diff mbox series

[09/23] fsmonitor--daemon: implement daemon command options

Message ID 2b291d805d59b54203d939e048bb568782d5e17b.1617291666.git.gitgitgadget@gmail.com (mailing list archive)
State New
Headers show
Series Builtin FSMonitor Feature | expand

Commit Message

Jeff Hostetler April 1, 2021, 3:40 p.m. UTC
From: Jeff Hostetler <jeffhost@microsoft.com>

Implement command options `--run` and `--start` to try to
begin listening for file system events.

This version defines the thread structure with a single
fsmonitor_fs_listen thread to watch for file system events
and a simple IPC thread pool to wait for connections from
Git clients over a well-known named pipe or Unix domain
socket.

This version does not actually do anything yet because the
backends are still just stubs.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 builtin/fsmonitor--daemon.c | 395 +++++++++++++++++++++++++++++++++++-
 fsmonitor--daemon.h         |  36 ++++
 2 files changed, 430 insertions(+), 1 deletion(-)
 create mode 100644 fsmonitor--daemon.h

Comments

Derrick Stolee April 26, 2021, 3:47 p.m. UTC | #1
On 4/1/21 11:40 AM, Jeff Hostetler via GitGitGadget wrote:
> From: Jeff Hostetler <jeffhost@microsoft.com>
...
> +	/* Prepare to (recursively) watch the <worktree-root> directory. */
> +	strbuf_init(&state.path_worktree_watch, 0);
> +	strbuf_addstr(&state.path_worktree_watch, absolute_path(get_git_work_tree()));
> +	state.nr_paths_watching = 1;

Yes, let's watch the working directory.

> +	/*
> +	 * If ".git" is not a directory, then <gitdir> is not inside the
> +	 * cone of <worktree-root>, so set up a second watch for it.
> +	 */
> +	strbuf_init(&state.path_gitdir_watch, 0);
> +	strbuf_addbuf(&state.path_gitdir_watch, &state.path_worktree_watch);
> +	strbuf_addstr(&state.path_gitdir_watch, "/.git");
> +	if (!is_directory(state.path_gitdir_watch.buf)) {
> +		strbuf_reset(&state.path_gitdir_watch);
> +		strbuf_addstr(&state.path_gitdir_watch, absolute_path(get_git_dir()));
> +		state.nr_paths_watching = 2;
> +	}

But why watch the .git directory, especially for a worktree (or
submodule I guess)? What benefit do we get from events within the
.git directory? I'm expecting any event within the .git directory
should be silently ignored.

> +
>  static int is_ipc_daemon_listening(void)
>  {
>  	return fsmonitor_ipc__get_state() == IPC_STATE__LISTENING;
>  }
>  
> +static int try_to_run_foreground_daemon(void)
> +{
> +	/*
> +	 * Technically, we don't need to probe for an existing daemon
> +	 * process, since we could just call `fsmonitor_run_daemon()`
> +	 * and let it fail if the pipe/socket is busy.
> +	 *
> +	 * However, this method gives us a nicer error message for a
> +	 * common error case.
> +	 */
> +	if (is_ipc_daemon_listening())
> +		die("fsmonitor--daemon is already running.");
Here, it seems like we only care about IPC_STATE_LISTENING, while
earlier I mentioned that I ended up in IPC_STATE__NOT_LISTENING,
and my manually running of the daemon helped.

> +	return !!fsmonitor_run_daemon();
> +}

You are ignoring the IPC_STATE__NOT_LISTENING and creating a new
process, which is good. I'm just wondering why that state exists
and what is the proper way to handle it?

> +
> +#ifndef GIT_WINDOWS_NATIVE

You are already creating a platform-specific mechanism for the
filesystem watcher. Shouldn't the implementation of this method
be part of that file in compat/fsmonitor/?

I guess the biggest reason is that macOS and Linux share this
implementation, so maybe this is the cleanest approach.

> +
> +/*
> + * This is adapted from `wait_or_whine()`.  Watch the child process and
> + * let it get started and begin listening for requests on the socket
> + * before reporting our success.
> + */
> +static int wait_for_background_startup(pid_t pid_child)
> +{
> +	int status;
> +	pid_t pid_seen;
> +	enum ipc_active_state s;
> +	time_t time_limit, now;
> +
> +	time(&time_limit);
> +	time_limit += fsmonitor__start_timeout_sec;
> +
> +	for (;;) {
> +		pid_seen = waitpid(pid_child, &status, WNOHANG);
> +
> +		if (pid_seen == -1)
> +			return error_errno(_("waitpid failed"));
> +
> +		else if (pid_seen == 0) {

There is some non-standard whitespace throughout this
if/else if/else:
...> +			continue;
> +		}
> +
> +		else if (pid_seen == pid_child) {
...
> +			return error(_("fsmonitor--daemon failed to start"));
> +		}
> +
> +		else
> +			return error(_("waitpid is confused"));

The rest of the glue in this patch looks good.

Thanks,
-Stolee
Derrick Stolee April 26, 2021, 4:12 p.m. UTC | #2
On 4/26/2021 11:47 AM, Derrick Stolee wrote:
> On 4/1/21 11:40 AM, Jeff Hostetler via GitGitGadget wrote:
>> From: Jeff Hostetler <jeffhost@microsoft.com>
> ...
>> +	/* Prepare to (recursively) watch the <worktree-root> directory. */
>> +	strbuf_init(&state.path_worktree_watch, 0);
>> +	strbuf_addstr(&state.path_worktree_watch, absolute_path(get_git_work_tree()));
>> +	state.nr_paths_watching = 1;
> 
> Yes, let's watch the working directory.
> 
>> +	/*
>> +	 * If ".git" is not a directory, then <gitdir> is not inside the
>> +	 * cone of <worktree-root>, so set up a second watch for it.
>> +	 */
>> +	strbuf_init(&state.path_gitdir_watch, 0);
>> +	strbuf_addbuf(&state.path_gitdir_watch, &state.path_worktree_watch);
>> +	strbuf_addstr(&state.path_gitdir_watch, "/.git");
>> +	if (!is_directory(state.path_gitdir_watch.buf)) {
>> +		strbuf_reset(&state.path_gitdir_watch);
>> +		strbuf_addstr(&state.path_gitdir_watch, absolute_path(get_git_dir()));
>> +		state.nr_paths_watching = 2;
>> +	}
> 
> But why watch the .git directory, especially for a worktree (or
> submodule I guess)? What benefit do we get from events within the
> .git directory? I'm expecting any event within the .git directory
> should be silently ignored.

I see in a following patch that we place a cookie file within the
.git directory. I'm reminded that this is done for a reason: other
filesystem watchers can get into a loop if we place the cookie
file outside of the .git directory. The classic example is VS Code
running 'git status' in a loop because Watchman writes a cookie
into the root of the working directory.

Thanks,
-Stolee
Jeff Hostetler April 30, 2021, 3:18 p.m. UTC | #3
On 4/26/21 12:12 PM, Derrick Stolee wrote:
> On 4/26/2021 11:47 AM, Derrick Stolee wrote:
>> On 4/1/21 11:40 AM, Jeff Hostetler via GitGitGadget wrote:
>>> From: Jeff Hostetler <jeffhost@microsoft.com>
>> ...
>>> +	/* Prepare to (recursively) watch the <worktree-root> directory. */
>>> +	strbuf_init(&state.path_worktree_watch, 0);
>>> +	strbuf_addstr(&state.path_worktree_watch, absolute_path(get_git_work_tree()));
>>> +	state.nr_paths_watching = 1;
>>
>> Yes, let's watch the working directory.
>>
>>> +	/*
>>> +	 * If ".git" is not a directory, then <gitdir> is not inside the
>>> +	 * cone of <worktree-root>, so set up a second watch for it.
>>> +	 */
>>> +	strbuf_init(&state.path_gitdir_watch, 0);
>>> +	strbuf_addbuf(&state.path_gitdir_watch, &state.path_worktree_watch);
>>> +	strbuf_addstr(&state.path_gitdir_watch, "/.git");
>>> +	if (!is_directory(state.path_gitdir_watch.buf)) {
>>> +		strbuf_reset(&state.path_gitdir_watch);
>>> +		strbuf_addstr(&state.path_gitdir_watch, absolute_path(get_git_dir()));
>>> +		state.nr_paths_watching = 2;
>>> +	}
>>
>> But why watch the .git directory, especially for a worktree (or
>> submodule I guess)? What benefit do we get from events within the
>> .git directory? I'm expecting any event within the .git directory
>> should be silently ignored.
> 
> I see in a following patch that we place a cookie file within the
> .git directory. I'm reminded that this is done for a reason: other
> filesystem watchers can get into a loop if we place the cookie
> file outside of the .git directory. The classic example is VS Code
> running 'git status' in a loop because Watchman writes a cookie
> into the root of the working directory.

Yes.  I'll add a comment explaining the need for the second watch.
Jeff Hostetler April 30, 2021, 3:59 p.m. UTC | #4
On 4/26/21 11:47 AM, Derrick Stolee wrote:
...
> 
>> +
>>   static int is_ipc_daemon_listening(void)
>>   {
>>   	return fsmonitor_ipc__get_state() == IPC_STATE__LISTENING;
>>   }
>>   
>> +static int try_to_run_foreground_daemon(void)
>> +{
>> +	/*
>> +	 * Technically, we don't need to probe for an existing daemon
>> +	 * process, since we could just call `fsmonitor_run_daemon()`
>> +	 * and let it fail if the pipe/socket is busy.
>> +	 *
>> +	 * However, this method gives us a nicer error message for a
>> +	 * common error case.
>> +	 */
>> +	if (is_ipc_daemon_listening())
>> +		die("fsmonitor--daemon is already running.");
> Here, it seems like we only care about IPC_STATE_LISTENING, while
> earlier I mentioned that I ended up in IPC_STATE__NOT_LISTENING,
> and my manually running of the daemon helped.
> 
>> +	return !!fsmonitor_run_daemon();
>> +}
> 
> You are ignoring the IPC_STATE__NOT_LISTENING and creating a new
> process, which is good. I'm just wondering why that state exists
> and what is the proper way to handle it?

I'll revisit this and clarify.

> 
>> +
>> +#ifndef GIT_WINDOWS_NATIVE
> 
> You are already creating a platform-specific mechanism for the
> filesystem watcher. Shouldn't the implementation of this method
> be part of that file in compat/fsmonitor/?
> 
> I guess the biggest reason is that macOS and Linux share this
> implementation, so maybe this is the cleanest approach.

This has to do with how to spawn a background process and
disassociate from the console and all that.

On Windows, the "git fsmonitor--daemon --start" process[1] must
start a child process[2] with "git fsmonitor--daemon --run" and
then the [1] can exit (to let the caller/shell continue) while
[2] is free to continue.

On Unix, the "git fsmonitor-daemon --start" process[1] can
fork() a child process[2] and [2] can just call the _run_daemon()
code.  We don't need to exec the child, so this is a bit faster.

This code is platform-specific, so maybe it should go elsewhere,
but it has knowledge of fsmonitor--daemon-specific command line
args and private functions (`fsmonitor_run_daemon()`) and it knows
that it is not a library function.  So it made sense to keep it
close to the fsmonitor--daemon main entry point.


It didn't feel right to make these 2 versions of
`spawn_background_fsmonitor_daemon()` more generic (such as putting
them near `daemonize()`), because they know too much about
fsmonitor--daemon.

I did the same thing in `t/helper/test-simple-ipc.c` where I
created variants of this that started the test-tool in the background.
36a7eb6876 (t0052: add simple-ipc tests and t/helper/test-simple-ipc 
tool, 2021-03-22)


I thought about putting them inside `compat/fsmonitor/*.c`
but that has different problems.  Those files are concerned strictly
with the FS layer and how to get FS notification events from the
kernel and translating them into something cross-platform.  They
are, in a sense, a "driver" for that FS.  Process spawning is outside
of their scope.

And as you say, MacOS and Linux can both use the same process
spawning code, but will have vastly different FS layers.

So I left them here.


Thanks,
Jeff
diff mbox series

Patch

diff --git a/builtin/fsmonitor--daemon.c b/builtin/fsmonitor--daemon.c
index 10434bce4b64..23a063707972 100644
--- a/builtin/fsmonitor--daemon.c
+++ b/builtin/fsmonitor--daemon.c
@@ -3,10 +3,14 @@ 
 #include "parse-options.h"
 #include "fsmonitor.h"
 #include "fsmonitor-ipc.h"
+#include "compat/fsmonitor/fsmonitor-fs-listen.h"
+#include "fsmonitor--daemon.h"
 #include "simple-ipc.h"
 #include "khash.h"
 
 static const char * const builtin_fsmonitor__daemon_usage[] = {
+	N_("git fsmonitor--daemon --start [<options>]"),
+	N_("git fsmonitor--daemon --run [<options>]"),
 	N_("git fsmonitor--daemon --stop"),
 	N_("git fsmonitor--daemon --is-running"),
 	N_("git fsmonitor--daemon --query <token>"),
@@ -16,6 +20,38 @@  static const char * const builtin_fsmonitor__daemon_usage[] = {
 };
 
 #ifdef HAVE_FSMONITOR_DAEMON_BACKEND
+/*
+ * Global state loaded from config.
+ */
+#define FSMONITOR__IPC_THREADS "fsmonitor.ipcthreads"
+static int fsmonitor__ipc_threads = 8;
+
+#define FSMONITOR__START_TIMEOUT "fsmonitor.starttimeout"
+static int fsmonitor__start_timeout_sec = 60;
+
+static int fsmonitor_config(const char *var, const char *value, void *cb)
+{
+	if (!strcmp(var, FSMONITOR__IPC_THREADS)) {
+		int i = git_config_int(var, value);
+		if (i < 1)
+			return error(_("value of '%s' out of range: %d"),
+				     FSMONITOR__IPC_THREADS, i);
+		fsmonitor__ipc_threads = i;
+		return 0;
+	}
+
+	if (!strcmp(var, FSMONITOR__START_TIMEOUT)) {
+		int i = git_config_int(var, value);
+		if (i < 0)
+			return error(_("value of '%s' out of range: %d"),
+				     FSMONITOR__START_TIMEOUT, i);
+		fsmonitor__start_timeout_sec = i;
+		return 0;
+	}
+
+	return git_default_config(var, value, cb);
+}
+
 /*
  * Acting as a CLIENT.
  *
@@ -113,15 +149,350 @@  static int do_as_client__send_flush(void)
 	return 0;
 }
 
+static ipc_server_application_cb handle_client;
+
+static int handle_client(void *data, const char *command,
+			 ipc_server_reply_cb *reply,
+			 struct ipc_server_reply_data *reply_data)
+{
+	/* struct fsmonitor_daemon_state *state = data; */
+	int result;
+
+	trace2_region_enter("fsmonitor", "handle_client", the_repository);
+	trace2_data_string("fsmonitor", the_repository, "request", command);
+
+	result = 0; /* TODO Do something here. */
+
+	trace2_region_leave("fsmonitor", "handle_client", the_repository);
+
+	return result;
+}
+
+static void *fsmonitor_fs_listen__thread_proc(void *_state)
+{
+	struct fsmonitor_daemon_state *state = _state;
+
+	trace2_thread_start("fsm-listen");
+
+	trace_printf_key(&trace_fsmonitor, "Watching: worktree '%s'",
+			 state->path_worktree_watch.buf);
+	if (state->nr_paths_watching > 1)
+		trace_printf_key(&trace_fsmonitor, "Watching: gitdir '%s'",
+				 state->path_gitdir_watch.buf);
+
+	fsmonitor_fs_listen__loop(state);
+
+	trace2_thread_exit();
+	return NULL;
+}
+
+static int fsmonitor_run_daemon_1(struct fsmonitor_daemon_state *state)
+{
+	struct ipc_server_opts ipc_opts = {
+		.nr_threads = fsmonitor__ipc_threads,
+
+		/*
+		 * We know that there are no other active threads yet,
+		 * so we can let the IPC layer temporarily chdir() if
+		 * it needs to when creating the server side of the
+		 * Unix domain socket.
+		 */
+		.uds_disallow_chdir = 0
+	};
+
+	/*
+	 * Start the IPC thread pool before the we've started the file
+	 * system event listener thread so that we have the IPC handle
+	 * before we need it.
+	 */
+	if (ipc_server_run_async(&state->ipc_server_data,
+				 fsmonitor_ipc__get_path(), &ipc_opts,
+				 handle_client, state))
+		return error(_("could not start IPC thread pool"));
+
+	/*
+	 * Start the fsmonitor listener thread to collect filesystem
+	 * events.
+	 */
+	if (pthread_create(&state->listener_thread, NULL,
+			   fsmonitor_fs_listen__thread_proc, state) < 0) {
+		ipc_server_stop_async(state->ipc_server_data);
+		ipc_server_await(state->ipc_server_data);
+
+		return error(_("could not start fsmonitor listener thread"));
+	}
+
+	/*
+	 * The daemon is now fully functional in background threads.
+	 * Wait for the IPC thread pool to shutdown (whether by client
+	 * request or from filesystem activity).
+	 */
+	ipc_server_await(state->ipc_server_data);
+
+	/*
+	 * The fsmonitor listener thread may have received a shutdown
+	 * event from the IPC thread pool, but it doesn't hurt to tell
+	 * it again.  And wait for it to shutdown.
+	 */
+	fsmonitor_fs_listen__stop_async(state);
+	pthread_join(state->listener_thread, NULL);
+
+	return state->error_code;
+}
+
+static int fsmonitor_run_daemon(void)
+{
+	struct fsmonitor_daemon_state state;
+	int err;
+
+	memset(&state, 0, sizeof(state));
+
+	pthread_mutex_init(&state.main_lock, NULL);
+	state.error_code = 0;
+	state.current_token_data = NULL;
+	state.test_client_delay_ms = 0;
+
+	/* Prepare to (recursively) watch the <worktree-root> directory. */
+	strbuf_init(&state.path_worktree_watch, 0);
+	strbuf_addstr(&state.path_worktree_watch, absolute_path(get_git_work_tree()));
+	state.nr_paths_watching = 1;
+
+	/*
+	 * If ".git" is not a directory, then <gitdir> is not inside the
+	 * cone of <worktree-root>, so set up a second watch for it.
+	 */
+	strbuf_init(&state.path_gitdir_watch, 0);
+	strbuf_addbuf(&state.path_gitdir_watch, &state.path_worktree_watch);
+	strbuf_addstr(&state.path_gitdir_watch, "/.git");
+	if (!is_directory(state.path_gitdir_watch.buf)) {
+		strbuf_reset(&state.path_gitdir_watch);
+		strbuf_addstr(&state.path_gitdir_watch, absolute_path(get_git_dir()));
+		state.nr_paths_watching = 2;
+	}
+
+	/*
+	 * Confirm that we can create platform-specific resources for the
+	 * filesystem listener before we bother starting all the threads.
+	 */
+	if (fsmonitor_fs_listen__ctor(&state)) {
+		err = error(_("could not initialize listener thread"));
+		goto done;
+	}
+
+	err = fsmonitor_run_daemon_1(&state);
+
+done:
+	pthread_mutex_destroy(&state.main_lock);
+	fsmonitor_fs_listen__dtor(&state);
+
+	ipc_server_free(state.ipc_server_data);
+
+	strbuf_release(&state.path_worktree_watch);
+	strbuf_release(&state.path_gitdir_watch);
+
+	return err;
+}
+
 static int is_ipc_daemon_listening(void)
 {
 	return fsmonitor_ipc__get_state() == IPC_STATE__LISTENING;
 }
 
+static int try_to_run_foreground_daemon(void)
+{
+	/*
+	 * Technically, we don't need to probe for an existing daemon
+	 * process, since we could just call `fsmonitor_run_daemon()`
+	 * and let it fail if the pipe/socket is busy.
+	 *
+	 * However, this method gives us a nicer error message for a
+	 * common error case.
+	 */
+	if (is_ipc_daemon_listening())
+		die("fsmonitor--daemon is already running.");
+
+	return !!fsmonitor_run_daemon();
+}
+
+#ifndef GIT_WINDOWS_NATIVE
+/*
+ * This is adapted from `daemonize()`.  Use `fork()` to directly create
+ * and run the daemon in a child process.  The fork-parent returns the
+ * child PID so that we can wait for the child to startup before exiting.
+ */
+static int spawn_background_fsmonitor_daemon(pid_t *pid)
+{
+	*pid = fork();
+
+	switch (*pid) {
+	case 0:
+		if (setsid() == -1)
+			error_errno(_("setsid failed"));
+		close(0);
+		close(1);
+		close(2);
+		sanitize_stdfds();
+
+		return !!fsmonitor_run_daemon();
+
+	case -1:
+		return error_errno(_("could not spawn fsmonitor--daemon in the background"));
+
+	default:
+		return 0;
+	}
+}
+#else
+/*
+ * Conceptually like `daemonize()` but different because Windows does not
+ * have `fork(2)`.  Spawn a normal Windows child process but without the
+ * limitations of `start_command()` and `finish_command()`.
+ */
+static int spawn_background_fsmonitor_daemon(pid_t *pid)
+{
+	char git_exe[MAX_PATH];
+	struct strvec args = STRVEC_INIT;
+	int in, out;
+
+	GetModuleFileNameA(NULL, git_exe, MAX_PATH);
+
+	in = open("/dev/null", O_RDONLY);
+	out = open("/dev/null", O_WRONLY);
+
+	strvec_push(&args, git_exe);
+	strvec_push(&args, "fsmonitor--daemon");
+	strvec_push(&args, "--run");
+
+	*pid = mingw_spawnvpe(args.v[0], args.v, NULL, NULL, in, out, out);
+	close(in);
+	close(out);
+
+	strvec_clear(&args);
+
+	if (*pid < 0)
+		return error(_("could not spawn fsmonitor--daemon in the background"));
+
+	return 0;
+}
+#endif
+
+/*
+ * This is adapted from `wait_or_whine()`.  Watch the child process and
+ * let it get started and begin listening for requests on the socket
+ * before reporting our success.
+ */
+static int wait_for_background_startup(pid_t pid_child)
+{
+	int status;
+	pid_t pid_seen;
+	enum ipc_active_state s;
+	time_t time_limit, now;
+
+	time(&time_limit);
+	time_limit += fsmonitor__start_timeout_sec;
+
+	for (;;) {
+		pid_seen = waitpid(pid_child, &status, WNOHANG);
+
+		if (pid_seen == -1)
+			return error_errno(_("waitpid failed"));
+
+		else if (pid_seen == 0) {
+			/*
+			 * The child is still running (this should be
+			 * the normal case).  Try to connect to it on
+			 * the socket and see if it is ready for
+			 * business.
+			 *
+			 * If there is another daemon already running,
+			 * our child will fail to start (possibly
+			 * after a timeout on the lock), but we don't
+			 * care (who responds) if the socket is live.
+			 */
+			s = fsmonitor_ipc__get_state();
+			if (s == IPC_STATE__LISTENING)
+				return 0;
+
+			time(&now);
+			if (now > time_limit)
+				return error(_("fsmonitor--daemon not online yet"));
+
+			continue;
+		}
+
+		else if (pid_seen == pid_child) {
+			/*
+			 * The new child daemon process shutdown while
+			 * it was starting up, so it is not listening
+			 * on the socket.
+			 *
+			 * Try to ping the socket in the odd chance
+			 * that another daemon started (or was already
+			 * running) while our child was starting.
+			 *
+			 * Again, we don't care who services the socket.
+			 */
+			s = fsmonitor_ipc__get_state();
+			if (s == IPC_STATE__LISTENING)
+				return 0;
+
+			/*
+			 * We don't care about the WEXITSTATUS() nor
+			 * any of the WIF*(status) values because
+			 * `cmd_fsmonitor__daemon()` does the `!!result`
+			 * trick on all function return values.
+			 *
+			 * So it is sufficient to just report the
+			 * early shutdown as an error.
+			 */
+			return error(_("fsmonitor--daemon failed to start"));
+		}
+
+		else
+			return error(_("waitpid is confused"));
+	}
+}
+
+static int try_to_start_background_daemon(void)
+{
+	pid_t pid_child;
+	int ret;
+
+	/*
+	 * Before we try to create a background daemon process, see
+	 * if a daemon process is already listening.  This makes it
+	 * easier for us to report an already-listening error to the
+	 * console, since our spawn/daemon can only report the success
+	 * of creating the background process (and not whether it
+	 * immediately exited).
+	 */
+	if (is_ipc_daemon_listening())
+		die("fsmonitor--daemon is already running.");
+
+	/*
+	 * Run the actual daemon in a background process.
+	 */
+	ret = spawn_background_fsmonitor_daemon(&pid_child);
+	if (pid_child <= 0)
+		return ret;
+
+	/*
+	 * Wait (with timeout) for the background child process get
+	 * started and begin listening on the socket/pipe.  This makes
+	 * the "start" command more synchronous and more reliable in
+	 * tests.
+	 */
+	ret = wait_for_background_startup(pid_child);
+
+	return ret;
+}
+
 int cmd_fsmonitor__daemon(int argc, const char **argv, const char *prefix)
 {
 	enum daemon_mode {
 		UNDEFINED_MODE,
+		START,
+		RUN,
 		STOP,
 		IS_RUNNING,
 		QUERY,
@@ -130,6 +501,11 @@  int cmd_fsmonitor__daemon(int argc, const char **argv, const char *prefix)
 	} mode = UNDEFINED_MODE;
 
 	struct option options[] = {
+		OPT_CMDMODE(0, "start", &mode,
+			    N_("run the daemon in the background"),
+			    START),
+		OPT_CMDMODE(0, "run", &mode,
+			    N_("run the daemon in the foreground"), RUN),
 		OPT_CMDMODE(0, "stop", &mode, N_("stop the running daemon"),
 			    STOP),
 
@@ -145,18 +521,35 @@  int cmd_fsmonitor__daemon(int argc, const char **argv, const char *prefix)
 			    QUERY_INDEX),
 		OPT_CMDMODE(0, "flush", &mode, N_("flush cached filesystem events"),
 			    FLUSH),
+
+		OPT_GROUP(N_("Daemon options")),
+		OPT_INTEGER(0, "ipc-threads",
+			    &fsmonitor__ipc_threads,
+			    N_("use <n> ipc worker threads")),
+		OPT_INTEGER(0, "start-timeout",
+			    &fsmonitor__start_timeout_sec,
+			    N_("Max seconds to wait for background daemon startup")),
 		OPT_END()
 	};
 
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(builtin_fsmonitor__daemon_usage, options);
 
-	git_config(git_default_config, NULL);
+	git_config(fsmonitor_config, NULL);
 
 	argc = parse_options(argc, argv, prefix, options,
 			     builtin_fsmonitor__daemon_usage, 0);
+	if (fsmonitor__ipc_threads < 1)
+		die(_("invalid 'ipc-threads' value (%d)"),
+		    fsmonitor__ipc_threads);
 
 	switch (mode) {
+	case START:
+		return !!try_to_start_background_daemon();
+
+	case RUN:
+		return !!try_to_run_foreground_daemon();
+
 	case STOP:
 		return !!do_as_client__send_stop();
 
diff --git a/fsmonitor--daemon.h b/fsmonitor--daemon.h
new file mode 100644
index 000000000000..09e4a6fb6675
--- /dev/null
+++ b/fsmonitor--daemon.h
@@ -0,0 +1,36 @@ 
+#ifndef FSMONITOR_DAEMON_H
+#define FSMONITOR_DAEMON_H
+
+#ifdef HAVE_FSMONITOR_DAEMON_BACKEND
+
+#include "cache.h"
+#include "dir.h"
+#include "run-command.h"
+#include "simple-ipc.h"
+#include "thread-utils.h"
+
+struct fsmonitor_batch;
+struct fsmonitor_token_data;
+
+struct fsmonitor_daemon_backend_data; /* opaque platform-specific data */
+
+struct fsmonitor_daemon_state {
+	pthread_t listener_thread;
+	pthread_mutex_t main_lock;
+
+	struct strbuf path_worktree_watch;
+	struct strbuf path_gitdir_watch;
+	int nr_paths_watching;
+
+	struct fsmonitor_token_data *current_token_data;
+
+	int error_code;
+	struct fsmonitor_daemon_backend_data *backend_data;
+
+	struct ipc_server_data *ipc_server_data;
+
+	int test_client_delay_ms;
+};
+
+#endif /* HAVE_FSMONITOR_DAEMON_BACKEND */
+#endif /* FSMONITOR_DAEMON_H */