known to affect
+performance or the remote URL so that different repos can be grouped
+differently.  For thread- or region-specific key/values pairs, see the
+`data` and `data_json` events.
+
+`def_repo`::
+	This event defines a repo-id and associates it with the root
+	of the worktree.  Subsequent repo-relative events will refer
+	to this repo-id.  Repo-id number 1 refers to the main
+	repository.
++
+------------
+{
+	"event":"def_repo",
+	...
+	"repo":1,
+	"worktree":"/Users/jeffhost/work/gfw"
+}
+------------
++
+As stated earlier, the repo-id is currently always 1, so there will
+only be one def_repo event.  Later, if in-proc submodules are
+supported, a def_repo event should be emitted for each submodule
+visited.
+
+`region_enter`::
+	This event is generated when entering a region.
+	It serves as a marker in the event stream.  It starts an activity timer
+	for the region.  And it pushes a frame on the region stack for the
+	current thread.
++
+------------
+{
+	"event":"region_enter",
+	...
+	"repo":1,
+	"nesting":1,		 # current region stack depth
+	"category":"index",	 # optional
+	"label":"do_read_index", # optional
+	"msg":".git/index"	 # optional
+}
+------------
++
+The `category` field may be used in a future enhancement to
+do category-based filtering.
++
+The `GIT_TR2_EVENT_NESTING` environment variable can be used to
+filter deeply nested regions and data events.  It defaults to "2".
+
+`region_leave`::
+	This event is generated when leaving a region.  It serves as a
+	marker in the event stream and reports the elapsed time spent
+	in the region.  It pops a frame off of the region stack for
+	the current thread.
++
+------------
+{
+	"event":"region_leave",
+	...
+	"repo":1,
+	"t_rel":0.002876,	 # time spent in region in seconds
+	"nesting":1,		 # region stack depth
+	"category":"index",	 # optional
+	"label":"do_read_index", # optional
+	"msg":".git/index"	 # optional
+}
+------------
++
+The examples in this document have generally shown the region_enter
+and region_leave events with the same category and label fields.  This
+is not a requirement, but makes it easier to understand the logs.
+
+`data`::
+	This event is generated to log a thread- and region-local
+	key/value pair.  For example, during a recursive treewalk,
+	logging the relative path and the number of entries in the
+	tree.
++
+------------
+{
+	"event":"data",
+	...
+	"repo":1,              # optional
+	"t_abs":0.024107,      # absolute elapsed time
+	"t_rel":0.001031,      # elapsed time in region/thread
+	"nesting":2,
+	"category":"index",
+	"key":"read/cache_nr",
+	"value":"3552"}
+}
+------------
++
+The `value` field may be an integer or a string.
+
+`data-json`::
+	This event is generated to log a pre-formatted JSON string
+	containing structured data.
++
+------------
+{
+	"event":"data_json",
+	...
+	"repo":1,              # optional
+	"t_abs":0.015905,
+	"t_rel":0.015905,
+	"nesting":1,
+	"category":"process",
+	"key":"windows/ancestry",
+	"value":["bash.exe","bash.exe"]
+}
+------------
+
+Appendix: Relationship to the Existing Trace Api (api-trace.txt)
+----------------------------------------------------------------
+
+Eventually, I would like Trace2 to replace the existing Trace1 API.
+
+Trace2 is a superset of Trace1 and the existing Trace1 API calls could
+be converted to Trace2, but that work has not yet been attempted.
+
+There are a few issues here:
+
+* Updating existing tests that assume GIT_TRACE format messages.
+
+* How to best handle custom GIT_TRACE_<key> messages?
+
+** The GIT_TRACE_<key> mechanism allows each <key> to write to a
+different file (in addition to just stderr).
+
+** Do we want to maintain that ability or simply write to the existing
+Trace2 targets (and convert <key> to a "category").
+

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 02/14] trace2: create new combined trace facility
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From: Jeff Hostetler <jeffhost@microsoft.com>

Create a new unified tracing facility for git.  The eventual intent is to
replace the current trace_printf* and trace_performance* routines with a
unified set of git_trace2* routines.

In addition to the usual printf-style API, trace2 provides higer-level
event verbs with fixed-fields allowing structured data to be written.
This makes post-processing and analysis easier for external tools.

Trace2 defines 3 output targets.  These are set using the environment
variables "GIT_TR2", "GIT_TR2_PERF", and "GIT_TR2_EVENT".  These may be
set to "1" or to an absolute pathname (just like the current GIT_TRACE).

* GIT_TR2 is intended to be a replacement for GIT_TRACE and logs command
  summary data.

* GIT_TR2_PERF is intended as a replacement for GIT_TRACE_PERFORMANCE.
  It extends the output with columns for the command process, thread,
  repo, absolute and relative elapsed times.  It reports events for
  child process start/stop, thread start/stop, and per-thread function
  nesting.

* GIT_TR2_EVENT is a new structured format. It writes event data as a
  series of JSON records.

Calls to trace2 functions log to any of the 3 output targets enabled
without the need to call different trace_printf* or trace_performance*
routines.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 Makefile                      |  10 +
 builtin/submodule--helper.c   |  11 +-
 cache.h                       |   1 +
 common-main.c                 |  12 +-
 compat/mingw.c                |  11 +-
 compat/mingw.h                |   3 +-
 config.c                      |   2 +
 exec-cmd.c                    |   2 +
 git-compat-util.h             |   7 +
 git.c                         |  65 +++
 remote-curl.c                 |   7 +
 repository.c                  |   2 +
 repository.h                  |   3 +
 run-command.c                 |  63 ++-
 run-command.h                 |  17 +-
 sh-i18n--envsubst.c           |   3 +
 submodule.c                   |  11 +-
 t/helper/test-parse-options.c |   3 +
 t/helper/test-tool.c          |   3 +
 t/t0001-init.sh               |   1 +
 trace2.c                      | 809 ++++++++++++++++++++++++++++++++++
 trace2.h                      | 390 ++++++++++++++++
 trace2/tr2_cfg.c              |  92 ++++
 trace2/tr2_cfg.h              |  19 +
 trace2/tr2_dst.c              |  90 ++++
 trace2/tr2_dst.h              |  34 ++
 trace2/tr2_sid.c              |  67 +++
 trace2/tr2_sid.h              |  18 +
 trace2/tr2_tbuf.c             |  32 ++
 trace2/tr2_tbuf.h             |  23 +
 trace2/tr2_tgt.h              | 126 ++++++
 trace2/tr2_tgt_event.c        | 606 +++++++++++++++++++++++++
 trace2/tr2_tgt_normal.c       | 331 ++++++++++++++
 trace2/tr2_tgt_perf.c         | 573 ++++++++++++++++++++++++
 trace2/tr2_tls.c              | 164 +++++++
 trace2/tr2_tls.h              |  95 ++++
 trace2/tr2_verb.c             |  30 ++
 trace2/tr2_verb.h             |  24 +
 usage.c                       |  31 ++
 39 files changed, 3774 insertions(+), 17 deletions(-)
 create mode 100644 trace2.c
 create mode 100644 trace2.h
 create mode 100644 trace2/tr2_cfg.c
 create mode 100644 trace2/tr2_cfg.h
 create mode 100644 trace2/tr2_dst.c
 create mode 100644 trace2/tr2_dst.h
 create mode 100644 trace2/tr2_sid.c
 create mode 100644 trace2/tr2_sid.h
 create mode 100644 trace2/tr2_tbuf.c
 create mode 100644 trace2/tr2_tbuf.h
 create mode 100644 trace2/tr2_tgt.h
 create mode 100644 trace2/tr2_tgt_event.c
 create mode 100644 trace2/tr2_tgt_normal.c
 create mode 100644 trace2/tr2_tgt_perf.c
 create mode 100644 trace2/tr2_tls.c
 create mode 100644 trace2/tr2_tls.h
 create mode 100644 trace2/tr2_verb.c
 create mode 100644 trace2/tr2_verb.h

diff --git a/Makefile b/Makefile
index 1a44c811aa..5d4e993cdc 100644
--- a/Makefile
+++ b/Makefile
@@ -996,6 +996,16 @@ LIB_OBJS += tempfile.o
 LIB_OBJS += thread-utils.o
 LIB_OBJS += tmp-objdir.o
 LIB_OBJS += trace.o
+LIB_OBJS += trace2.o
+LIB_OBJS += trace2/tr2_cfg.o
+LIB_OBJS += trace2/tr2_dst.o
+LIB_OBJS += trace2/tr2_sid.o
+LIB_OBJS += trace2/tr2_tbuf.o
+LIB_OBJS += trace2/tr2_tgt_event.o
+LIB_OBJS += trace2/tr2_tgt_normal.o
+LIB_OBJS += trace2/tr2_tgt_perf.o
+LIB_OBJS += trace2/tr2_tls.o
+LIB_OBJS += trace2/tr2_verb.o
 LIB_OBJS += trailer.o
 LIB_OBJS += transport.o
 LIB_OBJS += transport-helper.o
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index b45514be31..82716b5060 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1813,11 +1813,12 @@ static int update_submodules(struct submodule_update_clone *suc)
 {
 	int i;
 
-	run_processes_parallel(suc->max_jobs,
-			       update_clone_get_next_task,
-			       update_clone_start_failure,
-			       update_clone_task_finished,
-			       suc);
+	run_processes_parallel_tr2(suc->max_jobs,
+				   update_clone_get_next_task,
+				   update_clone_start_failure,
+				   update_clone_task_finished,
+				   suc,
+				   "submodule", "parallel/update");
 
 	/*
 	 * We saved the output and put it out all at once now.
diff --git a/cache.h b/cache.h
index 49713cc5a5..af996443ee 100644
--- a/cache.h
+++ b/cache.h
@@ -9,6 +9,7 @@
 #include "gettext.h"
 #include "convert.h"
 #include "trace.h"
+#include "trace2.h"
 #include "string-list.h"
 #include "pack-revindex.h"
 #include "hash.h"
diff --git a/common-main.c b/common-main.c
index 3728f66b4c..6dbdc4adf2 100644
--- a/common-main.c
+++ b/common-main.c
@@ -25,12 +25,18 @@ static void restore_sigpipe_to_default(void)
 
 int main(int argc, const char **argv)
 {
+	int result;
+
 	/*
 	 * Always open file descriptors 0/1/2 to avoid clobbering files
 	 * in die().  It also avoids messing up when the pipes are dup'ed
 	 * onto stdin/stdout/stderr in the child processes we spawn.
 	 */
 	sanitize_stdfds();
+	restore_sigpipe_to_default();
+
+	trace2_initialize();
+	trace2_cmd_start(argv);
 
 	git_resolve_executable_dir(argv[0]);
 
@@ -40,7 +46,9 @@ int main(int argc, const char **argv)
 
 	attr_start();
 
-	restore_sigpipe_to_default();
+	result = cmd_main(argc, argv);
+
+	trace2_cmd_exit(result);
 
-	return cmd_main(argc, argv);
+	return result;
 }
diff --git a/compat/mingw.c b/compat/mingw.c
index b459e1a291..111fe68baf 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1476,19 +1476,23 @@ static int try_shell_exec(const char *cmd, char *const *argv)
 		return 0;
 	prog = path_lookup(interpr, 1);
 	if (prog) {
+		int exec_id;
 		int argc = 0;
 		const char **argv2;
 		while (argv[argc]) argc++;
 		ALLOC_ARRAY(argv2, argc + 1);
 		argv2[0] = (char *)cmd;	/* full path to the script file */
 		memcpy(&argv2[1], &argv[1], sizeof(*argv) * argc);
+		exec_id = trace2_exec(prog, argv2);
 		pid = mingw_spawnv(prog, argv2, 1);
 		if (pid >= 0) {
 			int status;
 			if (waitpid(pid, &status, 0) < 0)
 				status = 255;
+			trace2_exec_result(exec_id, status);
 			exit(status);
 		}
+		trace2_exec_result(exec_id, -1);
 		pid = 1;	/* indicate that we tried but failed */
 		free(prog);
 		free(argv2);
@@ -1501,12 +1505,17 @@ int mingw_execv(const char *cmd, char *const *argv)
 	/* check if git_command is a shell script */
 	if (!try_shell_exec(cmd, argv)) {
 		int pid, status;
+		int exec_id;
 
+		exec_id = trace2_exec(cmd, (const char **)argv);
 		pid = mingw_spawnv(cmd, (const char **)argv, 0);
-		if (pid < 0)
+		if (pid < 0) {
+			trace2_exec_result(exec_id, -1);
 			return -1;
+		}
 		if (waitpid(pid, &status, 0) < 0)
 			status = 255;
+		trace2_exec_result(exec_id, status);
 		exit(status);
 	}
 	return -1;
diff --git a/compat/mingw.h b/compat/mingw.h
index 30d9fb3e36..4d73f8aa9d 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -147,8 +147,7 @@ static inline int fcntl(int fd, int cmd, ...)
 	errno = EINVAL;
 	return -1;
 }
-/* bash cannot reliably detect negative return codes as failure */
-#define exit(code) exit((code) & 0xff)
+
 #define sigemptyset(x) (void)0
 static inline int sigaddset(sigset_t *set, int signum)
 { return 0; }
diff --git a/config.c b/config.c
index ff521eb27a..4beeb7795a 100644
--- a/config.c
+++ b/config.c
@@ -2656,6 +2656,8 @@ int git_config_set_gently(const char *key, const char *value)
 void git_config_set(const char *key, const char *value)
 {
 	git_config_set_multivar(key, value, NULL, 0);
+
+	trace2_cmd_set_config(key, value);
 }
 
 /*
diff --git a/exec-cmd.c b/exec-cmd.c
index 4f81f44310..7deeab3039 100644
--- a/exec-cmd.c
+++ b/exec-cmd.c
@@ -209,6 +209,8 @@ static int git_get_exec_path(struct strbuf *buf, const char *argv0)
 		return -1;
 	}
 
+	trace2_cmd_path(buf->buf);
+
 	return 0;
 }
 
diff --git a/git-compat-util.h b/git-compat-util.h
index b8ace77410..a41b277929 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1238,6 +1238,13 @@ static inline int is_missing_file_error(int errno_)
 
 extern int cmd_main(int, const char **);
 
+/*
+ * Intercept all calls to exit() and route them to trace2 to
+ * optionally emit a message before calling the real exit().
+ */
+int trace2_cmd_exit_fl(const char *file, int line, int code);
+#define exit(code) exit(trace2_cmd_exit_fl(__FILE__, __LINE__, (code)))
+
 /*
  * You can mark a stack variable with UNLEAK(var) to avoid it being
  * reported as a leak by tools like LSAN or valgrind. The argument
diff --git a/git.c b/git.c
index 4d53a3d50d..2db81c775a 100644
--- a/git.c
+++ b/git.c
@@ -146,16 +146,20 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 				git_set_exec_path(cmd + 1);
 			else {
 				puts(git_exec_path());
+				trace2_cmd_verb("_query_");
 				exit(0);
 			}
 		} else if (!strcmp(cmd, "--html-path")) {
 			puts(system_path(GIT_HTML_PATH));
+			trace2_cmd_verb("_query_");
 			exit(0);
 		} else if (!strcmp(cmd, "--man-path")) {
 			puts(system_path(GIT_MAN_PATH));
+			trace2_cmd_verb("_query_");
 			exit(0);
 		} else if (!strcmp(cmd, "--info-path")) {
 			puts(system_path(GIT_INFO_PATH));
+			trace2_cmd_verb("_query_");
 			exit(0);
 		} else if (!strcmp(cmd, "-p") || !strcmp(cmd, "--paginate")) {
 			use_pager = 1;
@@ -284,6 +288,7 @@ static int handle_options(const char ***argv, int *argc, int *envchanged)
 			(*argv)++;
 			(*argc)--;
 		} else if (skip_prefix(cmd, "--list-cmds=", &cmd)) {
+			trace2_cmd_verb("_query_");
 			if (!strcmp(cmd, "parseopt")) {
 				struct string_list list = STRING_LIST_INIT_DUP;
 				int i;
@@ -331,9 +336,14 @@ static int handle_alias(int *argcp, const char ***argv)
 			commit_pager_choice();
 
 			child.use_shell = 1;
+			child.trace2_child_class = "shell_alias";
 			argv_array_push(&child.args, alias_string + 1);
 			argv_array_pushv(&child.args, (*argv) + 1);
 
+			trace2_cmd_alias(alias_command, child.args.argv);
+			trace2_cmd_list_config();
+			trace2_cmd_verb("_run_shell_alias_");
+
 			ret = run_command(&child);
 			if (ret >= 0)   /* normal exit */
 				exit(ret);
@@ -368,6 +378,9 @@ static int handle_alias(int *argcp, const char ***argv)
 		/* insert after command name */
 		memcpy(new_argv + count, *argv + 1, sizeof(char *) * *argcp);
 
+		trace2_cmd_alias(alias_command, new_argv);
+		trace2_cmd_list_config();
+
 		*argv = new_argv;
 		*argcp += count - 1;
 
@@ -416,6 +429,8 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 		setup_work_tree();
 
 	trace_argv_printf(argv, "trace: built-in: git");
+	trace2_cmd_verb(p->cmd);
+	trace2_cmd_list_config();
 
 	validate_cache_entries(&the_index);
 	status = p->fn(argc, argv, prefix);
@@ -665,7 +680,14 @@ static void execv_dashed_external(const char **argv)
 	cmd.clean_on_exit = 1;
 	cmd.wait_after_clean = 1;
 	cmd.silent_exec_failure = 1;
+	cmd.trace2_child_class = "dashed";
 
+	trace2_cmd_verb("_run_dashed_");
+
+	/*
+	 * The code in run_command() logs trace2 child_start/child_exit
+	 * events, so we do not need to report exec/exec_result events here.
+	 */
 	trace_argv_printf(cmd.args.argv, "trace: exec:");
 
 	/*
@@ -675,6 +697,12 @@ static void execv_dashed_external(const char **argv)
 	 * the program.
 	 */
 	status = run_command(&cmd);
+
+	/*
+	 * If the child process ran and we are now going to exit, emit a
+	 * generic string as our trace2 command verb to indicate that we
+	 * launched a dashed command.
+	 */
 	if (status >= 0)
 		exit(status);
 	else if (errno != ENOENT)
@@ -700,6 +728,43 @@ static int run_argv(int *argcp, const char ***argv)
 		if (!done_alias)
 			handle_builtin(*argcp, *argv);
 
+#if 0 // TODO In GFW, need to amend a7924b655e940b06cb547c235d6bed9767929673 to include trace2_ and _tr2 lines.
+		else if (get_builtin(**argv)) {
+			struct argv_array args = ARGV_ARRAY_INIT;
+			int i;
+
+			/*
+			 * The current process is committed to launching a
+			 * child process to run the command named in (**argv)
+			 * and exiting.  Log a generic string as the trace2
+			 * command verb to indicate this.  Note that the child
+			 * process will log the actual verb when it runs.
+			 */
+			trace2_cmd_verb("_run_git_alias_");
+
+			if (get_super_prefix())
+				die("%s doesn't support --super-prefix", **argv);
+
+			commit_pager_choice();
+
+			argv_array_push(&args, "git");
+			for (i = 0; i < *argcp; i++)
+				argv_array_push(&args, (*argv)[i]);
+
+			trace_argv_printf(args.argv, "trace: exec:");
+
+			/*
+			 * if we fail because the command is not found, it is
+			 * OK to return. Otherwise, we just pass along the status code.
+			 */
+			i = run_command_v_opt_tr2(args.argv, RUN_SILENT_EXEC_FAILURE |
+						  RUN_CLEAN_ON_EXIT, "git_alias");
+			if (i >= 0 || errno != ENOENT)
+				exit(i);
+			die("could not execute builtin %s", **argv);
+		}
+#endif // a7924b655e940b06cb547c235d6bed9767929673
+
 		/* .. then try the external ones */
 		execv_dashed_external(*argv);
 
diff --git a/remote-curl.c b/remote-curl.c
index 1220dffcdc..8950b5c383 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -1340,6 +1340,13 @@ int cmd_main(int argc, const char **argv)
 	string_list_init(&options.deepen_not, 1);
 	string_list_init(&options.push_options, 1);
 
+	/*
+	 * Just report "remote-curl" here (folding all the various aliases
+	 * ("git-remote-http", "git-remote-https", and etc.) here since they
+	 * are all just copies of the same actual executable.
+	 */
+	trace2_cmd_verb("remote-curl");
+
 	remote = remote_get(argv[1]);
 
 	if (argc > 2) {
diff --git a/repository.c b/repository.c
index 7b02e1dffa..b741089dbb 100644
--- a/repository.c
+++ b/repository.c
@@ -119,6 +119,8 @@ static int repo_init_gitdir(struct repository *repo, const char *gitdir)
 void repo_set_worktree(struct repository *repo, const char *path)
 {
 	repo->worktree = real_pathdup(path, 1);
+
+	trace2_def_repo(repo);
 }
 
 static int read_and_verify_repository_format(struct repository_format *format,
diff --git a/repository.h b/repository.h
index 9f16c42c1e..97b03846ba 100644
--- a/repository.h
+++ b/repository.h
@@ -90,6 +90,9 @@ struct repository {
 	/* Repository's current hash algorithm, as serialized on disk. */
 	const struct git_hash_algo *hash_algo;
 
+	/* A unique-id for tracing purposes. */
+	int trace2_repo_id;
+
 	/* Configurations */
 
 	/* Indicate if a repository has a different 'commondir' from 'gitdir' */
diff --git a/run-command.c b/run-command.c
index 3db26b7b0e..8948f6fc6f 100644
--- a/run-command.c
+++ b/run-command.c
@@ -219,9 +219,29 @@ static int exists_in_PATH(const char *file)
 
 int sane_execvp(const char *file, char * const argv[])
 {
+#ifndef GIT_WINDOWS_NATIVE
+	/*
+	 * execvp() doesn't return, so we all we can do is tell trace2
+	 * what we are about to do and let it leave a hint in the log
+	 * (unless of course the execvp() fails).
+	 *
+	 * we skip this for Windows because the compat layer already
+	 * has to emulate the execvp() call anyway.
+	 */
+	int exec_id = trace2_exec(file, (const char**)argv);
+#endif
+
 	if (!execvp(file, argv))
 		return 0; /* cannot happen ;-) */
 
+#ifndef GIT_WINDOWS_NATIVE
+	{
+		int ec = errno;
+		trace2_exec_result(exec_id, ec);
+		errno = ec;
+	}
+#endif
+
 	/*
 	 * When a command can't be found because one of the directories
 	 * listed in $PATH is unsearchable, execvp reports EACCES, but
@@ -712,6 +732,7 @@ int start_command(struct child_process *cmd)
 		cmd->err = fderr[0];
 	}
 
+	trace2_child_start(cmd);
 	trace_run_command(cmd);
 
 	fflush(NULL);
@@ -926,6 +947,8 @@ int start_command(struct child_process *cmd)
 #endif
 
 	if (cmd->pid < 0) {
+		trace2_child_exit(cmd, -1);
+
 		if (need_in)
 			close_pair(fdin);
 		else if (cmd->in)
@@ -964,13 +987,16 @@ int start_command(struct child_process *cmd)
 int finish_command(struct child_process *cmd)
 {
 	int ret = wait_or_whine(cmd->pid, cmd->argv[0], 0);
+	trace2_child_exit(cmd, ret);
 	child_process_clear(cmd);
 	return ret;
 }
 
 int finish_command_in_signal(struct child_process *cmd)
 {
-	return wait_or_whine(cmd->pid, cmd->argv[0], 1);
+	int ret = wait_or_whine(cmd->pid, cmd->argv[0], 1);
+	trace2_child_exit(cmd, ret);
+	return ret;
 }
 
 
@@ -992,7 +1018,18 @@ int run_command_v_opt(const char **argv, int opt)
 	return run_command_v_opt_cd_env(argv, opt, NULL, NULL);
 }
 
+int run_command_v_opt_tr2(const char **argv, int opt, const char *tr2_class)
+{
+	return run_command_v_opt_cd_env_tr2(argv, opt, NULL, NULL, tr2_class);
+}
+
 int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const char *const *env)
+{
+	return run_command_v_opt_cd_env_tr2(argv, opt, dir, env, NULL);
+}
+
+int run_command_v_opt_cd_env_tr2(const char **argv, int opt, const char *dir, const char *const *env,
+				 const char *tr2_class)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	cmd.argv = argv;
@@ -1004,6 +1041,7 @@ int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const
 	cmd.clean_on_exit = opt & RUN_CLEAN_ON_EXIT ? 1 : 0;
 	cmd.dir = dir;
 	cmd.env = env;
+	cmd.trace2_child_class = tr2_class;
 	return run_command(&cmd);
 }
 
@@ -1319,6 +1357,7 @@ int run_hook_ve(const char *const *env, const char *name, va_list args)
 	hook.env = env;
 	hook.no_stdin = 1;
 	hook.stdout_to_stderr = 1;
+	hook.trace2_hook_name = name;
 
 	return run_command(&hook);
 }
@@ -1807,3 +1846,25 @@ int run_processes_parallel(int n,
 	pp_cleanup(&pp);
 	return 0;
 }
+
+int run_processes_parallel_tr2(int n,
+			       get_next_task_fn get_next_task,
+			       start_failure_fn start_failure,
+			       task_finished_fn task_finished,
+			       void *pp_cb,
+			       const char *tr2_category,
+			       const char *tr2_label)
+{
+	int result;
+
+	trace2_region_enter_printf(tr2_category, tr2_label, NULL,
+				   "max:%d", ((n < 1) ? online_cpus() : n));
+
+	result = run_processes_parallel(n, get_next_task, start_failure,
+					task_finished, pp_cb);
+
+	trace2_region_leave(tr2_category, tr2_label, NULL);
+
+	return result;
+}
+
diff --git a/run-command.h b/run-command.h
index 68f5369fc2..91ac29102d 100644
--- a/run-command.h
+++ b/run-command.h
@@ -10,6 +10,12 @@ struct child_process {
 	struct argv_array args;
 	struct argv_array env_array;
 	pid_t pid;
+
+	int trace2_child_id;
+	uint64_t trace2_child_us_start;
+	const char *trace2_child_class;
+	const char *trace2_hook_name;
+
 	/*
 	 * Using .in, .out, .err:
 	 * - Specify 0 for no redirections (child inherits stdin, stdout,
@@ -73,12 +79,14 @@ extern int run_hook_ve(const char *const *env, const char *name, va_list args);
 #define RUN_USING_SHELL 16
 #define RUN_CLEAN_ON_EXIT 32
 int run_command_v_opt(const char **argv, int opt);
-
+int run_command_v_opt_tr2(const char **argv, int opt, const char *tr2_class);
 /*
  * env (the environment) is to be formatted like environ: "VAR=VALUE".
  * To unset an environment variable use just "VAR".
  */
 int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const char *const *env);
+int run_command_v_opt_cd_env_tr2(const char **argv, int opt, const char *dir, const char *const *env,
+				 const char *tr2_class);
 
 /**
  * Execute the given command, sending "in" to its stdin, and capturing its
@@ -220,5 +228,12 @@ int run_processes_parallel(int n,
 			   start_failure_fn,
 			   task_finished_fn,
 			   void *pp_cb);
+int run_processes_parallel_tr2(int n,
+			       get_next_task_fn,
+			       start_failure_fn,
+			       task_finished_fn,
+			       void *pp_cb,
+			       const char *tr2_category,
+			       const char *tr2_label);
 
 #endif
diff --git a/sh-i18n--envsubst.c b/sh-i18n--envsubst.c
index 09c6b445b9..2296ba1a2d 100644
--- a/sh-i18n--envsubst.c
+++ b/sh-i18n--envsubst.c
@@ -14,6 +14,7 @@
  */
 
 #include "git-compat-util.h"
+#include "trace2.h"
 
 /* Substitution of environment variables in shell format strings.
    Copyright (C) 2003-2007 Free Software Foundation, Inc.
@@ -67,6 +68,8 @@ cmd_main (int argc, const char *argv[])
   /* Default values for command line options.  */
   /* unsigned short int show_variables = 0; */
 
+  trace2_cmd_verb("sh-i18n--envsubst");
+
   switch (argc)
 	{
 	case 1:
diff --git a/submodule.c b/submodule.c
index 6415cc5580..acc96e133b 100644
--- a/submodule.c
+++ b/submodule.c
@@ -1374,11 +1374,12 @@ int fetch_populated_submodules(struct repository *r,
 	/* default value, "--submodule-prefix" and its value are added later */
 
 	calculate_changed_submodule_paths(r);
-	run_processes_parallel(max_parallel_jobs,
-			       get_next_submodule,
-			       fetch_start_failure,
-			       fetch_finish,
-			       &spf);
+	run_processes_parallel_tr2(max_parallel_jobs,
+				   get_next_submodule,
+				   fetch_start_failure,
+				   fetch_finish,
+				   &spf,
+				   "submodule", "parallel/fetch");
 
 	argv_array_clear(&spf.args);
 out:
diff --git a/t/helper/test-parse-options.c b/t/helper/test-parse-options.c
index 47fee660b8..5318681739 100644
--- a/t/helper/test-parse-options.c
+++ b/t/helper/test-parse-options.c
@@ -2,6 +2,7 @@
 #include "cache.h"
 #include "parse-options.h"
 #include "string-list.h"
+#include "trace2.h"
 
 static int boolean = 0;
 static int integer = 0;
@@ -153,6 +154,8 @@ int cmd__parse_options(int argc, const char **argv)
 	int i;
 	int ret = 0;
 
+	trace2_cmd_verb("_parse_");
+
 	argc = parse_options(argc, (const char **)argv, prefix, options, usage, 0);
 
 	if (length_cb.called) {
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index bfb195b1a8..85d1f812fe 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "test-tool.h"
+#include "trace2.h"
 
 struct test_cmd {
 	const char *name;
@@ -78,6 +79,8 @@ int cmd_main(int argc, const char **argv)
 		if (!strcmp(cmds[i].name, argv[1])) {
 			argv++;
 			argc--;
+			trace2_cmd_verb(cmds[i].name);
+			trace2_cmd_list_config();
 			return cmds[i].fn(argc, argv);
 		}
 	}
diff --git a/t/t0001-init.sh b/t/t0001-init.sh
index 42a263cada..5e27604b24 100755
--- a/t/t0001-init.sh
+++ b/t/t0001-init.sh
@@ -93,6 +93,7 @@ test_expect_success 'No extra GIT_* on alias scripts' '
 		sed -n \
 			-e "/^GIT_PREFIX=/d" \
 			-e "/^GIT_TEXTDOMAINDIR=/d" \
+			-e "/^GIT_TR2_PARENT/d" \
 			-e "/^GIT_/s/=.*//p" |
 		sort
 	EOF
diff --git a/trace2.c b/trace2.c
new file mode 100644
index 0000000000..72af89ba73
--- /dev/null
+++ b/trace2.c
@@ -0,0 +1,809 @@
+#include "cache.h"
+#include "config.h"
+#include "json-writer.h"
+#include "quote.h"
+#include "run-command.h"
+#include "sigchain.h"
+#include "thread-utils.h"
+#include "version.h"
+#include "trace2/tr2_cfg.h"
+#include "trace2/tr2_dst.h"
+#include "trace2/tr2_sid.h"
+#include "trace2/tr2_tgt.h"
+#include "trace2/tr2_tls.h"
+#include "trace2/tr2_verb.h"
+
+static int trace2_enabled;
+
+static int tr2_next_child_id; /* modify under lock */
+static int tr2_next_exec_id; /* modify under lock */
+static int tr2_next_repo_id = 1; /* modify under lock. zero is reserved */
+
+/*
+ * A table of the builtin TRACE2 targets.  Each of these may be independently
+ * enabled or disabled.  Each TRACE2 API method will try to write an event to
+ * *each* of the enabled targets.
+ */
+static struct tr2_tgt *tr2_tgt_builtins[] =
+{
+	&tr2_tgt_normal,
+	&tr2_tgt_perf,
+	&tr2_tgt_event,
+	NULL
+};
+
+#define for_each_builtin(j, tgt_j)			\
+	for (j = 0, tgt_j = tr2_tgt_builtins[j];	\
+	     tgt_j;					\
+	     j++, tgt_j = tr2_tgt_builtins[j])
+
+#define for_each_wanted_builtin(j, tgt_j)	\
+	for_each_builtin(j, tgt_j)		\
+	if (tr2_dst_trace_want(tgt_j->pdst))
+
+/*
+ * Force (rather than lazily) initialize any of the requested
+ * builtin TRACE2 targets at startup (and before we've seen an
+ * actual TRACE2 event call) so we can see if we need to setup
+ * the TR2 and TLS machinery.
+ *
+ * Return the number of builtin targets enabled.
+ */
+static int tr2_tgt_want_builtins(void)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	int sum = 0;
+
+	for_each_builtin(j, tgt_j) {
+		if (tgt_j->pfn_init())
+			sum++;
+	}
+
+	return sum;
+}
+
+/*
+ * Properly terminate each builtin target.  Give each target
+ * a chance to write a summary event and/or flush if necessary
+ * and then close the fd.
+ */
+static void tr2_tgt_disable_builtins(void)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	for_each_builtin(j, tgt_j) {
+		tgt_j->pfn_term();
+	}
+}
+
+static int tr2main_exit_code;
+
+/*
+ * Our atexit routine should run after everything has finished.
+ *
+ * Note that events generated here might not actually appear if
+ * we are writing to fd 1 or 2 and our atexit routine runs after
+ * the pager's atexit routine (since it closes them to shutdown
+ * the pipes).
+ */
+static void tr2main_atexit_handler(void)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	/*
+	 * Clear any unbalanced regions so that our atexit message
+	 * does not appear nested.  This improves the appearance of
+	 * the trace output if someone calls die(), for example.
+	 */
+	tr2tls_pop_unwind_self();
+	
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_atexit)
+			tgt_j->pfn_atexit(us_elapsed_absolute,
+					  tr2main_exit_code);
+	}
+
+	tr2_tgt_disable_builtins();
+
+	tr2tls_release();
+	tr2_sid_release();
+	tr2_verb_release();
+	tr2_cfg_free_patterns();
+
+	trace2_enabled = 0;
+}
+
+static void tr2main_signal_handler(int signo)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_signal)
+			tgt_j->pfn_signal(us_elapsed_absolute, signo);
+	}
+
+	sigchain_pop(signo);
+	raise(signo);
+}
+
+void trace2_initialize_fl(const char *file, int line)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (trace2_enabled)
+		return;
+
+	if (!tr2_tgt_want_builtins())
+		return;
+	trace2_enabled = 1;
+
+	tr2_sid_get();
+
+	atexit(tr2main_atexit_handler);
+	sigchain_push(SIGPIPE, tr2main_signal_handler);
+	tr2tls_init();
+
+	/*
+	 * Emit 'version' message on each active builtin target.
+	 */
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_version_fl)
+			tgt_j->pfn_version_fl(file, line);
+	}
+}
+
+int trace2_is_enabled(void)
+{
+	return trace2_enabled;
+}
+
+void trace2_cmd_start_fl(const char *file, int line, const char **argv)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_start_fl)
+			tgt_j->pfn_start_fl(file, line, argv);
+	}
+}
+
+int trace2_cmd_exit_fl(const char *file, int line, int code)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	code &= 0xff;
+
+	if (!trace2_enabled)
+		return code;
+
+	tr2main_exit_code = code;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_exit_fl)
+			tgt_j->pfn_exit_fl(file, line, us_elapsed_absolute, code);
+	}
+
+	return code;
+}
+
+void trace2_cmd_error_va_fl(const char *file, int line,
+			    const char *fmt, va_list ap)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	/*
+	 * We expect each target function to treat 'ap' as constant
+	 * and use va_copy (because an 'ap' can only be walked once).
+	 */
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_error_va_fl)
+			tgt_j->pfn_error_va_fl(file, line, fmt, ap);
+	}
+}
+
+void trace2_cmd_path_fl(const char *file, int line, const char *pathname)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_command_path_fl)
+			tgt_j->pfn_command_path_fl(file, line, pathname);
+	}
+}
+
+void trace2_cmd_verb_fl(const char *file, int line, const char *command_verb)
+{
+	struct tr2_tgt *tgt_j;
+	const char *hierarchy;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	tr2_verb_append_hierarchy(command_verb);
+	hierarchy = tr2_verb_get_hierarchy();
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_command_verb_fl)
+			tgt_j->pfn_command_verb_fl(file, line, command_verb,
+						   hierarchy);
+	}
+}
+
+void trace2_cmd_subverb_fl(const char *file, int line,
+			   const char *command_subverb)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_command_subverb_fl)
+			tgt_j->pfn_command_subverb_fl(file, line,
+						      command_subverb);
+	}
+}
+
+void trace2_cmd_alias_fl(const char *file, int line,
+			 const char *alias, const char **argv)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_alias_fl)
+			tgt_j->pfn_alias_fl(file, line, alias, argv);
+	}
+}
+
+void trace2_cmd_list_config_fl(const char *file, int line)
+{
+	if (!trace2_enabled)
+		return;
+
+	tr2_cfg_list_config_fl(file, line);
+}
+
+void trace2_cmd_set_config_fl(const char *file, int line,
+			      const char *key, const char *value)
+{
+	if (!trace2_enabled)
+		return;
+
+	tr2_cfg_set_fl(file, line, key, value);
+}
+
+void trace2_child_start_fl(const char *file, int line,
+			   struct child_process *cmd)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	cmd->trace2_child_id = tr2tls_locked_increment(&tr2_next_child_id);
+	cmd->trace2_child_us_start = us_now;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_child_start_fl)
+			tgt_j->pfn_child_start_fl(file, line,
+						  us_elapsed_absolute, cmd);
+	}
+}
+
+void trace2_child_exit_fl(const char *file, int line,
+			  struct child_process *cmd,
+			  int child_exit_code)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+	uint64_t us_elapsed_child;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+	
+	if (cmd->trace2_child_us_start)
+		us_elapsed_child = us_now - cmd->trace2_child_us_start;
+	else
+		us_elapsed_child = 0;
+	
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_child_exit_fl)
+			tgt_j->pfn_child_exit_fl(
+				file, line, us_elapsed_absolute,
+				cmd->trace2_child_id,
+				cmd->pid,
+				child_exit_code, us_elapsed_child);
+	}
+}
+
+int trace2_exec_fl(const char *file, int line,
+		   const char *exe, const char **argv)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	int exec_id;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return -1;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	exec_id = tr2tls_locked_increment(&tr2_next_exec_id);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_exec_fl)
+			tgt_j->pfn_exec_fl(file, line, us_elapsed_absolute,
+					   exec_id, exe, argv);
+	}
+
+	return exec_id;
+}
+
+void trace2_exec_result_fl(const char *file, int line, int exec_id, int code)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_exec_result_fl)
+			tgt_j->pfn_exec_result_fl(file, line,
+						  us_elapsed_absolute,
+						  exec_id, code);
+	}
+}
+
+void trace2_thread_start_fl(const char *file, int line,
+			    const char *thread_name)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return;
+
+	if (tr2tls_is_main_thread())
+	{
+		/*
+		 * We should only be called from the new thread's thread-proc,
+		 * so this is technically a bug.  But in those cases where the
+		 * main thread also runs the thread-proc function (or when we
+		 * are built with threading disabled), we need to allow it.
+		 *
+		 * Convert this call to a region-enter so the nesting looks
+		 * looks correct.  
+		 */
+		trace2_region_enter_printf_fl(file, line, NULL, NULL, NULL,
+					      "thread-proc on main: %s",
+					      thread_name);
+		return;
+	}
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	tr2tls_create_self(thread_name);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_thread_start_fl)
+			tgt_j->pfn_thread_start_fl(file, line,
+						   us_elapsed_absolute);
+	}
+}
+
+void trace2_thread_exit_fl(const char *file, int line)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+	uint64_t us_elapsed_thread;
+
+	if (!trace2_enabled)
+		return;
+
+	if (tr2tls_is_main_thread())
+	{
+		/*
+		 * We should only be called from the exiting thread's thread-proc,
+		 * so this is technically a bug.  But in those cases where the
+		 * main thread also runs the thread-proc function (or when we
+		 * are built with threading disabled), we need to allow it.
+		 *
+		 * Convert this call to a region-leave so the nesting looks
+		 * looks correct.  
+		 */
+		trace2_region_leave_printf_fl(file, line, NULL, NULL, NULL,
+					      "thread-proc on main");
+		return;
+	}
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	/*
+	 * Clear any unbalanced regions and then get the relative time
+	 * for the outer-most region (which we pushed when the thread
+	 * started).  This gives us the run time of the thread.
+	 */
+	tr2tls_pop_unwind_self();
+	us_elapsed_thread = tr2tls_region_elasped_self(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_thread_exit_fl)
+			tgt_j->pfn_thread_exit_fl(file, line,
+						  us_elapsed_absolute,
+						  us_elapsed_thread);
+	}
+
+	tr2tls_unset_self();
+}
+
+void trace2_def_param_fl(const char *file, int line,
+			 const char *param, const char *value)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_param_fl)
+			tgt_j->pfn_param_fl(file, line, param, value);
+	}
+}
+
+void trace2_def_repo_fl(const char *file, int line,
+			struct repository *repo)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+
+	if (!trace2_enabled)
+		return;
+
+	if (repo->trace2_repo_id)
+		return;
+
+	repo->trace2_repo_id = tr2tls_locked_increment(&tr2_next_repo_id);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_repo_fl)
+			tgt_j->pfn_repo_fl(file, line, repo);
+	}
+}
+
+void trace2_region_enter_printf_va_fl(const char *file, int line,
+				      const char *category,
+				      const char *label,
+				      const struct repository *repo,
+				      const char *fmt, va_list ap)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	/*
+	 * Print the region-enter message at the current nesting
+	 * (indentation) level and then push a new level.
+	 *
+	 * We expect each target function to treat 'ap' as constant
+	 * and use va_copy.
+	 */
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_region_enter_printf_va_fl)
+			tgt_j->pfn_region_enter_printf_va_fl(
+				file, line, us_elapsed_absolute,
+				category, label, repo, fmt, ap);
+	}
+
+	tr2tls_push_self(us_now);
+}
+
+void trace2_region_enter_fl(const char *file, int line,
+			    const char *category,
+			    const char *label,
+			    const struct repository *repo)
+{
+	trace2_region_enter_printf_va_fl(file, line,
+					 category, label, repo,
+					 NULL, NULL);
+}
+
+void trace2_region_enter_printf_fl(const char *file, int line,
+				   const char *category,
+				   const char *label,
+				   const struct repository *repo,
+				   const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_region_enter_printf_va_fl(file, line,
+					 category, label, repo,
+					 fmt, ap);
+	va_end(ap);
+}
+
+#ifndef HAVE_VARIADIC_MACROS
+void trace2_region_enter_printf(const char *category, const char *label,
+				const struct repository *repo,
+				const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_region_enter_printf_va_fl(NULL, 0,
+					 category, label, repo,
+					 fmt, ap);
+	va_end(ap);
+}
+#endif
+
+void trace2_region_leave_printf_va_fl(const char *file, int line,
+				      const char *category,
+				      const char *label,
+				      const struct repository *repo,
+				      const char *fmt, va_list ap)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+	uint64_t us_elapsed_region;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	/*
+	 * Get the elapsed time in the current region before we
+	 * pop it off the stack.  Pop the stack.  And then print
+	 * the perf message at the new (shallower) level so that
+	 * it lines up with the corresponding push/enter.
+	 */
+	us_elapsed_region = tr2tls_region_elasped_self(us_now);
+	
+	tr2tls_pop_self();
+
+	/*
+	 * We expect each target function to treat 'ap' as constant
+	 * and use va_copy.
+	 */
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_region_leave_printf_va_fl)
+			tgt_j->pfn_region_leave_printf_va_fl(
+				file, line, us_elapsed_absolute,
+				us_elapsed_region,
+				category, label, repo,
+				fmt, ap);
+	}
+}
+
+void trace2_region_leave_fl(const char *file, int line,
+			    const char *category,
+			    const char *label,
+			    const struct repository *repo)
+{
+	trace2_region_leave_printf_va_fl(file, line,
+					 category, label, repo,
+					 NULL, NULL);
+}
+
+void trace2_region_leave_printf_fl(const char *file, int line,
+				   const char *category,
+				   const char *label,
+				   const struct repository *repo,
+				   const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_region_leave_printf_va_fl(file, line,
+					 category, label, repo,
+					 fmt, ap);
+	va_end(ap);
+}
+
+#ifndef HAVE_VARIADIC_MACROS
+void trace2_region_leave_printf(const char *category, const char *label,
+				const struct repository *repo,
+				const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_region_leave_printf_va_fl(NULL, 0,
+					 category, label, repo,
+					 fmt, ap);
+	va_end(ap);
+}
+#endif
+
+void trace2_data_string_fl(const char *file, int line,
+			   const char *category,
+			   const struct repository *repo,
+			   const char *key,
+			   const char *value)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+	uint64_t us_elapsed_region;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+	us_elapsed_region = tr2tls_region_elasped_self(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_data_fl)
+			tgt_j->pfn_data_fl(file, line, us_elapsed_absolute,
+					   us_elapsed_region,
+					   category, repo, key, value);
+	}
+}
+
+void trace2_data_intmax_fl(const char *file, int line,
+			   const char *category,
+			   const struct repository *repo,
+			   const char *key,
+			   intmax_t value)
+{
+	struct strbuf buf_string = STRBUF_INIT;
+
+	if (!trace2_enabled)
+		return;
+
+	strbuf_addf(&buf_string, "%"PRIdMAX, value);
+	trace2_data_string_fl(file, line, category, repo, key, buf_string.buf);
+	strbuf_release(&buf_string);
+}
+
+void trace2_data_json_fl(const char *file, int line,
+			 const char *category,
+			 const struct repository *repo,
+			 const char *key,
+			 const struct json_writer *value)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+	uint64_t us_elapsed_region;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+	us_elapsed_region = tr2tls_region_elasped_self(us_now);
+
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_data_fl)
+			tgt_j->pfn_data_json_fl(file, line, us_elapsed_absolute,
+						us_elapsed_region,
+						category, repo, key, value);
+	}
+}
+
+void trace2_printf_va_fl(const char *file, int line,
+			 const char *fmt, va_list ap)
+{
+	struct tr2_tgt *tgt_j;
+	int j;
+	uint64_t us_now;
+	uint64_t us_elapsed_absolute;
+
+	if (!trace2_enabled)
+		return;
+
+	us_now = getnanotime() / 1000;
+	us_elapsed_absolute = tr2tls_absolute_elapsed(us_now);
+
+	/*
+	 * We expect each target function to treat 'ap' as constant
+	 * and use va_copy.
+	 */
+	for_each_wanted_builtin(j, tgt_j) {
+		if (tgt_j->pfn_printf_va_fl)
+			tgt_j->pfn_printf_va_fl(file, line, us_elapsed_absolute,
+						fmt, ap);
+	}
+}
+
+void trace2_printf_fl(const char *file, int line,
+		      const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_printf_va_fl(file, line, fmt, ap);
+	va_end(ap);
+}
+
+#ifndef HAVE_VARIADIC_MACROS
+void trace2_printf(const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	trace2_printf_va_fl(NULL, 0, fmt, ap);
+	va_end(ap);
+}
+#endif
diff --git a/trace2.h b/trace2.h
new file mode 100644
index 0000000000..7ab9f355f3
--- /dev/null
+++ b/trace2.h
@@ -0,0 +1,390 @@
+#ifndef TRACE2_H
+#define TRACE2_H
+
+struct child_process;
+struct repository;
+struct json_writer;
+
+/*
+ * The public TRACE2 routines are grouped into the following groups:
+ *
+ * [] trace2_initialize -- initialization.
+ * [] trace2_cmd_*      -- emit command/control messages.
+ * [] trace2_child*     -- emit child start/stop messages.
+ * [] trace2_exec*      -- emit exec start/stop messages.
+ * [] trace2_thread*    -- emit thread start/stop messages.
+ * [] trace2_def*       -- emit definition/parameter mesasges.
+ * [] trace2_region*    -- emit region nesting messages.
+ * [] trace2_data*      -- emit region/thread/repo data messages.
+ * [] trace2_printf*    -- legacy trace[1] messages.
+ */
+
+/*
+ * Initialize TRACE2 tracing facility if any of the builtin TRACE2
+ * targets are enabled in the environment.  Emits a 'version' event.
+ *
+ * Cleanup/Termination is handled automatically by a registered
+ * atexit() routine.
+ */
+void trace2_initialize_fl(const char *file, int line);
+
+#define trace2_initialize() trace2_initialize_fl(__FILE__, __LINE__)
+
+/*
+ * Return true if trace2 is enabled.
+ */
+int trace2_is_enabled(void);
+
+/*
+ * Emit a 'start' event with the original (unmodified) argv.
+ */
+void trace2_cmd_start_fl(const char *file, int line, const char **argv);
+
+#define trace2_cmd_start(argv) trace2_cmd_start_fl(__FILE__, __LINE__, (argv))
+
+/*
+ * Emit an 'exit' event.
+ *
+ * Write the exit-code that will be passed to exit() or returned
+ * from main().
+ *
+ * Use this prior to actually calling exit().
+ * See "#define exit()" in git-compat-util.h
+ */
+int trace2_cmd_exit_fl(const char *file, int line, int code);
+
+#define trace2_cmd_exit(code) (trace2_cmd_exit_fl(__FILE__, __LINE__, (code)))
+
+/*
+ * Emit an 'error' event.
+ *
+ * Write an error message to the TRACE2 targets.
+ */
+void trace2_cmd_error_va_fl(const char *file, int line,
+			    const char *fmt, va_list ap);
+
+#define trace2_cmd_error_va(fmt, ap) \
+	trace2_cmd_error_va_fl(__FILE__, __LINE__, (fmt), (ap))
+
+/*
+ * Emit a 'pathname' event with the canonical pathname of the current process
+ * This gives post-processors a simple field to identify the command without
+ * having to parse the argv.  For example, to distinguish invocations from
+ * installed versus debug executables.
+ */
+void trace2_cmd_path_fl(const char *file, int line, const char *pathname);
+
+#define trace2_cmd_path(p) \
+	trace2_cmd_path_fl(__FILE__, __LINE__, (p))
+
+/*
+ * Emit a 'cmd_verb' event with the canonical name of the (usually)
+ * builtin command.  This gives post-processors a simple field
+ * to identify the command verb without having to parse the argv.
+ */
+void trace2_cmd_verb_fl(const char *file, int line, const char *command_verb);
+
+#define trace2_cmd_verb(v) \
+	trace2_cmd_verb_fl(__FILE__, __LINE__, (v))
+
+/*
+ * Emit a 'cmd_subverb' event to further describe the command being run.
+ * For example, "checkout" can checkout a single file or can checkout a
+ * different branch.  This gives post-processors a simple field to compare
+ * equivalent commands without having to parse the argv.
+ */
+void trace2_cmd_subverb_fl(const char *file, int line,
+			   const char *command_subverb);
+
+#define trace2_cmd_subverb(sv) \
+	trace2_cmd_subverb_fl(__FILE__, __LINE__, (sv))
+
+/*
+ * Emit an 'alias' expansion event.
+ */
+void trace2_cmd_alias_fl(const char *file, int line,
+			 const char *alias, const char **argv);
+
+#define trace2_cmd_alias(alias, argv) \
+	trace2_cmd_alias_fl(__FILE__, __LINE__, (alias), (argv))
+
+/*
+ * Emit one or more 'def_param' events for "interesting" configuration
+ * settings.
+ *
+ * The environment variable "GIT_TR2_CONFIG_PARAMS" can be set to a
+ * list of patterns considered important.  For example:
+ *
+ *    GIT_TR2_CONFIG_PARAMS="core.*,remote.*.url"
+ *
+ * Note: this routine does a read-only iteration on the config data
+ * (using read_early_config()), so it must not be called until enough
+ * of the process environment has been established.  This includes the
+ * location of the git and worktree directories, expansion of any "-c"
+ * and "-C" command line options, and etc.
+ */
+void trace2_cmd_list_config_fl(const char *file, int line);
+
+#define trace2_cmd_list_config() \
+	trace2_cmd_list_config_fl(__FILE__, __LINE__)
+
+/*
+ * Emit a "def_param" event for the given config key/value pair IF
+ * we consider the key to be "interesting".
+ *
+ * Use this for new/updated config settings created/updated after
+ * trace2_cmd_list_config() is called.
+ */
+void trace2_cmd_set_config_fl(const char *file, int line,
+			      const char *key, const char *value);
+
+#define trace2_cmd_set_config(k, v) \
+	trace2_cmd_set_config_fl(__FILE__, __LINE__, (k), (v))
+
+/*
+ * Emit a 'child_start' event prior to spawning a child process.
+ *
+ * Before calling optionally set "cmd->trace2_child_class" to a string
+ * describing the type of the child process.  For example, "editor" or
+ * "pager".
+ */
+void trace2_child_start_fl(const char *file, int line,
+			   struct child_process *cmd);
+
+#define trace2_child_start(cmd) \
+	trace2_child_start_fl(__FILE__, __LINE__, (cmd))
+
+/*
+ * Emit a 'child_exit' event after the child process completes. 
+ */
+void trace2_child_exit_fl(const char *file, int line,
+			  struct child_process *cmd,
+			  int child_exit_code);
+
+#define trace2_child_exit(cmd, code) \
+	trace2_child_exit_fl(__FILE__, __LINE__, (cmd), (code))
+
+/*
+ * Emit an 'exec' event prior to calling one of exec(), execv(),
+ * execvp(), and etc.  On Unix-derived systems, this will be the
+ * last event emitted for the current process, unless the exec
+ * fails.  On Windows, exec() behaves like 'child_start' and a
+ * waitpid(), so additional events may be emitted.
+ *
+ * Returns the "exec_id".
+ */
+int trace2_exec_fl(const char *file, int line,
+		   const char *exe, const char **argv);
+
+#define trace2_exec(exe, argv) \
+	trace2_exec_fl(__FILE__, __LINE__, (exe), (argv))
+
+/*
+ * Emit an 'exec_result' when possible.  On Unix-derived systems,
+ * this should be called after exec() returns (which only happens
+ * when there is an error starting the new process).  On Windows,
+ * this should be called after the waitpid().
+ *
+ * The "exec_id" should be the value returned from trace2_exec().
+ */
+void trace2_exec_result_fl(const char *file, int line, int exec_id, int code);
+
+#define trace2_exec_result(id, code) \
+	trace2_exec_result_fl(__FILE__, __LINE__, (id), (code))
+
+/*
+ * Emit a 'thread_start' event.  This must be called from inside the
+ * thread-proc to set up the trace2 TLS data for the thread.
+ *
+ * Thread names should be descriptive, like "preload_index".
+ * Thread names will be decorated with an instance number automatically.
+ */
+void trace2_thread_start_fl(const char *file, int line,
+				 const char *thread_name);
+
+#define trace2_thread_start(thread_name) \
+	trace2_thread_start_fl(__FILE__, __LINE__, (thread_name))
+
+/*
+ * Emit a 'thread_exit' event.  This must be called from inside the
+ * thread-proc to report thread-specific data and cleanup TLS data
+ * for the thread.
+ */
+void trace2_thread_exit_fl(const char *file, int line);
+
+#define trace2_thread_exit() trace2_thread_exit_fl(__FILE__, __LINE__)
+
+/*
+ * Emit a 'param' event.
+ *
+ * Write a "<param> = <value>" pair describing some aspect of the
+ * run such as an important configuration setting or command line
+ * option that significantly changes command behavior.
+ */ 
+void trace2_def_param_fl(const char *file, int line,
+			 const char *param, const char *value);
+
+#define trace2_def_param(param, value) \
+	trace2_def_param_fl(__FILE__, __LINE__, (param), (value))
+
+/*
+ * Tell trace2 about a newly instantiated repo object and assign
+ * a trace2-repo-id to be used in subsequent activity events.
+ *
+ * Emits a 'worktree' event for this repo instance.
+ */
+void trace2_def_repo_fl(const char *file, int line,
+			struct repository *repo);
+
+#define trace2_def_repo(repo) \
+	trace2_def_repo_fl(__FILE__, __LINE__, repo)
+
+/*
+ * Emit a 'region_enter' event for <category>.<label> with optional
+ * repo-id and printf message.
+ *
+ * Enter a new nesting level on the current thread and remember the
+ * current time.  This controls the indenting of all subsequent events
+ * on this thread.
+ */
+void trace2_region_enter_fl(const char *file, int line,
+			    const char *category,
+			    const char *label,
+			    const struct repository *repo);
+
+#define trace2_region_enter(category, label, repo) \
+	trace2_region_enter_fl( \
+		__FILE__, __LINE__, (category), (label), (repo))
+
+void trace2_region_enter_printf_va_fl(const char *file, int line,
+				      const char *category,
+				      const char *label,
+				      const struct repository *repo,
+				      const char *fmt, va_list ap);
+
+#define trace2_region_enter_printf_va(category, label, repo, fmt, ap) \
+	trace2_region_enter_printf_va_fl( \
+		__FILE__, __LINE__, (category), (label), (repo), (fmt), (ap))
+
+void trace2_region_enter_printf_fl(const char *file, int line,
+				   const char *category, const char *label,
+				   const struct repository *repo,
+				   const char *fmt, ...);
+
+#ifdef HAVE_VARIADIC_MACROS
+#define trace2_region_enter_printf(category, label, repo, ...) \
+	trace2_region_enter_printf_fl( \
+		__FILE__, __LINE__, (category), (label), (repo), __VA_ARGS__)
+#else
+__attribute__((format (region_enter_printf, 4, 5)))
+void trace2_region_enter_printf(const char *category, const char *label,
+				const struct repository *repo,
+				const char *fmt, ...);
+#endif
+
+/*
+ * Emit a 'region_leave' event for <category>.<label> with optional
+ * repo-id and printf message.
+ *
+ * Leave current nesting level and report the elapsed time spent
+ * in this nesting level.
+ */
+void trace2_region_leave_fl(const char *file, int line,
+			    const char *category,
+			    const char *label,
+			    const struct repository *repo);
+
+#define trace2_region_leave(category, label, repo) \
+	trace2_region_leave_fl( \
+		__FILE__, __LINE__, (category), (label), (repo))
+
+void trace2_region_leave_printf_va_fl(const char *file, int line,
+				      const char *category,
+				      const char *label,
+				      const struct repository *repo,
+				      const char *fmt, va_list ap);
+
+#define trace2_region_leave_printf_va(category, label, repo, fmt, ap) \
+	trace2_region_leave_printf_va_fl( \
+		__FILE__, __LINE__, (category), (label), (repo), (fmt), (ap))
+
+void trace2_region_leave_printf_fl(const char *file, int line,
+				   const char *category, const char *label,
+				   const struct repository *repo,
+				   const char *fmt, ...);
+
+#ifdef HAVE_VARIADIC_MACROS
+#define trace2_region_leave_printf(category, label, repo, ...) \
+	trace2_region_leave_printf_fl( \
+		__FILE__, __LINE__, (category), (label), (repo), __VA_ARGS__)
+#else
+__attribute__((format (region_leave_printf, 4, 5)))
+void trace2_region_leave_printf(const char *category, const char *label,
+				const struct repository *repo,
+				const char *fmt, ...);
+#endif
+
+/*
+ * Emit a key-value pair 'data' event of the form <category>.<key> = <value>.
+ * This event implicitly contains information about thread, nesting region,
+ * and optional repo-id.
+ *
+ * On event-based TRACE2 targets, this generates a 'data' event suitable
+ * for post-processing.  On printf-based TRACE2 targets, this is converted
+ * into a fixed-format printf message.
+ */
+void trace2_data_string_fl(const char *file, int line,
+			   const char *category,
+			   const struct repository *repo,
+			   const char *key,
+			   const char *value);
+
+#define trace2_data_string(category, repo, key, value) \
+	trace2_data_string_fl( \
+		__FILE__, __LINE__, (category), (repo), (key), (value))
+
+void trace2_data_intmax_fl(const char *file, int line,
+			   const char *category,
+			   const struct repository *repo,
+			   const char *key,
+			   intmax_t value);
+
+#define trace2_data_intmax(category, repo, key, value) \
+	trace2_data_intmax_fl( \
+		__FILE__, __LINE__, (category), (repo), (key), (value))
+
+void trace2_data_json_fl(const char *file, int line,
+			 const char *category,
+			 const struct repository *repo,
+			 const char *key,
+			 const struct json_writer *jw);
+
+#define trace2_data_json(category, repo, key, value) \
+	trace2_data_json_fl( \
+		__FILE__, __LINE__, (category), (repo), (key), (value))
+
+/*
+ * Emit a 'printf' event.
+ *
+ * Write an arbitrary formatted message to the TRACE2 targets.  These
+ * text messages should be considered as human-readable strings without
+ * any formatting guidelines.  Post-processors may choose to ignore
+ * them.
+ */
+void trace2_printf_va_fl(const char *file, int line,
+			 const char *fmt, va_list ap);
+
+#define trace2_printf_va(fmt, ap) \
+	trace2_printf_va_fl(__FILE__, __LINE__, (fmt), (ap))
+
+void trace2_printf_fl(const char *file, int line, const char *fmt, ...);
+
+#ifdef HAVE_VARIADIC_MACROS
+#define trace2_printf(...) \
+	trace2_printf_fl(__FILE__, __LINE__, __VA_ARGS__)
+#else
+__attribute__((format (printf, 1, 2)))
+void trace2_printf(const char *fmt, ...);
+#endif
+
+#endif /* TRACE2_H */
diff --git a/trace2/tr2_cfg.c b/trace2/tr2_cfg.c
new file mode 100644
index 0000000000..7b8291010f
--- /dev/null
+++ b/trace2/tr2_cfg.c
@@ -0,0 +1,92 @@
+#include "cache.h"
+#include "config.h"
+#include "tr2_cfg.h"
+
+#define TR2_ENVVAR_CFG_PARAM "GIT_TR2_CONFIG_PARAMS"
+
+static struct strbuf **tr2_cfg_patterns;
+static int tr2_cfg_count_patterns;
+static int tr2_cfg_loaded;
+
+/*
+ * Parse a string containing a comma-delimited list of config keys
+ * or wildcard patterns into a list of strbufs.
+ */
+static int tr2_cfg_load_patterns(void)
+{
+	struct strbuf **s;
+	const char *envvar;
+
+	if (tr2_cfg_loaded)
+		return tr2_cfg_count_patterns;
+	tr2_cfg_loaded = 1;
+
+	envvar = getenv(TR2_ENVVAR_CFG_PARAM);
+	if (!envvar || !*envvar)
+		return tr2_cfg_count_patterns;
+
+	tr2_cfg_patterns = strbuf_split_buf(envvar, strlen(envvar), ',', -1);
+	for (s = tr2_cfg_patterns; *s; s++) {
+		struct strbuf *buf = *s;
+
+		if (buf->len && buf->buf[buf->len - 1] == ',')
+			strbuf_setlen(buf, buf->len - 1);
+		strbuf_trim_trailing_newline(*s);
+		strbuf_trim(*s);
+	}
+
+	tr2_cfg_count_patterns = s - tr2_cfg_patterns;
+	return tr2_cfg_count_patterns;
+}
+
+void tr2_cfg_free_patterns(void)
+{
+	if (tr2_cfg_patterns)
+		strbuf_list_free(tr2_cfg_patterns);
+	tr2_cfg_count_patterns = 0;
+	tr2_cfg_loaded = 0;
+}
+
+struct tr2_cfg_data
+{
+	const char *file;
+	int line;
+};
+
+/*
+ * See if the given config key matches any of our patterns of interest.
+ */
+static int tr2_cfg_cb(const char *key, const char *value, void *d)
+{
+	struct strbuf **s;
+	struct tr2_cfg_data *data = (struct tr2_cfg_data *)d;
+
+	for (s = tr2_cfg_patterns; *s; s++) {
+		struct strbuf *buf = *s;
+		int wm = wildmatch(buf->buf, key, WM_CASEFOLD);
+		if (wm == WM_MATCH) {
+			trace2_def_param_fl(data->file, data->line, key, value);
+			return 0;
+		}
+	}
+
+	return 0;
+}
+
+void tr2_cfg_list_config_fl(const char *file, int line)
+{
+	struct tr2_cfg_data data = { file, line };
+
+	if (tr2_cfg_load_patterns() > 0)
+		read_early_config(tr2_cfg_cb, &data);
+}
+
+void tr2_cfg_set_fl(const char *file, int line,
+		    const char *key, const char *value)
+{
+	struct tr2_cfg_data data = { file, line };
+
+	if (tr2_cfg_load_patterns() > 0)
+		tr2_cfg_cb(key, value, &data);
+}
+
diff --git a/trace2/tr2_cfg.h b/trace2/tr2_cfg.h
new file mode 100644
index 0000000000..0e7ca4eace
--- /dev/null
+++ b/trace2/tr2_cfg.h
@@ -0,0 +1,19 @@
+#ifndef TR2_CFG_H
+#define TR2_CFG_H
+
+/*
+ * Iterate over all config settings and emit 'def_param' events for the
+ * "interesting" ones to TRACE2.
+ */
+void tr2_cfg_list_config_fl(const char *file, int line);
+
+/*
+ * Emit a "def_param" event for the given key/value pair IF we consider
+ * the key to be "interesting".
+ */
+void tr2_cfg_set_fl(const char *file, int line,
+		    const char *key, const char *value);
+
+void tr2_cfg_free_patterns(void);
+
+#endif /* TR2_CFG_H */
diff --git a/trace2/tr2_dst.c b/trace2/tr2_dst.c
new file mode 100644
index 0000000000..485b2f8851
--- /dev/null
+++ b/trace2/tr2_dst.c
@@ -0,0 +1,90 @@
+#include "cache.h"
+#include "trace2/tr2_dst.h"
+
+void tr2_dst_trace_disable(struct tr2_dst *dst)
+{
+	if (dst->need_close)
+		close(dst->fd);
+	dst->fd = 0;
+	dst->initialized = 1;
+	dst->need_close = 0;
+}
+
+int tr2_dst_get_trace_fd(struct tr2_dst *dst)
+{
+	const char *trace;
+
+	/* don't open twice */
+	if (dst->initialized)
+		return dst->fd;
+
+	trace = getenv(dst->env_var_name);
+
+	if (!trace || !strcmp(trace, "") ||
+	    !strcmp(trace, "0") || !strcasecmp(trace, "false"))
+		dst->fd = 0;
+	else if (!strcmp(trace, "1") || !strcasecmp(trace, "true"))
+		dst->fd = STDERR_FILENO;
+	else if (strlen(trace) == 1 && isdigit(*trace))
+		dst->fd = atoi(trace);
+	else if (is_absolute_path(trace)) {
+		int fd = open(trace, O_WRONLY | O_APPEND | O_CREAT, 0666);
+		if (fd == -1) {
+			/*
+			 * Silently eat the error and disable tracing on this
+			 * destination.  This will cause us to lose events,
+			 * but that is better than causing a stream of warnings
+			 * for each git command they run.
+			 *
+			 * warning("could not open '%s' for tracing: %s",
+			 *         trace, strerror(errno));
+			 */
+			tr2_dst_trace_disable(dst);
+		} else {
+			dst->fd = fd;
+			dst->need_close = 1;
+		}
+	} else {
+		warning("unknown trace value for '%s': %s\n"
+			"         If you want to trace into a file, then please set %s\n"
+			"         to an absolute pathname (starting with /)",
+			dst->env_var_name, trace, dst->env_var_name);
+		tr2_dst_trace_disable(dst);
+	}
+
+	dst->initialized = 1;
+	return dst->fd;
+}
+
+int tr2_dst_trace_want(struct tr2_dst *dst)
+{
+	return !!tr2_dst_get_trace_fd(dst);
+}
+
+void tr2_dst_write_line(struct tr2_dst *dst, struct strbuf *buf_line)
+{
+	strbuf_complete_line(buf_line); /* ensure final NL on buffer */
+
+	/*
+	 * We do not use write_in_full() because we do not want
+	 * a short-write to try again.  We are using O_APPEND mode
+	 * files and the kernel handles the atomic seek+write. If
+	 * another thread or git process is concurrently writing to
+	 * this fd or file, our remainder-write may not be contiguous
+	 * with our initial write of this message.  And that will
+	 * confuse readers.  So just don't bother.
+	 *
+	 * It is assumed that TRACE2 messages are short enough that
+	 * the system can write them in 1 attempt and we won't see
+	 * a short-write.
+	 *
+	 * If we get an IO error, just close the trace dst.
+	 */
+
+	if (write(tr2_dst_get_trace_fd(dst),
+		  buf_line->buf, buf_line->len) < 0) {
+		warning("unable to write trace for '%s': %s",
+			dst->env_var_name, strerror(errno));
+		tr2_dst_trace_disable(dst);
+	}
+}
diff --git a/trace2/tr2_dst.h b/trace2/tr2_dst.h
new file mode 100644
index 0000000000..3eea3f2d1a
--- /dev/null
+++ b/trace2/tr2_dst.h
@@ -0,0 +1,34 @@
+#ifndef TR2_DST_H
+#define TR2_DST_H
+
+struct tr2_dst {
+	const char * const env_var_name;
+	int fd;
+	unsigned int initialized : 1;
+	unsigned int  need_close : 1;
+};
+
+/*
+ * Disable TRACE2 on the destination.  In TRACE2 a destination (DST)
+ * wraps a file descriptor; it is associated with a TARGET which
+ * defines the formatting.
+ */
+void tr2_dst_trace_disable(struct tr2_dst *dst);
+
+/*
+ * Return the file descriptor for the DST.
+ * If 0, the dst is closed or disabled.
+ */
+int tr2_dst_get_trace_fd(struct tr2_dst *dst);
+
+/*
+ * Return true if the DST is opened for writing.
+ */
+int tr2_dst_trace_want(struct tr2_dst *dst);
+
+/*
+ * Write a single line/message to the trace file.
+ */
+void tr2_dst_write_line(struct tr2_dst *dst, struct strbuf *buf_line);
+
+#endif /* TR2_DST_H */
diff --git a/trace2/tr2_sid.c b/trace2/tr2_sid.c
new file mode 100644
index 0000000000..43e8a3e184
--- /dev/null
+++ b/trace2/tr2_sid.c
@@ -0,0 +1,67 @@
+#include "cache.h"
+#include "trace2/tr2_sid.h"
+
+#define TR2_ENVVAR_PARENT_SID "GIT_TR2_PARENT_SID"
+
+static struct strbuf tr2sid_buf = STRBUF_INIT;
+static int tr2sid_nr_git_parents;
+
+/*
+ * Compute a "unique" session id (SID) for the current process.  This allows
+ * all events from this process to have a single label (much like a PID).
+ *
+ * Export this into our environment so that all child processes inherit it.
+ *
+ * If we were started by another git instance, use our parent's SID as a
+ * prefix.  (This lets us track parent/child relationships even if there
+ * is an intermediate shell process.)
+ *
+ * Additionally, count the number of nested git processes.
+ */
+static void tr2_sid_compute(void)
+{
+	uint64_t us_now;
+	const char *parent_sid;
+
+	if (tr2sid_buf.len)
+		return;
+
+	parent_sid = getenv(TR2_ENVVAR_PARENT_SID);
+	if (parent_sid && *parent_sid) {
+		const char *p;
+		for (p = parent_sid; *p; p++)
+			if (*p == '/')
+				tr2sid_nr_git_parents++;
+
+		strbuf_addstr(&tr2sid_buf, parent_sid);
+		strbuf_addch(&tr2sid_buf, '/');
+		tr2sid_nr_git_parents++;
+	}
+
+	us_now = getnanotime() / 1000;
+	strbuf_addf(&tr2sid_buf, "%"PRIuMAX"-%"PRIdMAX,
+		    (uintmax_t)us_now, (intmax_t)getpid());
+
+	setenv(TR2_ENVVAR_PARENT_SID, tr2sid_buf.buf, 1);
+}
+
+const char *tr2_sid_get(void)
+{
+	if (!tr2sid_buf.len)
+		tr2_sid_compute();
+
+	return tr2sid_buf.buf;
+}
+
+int tr2_sid_depth(void)
+{
+	if (!tr2sid_buf.len)
+		tr2_sid_compute();
+
+	return tr2sid_nr_git_parents;
+}
+
+void tr2_sid_release(void)
+{
+	strbuf_release(&tr2sid_buf);
+}
diff --git a/trace2/tr2_sid.h b/trace2/tr2_sid.h
new file mode 100644
index 0000000000..9bef321708
--- /dev/null
+++ b/trace2/tr2_sid.h
@@ -0,0 +1,18 @@
+#ifndef TR2_SID_H
+#define TR2_SID_H
+
+/*
+ * Get our session id. Compute if necessary.
+ */
+const char *tr2_sid_get(void);
+
+/*
+ * Get our process depth.  A top-level git process invoked from the
+ * command line will have depth=0.  A child git process will have
+ * depth=1 and so on.
+ */
+int tr2_sid_depth(void);
+
+void tr2_sid_release(void);
+
+#endif /* TR2_SID_H */
diff --git a/trace2/tr2_tbuf.c b/trace2/tr2_tbuf.c
new file mode 100644
index 0000000000..452b4d0520
--- /dev/null
+++ b/trace2/tr2_tbuf.c
@@ -0,0 +1,32 @@
+#include "cache.h"
+#include "tr2_tbuf.h"
+
+void tr2_tbuf_local_time(struct tr2_tbuf *tb)
+{
+	struct timeval tv;
+	struct tm tm;
+	time_t secs;
+
+	gettimeofday(&tv, NULL);
+	secs = tv.tv_sec;
+	localtime_r(&secs, &tm);
+
+	xsnprintf(tb->buf, sizeof(tb->buf), "%02d:%02d:%02d.%06ld",
+		  tm.tm_hour, tm.tm_min, tm.tm_sec, (long)tv.tv_usec);
+}
+
+void tr2_tbuf_utc_time(struct tr2_tbuf *tb)
+{
+	struct timeval tv;
+	struct tm tm;
+	time_t secs;
+
+	gettimeofday(&tv, NULL);
+	secs = tv.tv_sec;
+	gmtime_r(&secs, &tm);
+
+	xsnprintf(tb->buf, sizeof(tb->buf), "%4d-%02d-%02d %02d:%02d:%02d.%06ld",
+		  tm.tm_year+1900, tm.tm_mon+1, tm.tm_mday,
+		  tm.tm_hour, tm.tm_min, tm.tm_sec, (long)tv.tv_usec);
+}
+
diff --git a/trace2/tr2_tbuf.h b/trace2/tr2_tbuf.h
new file mode 100644
index 0000000000..9cdefa3957
--- /dev/null
+++ b/trace2/tr2_tbuf.h
@@ -0,0 +1,23 @@
+#ifndef TR2_TBUF_H
+#define TR2_TBUF_H
+
+/*
+ * A simple wrapper around a fixed buffer to avoid C syntax
+ * quirks and the need to pass around an additional size_t
+ * argument.
+ */
+struct tr2_tbuf {
+	char buf[32];
+};
+
+/*
+ * Fill buffer with formatted local time string.
+ */
+void tr2_tbuf_local_time(struct tr2_tbuf *tb);
+
+/*
+ * Fill buffer with formatted UTC time string.
+ */
+void tr2_tbuf_utc_time(struct tr2_tbuf *tb);
+
+#endif /* TR2_TBUF_H */
diff --git a/trace2/tr2_tgt.h b/trace2/tr2_tgt.h
new file mode 100644
index 0000000000..4fdf253b57
--- /dev/null
+++ b/trace2/tr2_tgt.h
@@ -0,0 +1,126 @@
+#ifndef TR2_TGT_H
+#define TR2_TGT_H
+
+/*
+ * Function prototypes for a TRACE2 "target" vtable.
+ */
+
+typedef int (tr2_tgt_init_t)(void);
+typedef void (tr2_tgt_term_t)(void);
+
+typedef void (tr2_tgt_evt_version_fl_t)
+	(const char *file, int line);
+
+typedef void (tr2_tgt_evt_start_fl_t)
+	(const char *file, int line, const char **argv);
+typedef void (tr2_tgt_evt_exit_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute, int code);
+typedef void (tr2_tgt_evt_signal_t)
+	(uint64_t us_elapsed_absolute, int signo);
+typedef void (tr2_tgt_evt_atexit_t)
+	(uint64_t us_elapsed_absolute, int code);
+
+typedef void (tr2_tgt_evt_error_va_fl_t)
+	(const char *file, int line, const char *fmt, va_list ap);
+
+typedef void (tr2_tgt_evt_command_path_fl_t)
+	(const char *file, int line, const char *command_path);
+typedef void (tr2_tgt_evt_command_verb_fl_t)
+	(const char *file, int line, const char *command_verb,
+	 const char *hierarchy);
+typedef void (tr2_tgt_evt_command_subverb_fl_t)
+	(const char *file, int line, const char *command_subverb);
+
+typedef void (tr2_tgt_evt_alias_fl_t)
+	(const char *file, int line, const char *alias, const char **argv);
+
+typedef void (tr2_tgt_evt_child_start_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 const struct child_process *cmd);
+typedef void (tr2_tgt_evt_child_exit_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute, int cid,
+	 int pid, int code, uint64_t us_elapsed_child);
+
+typedef void (tr2_tgt_evt_thread_start_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute);
+typedef void (tr2_tgt_evt_thread_exit_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 uint64_t us_elapsed_thread);
+
+typedef void (tr2_tgt_evt_exec_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 int exec_id, const char *exe, const char **argv);
+typedef void (tr2_tgt_evt_exec_result_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 int exec_id, int code);
+
+typedef void (tr2_tgt_evt_param_fl_t)
+	(const char *file, int line, const char *param, const char *value);
+
+typedef void (tr2_tgt_evt_repo_fl_t)
+	(const char *file, int line, const struct repository *repo);
+
+typedef void (tr2_tgt_evt_region_enter_printf_va_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 const char *category, const char *label, const struct repository *repo,
+	 const char *fmt, va_list ap);
+typedef void (tr2_tgt_evt_region_leave_printf_va_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 uint64_t us_elapsed_region, const char *category, const char *label,
+	 const struct repository *repo, const char *fmt, va_list ap);
+
+typedef void (tr2_tgt_evt_data_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 uint64_t us_elapsed_region, const char *category,
+	 const struct repository *repo, const char *key, const char *value);
+typedef void (tr2_tgt_evt_data_json_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 uint64_t us_elapsed_region, const char *category,
+	 const struct repository *repo,
+	 const char *key, const struct json_writer *value);
+
+typedef void (tr2_tgt_evt_printf_va_fl_t)
+	(const char *file, int line, uint64_t us_elapsed_absolute,
+	 const char *fmt, va_list ap);
+
+/*
+ * "vtable" for a TRACE2 target.  Use NULL if a target does not want
+ * to emit that message.
+ */
+struct tr2_tgt
+{
+	struct tr2_dst                          *pdst;
+
+	tr2_tgt_init_t                          *pfn_init;
+	tr2_tgt_term_t                          *pfn_term;
+
+	tr2_tgt_evt_version_fl_t                *pfn_version_fl;
+	tr2_tgt_evt_start_fl_t                  *pfn_start_fl;
+	tr2_tgt_evt_exit_fl_t                   *pfn_exit_fl;
+	tr2_tgt_evt_signal_t                    *pfn_signal;
+	tr2_tgt_evt_atexit_t                    *pfn_atexit;
+	tr2_tgt_evt_error_va_fl_t               *pfn_error_va_fl;
+	tr2_tgt_evt_command_path_fl_t           *pfn_command_path_fl;
+	tr2_tgt_evt_command_verb_fl_t           *pfn_command_verb_fl;
+	tr2_tgt_evt_command_subverb_fl_t        *pfn_command_subverb_fl;
+	tr2_tgt_evt_alias_fl_t                  *pfn_alias_fl;
+	tr2_tgt_evt_child_start_fl_t            *pfn_child_start_fl;
+	tr2_tgt_evt_child_exit_fl_t             *pfn_child_exit_fl;
+	tr2_tgt_evt_thread_start_fl_t           *pfn_thread_start_fl;
+	tr2_tgt_evt_thread_exit_fl_t            *pfn_thread_exit_fl;
+	tr2_tgt_evt_exec_fl_t                   *pfn_exec_fl;
+	tr2_tgt_evt_exec_result_fl_t            *pfn_exec_result_fl;
+	tr2_tgt_evt_param_fl_t                  *pfn_param_fl;
+	tr2_tgt_evt_repo_fl_t                   *pfn_repo_fl;
+	tr2_tgt_evt_region_enter_printf_va_fl_t *pfn_region_enter_printf_va_fl;
+	tr2_tgt_evt_region_leave_printf_va_fl_t *pfn_region_leave_printf_va_fl;
+	tr2_tgt_evt_data_fl_t                   *pfn_data_fl;
+	tr2_tgt_evt_data_json_fl_t              *pfn_data_json_fl;
+	tr2_tgt_evt_printf_va_fl_t              *pfn_printf_va_fl;
+};
+
+extern struct tr2_tgt tr2_tgt_event;
+extern struct tr2_tgt tr2_tgt_normal;
+extern struct tr2_tgt tr2_tgt_perf;
+
+#endif /* TR2_TGT_H */
diff --git a/trace2/tr2_tgt_event.c b/trace2/tr2_tgt_event.c
new file mode 100644
index 0000000000..495171ae6d
--- /dev/null
+++ b/trace2/tr2_tgt_event.c
@@ -0,0 +1,606 @@
+#include "cache.h"
+#include "config.h"
+#include "json-writer.h"
+#include "run-command.h"
+#include "version.h"
+#include "trace2/tr2_dst.h"
+#include "trace2/tr2_tbuf.h"
+#include "trace2/tr2_sid.h"
+#include "trace2/tr2_tgt.h"
+#include "trace2/tr2_tls.h"
+
+static struct tr2_dst tr2dst_event  = { "GIT_TR2_EVENT", 0, 0, 0 };
+
+/*
+ * The version number of the JSON data generated by the EVENT target
+ * in this source file.  Update this if you make a significant change
+ * to the JSON fields or message structure.  You probably do not need
+ * to update this if you just add another call to one of the existing
+ * TRACE2 API methods.
+ */
+#define TR2_EVENT_VERSION "1"
+
+/*
+ * Region nesting limit for messages written to the event target.
+ *
+ * The "region_enter" and "region_leave" messages (especially recursive
+ * messages such as those produced while diving the worktree or index)
+ * are primarily intended for the performance target during debugging.
+ *
+ * Some of the outer-most messages, however, may be of interest to the
+ * event target.  Set this environment variable to a larger integer for
+ * more detail in the event target.
+ */
+#define TR2_ENVVAR_EVENT_NESTING "GIT_TR2_EVENT_NESTING"
+static int tr2env_event_nesting_wanted = 2;
+
+/*
+ * Set this environment variable to true to omit the <time>, <file>, and
+ * <line> fields from most events.
+ */
+#define TR2_ENVVAR_EVENT_BRIEF "GIT_TR2_EVENT_BRIEF"
+static int tr2env_event_brief;
+
+static int fn_init(void)
+{
+	int want = tr2_dst_trace_want(&tr2dst_event);
+	int want_nesting;
+	int want_brief;
+	char *nesting;
+	char *brief;
+
+	if (!want)
+		return want;
+
+	nesting = getenv(TR2_ENVVAR_EVENT_NESTING);
+	if (nesting && ((want_nesting = atoi(nesting)) > 0))
+		tr2env_event_nesting_wanted = want_nesting;
+
+	brief = getenv(TR2_ENVVAR_EVENT_BRIEF);
+	if (brief && ((want_brief = atoi(brief)) > 0))
+		tr2env_event_brief = want_brief;
+
+	return want;
+}
+
+static void fn_term(void)
+{
+	tr2_dst_trace_disable(&tr2dst_event);
+}
+
+/*
+ * Append common key-value pairs to the currently open JSON object.
+ *     "event:"<event_name>"
+ *      "sid":"<sid>"
+ *   "thread":"<thread_name>"
+ *     "time":"<time>"
+ *     "file":"<filename>"
+ *     "line":<line_number>
+ *     "repo":<repo_id>
+ */
+static void event_fmt_prepare(const char *event_name,
+			      const char *file, int line,
+			      const struct repository *repo,
+			      struct json_writer *jw)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	struct tr2_tbuf tb_now;
+
+	jw_object_string(jw, "event", event_name);
+	jw_object_string(jw, "sid", tr2_sid_get());
+	jw_object_string(jw, "thread", ctx->thread_name.buf);
+
+	/*
+	 * In brief mode, only emit <time> on these 2 event types.
+	 */
+	if (!tr2env_event_brief ||
+	    !strcmp(event_name, "version") ||
+	    !strcmp(event_name, "atexit")) {
+		tr2_tbuf_utc_time(&tb_now);
+		jw_object_string(jw, "time", tb_now.buf);
+	}
+
+	if (!tr2env_event_brief && file && *file) {
+		jw_object_string(jw, "file", file);
+		jw_object_intmax(jw, "line", line);
+	}
+
+	if (repo)
+		jw_object_intmax(jw, "repo", repo->trace2_repo_id);
+}
+
+static void fn_version_fl(const char *file, int line)
+{
+	const char *event_name = "version";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "evt", TR2_EVENT_VERSION);
+	jw_object_string(&jw, "exe", git_version_string);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_start_fl(const char *file, int line, const char **argv)
+{
+	const char *event_name = "start";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_inline_begin_array(&jw, "argv");
+	jw_array_argv(&jw, argv);
+	jw_end(&jw);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_exit_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute, int code)
+{
+	const char *event_name = "exit";
+	struct json_writer jw = JSON_WRITER_INIT;
+	double t_abs = (double)us_elapsed_absolute / 1000000.0;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_double(&jw, "t_abs", 6, t_abs);
+	jw_object_intmax(&jw, "code", code);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_signal(uint64_t us_elapsed_absolute, int signo)
+{
+	const char *event_name = "signal";
+	struct json_writer jw = JSON_WRITER_INIT;
+	double t_abs = (double)us_elapsed_absolute / 1000000.0;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, __FILE__, __LINE__, NULL, &jw);
+	jw_object_double(&jw, "t_abs", 6, t_abs);
+	jw_object_intmax(&jw, "signo", signo);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_atexit(uint64_t us_elapsed_absolute, int code)
+{
+	const char *event_name = "atexit";
+	struct json_writer jw = JSON_WRITER_INIT;
+	double t_abs = (double)us_elapsed_absolute / 1000000.0;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, __FILE__, __LINE__, NULL, &jw);
+	jw_object_double(&jw, "t_abs", 6, t_abs);
+	jw_object_intmax(&jw, "code", code);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void maybe_add_string_va(struct json_writer *jw,
+				const char *field_name,
+				const char *fmt, va_list ap)
+{
+	if (fmt && *fmt && ap) {
+		va_list copy_ap;
+		struct strbuf buf = STRBUF_INIT;
+
+		va_copy(copy_ap, ap);
+		strbuf_vaddf(&buf, fmt, copy_ap);
+		va_end(copy_ap);
+
+		jw_object_string(jw, field_name, buf.buf);
+		strbuf_release(&buf);
+		return;
+	}
+
+	if (fmt && *fmt) {
+		jw_object_string(jw, field_name, fmt);
+		return;
+	}
+}
+
+static void fn_error_va_fl(const char *file, int line,
+			   const char *fmt, va_list ap)
+{
+	const char *event_name = "error";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	maybe_add_string_va(&jw, "msg", fmt, ap);
+	/*
+	 * Also emit the format string as a field in case
+	 * post-processors want to aggregate common error
+	 * messages by type without argument fields (such
+	 * as pathnames or branch names) cluttering it up.
+	 */
+	if (fmt && *fmt)
+		jw_object_string(&jw, "fmt", fmt);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_command_path_fl(const char *file, int line,
+			       const char *pathname)
+{
+	const char *event_name = "cmd_path";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "path", pathname);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_command_verb_fl(const char *file, int line,
+			       const char *command_verb,
+			       const char *verb_hierarchy)
+{
+	const char *event_name = "cmd_verb";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "name", command_verb);
+	if (verb_hierarchy && *verb_hierarchy)
+		jw_object_string(&jw, "hierarchy", verb_hierarchy);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_command_subverb_fl(const char *file, int line,
+			       const char *command_subverb)
+{
+	const char *event_name = "cmd_subverb";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "name", command_subverb);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_alias_fl(const char *file, int line,
+			const char *alias, const char **argv)
+{
+	const char *event_name = "alias";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "alias", alias);
+	jw_object_inline_begin_array(&jw, "argv");
+	jw_array_argv(&jw, argv);
+	jw_end(&jw);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_child_start_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      const struct child_process *cmd)
+{
+	const char *event_name = "child_start";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_intmax(&jw, "child_id", cmd->trace2_child_id);
+	if (cmd->trace2_hook_name) {
+		jw_object_string(&jw, "child_class", "hook");
+		jw_object_string(&jw, "hook_name", cmd->trace2_hook_name);
+	} else {
+		const char *child_class = ((cmd->trace2_child_class) ?
+					   cmd->trace2_child_class : "?");
+		jw_object_string(&jw, "child_class", child_class);
+	}
+	if (cmd->dir)
+		jw_object_string(&jw, "cd", cmd->dir);
+	jw_object_bool(&jw, "use_shell", cmd->use_shell);
+	jw_object_inline_begin_array(&jw, "argv");
+	if (cmd->git_cmd)
+		jw_array_string(&jw, "git");
+	jw_array_argv(&jw, cmd->argv);
+	jw_end(&jw);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_child_exit_fl(const char *file, int line,
+			     uint64_t us_elapsed_absolute,
+			     int cid, int pid, int code,
+			     uint64_t us_elapsed_child)
+{
+	const char *event_name = "child_exit";
+	struct json_writer jw = JSON_WRITER_INIT;
+	double t_rel = (double)us_elapsed_child / 1000000.0;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_intmax(&jw, "child_id", cid);
+	jw_object_intmax(&jw, "pid", pid);
+	jw_object_intmax(&jw, "code", code);
+	jw_object_double(&jw, "t_rel", 6, t_rel);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+
+	jw_release(&jw);
+}
+
+static void fn_thread_start_fl(const char *file, int line,
+			       uint64_t us_elapsed_absolute)
+{
+	const char *event_name = "thread_start";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_thread_exit_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      uint64_t us_elapsed_thread)
+{
+	const char *event_name = "thread_exit";
+	struct json_writer jw = JSON_WRITER_INIT;
+	double t_rel = (double)us_elapsed_thread / 1000000.0;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_double(&jw, "t_rel", 6, t_rel);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_exec_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute,
+		       int exec_id, const char *exe, const char **argv)
+{
+	const char *event_name = "exec";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_intmax(&jw, "exec_id", exec_id);
+	if (exe)
+		jw_object_string(&jw, "exe", exe);
+	jw_object_inline_begin_array(&jw, "argv");
+	jw_array_argv(&jw, argv);
+	jw_end(&jw);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_exec_result_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      int exec_id, int code)
+{
+	const char *event_name = "exec_result";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_intmax(&jw, "exec_id", exec_id);
+	jw_object_intmax(&jw, "code", code);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_param_fl(const char *file, int line,
+			const char *param, const char *value)
+{
+	const char *event_name = "def_param";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, NULL, &jw);
+	jw_object_string(&jw, "param", param);
+	jw_object_string(&jw, "value", value);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_repo_fl(const char *file, int line,
+		       const struct repository *repo)
+{
+	const char *event_name = "def_repo";
+	struct json_writer jw = JSON_WRITER_INIT;
+
+	jw_object_begin(&jw, 0);
+	event_fmt_prepare(event_name, file, line, repo, &jw);
+	jw_object_string(&jw, "worktree", repo->worktree);
+	jw_end(&jw);
+
+	tr2_dst_write_line(&tr2dst_event, &jw.json);
+	jw_release(&jw);
+}
+
+static void fn_region_enter_printf_va_fl(const char *file, int line,
+					 uint64_t us_elapsed_absolute,
+					 const char *category,
+					 const char *label,
+					 const struct repository *repo,
+					 const char *fmt, va_list ap)
+{
+	const char *event_name = "region_enter";
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	if (ctx->nr_open_regions <= tr2env_event_nesting_wanted) {
+		struct json_writer jw = JSON_WRITER_INIT;
+
+		jw_object_begin(&jw, 0);
+		event_fmt_prepare(
+			event_name, file, line, repo, &jw);
+		jw_object_intmax(&jw, "nesting", ctx->nr_open_regions);
+		if (category)
+			jw_object_string(&jw, "category", category);
+		if (label)
+			jw_object_string(&jw, "label", label);
+		maybe_add_string_va(&jw, "msg", fmt, ap);
+		jw_end(&jw);
+
+		tr2_dst_write_line(&tr2dst_event, &jw.json);
+		jw_release(&jw);
+	}
+}
+
+static void fn_region_leave_printf_va_fl(const char *file, int line,
+					 uint64_t us_elapsed_absolute,
+					 uint64_t us_elapsed_region,
+					 const char *category,
+					 const char *label,
+					 const struct repository *repo,
+					 const char *fmt, va_list ap)
+{
+	const char *event_name = "region_leave";
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	if (ctx->nr_open_regions <= tr2env_event_nesting_wanted) {
+		struct json_writer jw = JSON_WRITER_INIT;
+		double t_rel = (double)us_elapsed_region / 1000000.0;
+
+		jw_object_begin(&jw, 0);
+		event_fmt_prepare(event_name, file, line, repo, &jw);
+		jw_object_double(&jw, "t_rel", 6, t_rel);
+		jw_object_intmax(&jw, "nesting", ctx->nr_open_regions);
+		if (category)
+			jw_object_string(&jw, "category", category);
+		if (label)
+			jw_object_string(&jw, "label", label);
+		maybe_add_string_va(&jw, "msg", fmt, ap);
+		jw_end(&jw);
+
+		tr2_dst_write_line(&tr2dst_event, &jw.json);
+		jw_release(&jw);
+	}
+}
+
+static void fn_data_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute,
+		       uint64_t us_elapsed_region,
+		       const char *category,
+		       const struct repository *repo,
+		       const char *key,
+		       const char *value)
+{
+	const char *event_name = "data";
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	if (ctx->nr_open_regions <= tr2env_event_nesting_wanted) {
+		struct json_writer jw = JSON_WRITER_INIT;
+		double t_abs = (double)us_elapsed_absolute / 1000000.0;
+		double t_rel = (double)us_elapsed_region / 1000000.0;
+
+		jw_object_begin(&jw, 0);
+		event_fmt_prepare(event_name, file, line, repo, &jw);
+		jw_object_double(&jw, "t_abs", 6, t_abs);
+		jw_object_double(&jw, "t_rel", 6, t_rel);
+		jw_object_intmax(&jw, "nesting", ctx->nr_open_regions);
+		jw_object_string(&jw, "category", category);
+		jw_object_string(&jw, "key", key);
+		jw_object_string(&jw, "value", value);
+		jw_end(&jw);
+
+		tr2_dst_write_line(&tr2dst_event, &jw.json);
+		jw_release(&jw);
+	}
+}
+
+static void fn_data_json_fl(const char *file, int line,
+			    uint64_t us_elapsed_absolute,
+			    uint64_t us_elapsed_region,
+			    const char *category,
+			    const struct repository *repo,
+			    const char *key,
+			    const struct json_writer *value)
+{
+	const char *event_name = "data_json";
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	if (ctx->nr_open_regions <= tr2env_event_nesting_wanted) {
+		struct json_writer jw = JSON_WRITER_INIT;
+		double t_abs = (double)us_elapsed_absolute / 1000000.0;
+		double t_rel = (double)us_elapsed_region / 1000000.0;
+
+		jw_object_begin(&jw, 0);
+		event_fmt_prepare(event_name, file, line, repo, &jw);
+		jw_object_double(&jw, "t_abs", 6, t_abs);
+		jw_object_double(&jw, "t_rel", 6, t_rel);
+		jw_object_intmax(&jw, "nesting", ctx->nr_open_regions);
+		jw_object_string(&jw, "category", category);
+		jw_object_string(&jw, "key", key);
+		jw_object_sub_jw(&jw, "value", value);
+		jw_end(&jw);
+
+		tr2_dst_write_line(&tr2dst_event, &jw.json);
+		jw_release(&jw);
+	}
+}
+
+struct tr2_tgt tr2_tgt_event =
+{
+	&tr2dst_event,
+
+	fn_init,
+	fn_term,
+
+	fn_version_fl,
+	fn_start_fl,
+	fn_exit_fl,
+	fn_signal,
+	fn_atexit,
+	fn_error_va_fl,
+	fn_command_path_fl,
+	fn_command_verb_fl,
+	fn_command_subverb_fl,
+	fn_alias_fl,
+	fn_child_start_fl,
+	fn_child_exit_fl,
+	fn_thread_start_fl,
+	fn_thread_exit_fl,
+	fn_exec_fl,
+	fn_exec_result_fl,
+	fn_param_fl,
+	fn_repo_fl,
+	fn_region_enter_printf_va_fl,
+	fn_region_leave_printf_va_fl,
+	fn_data_fl,
+	fn_data_json_fl,
+	NULL, /* printf */
+};
diff --git a/trace2/tr2_tgt_normal.c b/trace2/tr2_tgt_normal.c
new file mode 100644
index 0000000000..602eba5112
--- /dev/null
+++ b/trace2/tr2_tgt_normal.c
@@ -0,0 +1,331 @@
+#include "cache.h"
+#include "config.h"
+#include "run-command.h"
+#include "quote.h"
+#include "version.h"
+#include "trace2/tr2_dst.h"
+#include "trace2/tr2_tbuf.h"
+#include "trace2/tr2_tgt.h"
+#include "trace2/tr2_tls.h"
+
+static struct tr2_dst tr2dst_normal = { "GIT_TR2", 0, 0, 0 };
+
+/*
+ * Set this environment variable to true to omit the "<time> <file>:<line>"
+ * fields from each line written to the builtin normal target.
+ *
+ * Unit tests may want to use this to help with testing.
+ */
+#define TR2_ENVVAR_NORMAL_BRIEF "GIT_TR2_BRIEF"
+static int tr2env_normal_brief;
+
+#define TR2FMT_NORMAL_FL_WIDTH       (50)
+
+static int fn_init(void)
+{
+	int want = tr2_dst_trace_want(&tr2dst_normal);
+	int want_brief;
+	char *brief;
+
+	if (!want)
+		return want;
+
+	brief = getenv(TR2_ENVVAR_NORMAL_BRIEF);
+	if (brief && *brief &&
+	    ((want_brief = git_parse_maybe_bool(brief)) != -1))
+		tr2env_normal_brief = want_brief;
+
+	return want;
+}
+
+static void fn_term(void)
+{
+	tr2_dst_trace_disable(&tr2dst_normal);
+}
+
+static void normal_fmt_prepare(const char *file, int line, struct strbuf *buf)
+{
+	strbuf_setlen(buf,0);
+
+	if (!tr2env_normal_brief) {
+		struct tr2_tbuf tb_now;
+
+		tr2_tbuf_local_time(&tb_now);
+		strbuf_addstr(buf, tb_now.buf);
+		strbuf_addch(buf, ' ');
+
+		if (file && *file)
+			strbuf_addf(buf, "%s:%d ", file, line);
+		while (buf->len < TR2FMT_NORMAL_FL_WIDTH)
+			strbuf_addch(buf, ' ');
+	}
+}
+
+static void normal_io_write_fl(const char *file, int line,
+			       const struct strbuf *buf_payload)
+{
+	struct strbuf buf_line = STRBUF_INIT;
+
+	normal_fmt_prepare(file, line, &buf_line);
+	strbuf_addbuf(&buf_line, buf_payload);
+	tr2_dst_write_line(&tr2dst_normal, &buf_line);
+	strbuf_release(&buf_line);
+}
+
+static void fn_version_fl(const char *file, int line)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "version %s", git_version_string);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_start_fl(const char *file, int line, const char **argv)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, "start ");
+	sq_quote_argv_pretty(&buf_payload, argv);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exit_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute, int code)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+	double elapsed = (double)us_elapsed_absolute / 1000000.0;
+
+	strbuf_addf(&buf_payload, "exit elapsed:%.6f code:%d",
+		    elapsed, code);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_signal(uint64_t us_elapsed_absolute, int signo)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+	double elapsed = (double)us_elapsed_absolute / 1000000.0;
+
+	strbuf_addf(&buf_payload, "signal elapsed:%.6f code:%d",
+		    elapsed, signo);
+	normal_io_write_fl(__FILE__, __LINE__, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_atexit(uint64_t us_elapsed_absolute, int code)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+	double elapsed = (double)us_elapsed_absolute / 1000000.0;
+
+	strbuf_addf(&buf_payload, "atexit elapsed:%.6f code:%d",
+		    elapsed, code);
+	normal_io_write_fl(__FILE__, __LINE__, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void maybe_append_string_va(struct strbuf *buf,
+				   const char *fmt, va_list ap)
+{
+	if (fmt && *fmt && ap) {
+		va_list copy_ap;
+
+		va_copy(copy_ap, ap);
+		strbuf_vaddf(buf, fmt, copy_ap);
+		va_end(copy_ap);
+		return;
+	}
+
+	if (fmt && *fmt) {
+		strbuf_addstr(buf, fmt);
+		return;
+	}
+}
+
+static void fn_error_va_fl(const char *file, int line,
+			   const char *fmt, va_list ap)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, "error ");
+	maybe_append_string_va(&buf_payload, fmt, ap);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_path_fl(const char *file, int line,
+			       const char *pathname)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "cmd_path %s", pathname);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_verb_fl(const char *file, int line,
+			       const char *command_verb,
+			       const char *verb_hierarchy)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "cmd_verb %s", command_verb);
+	if (verb_hierarchy && *verb_hierarchy)
+		strbuf_addf(&buf_payload, " (%s)", verb_hierarchy);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_subverb_fl(const char *file, int line,
+			       const char *command_subverb)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "cmd_subverb %s", command_subverb);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_alias_fl(const char *file, int line, const char *alias,
+			const char **argv)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "alias %s ->", alias);
+	sq_quote_argv_pretty(&buf_payload, argv);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_child_start_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      const struct child_process *cmd)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "child_start[%d] ", cmd->trace2_child_id);
+
+	if (cmd->dir) {
+		strbuf_addstr(&buf_payload, " cd");
+		sq_quote_buf_pretty(&buf_payload, cmd->dir);
+		strbuf_addstr(&buf_payload, "; ");
+	}
+
+	/*
+	 * TODO if (cmd->env) { Consider dumping changes to environment. }
+	 * See trace_add_env() in run-command.c as used by original trace.c
+	 */
+
+	if (cmd->git_cmd)
+		strbuf_addstr(&buf_payload, "git");
+	sq_quote_argv_pretty(&buf_payload, cmd->argv);
+
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_child_exit_fl(const char *file, int line,
+			     uint64_t us_elapsed_absolute,
+			     int cid, int pid, int code,
+			     uint64_t us_elapsed_child)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+	double elapsed = (double)us_elapsed_child / 1000000.0;
+
+	strbuf_addf(&buf_payload, "child_exit[%d] pid:%d code:%d elapsed:%.6f",
+		    cid, pid, code, elapsed);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exec_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute,
+		       int exec_id, const char *exe, const char **argv)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "exec[%d] ", exec_id);
+	if (exe)
+		strbuf_addstr(&buf_payload, exe);
+	sq_quote_argv_pretty(&buf_payload, argv);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exec_result_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      int exec_id, int code)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "exec_result[%d] code:%d", exec_id, code);
+	if (code > 0)
+		strbuf_addf(&buf_payload, " err:%s", strerror(code));
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_param_fl(const char *file, int line, const char *param,
+			const char *value)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "def_param %s=%s", param, value);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_repo_fl(const char *file, int line,
+		       const struct repository *repo)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, "worktree ");
+	sq_quote_buf_pretty(&buf_payload, repo->worktree);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_printf_va_fl(const char *file, int line,
+			    uint64_t us_elapsed_absolute,
+			    const char *fmt, va_list ap)
+{
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	maybe_append_string_va(&buf_payload, fmt, ap);
+	normal_io_write_fl(file, line, &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+struct tr2_tgt tr2_tgt_normal =
+{
+	&tr2dst_normal,
+
+	fn_init,
+	fn_term,
+
+	fn_version_fl,
+	fn_start_fl,
+	fn_exit_fl,
+	fn_signal,
+	fn_atexit,
+	fn_error_va_fl,
+	fn_command_path_fl,
+	fn_command_verb_fl,
+	fn_command_subverb_fl,
+	fn_alias_fl,
+	fn_child_start_fl,
+	fn_child_exit_fl,
+	NULL, /* thread_start */
+	NULL, /* thread_exit */
+	fn_exec_fl,
+	fn_exec_result_fl,
+	fn_param_fl,
+	fn_repo_fl,
+	NULL, /* region_enter */
+	NULL, /* region_leave */
+	NULL, /* data */
+	NULL, /* data_json */
+	fn_printf_va_fl,
+};
diff --git a/trace2/tr2_tgt_perf.c b/trace2/tr2_tgt_perf.c
new file mode 100644
index 0000000000..035855d883
--- /dev/null
+++ b/trace2/tr2_tgt_perf.c
@@ -0,0 +1,573 @@
+#include "cache.h"
+#include "config.h"
+#include "run-command.h"
+#include "quote.h"
+#include "version.h"
+#include "json-writer.h"
+#include "trace2/tr2_dst.h"
+#include "trace2/tr2_sid.h"
+#include "trace2/tr2_tbuf.h"
+#include "trace2/tr2_tgt.h"
+#include "trace2/tr2_tls.h"
+
+static struct tr2_dst tr2dst_perf   = { "GIT_TR2_PERF", 0, 0, 0 };
+
+/*
+ * Set this environment variable to true to omit the "<time> <file>:<line>"
+ * fields from each line written to the builtin performance target.
+ *
+ * Unit tests may want to use this to help with testing.
+ */
+#define TR2_ENVVAR_PERF_BRIEF "GIT_TR2_PERF_BRIEF"
+static int tr2env_perf_brief;
+
+#define TR2FMT_PERF_FL_WIDTH       (50)
+#define TR2FMT_PERF_MAX_EVENT_NAME (12)
+#define TR2FMT_PERF_REPO_WIDTH     (4)
+#define TR2FMT_PERF_CATEGORY_WIDTH (10)
+
+#define TR2_DOTS_BUFFER_SIZE (100)
+#define TR2_INDENT (2)
+#define TR2_INDENT_LENGTH(ctx) (((ctx)->nr_open_regions - 1) * TR2_INDENT)
+
+static struct strbuf dots = STRBUF_INIT;
+
+static int fn_init(void)
+{
+	int want = tr2_dst_trace_want(&tr2dst_perf);
+	int want_brief;
+	char *brief;
+
+	if (!want)
+		return want;
+
+	strbuf_addchars(&dots, '.', TR2_DOTS_BUFFER_SIZE);
+
+	brief = getenv(TR2_ENVVAR_PERF_BRIEF);
+	if (brief && *brief &&
+	    ((want_brief = git_parse_maybe_bool(brief)) != -1))
+		tr2env_perf_brief = want_brief;
+
+	return want;
+}
+
+static void fn_term(void)
+{
+	tr2_dst_trace_disable(&tr2dst_perf);
+
+	strbuf_release(&dots);
+}
+
+/*
+ * Format trace line prefix in human-readable classic format for
+ * the performance target:
+ *     "[<time> [<file>:<line>] <bar>] <nr_parents> <bar>
+ *         <thread_name> <bar> <event_name> <bar> [<repo>] <bar>
+ *         [<elapsed_absolute>] [<elapsed_relative>] <bar>
+ *         [<category>] <bar> [<dots>] "
+ */
+static void perf_fmt_prepare(
+	const char *event_name, struct tr2tls_thread_ctx *ctx,
+	const char *file, int line, const struct repository *repo,
+	uint64_t *p_us_elapsed_absolute, uint64_t *p_us_elapsed_relative,
+	const char *category, struct strbuf *buf)
+{
+	int len;
+
+	strbuf_setlen(buf,0);
+
+	if (!tr2env_perf_brief) {
+
+		struct tr2_tbuf tb_now;
+
+		tr2_tbuf_local_time(&tb_now);
+		strbuf_addstr(buf, tb_now.buf);
+		strbuf_addch(buf, ' ');
+
+		if (file && *file)
+			strbuf_addf(buf, "%s:%d ", file, line);
+		while (buf->len < TR2FMT_PERF_FL_WIDTH)
+			strbuf_addch(buf, ' ');
+
+		strbuf_addstr(buf, "| ");
+	}
+
+	strbuf_addf(buf, "d%d | ", tr2_sid_depth());
+	strbuf_addf(buf, "%-*s | %-*s | ",
+		    TR2_MAX_THREAD_NAME, ctx->thread_name.buf,
+		    TR2FMT_PERF_MAX_EVENT_NAME, event_name);
+
+	len = buf->len + TR2FMT_PERF_REPO_WIDTH;
+	if (repo)
+		strbuf_addf(buf, "r%d ", repo->trace2_repo_id);
+	while (buf->len < len)
+		strbuf_addch(buf, ' ' );
+	strbuf_addstr(buf, "| ");
+
+	if (p_us_elapsed_absolute)
+		strbuf_addf(buf, "%9.6f | ",
+			    ((double)(*p_us_elapsed_absolute)) / 1000000.0);
+	else
+		strbuf_addf(buf, "%9s | ", " ");
+
+	if (p_us_elapsed_relative)
+		strbuf_addf(buf, "%9.6f | ",
+			    ((double)(*p_us_elapsed_relative)) / 1000000.0);
+	else
+		strbuf_addf(buf, "%9s | ", " ");
+		
+	strbuf_addf(buf, "%-*s | ", TR2FMT_PERF_CATEGORY_WIDTH,
+		    (category ? category : ""));
+
+	if (ctx->nr_open_regions > 0) {
+		int len_indent = TR2_INDENT_LENGTH(ctx);
+		while (len_indent > dots.len) {
+			strbuf_addf(buf, "%s", dots.buf);
+			len_indent -= dots.len;
+		}
+		strbuf_addf(buf, "%.*s", len_indent, dots.buf);
+	}
+}
+
+static void perf_io_write_fl(
+	const char *file, int line,
+	const char *event_name,
+	const struct repository *repo,
+	uint64_t *p_us_elapsed_absolute,
+	uint64_t *p_us_elapsed_relative,
+	const char *category,
+	const struct strbuf *buf_payload)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+	struct strbuf buf_line = STRBUF_INIT;
+
+	perf_fmt_prepare(event_name, ctx, file, line, repo,
+			 p_us_elapsed_absolute, p_us_elapsed_relative,
+			 category, &buf_line);
+	strbuf_addbuf(&buf_line, buf_payload);
+	tr2_dst_write_line(&tr2dst_perf, &buf_line);
+	strbuf_release(&buf_line);
+}
+
+static void fn_version_fl(const char *file, int line)
+{
+	const char *event_name = "version";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "%s", git_version_string);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_start_fl(const char *file, int line, const char **argv)
+{
+	const char *event_name = "start";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	sq_quote_argv_pretty(&buf_payload, argv);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exit_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute, int code)
+{
+	const char *event_name = "exit";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "code:%d", code);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_signal(uint64_t us_elapsed_absolute, int signo)
+{
+	const char *event_name = "signal";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "signo:%d", signo);
+
+	perf_io_write_fl(__FILE__, __LINE__, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_atexit(uint64_t us_elapsed_absolute, int code)
+{
+	const char *event_name = "atexit";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "code:%d", code);
+
+	perf_io_write_fl(__FILE__, __LINE__, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void maybe_append_string_va(struct strbuf *buf,
+				   const char *fmt, va_list ap)
+{
+	if (fmt && *fmt && ap) {
+		va_list copy_ap;
+
+		va_copy(copy_ap, ap);
+		strbuf_vaddf(buf, fmt, copy_ap);
+		va_end(copy_ap);
+		return;
+	}
+
+	if (fmt && *fmt) {
+		strbuf_addstr(buf, fmt);
+		return;
+	}
+}
+
+static void fn_error_va_fl(const char *file, int line,
+			   const char *fmt, va_list ap)
+{
+	const char *event_name = "error";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	maybe_append_string_va(&buf_payload, fmt, ap);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_path_fl(const char *file, int line,
+			       const char *pathname)
+{
+	const char *event_name = "cmd_path";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, pathname);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_verb_fl(const char *file, int line,
+			       const char *command_verb,
+			       const char *verb_hierarchy)
+{
+	const char *event_name = "cmd_verb";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, command_verb);
+	if (verb_hierarchy && *verb_hierarchy)
+		strbuf_addf(&buf_payload, " (%s)", verb_hierarchy);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_command_subverb_fl(const char *file, int line,
+			       const char *command_subverb)
+{
+	const char *event_name = "cmd_subverb";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, command_subverb);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_alias_fl(const char *file, int line, const char *alias,
+			const char **argv)
+{
+	const char *event_name = "alias";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "alias:%s argv:", alias);
+	sq_quote_argv_pretty(&buf_payload, argv);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_child_start_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      const struct child_process *cmd)
+{
+	const char *event_name = "child_start";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	if (cmd->trace2_hook_name) {
+		strbuf_addf(&buf_payload, "[ch%d] class:hook hook:%s",
+			    cmd->trace2_child_id, cmd->trace2_hook_name);
+	} else {
+		const char *child_class = ((cmd->trace2_child_class) ?
+					   cmd->trace2_child_class : "?");
+		strbuf_addf(&buf_payload, "[ch%d] class:%s",
+			    cmd->trace2_child_id, child_class);
+	}
+
+	if (cmd->dir) {
+		strbuf_addstr(&buf_payload, " cd:");
+		sq_quote_buf_pretty(&buf_payload, cmd->dir);
+	}
+
+	strbuf_addstr(&buf_payload, " argv:");
+	if (cmd->git_cmd)
+		strbuf_addstr(&buf_payload, " git");
+	sq_quote_argv_pretty(&buf_payload, cmd->argv);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_child_exit_fl(const char *file, int line,
+			     uint64_t us_elapsed_absolute,
+			     int cid, int pid, int code,
+			     uint64_t us_elapsed_child)
+{
+	const char *event_name = "child_exit";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "[ch%d] pid:%d code:%d", cid, pid, code);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, &us_elapsed_child, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_thread_start_fl(const char *file, int line,
+				     uint64_t us_elapsed_absolute)
+{
+	const char *event_name = "thread_start";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_thread_exit_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      uint64_t us_elapsed_thread)
+{
+	const char *event_name = "thread_exit";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, &us_elapsed_thread, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exec_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute,
+		       int exec_id, const char *exe, const char **argv)
+{
+	const char *event_name = "exec";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "id:%d ", exec_id);
+	strbuf_addstr(&buf_payload, "argv:");
+	if (exe)
+		strbuf_addf(&buf_payload, " %s", exe);
+	sq_quote_argv_pretty(&buf_payload, argv);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_exec_result_fl(const char *file, int line,
+			      uint64_t us_elapsed_absolute,
+			      int exec_id, int code)
+{
+	const char *event_name = "exec_result";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "id:%d code:%d", exec_id, code);
+	if (code > 0)
+		strbuf_addf(&buf_payload, " err:%s", strerror(code));
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_param_fl(const char *file, int line,
+			const char *param, const char *value)
+{
+	const char *event_name = "def_param";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "%s:%s", param, value);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_repo_fl(const char *file, int line,
+		       const struct repository *repo)
+{
+	const char *event_name = "def_repo";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addstr(&buf_payload, "worktree:");
+	sq_quote_buf_pretty(&buf_payload, repo->worktree);
+
+	perf_io_write_fl(file, line, event_name, repo,
+			 NULL, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_region_enter_printf_va_fl(const char *file, int line,
+					 uint64_t us_elapsed_absolute,
+					 const char *category,
+					 const char *label,
+					 const struct repository *repo,
+					 const char *fmt, va_list ap)
+{
+	const char *event_name = "region_enter";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	if (label)
+		strbuf_addf(&buf_payload, "label:%s ", label);
+	maybe_append_string_va(&buf_payload, fmt, ap);
+
+	perf_io_write_fl(file, line, event_name, repo,
+			 &us_elapsed_absolute, NULL, category,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_region_leave_printf_va_fl(const char *file, int line,
+					 uint64_t us_elapsed_absolute,
+					 uint64_t us_elapsed_region,
+					 const char *category,
+					 const char *label,
+					 const struct repository *repo,
+					 const char *fmt, va_list ap)
+{
+	const char *event_name = "region_leave";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	if (label)
+		strbuf_addf(&buf_payload, "label:%s ", label);
+	maybe_append_string_va(&buf_payload, fmt, ap);
+
+	perf_io_write_fl(file, line, event_name, repo,
+			 &us_elapsed_absolute, &us_elapsed_region, category,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_data_fl(const char *file, int line,
+		       uint64_t us_elapsed_absolute,
+		       uint64_t us_elapsed_region,
+		       const char *category,
+		       const struct repository *repo,
+		       const char *key,
+		       const char *value)
+{
+	const char *event_name = "data";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "%s:%s", key, value);
+
+	perf_io_write_fl(file, line, event_name, repo,
+			 &us_elapsed_absolute, &us_elapsed_region, category,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_data_json_fl(const char *file, int line,
+			    uint64_t us_elapsed_absolute,
+			    uint64_t us_elapsed_region,
+			    const char *category,
+			    const struct repository *repo,
+			    const char *key,
+			    const struct json_writer *value)
+{
+	const char *event_name = "data_json";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	strbuf_addf(&buf_payload, "%s:%s", key, value->json.buf);
+
+	perf_io_write_fl(file, line, event_name, repo,
+			 &us_elapsed_absolute, &us_elapsed_region, category,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+static void fn_printf_va_fl(const char *file, int line,
+			    uint64_t us_elapsed_absolute,
+			    const char *fmt, va_list ap)
+{
+	const char *event_name = "printf";
+	struct strbuf buf_payload = STRBUF_INIT;
+
+	maybe_append_string_va(&buf_payload, fmt, ap);
+
+	perf_io_write_fl(file, line, event_name, NULL,
+			 &us_elapsed_absolute, NULL, NULL,
+			 &buf_payload);
+	strbuf_release(&buf_payload);
+}
+
+struct tr2_tgt tr2_tgt_perf =
+{
+	&tr2dst_perf,
+
+	fn_init,
+	fn_term,
+
+	fn_version_fl,
+	fn_start_fl,
+	fn_exit_fl,
+	fn_signal,
+	fn_atexit,
+	fn_error_va_fl,
+	fn_command_path_fl,
+	fn_command_verb_fl,
+	fn_command_subverb_fl,
+	fn_alias_fl,
+	fn_child_start_fl,
+	fn_child_exit_fl,
+	fn_thread_start_fl,
+	fn_thread_exit_fl,
+	fn_exec_fl,
+	fn_exec_result_fl,
+	fn_param_fl,
+	fn_repo_fl,
+	fn_region_enter_printf_va_fl,
+	fn_region_leave_printf_va_fl,
+	fn_data_fl,
+	fn_data_json_fl,
+	fn_printf_va_fl,
+};
diff --git a/trace2/tr2_tls.c b/trace2/tr2_tls.c
new file mode 100644
index 0000000000..f5e304428c
--- /dev/null
+++ b/trace2/tr2_tls.c
@@ -0,0 +1,164 @@
+#include "cache.h"
+#include "thread-utils.h"
+#include "trace2/tr2_tls.h"
+
+/*
+ * Initialize size of the thread stack for nested regions.
+ * This is used to store nested region start times.  Note that
+ * this stack is per-thread and not per-trace-key.
+ */
+#define TR2_REGION_NESTING_INITIAL_SIZE (100)
+
+static struct tr2tls_thread_ctx *tr2tls_thread_main;
+static uint64_t tr2tls_us_start_main;
+
+static pthread_mutex_t tr2tls_mutex;
+static pthread_key_t tr2tls_key;
+
+static int tr2_next_thread_id; /* modify under lock */
+
+struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name)
+{
+	uint64_t us_now = getnanotime() / 1000;
+	struct tr2tls_thread_ctx *ctx = xcalloc(1, sizeof(*ctx));
+
+	/*
+	 * Implicitly "tr2tls_push_self()" to capture the thread's start
+	 * time in array_us_start[0].  For the main thread this gives us the
+	 * application run time.
+	 */
+	ctx->alloc = TR2_REGION_NESTING_INITIAL_SIZE;
+	ctx->array_us_start = (uint64_t*)xcalloc(ctx->alloc, sizeof(uint64_t));
+	ctx->array_us_start[ctx->nr_open_regions++] = us_now;
+
+	ctx->thread_id = tr2tls_locked_increment(&tr2_next_thread_id);
+	
+	strbuf_init(&ctx->thread_name, 0);
+	if (ctx->thread_id)
+		strbuf_addf(&ctx->thread_name, "th%02d:", ctx->thread_id);
+	strbuf_addstr(&ctx->thread_name, thread_name);
+	if (ctx->thread_name.len > TR2_MAX_THREAD_NAME)
+		strbuf_setlen(&ctx->thread_name, TR2_MAX_THREAD_NAME);
+
+	pthread_setspecific(tr2tls_key, ctx);
+
+	return ctx;
+}
+
+struct tr2tls_thread_ctx *tr2tls_get_self(void)
+{
+	struct tr2tls_thread_ctx * ctx = pthread_getspecific(tr2tls_key);
+
+	/*
+	 * If the thread-proc did not call trace2_thread_start(), we won't
+	 * have any TLS data associated with the current thread.  Fix it
+	 * here and silently continue.
+	 */
+	if (!ctx)
+		ctx = tr2tls_create_self("unknown");
+
+	return ctx;
+}
+
+int tr2tls_is_main_thread(void)
+{
+	struct tr2tls_thread_ctx * ctx = pthread_getspecific(tr2tls_key);
+
+	return ctx == tr2tls_thread_main;
+}
+
+void tr2tls_unset_self(void)
+{
+	struct tr2tls_thread_ctx * ctx;
+
+	ctx = tr2tls_get_self();
+
+	pthread_setspecific(tr2tls_key, NULL);
+
+	free(ctx->array_us_start);
+	free(ctx);
+}
+
+void tr2tls_push_self(uint64_t us_now)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+
+	ALLOC_GROW(ctx->array_us_start, ctx->nr_open_regions + 1, ctx->alloc);
+	ctx->array_us_start[ctx->nr_open_regions++] = us_now;
+}
+
+void tr2tls_pop_self(void)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+
+	if (!ctx->nr_open_regions)
+		BUG("no open regions in thread '%s'", ctx->thread_name.buf);
+
+	ctx->nr_open_regions--;
+}
+
+void tr2tls_pop_unwind_self(void)
+{
+	struct tr2tls_thread_ctx *ctx = tr2tls_get_self();
+
+	while (ctx->nr_open_regions > 1)
+		tr2tls_pop_self();
+}
+
+uint64_t tr2tls_region_elasped_self(uint64_t us)
+{
+	struct tr2tls_thread_ctx *ctx;
+	uint64_t us_start;
+
+	ctx = tr2tls_get_self();
+	if (!ctx->nr_open_regions)
+		return 0;
+	
+	us_start = ctx->array_us_start[ctx->nr_open_regions - 1];
+
+	return us - us_start;
+}
+
+uint64_t tr2tls_absolute_elapsed(uint64_t us)
+{
+	if (!tr2tls_thread_main)
+		return 0;
+
+	return us - tr2tls_us_start_main;
+}
+
+void tr2tls_init(void)
+{
+	pthread_key_create(&tr2tls_key, NULL);
+	init_recursive_mutex(&tr2tls_mutex);
+
+	tr2tls_thread_main = tr2tls_create_self("main");
+	/*
+	 * Keep a copy of the absolute start time of the main thread
+	 * in a fixed variable since other threads need to access it.
+	 * This also eliminates the need to lock accesses to the main
+	 * thread's array (because of reallocs).
+	 */
+	tr2tls_us_start_main = tr2tls_thread_main->array_us_start[0];
+}
+
+void tr2tls_release(void)
+{
+	tr2tls_unset_self();
+	tr2tls_thread_main = NULL;
+
+	pthread_mutex_destroy(&tr2tls_mutex);
+	pthread_key_delete(tr2tls_key);
+}
+
+int tr2tls_locked_increment(int *p)
+{
+	int current_value;
+
+	pthread_mutex_lock(&tr2tls_mutex);
+	current_value = *p;
+	*p = current_value + 1;
+	pthread_mutex_unlock(&tr2tls_mutex);
+
+	return current_value;
+}
diff --git a/trace2/tr2_tls.h b/trace2/tr2_tls.h
new file mode 100644
index 0000000000..99ea9018ce
--- /dev/null
+++ b/trace2/tr2_tls.h
@@ -0,0 +1,95 @@
+#ifndef TR2_TLS_H
+#define TR2_TLS_H
+
+/*
+ * Arbitry limit for thread names for column alignment.
+ */
+#define TR2_MAX_THREAD_NAME (24)
+
+struct tr2tls_thread_ctx {
+	struct strbuf thread_name;
+	uint64_t *array_us_start;
+	int alloc;
+	int nr_open_regions; /* plays role of "nr" in ALLOC_GROW */
+	int thread_id;
+};
+
+/*
+ * Create TLS data for the current thread.  This gives us a place to
+ * put per-thread data, such as thread start time, function nesting
+ * and a per-thread label for our messages.
+ *
+ * We assume the first thread is "main".  Other threads are given
+ * non-zero thread-ids to help distinguish messages from concurrent
+ * threads.
+ *
+ * Truncate the thread name if necessary to help with column alignment
+ * in printf-style messages.
+ *
+ * In this and all following functions the term "self" refers to the
+ * current thread.
+ */
+struct tr2tls_thread_ctx *tr2tls_create_self(const char *thread_name);
+
+/*
+ * Get our TLS data.
+ */
+struct tr2tls_thread_ctx *tr2tls_get_self(void);
+
+/*
+ * return true if the current thread is the main thread.
+ */
+int tr2tls_is_main_thread(void);
+
+/*
+ * Free our TLS data.
+ */
+void tr2tls_unset_self(void);
+
+/*
+ * Begin a new nested region and remember the start time.
+ */
+void tr2tls_push_self(uint64_t us_now);
+
+/*
+ * End the innermost nested region.
+ */
+void tr2tls_pop_self(void);
+
+/*
+ * Pop any extra (above the first) open regions on the current
+ * thread and discard.  During a thread-exit, we should only
+ * have region[0] that was pushed in trace2_thread_start() if
+ * the thread exits normally.
+ */
+void tr2tls_pop_unwind_self(void);
+
+/*
+ * Compute the elapsed time since the innermost region in the
+ * current thread started and the given time (usually now).
+ */
+uint64_t tr2tls_region_elasped_self(uint64_t us);
+
+/*
+ * Compute the elapsed time since the main thread started
+ * and the given time (usually now).  This is assumed to
+ * be the absolute run time of the process.
+ */
+uint64_t tr2tls_absolute_elapsed(uint64_t us);
+
+/*
+ * Initialize the tr2 TLS system.
+ */
+void tr2tls_init(void);
+
+/*
+ * Free all tr2 TLS resources.
+ */
+void tr2tls_release(void);
+
+/*
+ * Protected increment of an integer.
+ */
+int tr2tls_locked_increment(int *p);
+
+#endif /* TR2_TLS_H */
diff --git a/trace2/tr2_verb.c b/trace2/tr2_verb.c
new file mode 100644
index 0000000000..52be27c29f
--- /dev/null
+++ b/trace2/tr2_verb.c
@@ -0,0 +1,30 @@
+#include "cache.h"
+#include "trace2/tr2_verb.h"
+
+#define TR2_ENVVAR_PARENT_VERB "GIT_TR2_PARENT_VERB"
+
+static struct strbuf tr2verb_hierarchy = STRBUF_INIT;
+
+void tr2_verb_append_hierarchy(const char *verb)
+{
+	const char *parent_verb = getenv(TR2_ENVVAR_PARENT_VERB);
+
+	strbuf_reset(&tr2verb_hierarchy);
+	if (parent_verb && *parent_verb) {
+		strbuf_addstr(&tr2verb_hierarchy, parent_verb);
+		strbuf_addch(&tr2verb_hierarchy, '/');
+	}
+	strbuf_addstr(&tr2verb_hierarchy, verb);
+
+	setenv(TR2_ENVVAR_PARENT_VERB, tr2verb_hierarchy.buf, 1);
+}
+
+const char *tr2_verb_get_hierarchy(void)
+{
+	return tr2verb_hierarchy.buf;
+}
+
+void tr2_verb_release(void)
+{
+	strbuf_release(&tr2verb_hierarchy);
+}
diff --git a/trace2/tr2_verb.h b/trace2/tr2_verb.h
new file mode 100644
index 0000000000..f84f873463
--- /dev/null
+++ b/trace2/tr2_verb.h
@@ -0,0 +1,24 @@
+#ifndef TR2_VERB_H
+#define TR2_VERB_H
+
+/*
+ * Append the current git command's "verb" to the list being maintained
+ * in the environment.
+ *
+ * The hierarchy for a top-level git command is just the current verb.
+ * For a child git process, the hierarchy lists the verbs of the parent
+ * git processes (much like the SID).
+ *
+ * The hierarchy for the current process will be exported to the environment
+ * and inherited by child processes.
+ */
+void tr2_verb_append_hierarchy(const char *verb);
+
+/*
+ * Get the verb hierarchy for the current process.
+ */
+const char *tr2_verb_get_hierarchy(void);
+
+void tr2_verb_release(void);
+
+#endif /* TR2_VERB_H */
diff --git a/usage.c b/usage.c
index cc803336bd..5ec7511e11 100644
--- a/usage.c
+++ b/usage.c
@@ -22,17 +22,48 @@ void vreportf(const char *prefix, const char *err, va_list params)
 static NORETURN void usage_builtin(const char *err, va_list params)
 {
 	vreportf("usage: ", err, params);
+
+	/*
+	 * When we detect a usage error *before* the command dispatch in
+	 * cmd_main(), we don't know what verb to report.  Force it to this
+	 * to facilitate post-processing.
+	 */
+	trace2_cmd_verb("_usage_");
+
+	/*
+	 * Currently, the (err, params) are usually just the static usage
+	 * string which isn't very useful here.  Usually, the call site
+	 * manually calls fprintf(stderr,...) with the actual detailed
+	 * syntax error before calling usage().
+	 *
+	 * TODO It would be nice to update the call sites to pass both
+	 * the static usage string and the detailed error message.
+	 */
+
 	exit(129);
 }
 
 static NORETURN void die_builtin(const char *err, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(err, params);
+
 	vreportf("fatal: ", err, params);
+
 	exit(128);
 }
 
 static void error_builtin(const char *err, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(err, params);
+
 	vreportf("error: ", err, params);
 }
 

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 03/14] trace2: collect platform-specific process information
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add optional platform-specific code to log information about
the current process.

On Windows, this includes whether git.exe is running under a
debugger and information about the ancestors of the process.

The purpose of this information is to help indicate if the
process was launched interactively or in the background by
an IDE, for example.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 common-main.c           |   1 +
 compat/win32/ancestry.c | 102 ++++++++++++++++++++++++++++++++++++++++
 config.mak.uname        |   2 +
 trace2.h                |  13 +++++
 4 files changed, 118 insertions(+)
 create mode 100644 compat/win32/ancestry.c

diff --git a/common-main.c b/common-main.c
index 6dbdc4adf2..d484aec209 100644
--- a/common-main.c
+++ b/common-main.c
@@ -37,6 +37,7 @@ int main(int argc, const char **argv)
 
 	trace2_initialize();
 	trace2_cmd_start(argv);
+	trace2_collect_process_info();
 
 	git_resolve_executable_dir(argv[0]);
 
diff --git a/compat/win32/ancestry.c b/compat/win32/ancestry.c
new file mode 100644
index 0000000000..629d8214b9
--- /dev/null
+++ b/compat/win32/ancestry.c
@@ -0,0 +1,102 @@
+#include "../../cache.h"
+#include "../../json-writer.h"
+#include <Psapi.h>
+#include <tlHelp32.h>
+
+/*
+ * Find the process data for the given PID in the given snapshot
+ * and update the PROCESSENTRY32 data.
+ */
+static int find_pid(DWORD pid, HANDLE hSnapshot, PROCESSENTRY32 *pe32)
+{
+	pe32->dwSize = sizeof(PROCESSENTRY32);
+
+	if (Process32First(hSnapshot, pe32)) {
+		do {
+			if (pe32->th32ProcessID == pid)
+				return 1;
+		} while (Process32Next(hSnapshot, pe32));
+	}
+	return 0;
+}
+
+/*
+ * Accumulate JSON array:
+ *     [
+ *         exe-name-parent,
+ *         exe-name-grand-parent,
+ *         ...
+ *     ]
+ *
+ * Note: we only report the filename of the process executable; the
+ *       only way to get its full pathname is to use OpenProcess()
+ *       and GetModuleFileNameEx() or QueryfullProcessImageName()
+ *       and that seems rather expensive (on top of the cost of
+ *       getting the snapshot).
+ */
+static void get_processes(struct json_writer *jw, HANDLE hSnapshot)
+{
+	PROCESSENTRY32 pe32;
+	DWORD pid;
+
+	pid = GetCurrentProcessId();
+
+	/* We only want parent processes, so skip self. */
+	if (!find_pid(pid, hSnapshot, &pe32))
+		return;
+	pid = pe32.th32ParentProcessID;
+
+	while (find_pid(pid, hSnapshot, &pe32)) {
+
+		jw_array_string(jw, pe32.szExeFile);
+
+		pid = pe32.th32ParentProcessID;
+	}
+}
+
+/*
+ * Emit JSON data for the current and parent processes.  Individual
+ * trace2 targets can decide how to actually print it.
+ */
+static void get_ancestry(void)
+{
+	HANDLE hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
+
+	if (hSnapshot != INVALID_HANDLE_VALUE) {
+		struct json_writer jw = JSON_WRITER_INIT;
+
+		jw_array_begin(&jw, 0);
+		get_processes(&jw, hSnapshot);
+		jw_end(&jw);
+
+		trace2_data_json("process", the_repository,
+				 "windows/ancestry", &jw);
+
+		jw_release(&jw);
+		CloseHandle(hSnapshot);
+	}
+}
+
+/*
+ * Is a debugger attached to the current process?
+ *
+ * This will catch debug runs (where the debugger started the process).
+ * This is the normal case.  Since this code is called during our startup,
+ * it will not report instances where a debugger is attached dynamically
+ * to a running git process, but that is relatively rare.
+ */
+static void get_is_being_debugged(void)
+{
+	if (IsDebuggerPresent())
+		trace2_data_intmax("process", the_repository,
+			   "windows/debugger_present", 1);
+}
+
+void trace2_collect_process_info(void)
+{
+	if (!trace2_is_enabled())
+		return;
+
+	get_is_being_debugged();
+	get_ancestry();
+}
diff --git a/config.mak.uname b/config.mak.uname
index e391431041..13d7c5410f 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -391,6 +391,7 @@ ifeq ($(uname_S),Windows)
 	BASIC_CFLAGS = -nologo -I. -I../zlib -Icompat/vcbuild -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE
 	COMPAT_OBJS = compat/msvc.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
+		compat/win32/ancestry.o \
 		compat/win32/dirent.o
 	COMPAT_CFLAGS = -D__USE_MINGW_ACCESS -DNOGDI -DHAVE_STRING_H -Icompat -Icompat/regex -Icompat/win32 -DSTRIP_EXTENSION=\".exe\"
 	BASIC_LDFLAGS = -IGNORE:4217 -IGNORE:4049 -NOLOGO -SUBSYSTEM:CONSOLE
@@ -528,6 +529,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	COMPAT_CFLAGS += -DNOGDI -Icompat -Icompat/win32
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
+		compat/win32/ancestry.o \
 		compat/win32/path-utils.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/dirent.o
diff --git a/trace2.h b/trace2.h
index 7ab9f355f3..a4a075d060 100644
--- a/trace2.h
+++ b/trace2.h
@@ -387,4 +387,17 @@ __attribute__((format (printf, 1, 2)))
 void trace2_printf(const char *fmt, ...);
 #endif
 
+/*
+ * Optional platform-specific code to dump information about the
+ * current and any parent process(es).  This is intended to allow
+ * post-processors to know who spawned this git instance and anything
+ * else the platform may be able to tell us about the current process.
+ */
+#if defined(GIT_WINDOWS_NATIVE)
+void trace2_collect_process_info(void);
+#else
+#define trace2_collect_process_info() \
+	do {} while (0)
+#endif
+
 #endif /* TRACE2_H */

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 04/14] trace2:data: add trace2 regions to wt-status
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add trace2_region_enter() and trace2_region_leave() calls around the
various phases of a status scan.  This gives elapsed time for each
phase in the GIT_TR2_PERF and GIT_TR2_EVENT trace target.

Also, these Trace2 calls now use s->repo rather than the_repository.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 wt-status.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/wt-status.c b/wt-status.c
index 0fe3bcd4cd..7c5ba4e902 100644
--- a/wt-status.c
+++ b/wt-status.c
@@ -748,12 +748,23 @@ static int has_unmerged(struct wt_status *s)
 
 void wt_status_collect(struct wt_status *s)
 {
+	trace2_region_enter("status", "worktrees", s->repo);
 	wt_status_collect_changes_worktree(s);
-	if (s->is_initial)
+	trace2_region_leave("status", "worktrees", s->repo);
+
+	if (s->is_initial) {
+		trace2_region_enter("status", "initial", s->repo);
 		wt_status_collect_changes_initial(s);
-	else
+		trace2_region_leave("status", "initial", s->repo);
+	} else {
+		trace2_region_enter("status", "index", s->repo);
 		wt_status_collect_changes_index(s);
+		trace2_region_leave("status", "index", s->repo);
+	}
+
+	trace2_region_enter("status", "untracked", s->repo);
 	wt_status_collect_untracked(s);
+	trace2_region_leave("status", "untracked", s->repo);
 
 	wt_status_get_state(s->repo, &s->state, s->branch && !strcmp(s->branch, "HEAD"));
 	if (s->state.merge_in_progress && !has_unmerged(s))
@@ -2291,6 +2302,12 @@ static void wt_porcelain_v2_print(struct wt_status *s)
 
 void wt_status_print(struct wt_status *s)
 {
+	trace2_data_intmax("status", s->repo, "count/changed", s->change.nr);
+	trace2_data_intmax("status", s->repo, "count/untracked", s->untracked.nr);
+	trace2_data_intmax("status", s->repo, "count/ignored", s->ignored.nr);
+
+	trace2_region_enter("status", "print", s->repo);
+
 	switch (s->status_format) {
 	case STATUS_FORMAT_SHORT:
 		wt_shortstatus_print(s);
@@ -2309,6 +2326,8 @@ void wt_status_print(struct wt_status *s)
 		wt_longstatus_print(s);
 		break;
 	}
+
+	trace2_region_leave("status", "print", s->repo);
 }
 
 /**

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 05/14] trace2:data: add editor/pager child classification
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add trace2 process classification for editor and pager
child processes.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 editor.c | 1 +
 pager.c  | 1 +
 2 files changed, 2 insertions(+)

diff --git a/editor.c b/editor.c
index c985eee1f9..71547674ab 100644
--- a/editor.c
+++ b/editor.c
@@ -78,6 +78,7 @@ static int launch_specified_editor(const char *editor, const char *path,
 		p.argv = args;
 		p.env = env;
 		p.use_shell = 1;
+		p.trace2_child_class = "editor";
 		if (start_command(&p) < 0)
 			return error("unable to start editor '%s'", editor);
 
diff --git a/pager.c b/pager.c
index a768797fcf..4168460ae9 100644
--- a/pager.c
+++ b/pager.c
@@ -100,6 +100,7 @@ void prepare_pager_args(struct child_process *pager_process, const char *pager)
 	argv_array_push(&pager_process->args, pager);
 	pager_process->use_shell = 1;
 	setup_pager_env(&pager_process->env_array);
+	pager_process->trace2_child_class = "pager";
 }
 
 void setup_pager(void)

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Subject: [PATCH 06/14] trace2:data: add trace2 sub-process classification
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add trace2 classification for long-running processes
started in sub-process.c

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 sub-process.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sub-process.c b/sub-process.c
index 8d2a1707cf..3f4af93555 100644
--- a/sub-process.c
+++ b/sub-process.c
@@ -88,6 +88,7 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
 	process->out = -1;
 	process->clean_on_exit = 1;
 	process->clean_on_exit_handler = subprocess_exit_handler;
+	process->trace2_child_class = "subprocess";
 
 	err = start_command(process);
 	if (err) {

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Subject: [PATCH 07/14] trace2:data: add trace2 transport child classification
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add trace2 child classification for transport processes.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 connect.c          | 3 +++
 transport-helper.c | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/connect.c b/connect.c
index 24281b6082..3c6f829a05 100644
--- a/connect.c
+++ b/connect.c
@@ -1251,6 +1251,7 @@ struct child_process *git_connect(int fd[2], const char *url,
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
 		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn->trace2_child_class = "transport/git";
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,9 +1294,11 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
+			conn->trace2_child_class = "transport/ssh";
 			fill_ssh_args(conn, ssh_host, port, version, flags);
 		} else {
 			transport_check_allowed("file");
+			conn->trace2_child_class = "transport/file";
 			if (version > 0) {
 				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
 						 version);
diff --git a/transport-helper.c b/transport-helper.c
index bf225c698f..1eb656b687 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -127,6 +127,8 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	helper->trace2_child_class = helper->args.argv[0]; /* "remote-<name>" */
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 08/14] trace2:data: add trace2 hook classification
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From: Jeff Hostetler <jeffhost@microsoft.com>

Classify certain child processes as hooks.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 builtin/am.c           | 1 +
 builtin/receive-pack.c | 4 ++++
 builtin/worktree.c     | 1 +
 sequencer.c            | 2 ++
 transport.c            | 1 +
 5 files changed, 9 insertions(+)

diff --git a/builtin/am.c b/builtin/am.c
index 95370313b6..a81e61c1bc 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -468,6 +468,7 @@ static int run_post_rewrite_hook(const struct am_state *state)
 
 	cp.in = xopen(am_path(state, "rewritten"), O_RDONLY);
 	cp.stdout_to_stderr = 1;
+	cp.trace2_hook_name = "post-rewrite";
 
 	ret = run_command(&cp);
 
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 33187bd8e9..6eef8575b9 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -694,6 +694,8 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed,
 	proc.argv = argv;
 	proc.in = -1;
 	proc.stdout_to_stderr = 1;
+	proc.trace2_hook_name = hook_name;
+
 	if (feed_state->push_options) {
 		int i;
 		for (i = 0; i < feed_state->push_options->nr; i++)
@@ -807,6 +809,7 @@ static int run_update_hook(struct command *cmd)
 	proc.stdout_to_stderr = 1;
 	proc.err = use_sideband ? -1 : 0;
 	proc.argv = argv;
+	proc.trace2_hook_name = "update";
 
 	code = start_command(&proc);
 	if (code)
@@ -1190,6 +1193,7 @@ static void run_update_post_hook(struct command *commands)
 	proc.no_stdin = 1;
 	proc.stdout_to_stderr = 1;
 	proc.err = use_sideband ? -1 : 0;
+	proc.trace2_hook_name = "post-update";
 
 	if (!start_command(&proc)) {
 		if (use_sideband)
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 5e84026177..32e6f1b341 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -401,6 +401,7 @@ static int add_worktree(const char *path, const char *refname,
 			cp.dir = path;
 			cp.env = env;
 			cp.argv = NULL;
+			cp.trace2_hook_name = "post-checkout";
 			argv_array_pushl(&cp.args, absolute_path(hook),
 					 oid_to_hex(&null_oid),
 					 oid_to_hex(&commit->object.oid),
diff --git a/sequencer.c b/sequencer.c
index b68bca0bef..9a6ec7fcd4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -1102,6 +1102,7 @@ static int run_rewrite_hook(const struct object_id *oldoid,
 	proc.argv = argv;
 	proc.in = -1;
 	proc.stdout_to_stderr = 1;
+	proc.trace2_hook_name = "post-rewrite";
 
 	code = start_command(&proc);
 	if (code)
@@ -3771,6 +3772,7 @@ static int pick_commits(struct repository *r,
 				hook.in = open(rebase_path_rewritten_list(),
 					O_RDONLY);
 				hook.stdout_to_stderr = 1;
+				hook.trace2_hook_name = "post-rewrite";
 				argv_array_push(&hook.args, post_rewrite_hook);
 				argv_array_push(&hook.args, "rebase");
 				/* we don't care if this hook failed */
diff --git a/transport.c b/transport.c
index 99678153c1..ed5a733c4a 100644
--- a/transport.c
+++ b/transport.c
@@ -1061,6 +1061,7 @@ static int run_pre_push_hook(struct transport *transport,
 
 	proc.argv = argv;
 	proc.in = -1;
+	proc.trace2_hook_name = "pre-push";
 
 	if (start_command(&proc)) {
 		finish_command(&proc);

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Subject: [PATCH 09/14] trace2:data: add trace2 instrumentation to index
 read/write
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From: Jeff Hostetler <jeffhost@microsoft.com>

Add trace2 events to measure reading and writing the index.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 read-cache.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index bfff271a3d..f6d84bd7ae 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2232,6 +2232,14 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 		load_index_extensions(&p);
 	}
 	munmap((void *)mmap, mmap_size);
+
+	/*
+	 * TODO trace2: replace "the_repository" with the actual repo instance
+	 * that is associated with the given "istate".
+	 */
+	trace2_data_intmax("index", the_repository, "read/version", istate->version);
+	trace2_data_intmax("index", the_repository, "read/cache_nr", istate->cache_nr);
+
 	return istate->cache_nr;
 
 unmap:
@@ -2263,9 +2271,17 @@ int read_index_from(struct index_state *istate, const char *path,
 	if (istate->initialized)
 		return istate->cache_nr;
 
+	/*
+	 * TODO trace2: replace "the_repository" with the actual repo instance
+	 * that is associated with the given "istate".
+	 */
+	trace2_region_enter_printf("index", "do_read_index", the_repository,
+				   "%s", path);
 	trace_performance_enter();
 	ret = do_read_index(istate, path, 0);
 	trace_performance_leave("read cache %s", path);
+	trace2_region_leave_printf("index", "do_read_index", the_repository,
+				   "%s", path);
 
 	split_index = istate->split_index;
 	if (!split_index || is_null_oid(&split_index->base_oid)) {
@@ -2281,7 +2297,11 @@ int read_index_from(struct index_state *istate, const char *path,
 
 	base_oid_hex = oid_to_hex(&split_index->base_oid);
 	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
+	trace2_region_enter_printf("index", "shared/do_read_index", the_repository,
+				   "%s", base_path);
 	ret = do_read_index(split_index->base, base_path, 1);
+	trace2_region_leave_printf("index", "shared/do_read_index", the_repository,
+				   "%s", base_path);
 	if (!oideq(&split_index->base_oid, &split_index->base->oid))
 		die(_("broken index, expect %s in %s, got %s"),
 		    base_oid_hex, base_path,
@@ -2988,6 +3008,14 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 	istate->timestamp.sec = (unsigned int)st.st_mtime;
 	istate->timestamp.nsec = ST_MTIME_NSEC(st);
 	trace_performance_since(start, "write index, changed mask = %x", istate->cache_changed);
+
+	/*
+	 * TODO trace2: replace "the_repository" with the actual repo instance
+	 * that is associated with the given "istate".
+	 */
+	trace2_data_intmax("index", the_repository, "write/version", istate->version);
+	trace2_data_intmax("index", the_repository, "write/cache_nr", istate->cache_nr);
+
 	return 0;
 }
 
@@ -3007,7 +3035,18 @@ static int commit_locked_index(struct lock_file *lk)
 static int do_write_locked_index(struct index_state *istate, struct lock_file *lock,
 				 unsigned flags)
 {
-	int ret = do_write_index(istate, lock->tempfile, 0);
+	int ret;
+
+	/*
+	 * TODO trace2: replace "the_repository" with the actual repo instance
+	 * that is associated with the given "istate".
+	 */
+	trace2_region_enter_printf("index", "do_write_index", the_repository,
+				   "%s", lock->tempfile->filename.buf);
+	ret = do_write_index(istate, lock->tempfile, 0);
+	trace2_region_leave_printf("index", "do_write_index", the_repository,
+				   "%s", lock->tempfile->filename.buf);
+
 	if (ret)
 		return ret;
 	if (flags & COMMIT_LOCK)
@@ -3092,7 +3131,13 @@ static int write_shared_index(struct index_state *istate,
 	int ret;
 
 	move_cache_to_base_index(istate);
+
+	trace2_region_enter_printf("index", "shared/do_write_index",
+				   the_repository, "%s", (*temp)->filename.buf);
 	ret = do_write_index(si->base, *temp, 1);
+	trace2_region_enter_printf("index", "shared/do_write_index",
+				   the_repository, "%s", (*temp)->filename.buf);
+
 	if (ret)
 		return ret;
 	ret = adjust_shared_perm(get_tempfile_path(*temp));

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From: "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 10/14] pack-objects: add trace2 regions
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From: Derrick Stolee <dstolee@microsoft.com>

When studying the performance of 'git push' we would like to know
how much time is spent at various parts of the command. One area
that could cause performance trouble is 'git pack-objects'.

Add trace2 regions around the three main actions taken in this
command:

1. Enumerate objects.
2. Prepare pack.
3. Write pack-file.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 builtin/pack-objects.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 889df2c755..6708529e3c 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -33,6 +33,7 @@
 #include "object-store.h"
 #include "dir.h"
 #include "midx.h"
+#include "trace2.h"
 
 #define IN_PACK(obj) oe_in_pack(&to_pack, obj)
 #define SIZE(obj) oe_size(&to_pack, obj)
@@ -3472,6 +3473,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	trace2_region_enter("pack-objects", "enumerate-objects", the_repository);
 	prepare_packing_data(the_repository, &to_pack);
 
 	if (progress)
@@ -3486,12 +3488,20 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (include_tag && nr_result)
 		for_each_ref(add_ref_tag, NULL);
 	stop_progress(&progress_state);
+	trace2_region_leave("pack-objects", "enumerate-objects", the_repository);
 
 	if (non_empty && !nr_result)
 		return 0;
-	if (nr_result)
+	if (nr_result) {
+		trace2_region_enter("pack-objects", "prepare-pack", the_repository);
 		prepare_pack(window, depth);
+		trace2_region_leave("pack-objects", "prepare-pack", the_repository);
+	}
+
+	trace2_region_enter("pack-objects", "write-pack-file", the_repository);
 	write_pack_file();
+	trace2_region_leave("pack-objects", "write-pack-file", the_repository);
+
 	if (progress)
 		fprintf_ln(stderr,
 			   _("Total %"PRIu32" (delta %"PRIu32"),"

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 11/14] trace2:data: add subverb to checkout command
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From: Jeff Hostetler <jeffhost@microsoft.com>

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 builtin/checkout.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 6fadf412e8..8939ae99ed 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -262,6 +262,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 	struct lock_file lock_file = LOCK_INIT;
 	int nr_checkouts = 0;
 
+	trace2_cmd_subverb(opts->patch_mode ? "patch" : "path");
+
 	if (opts->track != BRANCH_TRACK_UNSPECIFIED)
 		die(_("'%s' cannot be used with updating paths"), "--track");
 
@@ -952,6 +954,9 @@ static int switch_branches(const struct checkout_opts *opts,
 	void *path_to_free;
 	struct object_id rev;
 	int flag, writeout_error = 0;
+
+	trace2_cmd_subverb("branch");
+
 	memset(&old_branch_info, 0, sizeof(old_branch_info));
 	old_branch_info.path = path_to_free = resolve_refdup("HEAD", 0, &rev, &flag);
 	if (old_branch_info.path)
@@ -1189,6 +1194,8 @@ static int switch_unborn_to_new_branch(const struct checkout_opts *opts)
 	int status;
 	struct strbuf branch_ref = STRBUF_INIT;
 
+	trace2_cmd_subverb("unborn");
+
 	if (!opts->new_branch)
 		die(_("You are on a branch yet to be born"));
 	strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 12/14] trace2:data: add subverb to reset command
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From: Jeff Hostetler <jeffhost@microsoft.com>

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 builtin/reset.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/builtin/reset.c b/builtin/reset.c
index 59898c972e..b65b4a66db 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -340,6 +340,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (patch_mode) {
 		if (reset_type != NONE)
 			die(_("--patch is incompatible with --{hard,mixed,soft}"));
+		trace2_cmd_subverb("patch-interactive");
 		return run_add_interactive(rev, "--patch=reset", &pathspec);
 	}
 
@@ -356,6 +357,11 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (reset_type == NONE)
 		reset_type = MIXED; /* by default */
 
+	if (pathspec.nr)
+		trace2_cmd_subverb("path");
+	else
+		trace2_cmd_subverb(reset_type_names[reset_type]);
+
 	if (reset_type != SOFT && (reset_type != MIXED || get_git_work_tree()))
 		setup_work_tree();
 

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Subject: [PATCH 13/14] trace2:data: add subverb for rebase
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From: Jeff Hostetler <jeffhost@microsoft.com>

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 builtin/rebase.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 00de70365e..aac0d52ade 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -819,6 +819,16 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		ACTION_EDIT_TODO,
 		ACTION_SHOW_CURRENT_PATCH,
 	} action = NO_ACTION;
+	static const char *action_names[] = {
+		N_("undefined"),
+		N_("continue"),
+		N_("skip"),
+		N_("abort"),
+		N_("quit"),
+		N_("edit_todo"),
+		N_("show_current_patch"),
+		NULL
+	};
 	const char *gpg_sign = NULL;
 	struct string_list exec = STRING_LIST_INIT_NODUP;
 	const char *rebase_merges = NULL;
@@ -1001,6 +1011,15 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		die(_("The --edit-todo action can only be used during "
 		      "interactive rebase."));
 
+	if (trace2_is_enabled()) {
+		if (is_interactive(&options))
+			trace2_cmd_subverb("interactive");
+		else if (exec.nr)
+			trace2_cmd_subverb("interactive-exec");
+		else
+			trace2_cmd_subverb(action_names[action]);
+	}
+
 	switch (action) {
 	case ACTION_CONTINUE: {
 		struct object_id head;

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From: "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com>
Subject: [PATCH 14/14] trace2: t/helper/test-trace2, t0210.sh, t0211.sh,
 t0212.sh
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From: Jeff Hostetler <jeffhost@microsoft.com>

Create unit tests for Trace2.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 Makefile                  |   1 +
 t/helper/test-tool.c      |   1 +
 t/helper/test-tool.h      |   1 +
 t/helper/test-trace2.c    | 273 ++++++++++++++++++++++++++++++++++++++
 t/t0210-trace2-normal.sh  | 135 +++++++++++++++++++
 t/t0210/scrub_normal.perl |  48 +++++++
 t/t0211-trace2-perf.sh    | 153 +++++++++++++++++++++
 t/t0211/scrub_perf.perl   |  76 +++++++++++
 t/t0212-trace2-event.sh   | 237 +++++++++++++++++++++++++++++++++
 t/t0212/parse_events.perl | 251 +++++++++++++++++++++++++++++++++++
 10 files changed, 1176 insertions(+)
 create mode 100644 t/helper/test-trace2.c
 create mode 100755 t/t0210-trace2-normal.sh
 create mode 100644 t/t0210/scrub_normal.perl
 create mode 100755 t/t0211-trace2-perf.sh
 create mode 100644 t/t0211/scrub_perf.perl
 create mode 100755 t/t0212-trace2-event.sh
 create mode 100644 t/t0212/parse_events.perl

diff --git a/Makefile b/Makefile
index 5d4e993cdc..8100364023 100644
--- a/Makefile
+++ b/Makefile
@@ -753,6 +753,7 @@ TEST_BUILTINS_OBJS += test-string-list.o
 TEST_BUILTINS_OBJS += test-submodule-config.o
 TEST_BUILTINS_OBJS += test-submodule-nested-repo-config.o
 TEST_BUILTINS_OBJS += test-subprocess.o
+TEST_BUILTINS_OBJS += test-trace2.o
 TEST_BUILTINS_OBJS += test-urlmatch-normalization.o
 TEST_BUILTINS_OBJS += test-wildmatch.o
 TEST_BUILTINS_OBJS += test-windows-named-pipe.o
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index 85d1f812fe..3d591c00ac 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -49,6 +49,7 @@ static struct test_cmd cmds[] = {
 	{ "submodule-config", cmd__submodule_config },
 	{ "submodule-nested-repo-config", cmd__submodule_nested_repo_config },
 	{ "subprocess", cmd__subprocess },
+	{ "trace2", cmd__trace2 },
 	{ "urlmatch-normalization", cmd__urlmatch_normalization },
 	{ "wildmatch", cmd__wildmatch },
 #ifdef GIT_WINDOWS_NATIVE
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index 042f12464b..226b86aca9 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -44,6 +44,7 @@ int cmd__string_list(int argc, const char **argv);
 int cmd__submodule_config(int argc, const char **argv);
 int cmd__submodule_nested_repo_config(int argc, const char **argv);
 int cmd__subprocess(int argc, const char **argv);
+int cmd__trace2(int argc, const char **argv);
 int cmd__urlmatch_normalization(int argc, const char **argv);
 int cmd__wildmatch(int argc, const char **argv);
 #ifdef GIT_WINDOWS_NATIVE
diff --git a/t/helper/test-trace2.c b/t/helper/test-trace2.c
new file mode 100644
index 0000000000..b867f58d32
--- /dev/null
+++ b/t/helper/test-trace2.c
@@ -0,0 +1,273 @@
+#include "test-tool.h"
+#include "cache.h"
+#include "argv-array.h"
+#include "run-command.h"
+#include "exec-cmd.h"
+#include "config.h"
+
+typedef int (fn_unit_test)(int argc, const char **argv);
+
+struct unit_test 
+{
+	fn_unit_test *ut_fn;
+	const char   *ut_name;
+	const char   *ut_usage;
+};
+
+#define MyOk    0
+#define MyError 1
+
+static int get_i(int *p_value, const char *data)
+{
+	char *endptr;
+
+	if (!data || !*data)
+		return MyError;
+
+	*p_value = strtol(data, &endptr, 10);
+	if (*endptr || errno == ERANGE)
+		return MyError;
+
+	return MyOk;
+}
+
+/*
+ * Cause process to exit with the requested value via "return".
+ *
+ * Rely on test-tool.c:cmd_main() to call trace2_cmd_exit()
+ * with our result.
+ *
+ * Test harness can confirm:
+ * [] the process-exit value.
+ * [] the "code" field in the "exit" trace2 event.
+ * [] the "code" field in the "atexit" trace2 event.
+ * [] the "name" field in the "cmd_verb" trace2 event.
+ * [] "def_param" events for all of the "interesting" pre-defined config settings.
+ */
+static int ut_001return(int argc, const char **argv)
+{
+	int rc;
+
+	if (get_i(&rc, argv[0]))
+		die("expect <exit_code>");
+
+	return rc;
+}
+
+/*
+ * Cause the process to exit with the requested value via "exit()".
+ *
+ * Test harness can confirm:
+ * [] the "code" field in the "exit" trace2 event.
+ * [] the "code" field in the "atexit" trace2 event.
+ * [] the "name" field in the "cmd_verb" trace2 event.
+ * [] "def_param" events for all of the "interesting" pre-defined config settings.
+ */
+static int ut_002exit(int argc, const char **argv)
+{
+	int rc;
+
+	if (get_i(&rc, argv[0]))
+		die("expect <exit_code>");
+
+	exit(rc);
+}
+
+/*
+ * Send an "error" event with each value in argv.  Normally, git only issues
+ * a single "error" event immediately before issuing an "exit" event (such
+ * as in die() or BUG()), but multiple "error" events are allowed.
+ *
+ * Test harness can confirm:
+ * [] a trace2 "error" event for each value in argv.
+ * [] the "name" field in the "cmd_verb" trace2 event.
+ * [] (optional) the file:line in the "exit" event refers to this function.
+ */
+static int ut_003error(int argc, const char **argv)
+{
+	int k;
+
+	if (!argv[0] || !*argv[0])
+		die("expect <error_message>");
+
+	for (k = 0; k < argc; k++)
+		error("%s", argv[k]);
+
+	return 0;
+}
+
+/*
+ * Run a child process and wait for it to finish and exit with its return code.
+ * test-tool trace2 004child [<child-command-line>]
+ *
+ * For example:
+ * test-tool trace2 004child git version
+ * test-tool trace2 004child test-tool trace2 001return 0
+ * test-tool trace2 004child test-tool trace2 004child test-tool trace2 004child
+ * test-tool trace2 004child git -c alias.xyz=version xyz
+ *
+ * Test harness can confirm:
+ * [] the "name" field in the "cmd_verb" trace2 event.
+ * [] that the outer process has a single component SID (or depth "d0" in
+ *    the PERF stream).
+ * [] that "child_start" and "child_exit" events are generated for the child.
+ * [] if the child process is an instrumented executable:
+ *    [] that "version", "start", ..., "exit", and "atexit" events are
+ *       generated by the child process.
+ *    [] that the child process events have a multiple component SID (or
+ *       depth "dN+1" in the PERF stream).
+ * [] that the child exit code is propagated to the parent process "exit"
+ *    and "atexit" events..
+ * [] (optional) that the "t_abs" field in the child process "atexit" event
+ *    is less than the "t_rel" field in the "child_exit" event of the parent
+ *    process.
+ * [] if the child process is like the alias example above,
+ *    [] (optional) the child process attempts to run "git-xyx" as a dashed
+ *       command.
+ *    [] the child process emits an "alias" event with "xyz" => "version"
+ *    [] the child process runs "git version" as a child process.
+ *    [] the child process has a 3 component SID (or depth "d2" in the PERF
+ *       stream).
+ */
+static int ut_004child(int argc, const char **argv)
+{
+	int result;
+
+	/*
+	 * Allow empty <child_command_line> so we can do arbitrarily deep
+	 * command nesting and let the last one be null.
+	 */
+	if (!argc)
+		return 0;
+
+	result = run_command_v_opt(argv, 0);
+	exit(result);
+}
+
+/*
+ * Exec a git command.  This may either create a child process (Windows)
+ * or replace the existing process.
+ * test-tool trace2 005exec <git_command_args>
+ *
+ * For example:
+ * test-tool trace2 005exec version
+ *
+ * Test harness can confirm (on Windows):
+ * [] the "name" field in the "cmd_verb" trace2 event.
+ * [] that the outer process has a single component SID (or depth "d0" in
+ *    the PERF stream).
+ * [] that "exec" and "exec_result" events are generated for the child
+ *    process (since the Windows compatibility layer fakes an exec() with
+ *    a CreateProcess(), WaitForSingleObject(), and exit()).
+ * [] that the child process has multiple component SID (or depth "dN+1"
+ *    in the PERF stream).
+ *
+ * Test harness can confirm (on platforms with a real exec() function):
+ * [] TODO talk about process replacement and how it affects SID.
+ */ 
+static int ut_005exec(int argc, const char **argv)
+{
+	int result;
+
+	if (!argc)
+		return 0;
+
+	result = execv_git_cmd(argv);
+	return result;
+}
+
+static int ut_006data(int argc, const char **argv)
+{
+	const char *usage_error =
+		"expect <cat0> <k0> <v0> [<cat1> <k1> <v1> [...]]";
+
+	if (argc % 3 != 0)
+		die("%s", usage_error);
+
+	while (argc) {
+		if (!argv[0] || !*argv[0] ||
+		    !argv[1] || !*argv[1] ||
+		    !argv[2] || !*argv[2])
+			die("%s", usage_error);
+
+		trace2_data_string(argv[0], the_repository, argv[1], argv[2]);
+		argv += 3;
+		argc -= 3;
+	}
+
+	return 0;
+}
+
+/*
+ * Usage:
+ *     test-tool trace2 <ut_name_1> <ut_usage_1>
+ *     test-tool trace2 <ut_name_2> <ut_usage_2>
+ *     ...
+ */ 
+#define USAGE_PREFIX "test-tool trace2"
+
+static struct unit_test ut_table[] = {
+	{ ut_001return,   "001return", "<exit_code>" },
+	{ ut_002exit,     "002exit",   "<exit_code>" },
+	{ ut_003error,    "003error",  "<error_message>+" },
+	{ ut_004child,    "004child",  "[<child_command_line>]" },
+	{ ut_005exec,     "005exec",   "<git_command_args>" },
+	{ ut_006data,     "006data",   "[<category> <key> <value>]+" },
+};
+
+#define for_each_ut(k, ut_k)			\
+	for (k = 0, ut_k = &ut_table[k];	\
+	     k < ARRAY_SIZE(ut_table);		\
+	     k++, ut_k = &ut_table[k])
+
+static int print_usage(void)
+{
+	int k;
+	struct unit_test *ut_k;
+
+	fprintf(stderr, "usage:\n");
+	for_each_ut(k, ut_k) {
+		fprintf(stderr, "\t%s %s %s\n",
+			USAGE_PREFIX, ut_k->ut_name, ut_k->ut_usage);
+	}
+
+	return 129;
+}
+
+/*
+ * Issue various trace2 events for testing.
+ *
+ * We assume that these trace2 routines has already been called:
+ *    [] trace2_initialize()      [common-main.c:main()]
+ *    [] trace2_cmd_start()       [common-main.c:main()]
+ *    [] trace2_cmd_verb()        [test-tool.c:cmd_main()]
+ *    [] tracd2_cmd_list_config() [test-tool.c:cmd_main()]
+ * So that:
+ *    [] the various trace2 streams are open.
+ *    [] the process SID has been created.
+ *    [] the "version" event has been generated.
+ *    [] the "start" event has been generated.
+ *    [] the "verb" event has been generated.
+ *    [] this writes various "def_param" events for interesting config values.
+ *
+ * We further assume that if we return (rather than exit()), trace2_cmd_exit()
+ * will be called by test-tool.c:cmd_main().
+ */
+int cmd__trace2(int argc, const char **argv)
+{
+	int k;
+	struct unit_test *ut_k;
+
+	argc--;	/* skip over "trace2" arg */
+	argv++;
+
+	if (argc) {
+		for_each_ut(k, ut_k) {
+			if (!strcmp(argv[0], ut_k->ut_name))
+				return ut_k->ut_fn(argc - 1, argv + 1);
+		}
+	}
+
+	return print_usage();
+}
+
diff --git a/t/t0210-trace2-normal.sh b/t/t0210-trace2-normal.sh
new file mode 100755
index 0000000000..0b5b45b263
--- /dev/null
+++ b/t/t0210-trace2-normal.sh
@@ -0,0 +1,135 @@
+#!/bin/sh
+
+test_description='test trace2 facility (normal target)'
+. ./test-lib.sh
+
+# Add t/helper directory to PATH so that we can use a relative
+# path to run nested instances of test-tool.exe (see 004child).
+# This helps with HEREDOC comparisons later.
+TTDIR="$GIT_BUILD_DIR/t/helper/" && export TTDIR
+PATH="$TTDIR:$PATH" && export PATH
+
+TT="test-tool" && export TT
+
+# Warning: use of 'test_cmp' may run test-tool.exe and/or git.exe
+# Warning: to do the actual diff/comparison, so the HEREDOCs here
+# Warning: only cover our actual calls to test-tool and/or git.
+# Warning: So you may see extra lines in artifact files when
+# Warning: interactively debugging.
+
+# Turn off any inherited trace2 settings for this test.
+unset GIT_TR2 GIT_TR2_PERF GIT_TR2_EVENT
+unset GIT_TR2_BRIEF
+unset GIT_TR2_CONFIG_PARAMS
+
+V=$(git version | sed -e 's/^git version //') && export V
+
+# There are multiple trace2 targets: normal, perf, and event.
+# Trace2 events will/can be written to each active target (subject
+# to whatever filtering that target decides to do).
+# This script tests the normal target in isolation.
+
+# Enable "normal" trace2 target.
+GIT_TR2="$(pwd)/trace.normal" && export GIT_TR2
+# Enable "brief" feature which turns off "<clock> <file>:<line> " prefix.
+GIT_TR2_BRIEF=1 && export GIT_TR2_BRIEF
+
+# Basic tests of the trace2 normal stream.  Since this stream is used
+# primarily with printf-style debugging/tracing, we do limited testing
+# here.
+#
+# We do confirm the following API features:
+# [] the 'version <v>' event
+# [] the 'start <argv>' event
+# [] the 'cmd_verb <verb>' event
+# [] the 'exit <time> code:<code>' event
+# [] the 'atexit <time> code:<code>' event
+#
+# Fields of the form _FIELD_ are tokens that have been replaced (such
+# as the elapsed time).
+
+# Verb 001return
+#
+# Implicit return from cmd_<verb> function propagates <code>.
+
+test_expect_success 'normal stream, return code 0' '
+	test_when_finished "rm trace.normal actual expect" &&
+	$TT trace2 001return 0 &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 001return 0
+		cmd_verb trace2 (trace2)
+		exit elapsed:_TIME_ code:0
+		atexit elapsed:_TIME_ code:0
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'normal stream, return code 1' '
+	test_when_finished "rm trace.normal actual expect" &&
+	test_must_fail $TT trace2 001return 1 &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 001return 1
+		cmd_verb trace2 (trace2)
+		exit elapsed:_TIME_ code:1
+		atexit elapsed:_TIME_ code:1
+	EOF
+	test_cmp expect actual
+'
+
+# Verb 002exit
+#
+# Explicit exit(code) from within cmd_<verb> propagates <code>.
+
+test_expect_success 'normal stream, exit code 0' '
+	test_when_finished "rm trace.normal actual expect" &&
+	$TT trace2 002exit 0 &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 002exit 0
+		cmd_verb trace2 (trace2)
+		exit elapsed:_TIME_ code:0
+		atexit elapsed:_TIME_ code:0
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'normal stream, exit code 1' '
+	test_when_finished "rm trace.normal actual expect" &&
+	test_must_fail $TT trace2 002exit 1 &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 002exit 1
+		cmd_verb trace2 (trace2)
+		exit elapsed:_TIME_ code:1
+		atexit elapsed:_TIME_ code:1
+	EOF
+	test_cmp expect actual
+'
+
+# Verb 003error
+#
+# To the above, add multiple 'error <msg>' events
+
+test_expect_success 'normal stream, error event' '
+	test_when_finished "rm trace.normal actual expect" &&
+	$TT trace2 003error "hello world" "this is a test" &&
+	perl "$TEST_DIRECTORY/t0210/scrub_normal.perl" <trace.normal >actual &&
+	cat >expect <<-EOF &&
+		version $V
+		start _EXE_ trace2 003error '\''hello world'\'' '\''this is a test'\''
+		cmd_verb trace2 (trace2)
+		error hello world
+		error this is a test
+		exit elapsed:_TIME_ code:0
+		atexit elapsed:_TIME_ code:0
+	EOF
+	test_cmp expect actual
+'
+
+test_done
diff --git a/t/t0210/scrub_normal.perl b/t/t0210/scrub_normal.perl
new file mode 100644
index 0000000000..c65d1a815e
--- /dev/null
+++ b/t/t0210/scrub_normal.perl
@@ -0,0 +1,48 @@
+#!/usr/bin/perl
+#
+# Scrub the variable fields from the normal trace2 output to
+# make testing easier.
+
+use strict;
+use warnings;
+
+my $float = '[0-9]*\.[0-9]+([eE][-+]?[0-9]+)?';
+
+# This code assumes that the trace2 data was written with bare
+# turned on (which omits the "<clock> <file>:<line>" prefix.
+
+while (<>) {
+    # Various messages include an elapsed time in the middle
+    # of the message.  Replace the time with a placeholder to
+    # simplify our HEREDOC in the test script.
+    s/elapsed:$float/elapsed:_TIME_/g;
+
+    my $line = $_;
+
+    # we expect:
+    #    start <argv0> [<argv1> [<argv2> [...]]]
+    #
+    # where argv0 might be a relative or absolute path, with
+    # or without quotes, and platform dependent.  Replace argv0
+    # with a token for HEREDOC matching in the test script.
+
+    if ($line =~ m/^start/) {
+	$line =~ /^start\s+(.*)/;
+	my $argv = $1;
+	$argv =~ m/(\'[^\']*\'|[^ ]+)\s+(.*)/;
+	my $argv_0 = $1;
+	my $argv_rest = $2;
+
+	print "start _EXE_ $argv_rest\n";
+    }
+    elsif ($line =~ m/^cmd_path/) {
+	# Likewise, the 'cmd_path' message breaks out argv[0].
+	#
+	# This line is only emitted when RUNTIME_PREFIX is defined,
+	# so just omit it for testing purposes.
+	# print "cmd_path _EXE_\n";
+    }
+    else {
+	print "$line";
+    }
+}
diff --git a/t/t0211-trace2-perf.sh b/t/t0211-trace2-perf.sh
new file mode 100755
index 0000000000..6146c3f6e3
--- /dev/null
+++ b/t/t0211-trace2-perf.sh
@@ -0,0 +1,153 @@
+#!/bin/sh
+
+test_description='test trace2 facility (perf target)'
+. ./test-lib.sh
+
+# Add t/helper directory to PATH so that we can use a relative
+# path to run nested instances of test-tool.exe (see 004child).
+# This helps with HEREDOC comparisons later.
+TTDIR="$GIT_BUILD_DIR/t/helper/" && export TTDIR
+PATH="$TTDIR:$PATH" && export PATH
+
+TT="test-tool" && export TT
+
+# Warning: use of 'test_cmp' may run test-tool.exe and/or git.exe
+# Warning: to do the actual diff/comparison, so the HEREDOCs here
+# Warning: only cover our actual calls to test-tool and/or git.
+# Warning: So you may see extra lines in artifact files when
+# Warning: interactively debugging.
+
+# Turn off any inherited trace2 settings for this test.
+unset GIT_TR2 GIT_TR2_PERF GIT_TR2_EVENT
+unset GIT_TR2_PERF_BRIEF
+unset GIT_TR2_CONFIG_PARAMS
+
+V=$(git version | sed -e 's/^git version //') && export V
+
+# There are multiple trace2 targets: normal, perf, and event.
+# Trace2 events will/can be written to each active target (subject
+# to whatever filtering that target decides to do).
+# Test each target independently.
+
+# Enable "perf" trace2 target.
+GIT_TR2_PERF="$(pwd)/trace.perf" && export GIT_TR2_PERF
+# Enable "brief" feature which turns off the prefix:
+#     "<clock> <file>:<line> | <nr_parents> | "
+GIT_TR2_PERF_BRIEF=1 && export GIT_TR2_PERF_BRIEF
+
+# Repeat some of the t0210 tests using the perf target stream instead of
+# the normal stream.
+#
+# Tokens here of the form _FIELD_ have been replaced in the observed output.
+
+# Verb 001return
+#
+# Implicit return from cmd_<verb> function propagates <code>.
+
+test_expect_success 'perf stream, return code 0' '
+	test_when_finished "rm trace.perf actual expect" &&
+	$TT trace2 001return 0 &&
+	perl "$TEST_DIRECTORY/t0211/scrub_perf.perl" <trace.perf >actual &&
+	cat >expect <<-EOF &&
+		d0|main|version|||||$V
+		d0|main|start|||||_EXE_ trace2 001return 0
+		d0|main|cmd_verb|||||trace2 (trace2)
+		d0|main|exit||_T_ABS_|||code:0
+		d0|main|atexit||_T_ABS_|||code:0
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'perf stream, return code 1' '
+	test_when_finished "rm trace.perf actual expect" &&
+	test_must_fail $TT trace2 001return 1 &&
+	perl "$TEST_DIRECTORY/t0211/scrub_perf.perl" <trace.perf >actual &&
+	cat >expect <<-EOF &&
+		d0|main|version|||||$V
+		d0|main|start|||||_EXE_ trace2 001return 1
+		d0|main|cmd_verb|||||trace2 (trace2)
+		d0|main|exit||_T_ABS_|||code:1
+		d0|main|atexit||_T_ABS_|||code:1
+	EOF
+	test_cmp expect actual
+'
+
+# Verb 003error
+#
+# To the above, add multiple 'error <msg>' events
+
+test_expect_success 'perf stream, error event' '
+	test_when_finished "rm trace.perf actual expect" &&
+	$TT trace2 003error "hello world" "this is a test" &&
+	perl "$TEST_DIRECTORY/t0211/scrub_perf.perl" <trace.perf >actual &&
+	cat >expect <<-EOF &&
+		d0|main|version|||||$V
+		d0|main|start|||||_EXE_ trace2 003error '\''hello world'\'' '\''this is a test'\''
+		d0|main|cmd_verb|||||trace2 (trace2)
+		d0|main|error|||||hello world
+		d0|main|error|||||this is a test
+		d0|main|exit||_T_ABS_|||code:0
+		d0|main|atexit||_T_ABS_|||code:0
+	EOF
+	test_cmp expect actual
+'
+
+# Verb 004child
+#
+# Test nested spawning of child processes.
+#
+# Conceptually, this looks like:
+#    P1: TT trace2 004child
+#    P2: |--- TT trace2 004child
+#    P3:      |--- TT trace2 001return 0
+#
+# Which should generate events:
+#    P1: version
+#    P1: start
+#    P1: cmd_verb
+#    P1: child_start
+#        P2: version
+#        P2: start
+#        P2: cmd_verb
+#        P2: child_start
+#            P3: version
+#            P3: start
+#            P3: cmd_verb
+#            P3: exit
+#            P3: atexit
+#        P2: child_exit
+#        P2: exit
+#        P2: atexit
+#    P1: child_exit
+#    P1: exit
+#    P1: atexit
+
+test_expect_success 'perf stream, child processes' '
+	test_when_finished "rm trace.perf actual expect" &&
+	$TT trace2 004child $TT trace2 004child $TT trace2 001return 0 &&
+	perl "$TEST_DIRECTORY/t0211/scrub_perf.perl" <trace.perf >actual &&
+	cat >expect <<-EOF &&
+		d0|main|version|||||$V
+		d0|main|start|||||_EXE_ trace2 004child $TT trace2 004child $TT trace2 001return 0
+		d0|main|cmd_verb|||||trace2 (trace2)
+		d0|main|child_start||_T_ABS_|||[ch0] class:? argv: $TT trace2 004child $TT trace2 001return 0
+		d1|main|version|||||$V
+		d1|main|start|||||_EXE_ trace2 004child $TT trace2 001return 0
+		d1|main|cmd_verb|||||trace2 (trace2/trace2)
+		d1|main|child_start||_T_ABS_|||[ch0] class:? argv: $TT trace2 001return 0
+		d2|main|version|||||$V
+		d2|main|start|||||_EXE_ trace2 001return 0
+		d2|main|cmd_verb|||||trace2 (trace2/trace2/trace2)
+		d2|main|exit||_T_ABS_|||code:0
+		d2|main|atexit||_T_ABS_|||code:0
+		d1|main|child_exit||_T_ABS_|_T_REL_||[ch0] pid:_PID_ code:0
+		d1|main|exit||_T_ABS_|||code:0
+		d1|main|atexit||_T_ABS_|||code:0
+		d0|main|child_exit||_T_ABS_|_T_REL_||[ch0] pid:_PID_ code:0
+		d0|main|exit||_T_ABS_|||code:0
+		d0|main|atexit||_T_ABS_|||code:0
+	EOF
+	test_cmp expect actual
+'
+
+test_done
diff --git a/t/t0211/scrub_perf.perl b/t/t0211/scrub_perf.perl
new file mode 100644
index 0000000000..351af7844e
--- /dev/null
+++ b/t/t0211/scrub_perf.perl
@@ -0,0 +1,76 @@
+#!/usr/bin/perl
+#
+# Scrub the variable fields from the perf trace2 output to
+# make testing easier.
+
+use strict;
+use warnings;
+
+my $qpath = '\'[^\']*\'|[^ ]*';
+
+my $col_depth=0;
+my $col_thread=1;
+my $col_event=2;
+my $col_repo=3;
+my $col_t_abs=4;
+my $col_t_rel=5;
+my $col_category=6;
+my $col_rest=7;
+
+# This code assumes that the trace2 data was written with bare
+# turned on (which omits the "<clock> <file>:<line> | <parents>"
+# prefix.
+
+while (<>) {
+    my @tokens = split /\|/;
+
+    foreach my $col (@tokens) { $col =~ s/^\s+|\s+$//g; }
+
+    if ($tokens[$col_event] =~ m/^start/) {
+	# The 'start' message lists the contents of argv in $col_rest.
+	# On some platforms (Windows), argv[0] is *sometimes* a canonical
+	# absolute path to the EXE rather than the value passed in the
+	# shell script.  Replace it with a placeholder to simplify our
+	# HEREDOC in the test script.
+	my $argv0;
+	my $argvRest;
+	$tokens[$col_rest] =~ s/^($qpath)\W*(.*)/_EXE_ $2/;
+    }
+    elsif ($tokens[$col_event] =~ m/cmd_path/) {
+	# Likewise, the 'cmd_path' message breaks out argv[0].
+	#
+	# This line is only emitted when RUNTIME_PREFIX is defined,
+	# so just omit it for testing purposes.
+	# $tokens[$col_rest] = "_EXE_";
+	goto SKIP_LINE;
+    }
+    elsif ($tokens[$col_event] =~ m/child_exit/) {
+	$tokens[$col_rest] =~ s/ pid:\d* / pid:_PID_ /;
+    }
+    elsif ($tokens[$col_event] =~ m/data/) {
+	if ($tokens[$col_category] =~ m/process/) {
+	    # 'data' and 'data_json' events containing 'process'
+	    # category data are assumed to be platform-specific
+	    # and highly variable.  Just omit them.
+	    goto SKIP_LINE;
+	}
+    }
+
+    # t_abs and t_rel are either blank or a float.  Replace the float
+    # with a constant for matching the HEREDOC in the test script.
+    if ($tokens[$col_t_abs] =~ m/\d/) {
+	$tokens[$col_t_abs] = "_T_ABS_";
+    }
+    if ($tokens[$col_t_rel] =~ m/\d/) {
+	$tokens[$col_t_rel] = "_T_REL_";
+    }
+
+    my $out;
+
+    $out = join('|', @tokens);
+    print "$out\n";
+
+  SKIP_LINE:
+}
+
+
diff --git a/t/t0212-trace2-event.sh b/t/t0212-trace2-event.sh
new file mode 100755
index 0000000000..5a9527c34f
--- /dev/null
+++ b/t/t0212-trace2-event.sh
@@ -0,0 +1,237 @@
+#!/bin/sh
+
+test_description='test trace2 facility'
+. ./test-lib.sh
+
+# Add t/helper directory to PATH so that we can use a relative
+# path to run nested instances of test-tool.exe (see 004child).
+# This helps with HEREDOC comparisons later.
+TTDIR="$GIT_BUILD_DIR/t/helper/" && export TTDIR
+PATH="$TTDIR:$PATH" && export PATH
+
+TT="test-tool" && export TT
+
+# Warning: use of 'test_cmp' may run test-tool.exe and/or git.exe
+# Warning: to do the actual diff/comparison, so the HEREDOCs here
+# Warning: only cover our actual calls to test-tool and/or git.
+# Warning: So you may see extra lines in artifact files when
+# Warning: interactively debugging.
+
+# Turn off any inherited trace2 settings for this test.
+unset GIT_TR2 GIT_TR2_PERF GIT_TR2_EVENT
+unset GIT_TR2_BARE
+unset GIT_TR2_CONFIG_PARAMS
+
+V=$(git version | sed -e 's/^git version //') && export V
+
+# There are multiple trace2 targets: normal, perf, and event.
+# Trace2 events will/can be written to each active target (subject
+# to whatever filtering that target decides to do).
+# Test each target independently.
+
+# Enable "event" trace2 target.
+GIT_TR2_EVENT="$(pwd)/trace.event" && export GIT_TR2_EVENT
+
+# We don't bother repeating the 001return and 002exit tests, since they
+# have coverage in the normal and perf targets.
+
+# Verb 003error
+#
+# To the above, add multiple 'error <msg>' events
+
+test_expect_success 'event stream, error event' '
+	test_when_finished "rm trace.event actual expect" &&
+	$TT trace2 003error "hello world" "this is a test" &&
+	perl "$TEST_DIRECTORY/t0212/parse_events.perl" <trace.event >actual &&
+	sed -e "s/^|//" >expect <<-EOF &&
+	|VAR1 = {
+	|  "_SID0_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "003error",
+	|      "hello world",
+	|      "this is a test"
+	|    ],
+	|    "errors":[
+	|      "%s",
+	|      "%s"
+	|    ],
+	|    "exit_code":0,
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2",
+	|    "version":"$V"
+	|  }
+	|};
+	EOF
+	test_cmp expect actual
+'
+
+# Verb 004child
+#
+# Test nested spawning of child processes.
+#
+# Conceptually, this looks like:
+#    P1: TT trace2 004child
+#    P2: |--- TT trace2 004child
+#    P3:      |--- TT trace2 001return 0
+
+test_expect_success 'event stream, return code 0' '
+#	test_when_finished "rm trace.event actual expect" &&
+	$TT trace2 004child $TT trace2 004child $TT trace2 001return 0 &&
+	perl "$TEST_DIRECTORY/t0212/parse_events.perl" <trace.event >actual &&
+	sed -e "s/^|//" >expect <<-EOF &&
+	|VAR1 = {
+	|  "_SID0_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "004child",
+	|      "$TT",
+	|      "trace2",
+	|      "004child",
+	|      "$TT",
+	|      "trace2",
+	|      "001return",
+	|      "0"
+	|    ],
+	|    "child":{
+	|      "0":{
+	|        "child_argv":[
+	|          "_EXE_",
+	|          "trace2",
+	|          "004child",
+	|          "$TT",
+	|          "trace2",
+	|          "001return",
+	|          "0"
+	|        ],
+	|        "child_class":"?",
+	|        "child_code":0,
+	|        "use_shell":0
+	|      }
+	|    },
+	|    "exit_code":0,
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2",
+	|    "version":"$V"
+	|  },
+	|  "_SID0_/_SID1_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "004child",
+	|      "$TT",
+	|      "trace2",
+	|      "001return",
+	|      "0"
+	|    ],
+	|    "child":{
+	|      "0":{
+	|        "child_argv":[
+	|          "_EXE_",
+	|          "trace2",
+	|          "001return",
+	|          "0"
+	|        ],
+	|        "child_class":"?",
+	|        "child_code":0,
+	|        "use_shell":0
+	|      }
+	|    },
+	|    "exit_code":0,
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2/trace2",
+	|    "version":"$V"
+	|  },
+	|  "_SID0_/_SID1_/_SID2_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "001return",
+	|      "0"
+	|    ],
+	|    "exit_code":0,
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2/trace2/trace2",
+	|    "version":"$V"
+	|  }
+	|};
+	EOF
+	test_cmp expect actual
+'
+
+# Test listing of all "interesting" config settings.
+
+test_expect_success 'event stream, list config' '
+	test_when_finished "rm trace.event actual expect" &&
+	git config --local t0212.abc 1 &&
+	git config --local t0212.def "hello world" &&
+	# delete events generated by the above config commands
+	rm trace.event &&
+	GIT_TR2_CONFIG_PARAMS="t0212.*" $TT trace2 001return 0 &&
+	perl "$TEST_DIRECTORY/t0212/parse_events.perl" <trace.event >actual &&
+	sed -e "s/^|//" >expect <<-EOF &&
+	|VAR1 = {
+	|  "_SID0_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "001return",
+	|      "0"
+	|    ],
+	|    "exit_code":0,
+	|    "params":[
+	|      {
+	|        "param":"t0212.abc",
+	|        "value":"1"
+	|      },
+	|      {
+	|        "param":"t0212.def",
+	|        "value":"hello world"
+	|      }
+	|    ],
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2",
+	|    "version":"$V"
+	|  }
+	|};
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'basic trace2_data' '
+	test_when_finished "rm trace.event actual expect" &&
+	$TT trace2 006data test_category k1 v1 test_category k2 v2 &&
+	perl "$TEST_DIRECTORY/t0212/parse_events.perl" <trace.event >actual &&
+	sed -e "s/^|//" >expect <<-EOF &&
+	|VAR1 = {
+	|  "_SID0_":{
+	|    "argv":[
+	|      "_EXE_",
+	|      "trace2",
+	|      "006data",
+	|      "test_category",
+	|      "k1",
+	|      "v1",
+	|      "test_category",
+	|      "k2",
+	|      "v2"
+	|    ],
+	|    "data":{
+	|      "test_category":{
+	|        "k1":"v1",
+	|        "k2":"v2"
+	|      }
+	|    },
+	|    "exit_code":0,
+	|    "verb":"trace2",
+	|    "verb_hierarchy":"trace2",
+	|    "version":"$V"
+	|  }
+	|};
+	EOF
+	test_cmp expect actual
+'
+
+test_done
diff --git a/t/t0212/parse_events.perl b/t/t0212/parse_events.perl
new file mode 100644
index 0000000000..0e38fa6db6
--- /dev/null
+++ b/t/t0212/parse_events.perl
@@ -0,0 +1,251 @@
+#!/usr/bin/perl
+#
+# Parse event stream and convert individual events into a summary
+# record for the process.
+#
+# Git.exe generates one or more "event" records for each API method,
+# such as "start <argv>" and "exit <code>", during the life of the git
+# process.  Additionally, the input may contain interleaved events
+# from multiple concurrent git processes and/or multiple threads from
+# within a git process.
+#
+# Accumulate events for each process (based on its unique SID) in a
+# dictionary and emit process summary records.
+#
+# Convert some of the variable fields (such as elapsed time) into
+# placeholders (or omit them) to make HEREDOC comparisons easier in
+# the test scripts.
+#
+# We may also omit fields not (currently) useful for testing purposes.
+
+use strict;
+use warnings;
+use JSON::PP;
+use Data::Dumper;
+use Getopt::Long;
+
+# The version of the trace2 event target format that we understand.
+# This is reported in the 'version' event in the 'evt' field.
+# It comes from the GIT_TR2_EVENT_VERSION macro in trace2/tr2_tgt_event.c
+my $evt_version = '1';
+
+my $show_children = 1;
+my $show_exec     = 1;
+my $show_threads  = 1;
+
+# A hack to generate test HEREDOC data for pasting into the test script.
+# Usage:
+#    cd "t/trash directory.t0212-trace2-event"
+#    $TT trace ... >trace.event
+#    VV=$(../../git.exe version | sed -e 's/^git version //')
+#    perl ../t0212/parse_events.perl --HEREDOC --VERSION=$VV <trace.event >heredoc
+# Then paste heredoc into your new test.
+
+my $gen_heredoc = 0;
+my $gen_version = '';
+
+GetOptions("children!" => \$show_children,
+	   "exec!"     => \$show_exec,
+	   "threads!"  => \$show_threads,
+	   "HEREDOC!"  => \$gen_heredoc,
+	   "VERSION=s" => \$gen_version    )
+    or die("Error in command line arguments\n");
+
+
+# SIDs contains timestamps and PIDs of the process and its parents.
+# This makes it difficult to match up in a HEREDOC in the test script.
+# Build a map from actual SIDs to predictable constant values and yet
+# keep the parent/child relationships.  For example:
+# {..., "sid":"1539706952458276-8652", ...}
+# {..., "sid":"1539706952458276-8652/1539706952649493-15452", ...}
+# becomes:
+# {..., "sid":"_SID1_", ...}
+# {..., "sid":"_SID1_/_SID2_", ...}
+my $sid_map;
+my $sid_count = 0;
+
+my $processes;
+
+while (<>) {
+    my $line = decode_json( $_ );
+
+    my $sid = "";
+    my $sid_sep = "";
+
+    my $raw_sid = $line->{'sid'};
+    my @raw_sid_parts = split /\//, $raw_sid;
+    foreach my $raw_sid_k (@raw_sid_parts) {
+	if (!exists $sid_map->{$raw_sid_k}) {
+	    $sid_map->{$raw_sid_k} = '_SID' . $sid_count . '_';
+	    $sid_count++;
+	}
+	$sid = $sid . $sid_sep . $sid_map->{$raw_sid_k};
+	$sid_sep = '/';
+    }
+    
+    my $event = $line->{'event'};
+
+    if ($event eq 'version') {
+	$processes->{$sid}->{'version'} = $line->{'exe'};
+	if ($gen_heredoc == 1 && $gen_version eq $line->{'exe'}) {
+	    # If we are generating data FOR the test script, replace
+	    # the reported git.exe version with a reference to an
+	    # environment variable.  When our output is pasted into
+	    # the test script, it will then be expanded in future
+	    # test runs to the THEN current version of git.exe.
+	    # We assume that the test script uses env var $V.
+	    $processes->{$sid}->{'version'} = "\$V";
+	}
+    }
+
+    elsif ($event eq 'start') {
+	$processes->{$sid}->{'argv'} = $line->{'argv'};
+	$processes->{$sid}->{'argv'}[0] = "_EXE_";
+    }
+
+    elsif ($event eq 'exit') {
+	$processes->{$sid}->{'exit_code'} = $line->{'code'};
+    }
+
+    elsif ($event eq 'atexit') {
+	$processes->{$sid}->{'exit_code'} = $line->{'code'};
+    }
+
+    elsif ($event eq 'error') {
+	# For HEREDOC purposes, use the error message format string if
+	# available, rather than the formatted message (which probably
+	# has an absolute pathname).
+	if (exists $line->{'fmt'}) {
+	    push( @{$processes->{$sid}->{'errors'}}, $line->{'fmt'} );
+	}
+	elsif (exists $line->{'msg'}) {
+	    push( @{$processes->{$sid}->{'errors'}}, $line->{'msg'} );
+	}
+    }
+
+    elsif ($event eq 'cmd_path') {
+	## $processes->{$sid}->{'path'} = $line->{'path'};
+	#
+	# Like in the 'start' event, we need to replace the value of
+	# argv[0] with a token for HEREDOC purposes.  However, the
+	# event is only emitted when RUNTIME_PREFIX is defined, so
+	# just omit it for testing purposes.
+	# $processes->{$sid}->{'path'} = "_EXE_";
+    }
+    
+    elsif ($event eq 'cmd_verb') {
+	$processes->{$sid}->{'verb'} = $line->{'name'};
+	$processes->{$sid}->{'verb_hierarchy'} = $line->{'hierarchy'};
+    }
+
+    elsif ($event eq 'alias') {
+	$processes->{$sid}->{'alias'}->{'key'} = $line->{'alias'};
+	$processes->{$sid}->{'alias'}->{'argv'} = $line->{'argv'};
+    }
+
+    elsif ($event eq 'def_param') {
+	my $kv;
+	$kv->{'param'} = $line->{'param'};
+	$kv->{'value'} = $line->{'value'};
+	push( @{$processes->{$sid}->{'params'}}, $kv );
+    }
+
+    elsif ($event eq 'child_start') {
+	if ($show_children == 1) {
+	    $processes->{$sid}->{'child'}->{$line->{'child_id'}}->{'child_class'} = $line->{'child_class'};
+	    $processes->{$sid}->{'child'}->{$line->{'child_id'}}->{'child_argv'} = $line->{'argv'};
+	    $processes->{$sid}->{'child'}->{$line->{'child_id'}}->{'child_argv'}[0] = "_EXE_";
+	    $processes->{$sid}->{'child'}->{$line->{'child_id'}}->{'use_shell'} = $line->{'use_shell'} ? 1 : 0;
+	}
+    }
+
+    elsif ($event eq 'child_exit') {
+	if ($show_children == 1) {
+	    $processes->{$sid}->{'child'}->{$line->{'child_id'}}->{'child_code'} = $line->{'code'};
+	}
+    }
+
+    # TODO decide what information we want to test from thread events.
+
+    elsif ($event eq 'thread_start') {
+	if ($show_threads == 1) {
+	}
+    }
+
+    elsif ($event eq 'thread_exit') {
+	if ($show_threads == 1) {
+	}
+    }
+
+    # TODO decide what information we want to test from exec events.
+
+    elsif ($event eq 'exec') {
+	if ($show_exec == 1) {
+	}
+    }
+
+    elsif ($event eq 'exec_result') {
+	if ($show_exec == 1) {
+	}
+    }
+
+    elsif ($event eq 'def_param') {
+	# Accumulate parameter key/value pairs by key rather than in an array
+	# so that we get overwrite (last one wins) effects.
+	$processes->{$sid}->{'params'}->{$line->{'param'}} = $line->{'value'};
+    }
+
+    elsif ($event eq 'def_repo') {
+	# $processes->{$sid}->{'repos'}->{$line->{'repo'}} = $line->{'worktree'};
+	$processes->{$sid}->{'repos'}->{$line->{'repo'}} = "_WORKTREE_";
+    }
+
+    # A series of potentially nested and threaded region and data events
+    # is fundamentally incompatibile with the type of summary record we
+    # are building in this script.  Since they are intended for
+    # perf-trace-like analysis rather than a result summary, we ignore
+    # most of them here.
+
+    # elsif ($event eq 'region_enter') {
+    # }
+    # elsif ($event eq 'region_leave') {
+    # }
+
+    elsif ($event eq 'data') {
+	my $cat = $line->{'category'};
+	if ($cat eq 'test_category') {
+	    
+	    my $key = $line->{'key'};
+	    my $value = $line->{'value'};
+	    $processes->{$sid}->{'data'}->{$cat}->{$key} = $value;
+	}
+    }
+
+    # This trace2 target does not emit 'printf' events.
+    #
+    # elsif ($event eq 'printf') {
+    # }
+}
+
+# Dump the resulting hash into something that we can compare against
+# in the test script.  These options make Dumper output look a little
+# bit like JSON.  Also convert variable references of the form "$VAR*"
+# so that the matching HEREDOC doesn't need to escape it.
+
+$Data::Dumper::Sortkeys = 1;
+$Data::Dumper::Indent = 1;
+$Data::Dumper::Purity = 1;
+$Data::Dumper::Pair = ':';
+
+my $out = Dumper($processes);
+$out =~ s/'/"/g;
+$out =~ s/\$VAR/VAR/g;
+
+# Finally, if we're running this script to generate (manually confirmed)
+# data to add to the test script, guard the indentation.
+
+if ($gen_heredoc == 1) {
+    $out =~ s/^/\t\|/gms;
+}
+
+print $out;