[01/15] rebase: extend the options for handling of empty commits

Commit Message

Johannes Schindelin via GitGitGadget Dec. 20, 2019, 5:09 p.m. UTC

From: Elijah Newren <[e-mail]>

Extend the interactive machinery with the ability to handle the full
spread of options for how to handle commits that either start or become
empty (by "become empty" I mean the changes in a commit are a subset of
changes that exist upstream, so the net effect of applying the commit is
no changes).  Introduce a new command line flag for selecting the
desired behavior:
    --empty={drop,keep,ask}
with the definitions:
    drop: drop empty commits
    keep: keep empty commits
    ask:  provide the user a chance to interact and pick what to do with
          empty commits on a case-by-case basis

Note that traditionally, am-based rebases have always dropped commits
that either started or became empty, while interactive-based rebases
have defaulted to ask (and provided an option to keep commits that
started empty).  This difference made sense since users of an am-based
rebase just wanted to quickly batch apply a sequence of commits, while
users editing a todo list will likely want the chance to interact and
handle unusual cases on a case-by-case basis.  However, not all rebases
using the interactive machinery are explicitly interactive anymore.  In
particular --merge was always meant to behave more like --am: just
rebase a batch of commits without popping up a todo list.

If the --empty flag is not specified, pick defaults as follows:
    explicitly interactive: ask
    --exec: keep (exec is about checking existing commits, and often
                  used without actually changing the base.  Thus the
                  expectation is that the user doesn't necessarily want
                  anything to change; they just want to test).
    otherwise: drop

Also, this commit makes --keep-empty just imply --empty=keep, and hides
it from help so that we aren't confusing users with different ways to do
the same thing.  (I could have added a --drop-empty flag, but then that
invites users to specify both --keep-empty and --drop-empty and we have
to add sanity checking around that; it seems cleaner to have a single
multi-valued option.)  This actually fixes --keep-empty too; previously,
it only meant to sometimes keep empty commits, in particular commits
which started empty would be kept.  But it would still error out and ask
the user what to do with commits that became empty.  Now it keeps empty
commits, as instructed.

Signed-off-by: Elijah Newren <[e-mail]>
---
 Documentation/git-rebase.txt      | 35 ++++++------
 builtin/rebase.c                  | 87 +++++++++++++++++++++++++++---
 rebase-interactive.c              |  4 +-
 rebase-interactive.h              |  2 +-
 sequencer.c                       | 74 +++++++++++++++++++------
 sequencer.h                       |  6 ++-
 t/t3421-rebase-topology-linear.sh |  4 +-
 t/t3424-rebase-empty.sh           | 89 +++++++++++++++++++++++++++++++
 t/t3427-rebase-subtree.sh         | 16 +++---
 9 files changed, 267 insertions(+), 50 deletions(-)
 create mode 100755 t/t3424-rebase-empty.sh

Comments

Junio C Hamano Dec. 20, 2019, 9:29 p.m. UTC | #1

"Elijah Newren via GitGitGadget" <[e-mail]> writes:

> From: Elijah Newren <[e-mail]>
>
> Extend the interactive machinery with the ability to handle the full
> spread of options for how to handle commits that either start or become
> empty (by "become empty" I mean the changes in a commit are a subset of
> changes that exist upstream, so the net effect of applying the commit is
> no changes).  Introduce a new command line flag for selecting the
> desired behavior:
>     --empty={drop,keep,ask}
> with the definitions:
>     drop: drop empty commits
>     keep: keep empty commits
>     ask:  provide the user a chance to interact and pick what to do with
>           empty commits on a case-by-case basis

This looks like a logical and natural extension of the --keep-empty
option.

After seeing the stress on "empty from the beginning and ending up
to be empty" in the description, I somehow expected that we may be
able to specify what happens to the empty commit separately, but
that does not seem to be what the patch is about, which was somewhat
disappointing.

> +static long parse_empty_value(const char *value)
> +{
> +	if (!value)
> +		return EMPTY_UNSPECIFIED;
> +	else if (!strcasecmp(value, "drop"))
> +		return EMPTY_DROP;
> +	else if (!strcasecmp(value, "keep"))
> +		return EMPTY_KEEP;
> +	else if (!strcasecmp(value, "ask"))
> +		return EMPTY_ASK;

Not an error but just silently ignored?

> +	return EMPTY_UNSPECIFIED;
> +}
> +

> +static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
> +{
> +	struct rebase_options *options = opt->value;
> +	long value = parse_empty_value(arg);

Ahh, OK.

Wouldn't it be better to make the variable and the parsing helper
function of type "enum empty_type", not "long", just like the field
in the rebase_options struct?

> +	BUG_ON_OPT_NEG(unset);
> +	if (value < 0)
> +		return error(_("option empty accepts \"drop\", "
> +			       "\"keep\", and \"ask\""));
> +
> +	options->empty = value;
> +	return 0;
> +}

Elijah Newren Dec. 21, 2019, 12:32 a.m. UTC | #2

On Fri, Dec 20, 2019 at 1:29 PM Junio C Hamano <[e-mail]> wrote:
>
> "Elijah Newren via GitGitGadget" <[e-mail]> writes:
>
> > From: Elijah Newren <[e-mail]>
> >
> > Extend the interactive machinery with the ability to handle the full
> > spread of options for how to handle commits that either start or become
> > empty (by "become empty" I mean the changes in a commit are a subset of
> > changes that exist upstream, so the net effect of applying the commit is
> > no changes).  Introduce a new command line flag for selecting the
> > desired behavior:
> >     --empty={drop,keep,ask}
> > with the definitions:
> >     drop: drop empty commits
> >     keep: keep empty commits
> >     ask:  provide the user a chance to interact and pick what to do with
> >           empty commits on a case-by-case basis
>
> This looks like a logical and natural extension of the --keep-empty
> option.
>
> After seeing the stress on "empty from the beginning and ending up
> to be empty" in the description, I somehow expected that we may be
> able to specify what happens to the empty commit separately, but
> that does not seem to be what the patch is about, which was somewhat
> disappointing.

My main reason for stressing both was that I've commits and code that
talked about "empty" commits, but which often only dealt with one of
the two types (sometimes when I would expect both to have been
addressed), so I was just trying to be careful in my explanation of
what it was about.

If wanted, we could implement finer control, but I don't know what the
UI would look like or what the range of options we might want.  Do we
want to mix all possible permutations and have two flags, e.g.
--become-empty=drop --start-empty=keep or --start-empty=ask
--become-empty=keep?  Would --empty=<whatever> imply both
--become-empty=<whatever> and --start-empty=<whatever> or would users
need to specify both?  Do we think all the permutations make sense, or
would we want to limit users to some subset?

The current choice of three covered the things that I had wanted in
the past, and which I think made for good defaults for various
different flags, so I just implemented it.  We could always extend it
later.

>
> > +static long parse_empty_value(const char *value)
> > +{
> > +     if (!value)
> > +             return EMPTY_UNSPECIFIED;
> > +     else if (!strcasecmp(value, "drop"))
> > +             return EMPTY_DROP;
> > +     else if (!strcasecmp(value, "keep"))
> > +             return EMPTY_KEEP;
> > +     else if (!strcasecmp(value, "ask"))
> > +             return EMPTY_ASK;
>
> Not an error but just silently ignored?

Oops, I'll switch it to an error.

> > +     return EMPTY_UNSPECIFIED;
> > +}
> > +
>
> > +static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
> > +{
> > +     struct rebase_options *options = opt->value;
> > +     long value = parse_empty_value(arg);
>
> Ahh, OK.
>
> Wouldn't it be better to make the variable and the parsing helper
> function of type "enum empty_type", not "long", just like the field
> in the rebase_options struct?

Indeed, I'll fix this up.

Elijah Newren Dec. 21, 2019, 6:52 p.m. UTC | #3

On Fri, Dec 20, 2019 at 4:32 PM Elijah Newren <[e-mail]> wrote:
>
> On Fri, Dec 20, 2019 at 1:29 PM Junio C Hamano <[e-mail]> wrote:
> >
> > "Elijah Newren via GitGitGadget" <[e-mail]> writes:
> >

> > > +static long parse_empty_value(const char *value)
> > > +{
> > > +     if (!value)
> > > +             return EMPTY_UNSPECIFIED;
> > > +     else if (!strcasecmp(value, "drop"))
> > > +             return EMPTY_DROP;
> > > +     else if (!strcasecmp(value, "keep"))
> > > +             return EMPTY_KEEP;
> > > +     else if (!strcasecmp(value, "ask"))
> > > +             return EMPTY_ASK;
> >
> > Not an error but just silently ignored?
>
> Oops, I'll switch it to an error.

By the way, diff.c's parse_algorithm_value() and
diff_opt_diff_algorithm() behave the same way, where
parse_algorithm_value() will return a negative value if it's not one
of the valid known cases, and diff_opt_diff_algorithm() checks for
negative and throws an error.  Should I also modify those to put the
error in parse_algorithm_value()?  (Not part of this series, but as a
consistency cleanup since it clearly surprised you and even I forgot
that I had the calling function returning the error when I copied that
style?)

Junio C Hamano Dec. 21, 2019, 11:49 p.m. UTC | #4

Elijah Newren <[e-mail]> writes:

>> > +static long parse_empty_value(const char *value)
>> > +{
>> > +     if (!value)
>> > +             return EMPTY_UNSPECIFIED;
>> > +     else if (!strcasecmp(value, "drop"))
>> > +             return EMPTY_DROP;
>> > +     else if (!strcasecmp(value, "keep"))
>> > +             return EMPTY_KEEP;
>> > +     else if (!strcasecmp(value, "ask"))
>> > +             return EMPTY_ASK;
>>
>> Not an error but just silently ignored?
>
> Oops, I'll switch it to an error.

Not necessarily (see "Ahh, OK" below).

>> > +     return EMPTY_UNSPECIFIED;
>> > +}
>> > +
>>
>> > +static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
>> > +{
>> > +     struct rebase_options *options = opt->value;
>> > +     long value = parse_empty_value(arg);
>>
>> Ahh, OK.
>>
>> Wouldn't it be better to make the variable and the parsing helper
>> function of type "enum empty_type", not "long", just like the field
>> in the rebase_options struct?
>
> Indeed, I'll fix this up.

Thanks. I was wondering if there is something subtle I didn't see
going on around here.

Patch

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 1d0e2d27cc..ff32ca1080 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -258,9 +258,25 @@  See also INCOMPATIBLE OPTIONS below.
 	original branch. The index and working tree are also left
 	unchanged as a result.
 
+--empty={drop,keep,ask}::
+	How to handle commits that become empty (because they contain a
+	subset of already upstream changes) or start empty.  With drop
+	(the default), commits that start or become empty are dropped.
+	With keep (implied by --exec), such commits are kept.  With ask
+	(implied by --interactive), the rebase will halt when an empty
+	commit is applied allowing you to choose whether to drop it or
+	commit it.  Also with ask, if the rebase is interactive then
+	commits which start empty will be commented out in the todo
+	action list (giving you a chance to uncomment).
++
+Note that this has no effect on commits which are already upstream (as
+can be checked via `git log --cherry-mark ...`), which are always
+dropped by rebase.
++
+See also INCOMPATIBLE OPTIONS below.
+
 --keep-empty::
-	Keep the commits that do not change anything from its
-	parents in the result.
+	Deprecated alias for what is now known as --empty=keep.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -569,6 +585,7 @@  are incompatible with the following options:
  * --interactive
  * --exec
  * --keep-empty
+ * --empty=
  * --edit-todo
  * --root when used in combination with --onto
 
@@ -580,6 +597,7 @@  In addition, the following pairs of options are incompatible:
  * --preserve-merges and --ignore-whitespace
  * --preserve-merges and --committer-date-is-author-date
  * --preserve-merges and --ignore-date
+ * --preserve-merges and --empty=
  * --keep-base and --onto
  * --keep-base and --root
 
@@ -588,19 +606,6 @@  BEHAVIORAL DIFFERENCES
 
 There are some subtle differences how the backends behave.
 
-Empty commits
-~~~~~~~~~~~~~
-
-The am backend drops any "empty" commits, regardless of whether the
-commit started empty (had no changes relative to its parent to
-start with) or ended empty (all changes were already applied
-upstream in other commits).
-
-The interactive backend drops commits by default that
-started empty and halts if it hits a commit that ended up empty.
-The `--keep-empty` option exists for the interactive backend to allow
-it to keep commits that started empty.
-
 Directory rename detection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/builtin/rebase.c b/builtin/rebase.c
index ddf33bc9d4..96db10eaed 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -50,8 +50,16 @@  enum rebase_type {
 	REBASE_PRESERVE_MERGES
 };
 
+enum empty_type {
+	EMPTY_UNSPECIFIED = -1,
+	EMPTY_DROP,
+	EMPTY_KEEP,
+	EMPTY_ASK
+};
+
 struct rebase_options {
 	enum rebase_type type;
+	enum empty_type empty;
 	const char *state_dir;
 	struct commit *upstream;
 	const char *upstream_name;
@@ -77,7 +85,6 @@  struct rebase_options {
 	const char *action;
 	int signoff;
 	int allow_rerere_autoupdate;
-	int keep_empty;
 	int autosquash;
 	int ignore_whitespace;
 	char *gpg_sign_opt;
@@ -95,6 +102,7 @@  struct rebase_options {
 
 #define REBASE_OPTIONS_INIT {			  	\
 		.type = REBASE_UNSPECIFIED,	  	\
+		.empty = EMPTY_UNSPECIFIED,	  	\
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = ARGV_ARRAY_INIT,		\
 		.git_format_patch_opt = STRBUF_INIT	\
@@ -114,6 +122,10 @@  static struct replay_opts get_replay_opts(const struct rebase_options *opts)
 		replay.allow_rerere_auto = opts->allow_rerere_autoupdate;
 	replay.allow_empty = 1;
 	replay.allow_empty_message = opts->allow_empty_message;
+	replay.drop_redundant_commits = (opts->empty == EMPTY_DROP);
+	replay.keep_redundant_commits = (opts->empty == EMPTY_KEEP);
+	replay.ask_on_initially_empty = (opts->empty == EMPTY_ASK &&
+					 !(opts->flags & REBASE_INTERACTIVE_EXPLICIT));
 	replay.verbose = opts->flags & REBASE_VERBOSE;
 	replay.reschedule_failed_exec = opts->reschedule_failed_exec;
 	replay.committer_date_is_author_date =
@@ -389,7 +401,10 @@  static int run_rebase_interactive(struct rebase_options *opts,
 
 	git_config_get_bool("rebase.abbreviatecommands", &abbreviate_commands);
 
-	flags |= opts->keep_empty ? TODO_LIST_KEEP_EMPTY : 0;
+	flags |= (opts->empty == EMPTY_DROP) ? TODO_LIST_DROP_EMPTY : 0;
+	flags |= (opts->empty == EMPTY_ASK &&
+		  opts->flags & REBASE_INTERACTIVE_EXPLICIT) ?
+			TODO_LIST_ASK_EMPTY : 0;
 	flags |= abbreviate_commands ? TODO_LIST_ABBREVIATE_CMDS : 0;
 	flags |= opts->rebase_merges ? TODO_LIST_REBASE_MERGES : 0;
 	flags |= opts->rebase_cousins > 0 ? TODO_LIST_REBASE_COUSINS : 0;
@@ -453,6 +468,19 @@  static int run_rebase_interactive(struct rebase_options *opts,
 	return ret;
 }
 
+static int parse_opt_keep_empty(const struct option *opt, const char *arg,
+				int unset)
+{
+	struct rebase_options *opts = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+	BUG_ON_OPT_ARG(arg);
+
+	opts->empty = EMPTY_KEEP;
+	opts->type = REBASE_INTERACTIVE;
+	return 0;
+}
+
 static const char * const builtin_rebase_interactive_usage[] = {
 	N_("git rebase--interactive [<options>]"),
 	NULL
@@ -466,7 +494,10 @@  int cmd_rebase__interactive(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT_NEGBIT(0, "ff", &opts.flags, N_("allow fast-forward"),
 			   REBASE_FORCE),
-		OPT_BOOL(0, "keep-empty", &opts.keep_empty, N_("keep empty commits")),
+		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
+			N_("(DEPRECATED) keep empty commits"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG | PARSE_OPT_HIDDEN,
+			parse_opt_keep_empty },
 		OPT_BOOL(0, "allow-empty-message", &opts.allow_empty_message,
 			 N_("allow commits with empty messages")),
 		OPT_BOOL(0, "rebase-merges", &opts.rebase_merges, N_("rebase merge commits")),
@@ -1166,7 +1197,7 @@  static int run_specific_rebase(struct rebase_options *opts, enum action action)
 		opts->allow_rerere_autoupdate ?
 			opts->allow_rerere_autoupdate == RERERE_AUTOUPDATE ?
 			"--rerere-autoupdate" : "--no-rerere-autoupdate" : "");
-	add_var(&script_snippet, "keep_empty", opts->keep_empty ? "yes" : "");
+	add_var(&script_snippet, "empty", opts->empty == EMPTY_KEEP ? "yes" : "");
 	add_var(&script_snippet, "autosquash", opts->autosquash ? "t" : "");
 	add_var(&script_snippet, "gpg_sign_opt", opts->gpg_sign_opt);
 	add_var(&script_snippet, "cmd", opts->cmd);
@@ -1360,6 +1391,33 @@  static int parse_opt_interactive(const struct option *opt, const char *arg,
 	return 0;
 }
 
+static long parse_empty_value(const char *value)
+{
+	if (!value)
+		return EMPTY_UNSPECIFIED;
+	else if (!strcasecmp(value, "drop"))
+		return EMPTY_DROP;
+	else if (!strcasecmp(value, "keep"))
+		return EMPTY_KEEP;
+	else if (!strcasecmp(value, "ask"))
+		return EMPTY_ASK;
+	return EMPTY_UNSPECIFIED;
+}
+
+static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
+{
+	struct rebase_options *options = opt->value;
+	long value = parse_empty_value(arg);
+
+	BUG_ON_OPT_NEG(unset);
+	if (value < 0)
+		return error(_("option empty accepts \"drop\", "
+			       "\"keep\", and \"ask\""));
+
+	options->empty = value;
+	return 0;
+}
+
 static void NORETURN error_on_missing_default_upstream(void)
 {
 	struct branch *current_branch = branch_get(NULL);
@@ -1505,8 +1563,13 @@  int cmd_rebase(int argc, const char **argv, const char *prefix)
 				 "ignoring them"),
 			      REBASE_PRESERVE_MERGES, PARSE_OPT_HIDDEN),
 		OPT_RERERE_AUTOUPDATE(&options.allow_rerere_autoupdate),
-		OPT_BOOL('k', "keep-empty", &options.keep_empty,
-			 N_("preserve empty commits during rebase")),
+		OPT_CALLBACK_F(0, "empty", &options, N_("{drop,keep,ask}"),
+			       N_("how to handle empty commits"),
+			       PARSE_OPT_NONEG, parse_opt_empty),
+		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
+			N_("(DEPRECATED) keep empty commits"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG | PARSE_OPT_HIDDEN,
+			parse_opt_keep_empty },
 		OPT_BOOL(0, "autosquash", &options.autosquash,
 			 N_("move commits that begin with "
 			    "squash!/fixup! under -i")),
@@ -1770,8 +1833,8 @@  int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (!(options.flags & REBASE_NO_QUIET))
 		argv_array_push(&options.git_am_opts, "-q");
 
-	if (options.keep_empty)
-		imply_interactive(&options, "--keep-empty");
+	if (options.empty != EMPTY_UNSPECIFIED)
+		imply_interactive(&options, "--empty");
 
 	if (gpg_sign) {
 		free(options.gpg_sign_opt);
@@ -1856,6 +1919,14 @@  int cmd_rebase(int argc, const char **argv, const char *prefix)
 		break;
 	}
 
+	if (options.empty == EMPTY_UNSPECIFIED) {
+		if (options.flags & REBASE_INTERACTIVE_EXPLICIT)
+			options.empty = EMPTY_ASK;
+		else if (exec.nr > 0)
+			options.empty = EMPTY_KEEP;
+		else
+			options.empty = EMPTY_DROP;
+	}
 	if (reschedule_failed_exec > 0 && !is_interactive(&options))
 		die(_("--reschedule-failed-exec requires "
 		      "--exec or --interactive"));
diff --git a/rebase-interactive.c b/rebase-interactive.c
index aa18ae82b7..ad82bf77df 100644
--- a/rebase-interactive.c
+++ b/rebase-interactive.c
@@ -28,7 +28,7 @@  static enum missing_commit_check_level get_missing_commit_check_level(void)
 	return MISSING_COMMIT_CHECK_IGNORE;
 }
 
-void append_todo_help(unsigned keep_empty, int command_count,
+void append_todo_help(unsigned no_ask_empty, int command_count,
 		      const char *shortrevisions, const char *shortonto,
 		      struct strbuf *buf)
 {
@@ -81,7 +81,7 @@  void append_todo_help(unsigned keep_empty, int command_count,
 
 	strbuf_add_commented_lines(buf, msg, strlen(msg));
 
-	if (!keep_empty) {
+	if (!no_ask_empty) {
 		msg = _("Note that empty commits are commented out");
 		strbuf_add_commented_lines(buf, msg, strlen(msg));
 	}
diff --git a/rebase-interactive.h b/rebase-interactive.h
index 44dbb06311..f531e00ba7 100644
--- a/rebase-interactive.h
+++ b/rebase-interactive.h
@@ -5,7 +5,7 @@  struct strbuf;
 struct repository;
 struct todo_list;
 
-void append_todo_help(unsigned keep_empty, int command_count,
+void append_todo_help(unsigned no_ask_empty, int command_count,
 		      const char *shortrevisions, const char *shortonto,
 		      struct strbuf *buf);
 int edit_todo_list(struct repository *r, struct todo_list *todo_list,
diff --git a/sequencer.c b/sequencer.c
index 763ccbbc45..d2c11f34b7 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -160,6 +160,9 @@  static GIT_PATH_FUNC(rebase_path_strategy, "rebase-merge/strategy")
 static GIT_PATH_FUNC(rebase_path_strategy_opts, "rebase-merge/strategy_opts")
 static GIT_PATH_FUNC(rebase_path_allow_rerere_autoupdate, "rebase-merge/allow_rerere_autoupdate")
 static GIT_PATH_FUNC(rebase_path_reschedule_failed_exec, "rebase-merge/reschedule-failed-exec")
+static GIT_PATH_FUNC(rebase_path_drop_redundant_commits, "rebase-merge/drop_redundant_commits")
+static GIT_PATH_FUNC(rebase_path_keep_redundant_commits, "rebase-merge/keep_redundant_commits")
+static GIT_PATH_FUNC(rebase_path_ask_on_initially_empty, "rebase-merge/ask_on_initially_empty")
 
 static int git_sequencer_config(const char *k, const char *v, void *cb)
 {
@@ -1623,7 +1626,7 @@  static int allow_empty(struct repository *r,
 	empty_commit = is_original_commit_empty(commit);
 	if (empty_commit < 0)
 		return empty_commit;
-	if (!empty_commit)
+	if (!empty_commit || opts->ask_on_initially_empty)
 		return 0;
 	else
 		return 1;
@@ -1837,7 +1840,7 @@  static int do_pick_commit(struct repository *r,
 	char *author = NULL;
 	struct commit_message msg = { NULL, NULL, NULL, NULL };
 	struct strbuf msgbuf = STRBUF_INIT;
-	int res, unborn = 0, reword = 0, allow;
+	int res, unborn = 0, reword = 0, allow, drop_commit;
 
 	if (opts->no_commit) {
 		/*
@@ -2042,13 +2045,20 @@  static int do_pick_commit(struct repository *r,
 		goto leave;
 	}
 
-	allow = allow_empty(r, opts, commit);
-	if (allow < 0) {
-		res = allow;
-		goto leave;
-	} else if (allow)
-		flags |= ALLOW_EMPTY;
-	if (!opts->no_commit) {
+	drop_commit = 0;
+	if (opts->drop_redundant_commits && is_index_unchanged(r)) {
+		drop_commit = 1;
+		fprintf(stderr, _("No changes -- Patch already applied."));
+	} else {
+		allow = allow_empty(r, opts, commit);
+		if (allow < 0) {
+			res = allow;
+			goto leave;
+		} else if (allow) {
+			flags |= ALLOW_EMPTY;
+		}
+	}
+	if (!opts->no_commit && !drop_commit) {
 		if (author || command == TODO_REVERT || (flags & AMEND_MSG))
 			res = do_commit(r, msg_file, author, opts, flags);
 		else
@@ -2501,9 +2511,15 @@  static int populate_opts_cb(const char *key, const char *value, void *data)
 	else if (!strcmp(key, "options.allow-empty-message"))
 		opts->allow_empty_message =
 			git_config_bool_or_int(key, value, &error_flag);
+	else if (!strcmp(key, "options.drop-redundant-commits"))
+		opts->drop_redundant_commits =
+			git_config_bool_or_int(key, value, &error_flag);
 	else if (!strcmp(key, "options.keep-redundant-commits"))
 		opts->keep_redundant_commits =
 			git_config_bool_or_int(key, value, &error_flag);
+	else if (!strcmp(key, "options.ask_on_initially_empty"))
+		opts->ask_on_initially_empty =
+			git_config_bool_or_int(key, value, &error_flag);
 	else if (!strcmp(key, "options.signoff"))
 		opts->signoff = git_config_bool_or_int(key, value, &error_flag);
 	else if (!strcmp(key, "options.record-origin"))
@@ -2612,6 +2628,15 @@  static int read_populate_opts(struct replay_opts *opts)
 		if (file_exists(rebase_path_reschedule_failed_exec()))
 			opts->reschedule_failed_exec = 1;
 
+		if (file_exists(rebase_path_drop_redundant_commits()))
+			opts->drop_redundant_commits = 1;
+
+		if (file_exists(rebase_path_keep_redundant_commits()))
+			opts->keep_redundant_commits = 1;
+
+		if (file_exists(rebase_path_ask_on_initially_empty()))
+			opts->ask_on_initially_empty = 1;
+
 		read_strategy_opts(opts, &buf);
 		strbuf_release(&buf);
 
@@ -2695,6 +2720,12 @@  int write_basic_state(struct replay_opts *opts, const char *head_name,
 		write_file(rebase_path_cdate_is_adate(), "%s", "");
 	if (opts->ignore_date)
 		write_file(rebase_path_ignore_date(), "%s", "");
+	if (opts->drop_redundant_commits)
+		write_file(rebase_path_drop_redundant_commits(), "%s", "");
+	if (opts->keep_redundant_commits)
+		write_file(rebase_path_keep_redundant_commits(), "%s", "");
+	if (opts->ask_on_initially_empty)
+		write_file(rebase_path_ask_on_initially_empty(), "%s", "");
 	if (opts->reschedule_failed_exec)
 		write_file(rebase_path_reschedule_failed_exec(), "%s", "");
 
@@ -3033,9 +3064,15 @@  static int save_opts(struct replay_opts *opts)
 	if (opts->allow_empty_message)
 		res |= git_config_set_in_file_gently(opts_file,
 				"options.allow-empty-message", "true");
+	if (opts->drop_redundant_commits)
+		res |= git_config_set_in_file_gently(opts_file,
+				"options.drop-redundant-commits", "true");
 	if (opts->keep_redundant_commits)
 		res |= git_config_set_in_file_gently(opts_file,
 				"options.keep-redundant-commits", "true");
+	if (opts->ask_on_initially_empty)
+		res |= git_config_set_in_file_gently(opts_file,
+				"options.ask_on_initially_empty", "true");
 	if (opts->signoff)
 		res |= git_config_set_in_file_gently(opts_file,
 					"options.signoff", "true");
@@ -4691,7 +4728,8 @@  static int make_script_with_merges(struct pretty_print_context *pp,
 				   struct rev_info *revs, struct strbuf *out,
 				   unsigned flags)
 {
-	int keep_empty = flags & TODO_LIST_KEEP_EMPTY;
+	int drop_empty = flags & TODO_LIST_DROP_EMPTY;
+	int ask_empty = flags & TODO_LIST_ASK_EMPTY;
 	int rebase_cousins = flags & TODO_LIST_REBASE_COUSINS;
 	int root_with_onto = flags & TODO_LIST_ROOT_WITH_ONTO;
 	struct strbuf buf = STRBUF_INIT, oneline = STRBUF_INIT;
@@ -4746,6 +4784,8 @@  static int make_script_with_merges(struct pretty_print_context *pp,
 		is_empty = is_original_commit_empty(commit);
 		if (!is_empty && (commit->object.flags & PATCHSAME))
 			continue;
+		if (is_empty && drop_empty)
+			continue;
 
 		strbuf_reset(&oneline);
 		pretty_print_commit(pp, commit, &oneline);
@@ -4754,7 +4794,7 @@  static int make_script_with_merges(struct pretty_print_context *pp,
 		if (!to_merge) {
 			/* non-merge commit: easy case */
 			strbuf_reset(&buf);
-			if (!keep_empty && is_empty)
+			if (is_empty && ask_empty)
 				strbuf_addf(&buf, "%c ", comment_line_char);
 			strbuf_addf(&buf, "%s %s %s", cmd_pick,
 				    oid_to_hex(&commit->object.oid),
@@ -4922,7 +4962,8 @@  int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 	struct pretty_print_context pp = {0};
 	struct rev_info revs;
 	struct commit *commit;
-	int keep_empty = flags & TODO_LIST_KEEP_EMPTY;
+	int drop_empty = flags & TODO_LIST_DROP_EMPTY;
+	int ask_empty = flags & TODO_LIST_ASK_EMPTY;
 	const char *insn = flags & TODO_LIST_ABBREVIATE_CMDS ? "p" : "pick";
 	int rebase_merges = flags & TODO_LIST_REBASE_MERGES;
 
@@ -4958,11 +4999,13 @@  int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 		return make_script_with_merges(&pp, &revs, out, flags);
 
 	while ((commit = get_revision(&revs))) {
-		int is_empty  = is_original_commit_empty(commit);
+		int is_empty = is_original_commit_empty(commit);
 
 		if (!is_empty && (commit->object.flags & PATCHSAME))
 			continue;
-		if (!keep_empty && is_empty)
+		if (is_empty && drop_empty)
+			continue;
+		if (is_empty && ask_empty)
 			strbuf_addf(out, "%c ", comment_line_char);
 		strbuf_addf(out, "%s %s ", insn,
 			    oid_to_hex(&commit->object.oid));
@@ -5100,7 +5143,8 @@  int todo_list_write_to_file(struct repository *r, struct todo_list *todo_list,
 
 	todo_list_to_strbuf(r, todo_list, &buf, num, flags);
 	if (flags & TODO_LIST_APPEND_TODO_HELP)
-		append_todo_help(flags & TODO_LIST_KEEP_EMPTY, count_commands(todo_list),
+		append_todo_help(!(flags & TODO_LIST_ASK_EMPTY),
+				 count_commands(todo_list),
 				 shortrevisions, shortonto, &buf);
 
 	res = write_message(buf.buf, buf.len, file, 0);
diff --git a/sequencer.h b/sequencer.h
index e9a0e03ea2..1c3abb661c 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -39,7 +39,9 @@  struct replay_opts {
 	int allow_rerere_auto;
 	int allow_empty;
 	int allow_empty_message;
+	int drop_redundant_commits;
 	int keep_redundant_commits;
+	int ask_on_initially_empty;
 	int verbose;
 	int quiet;
 	int reschedule_failed_exec;
@@ -134,7 +136,7 @@  int sequencer_rollback(struct repository *repo, struct replay_opts *opts);
 int sequencer_skip(struct repository *repo, struct replay_opts *opts);
 int sequencer_remove_state(struct replay_opts *opts);
 
-#define TODO_LIST_KEEP_EMPTY (1U << 0)
+/* #define TODO_LIST_KEEP_EMPTY (1U << 0) */ /* No longer used */
 #define TODO_LIST_SHORTEN_IDS (1U << 1)
 #define TODO_LIST_ABBREVIATE_CMDS (1U << 2)
 #define TODO_LIST_REBASE_MERGES (1U << 3)
@@ -150,6 +152,8 @@  int sequencer_remove_state(struct replay_opts *opts);
  * `--onto`, we do not want to re-generate the root commits.
  */
 #define TODO_LIST_ROOT_WITH_ONTO (1U << 6)
+#define TODO_LIST_DROP_EMPTY (1U << 7)
+#define TODO_LIST_ASK_EMPTY (1U << 8)
 
 
 int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
index 325072b0a3..d23e0bf778 100755
--- a/t/t3421-rebase-topology-linear.sh
+++ b/t/t3421-rebase-topology-linear.sh
@@ -230,7 +230,7 @@  test_run_rebase () {
 test_run_rebase success ''
 test_run_rebase success -m
 test_run_rebase success -i
-test_have_prereq !REBASE_P || test_run_rebase failure -p
+test_have_prereq !REBASE_P || test_run_rebase success -p
 
 test_run_rebase () {
 	result=$1
@@ -245,7 +245,7 @@  test_run_rebase () {
 test_run_rebase success ''
 test_run_rebase success -m
 test_run_rebase success -i
-test_have_prereq !REBASE_P || test_run_rebase failure -p
+test_have_prereq !REBASE_P || test_run_rebase success -p
 test_run_rebase success --rebase-merges
 
 #       m
diff --git a/t/t3424-rebase-empty.sh b/t/t3424-rebase-empty.sh
new file mode 100755
index 0000000000..9d52e1417f
--- /dev/null
+++ b/t/t3424-rebase-empty.sh
@@ -0,0 +1,89 @@ 
+#!/bin/sh
+
+test_description='git rebase of commits that start or become empty'
+
+. ./test-lib.sh
+
+test_expect_success 'setup test repository' '
+	test_write_lines 1 2 3 4 5 6 7 8 9 10 >numbers &&
+	test_write_lines A B C D E F G H I J >letters &&
+	git add numbers letters &&
+	git commit -m A &&
+
+	git branch upstream &&
+	git branch localmods &&
+
+	git checkout upstream &&
+	test_write_lines A B C D E >letters &&
+	git add letters &&
+	git commit -m B &&
+
+	test_write_lines 1 2 3 4 five 6 7 8 9 ten >numbers &&
+	git add numbers &&
+	git commit -m C &&
+
+	git checkout localmods &&
+	test_write_lines 1 2 3 4 five 6 7 8 9 10 >numbers &&
+	git add numbers &&
+	git commit -m C2 &&
+
+	git commit --allow-empty -m D &&
+
+	test_write_lines A B C D E >letters &&
+	git add letters &&
+	git commit -m "Five letters ought to be enough for anybody"
+'
+
+test_expect_success 'rebase --merge --empty=drop' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=drop upstream &&
+
+	test_write_lines C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --merge --empty=keep' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=keep upstream &&
+
+	test_write_lines D C2 C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --merge --empty=ask' '
+	git checkout -B testing localmods &&
+	test_must_fail git rebase --merge --empty=ask upstream &&
+
+	test_must_fail git rebase --skip &&
+	git commit --allow-empty &&
+	git rebase --continue &&
+
+	test_write_lines D C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+GIT_SEQUENCE_EDITOR=: && export GIT_SEQUENCE_EDITOR
+
+test_expect_success 'rebase --interactive --empty=drop' '
+	git checkout -B testing localmods &&
+	git rebase --interactive --empty=drop upstream &&
+
+	test_write_lines C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --interactive --empty=keep' '
+	git checkout -B testing localmods &&
+	git rebase --interactive --empty=keep upstream &&
+
+	test_write_lines D C2 C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+
+test_done
diff --git a/t/t3427-rebase-subtree.sh b/t/t3427-rebase-subtree.sh
index bec48e6a1f..468ebc1bef 100755
--- a/t/t3427-rebase-subtree.sh
+++ b/t/t3427-rebase-subtree.sh
@@ -85,23 +85,27 @@  test_expect_failure REBASE_P 'Rebase -Xsubtree --keep-empty --preserve-merges --
 	verbose test "$(commit_message HEAD)" = "Empty commit"
 '
 
-test_expect_success 'Rebase -Xsubtree --keep-empty --onto commit' '
+test_expect_success 'Rebase -Xsubtree --empty=ask --onto commit' '
 	reset_rebase &&
 	git checkout -b rebase-onto to-rebase &&
-	test_must_fail git rebase -Xsubtree=files_subtree --keep-empty --onto files-master master &&
+	test_must_fail git rebase -Xsubtree=files_subtree --empty=ask --onto files-master master &&
 	: first pick results in no changes &&
-	git rebase --continue &&
+	test_must_fail git rebase --skip &&
+	: last pick was an empty commit that has no changes, but we want to keep it &&
+	git commit --allow-empty &&
 	verbose test "$(commit_message HEAD~2)" = "master4" &&
 	verbose test "$(commit_message HEAD~)" = "files_subtree/master5" &&
 	verbose test "$(commit_message HEAD)" = "Empty commit"
 '
 
-test_expect_success 'Rebase -Xsubtree --keep-empty --rebase-merges --onto commit' '
+test_expect_success 'Rebase -Xsubtree --empty=ask --rebase-merges --onto commit' '
 	reset_rebase &&
 	git checkout -b rebase-merges-onto to-rebase &&
-	test_must_fail git rebase -Xsubtree=files_subtree --keep-empty --rebase-merges --onto files-master --root &&
+	test_must_fail git rebase -Xsubtree=files_subtree --empty=ask --rebase-merges --onto files-master --root &&
 	: first pick results in no changes &&
-	git rebase --continue &&
+	test_must_fail git rebase --skip &&
+	: last pick was an empty commit that has no changes, but we want to keep it &&
+	git commit --allow-empty &&
 	verbose test "$(commit_message HEAD~2)" = "master4" &&
 	verbose test "$(commit_message HEAD~)" = "files_subtree/master5" &&
 	verbose test "$(commit_message HEAD)" = "Empty commit"