rebase: reinstate --no-keep-empty

Commit d48e5e21da ("rebase (interactive-backend): make --keep-empty the
default", 2020-02-15) turned --keep-empty (for keeping commits which
start empty) into the default.  The logic underpinning that commit was:

  1) 'git commit' errors out on the creation of empty commits without an
     override flag
  2) Once someone determines that the override is worthwhile, it's
     annoying and/or harmful to required them to take extra steps in
     order to keep such commits around (and to repeat such steps with
     every rebase).

While the logic on which the decision was made is sound, the result was
a bit of an overcorrection.  Instead of jumping to having --keep-empty
being the default, it jumped to making --keep-empty the only available
behavior.  There was a simple workaround, though, which was thought to
be good enough at the time.  People could still drop commits which
started empty the same way the could drop any commits: by firing up an
interactive rebase and picking out the commits they didn't want from the
list.  However, there are cases where external tools might create enough
empty commits that picking all of them out is painful.  As such, having
a flag to automatically remove start-empty commits may be beneficial.

Provide users a way to drop commits which start empty using a flag that
existed for years: --no-keep-empty.  Interpret --keep-empty as
countermanding any previous --no-keep-empty, but otherwise leaving
--keep-empty as the default.

This might lead to some slight weirdness since commands like
  git rebase --empty=drop --keep-empty
  git rebase --empty=keep --no-keep-empty
look really weird despite making perfect sense (the first will drop
commits which become empty, but keep commits that started empty; the
second will keep commits which become empty, but drop commits which
started empty).  However, --no-keep-empty was named years ago and we are
predominantly keeping it for backward compatibility; also we suspect it
will only be used rarely since folks already have a simple way to drop
commits they don't want with an interactive rebase.

Reported-by: Bryan Turner <bturner@atlassian.com>
Reported-by: Sami Boukortt <sami@boukortt.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -277,20 +277,32 @@ See also INCOMPATIBLE OPTIONS below.
 	Other options, like --exec, will use the default of drop unless
 	-i/--interactive is explicitly specified.
 +
-Note that commits which start empty are kept, and commits which are
-clean cherry-picks (as determined by `git log --cherry-mark ...`) are
-always dropped.
+Note that commits which start empty are kept (unless --no-keep-empty
+is specified), and commits which are clean cherry-picks (as determined
+by `git log --cherry-mark ...`) are always dropped.
 +
 See also INCOMPATIBLE OPTIONS below.
 
+--no-keep-empty::
 --keep-empty::
-	No-op.  Rebasing commits that started empty (had no change
-	relative to their parent) used to fail and this option would
-	override that behavior, allowing commits with empty changes to
-	be rebased.  Now commits with no changes do not cause rebasing
-	to halt.
+	Do not keep commits that start empty before the rebase
+	(i.e. that do not change anything from its parent) in the
+	result.  The default is to keep commits which start empty,
+	since creating such commits requires passing the --allow-empty
+	override flag to `git commit`, signifying that a user is very
+	intentionally creating such a commit and thus wants to keep
+	it.
 +
-See also BEHAVIORAL DIFFERENCES and INCOMPATIBLE OPTIONS below.
+Usage of this flag will probably be rare, since you can get rid of
+commits that start empty by just firing up an interactive rebase and
+removing the lines corresponding to the commits you don't want.  This
+flag exists as a convenient shortcut, such as for cases where external
+tools generate many empty commits and you want them all removed.
++
+For commits which do not start empty but become empty after rebasing,
+see the --empty flag.
++
+See also INCOMPATIBLE OPTIONS below.
 
 --allow-empty-message::
 	No-op.  Rebasing commits with an empty message used to fail
@@ -587,7 +599,7 @@ are incompatible with the following options:
  * --preserve-merges
  * --interactive
  * --exec
- * --keep-empty
+ * --no-keep-empty
  * --empty=
  * --edit-todo
  * --root when used in combination with --onto
@@ -620,13 +632,15 @@ commits that started empty, though these are rare in practice.  It
 also drops commits that become empty and has no option for controlling
 this behavior.
 
-The merge backend keeps intentionally empty commits (though with -i
-they are marked as empty in the todo list editor).  Similar to the
-apply backend, by default the merge backend drops commits that become
-empty unless -i/--interactive is specified (in which case it stops and
-asks the user what to do).  The merge backend also has an
---empty={drop,keep,ask} option for changing the behavior of handling
-commits that become empty.
+The merge backend keeps intentionally empty commits by default (though
+with -i they are marked as empty in the todo list editor, or they can
+be dropped automatically with --no-keep-empty).
+
+Similar to the apply backend, by default the merge backend drops
+commits that become empty unless -i/--interactive is specified (in
+which case it stops and asks the user what to do).  The merge backend
+also has an --empty={drop,keep,ask} option for changing the behavior
+of handling commits that become empty.
 
 Directory rename detection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

builtin/rebase.c

@@ -85,6 +85,7 @@ struct rebase_options {
 	const char *action;
 	int signoff;
 	int allow_rerere_autoupdate;
+	int keep_empty;
 	int autosquash;
 	char *gpg_sign_opt;
 	int autostash;
@@ -100,6 +101,7 @@ struct rebase_options {
 #define REBASE_OPTIONS_INIT {			  	\
 		.type = REBASE_UNSPECIFIED,	  	\
 		.empty = EMPTY_UNSPECIFIED,	  	\
+		.keep_empty = 1,			\
 		.default_backend = "merge",	  	\
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = ARGV_ARRAY_INIT,		\
@@ -379,6 +381,7 @@ static int run_sequencer_rebase(struct rebase_options *opts,
 
 	git_config_get_bool("rebase.abbreviatecommands", &abbreviate_commands);
 
+	flags |= opts->keep_empty ? TODO_LIST_KEEP_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;
@@ -442,6 +445,7 @@ static int run_sequencer_rebase(struct rebase_options *opts,
 	return ret;
 }
 
+static void imply_merge(struct rebase_options *opts, const char *option);
 static int parse_opt_keep_empty(const struct option *opt, const char *arg,
 				int unset)
 {
@@ -449,10 +453,8 @@ static int parse_opt_keep_empty(const struct option *opt, const char *arg,
 
 	BUG_ON_OPT_ARG(arg);
 
-	/*
-	 * If we ever want to remap --keep-empty to --empty=keep, insert:
-	 * 	opts->empty = unset ? EMPTY_UNSPECIFIED : EMPTY_KEEP;
-	 */
+	imply_merge(opts, unset ? "--no-keep-empty" : "--keep-empty");
+	opts->keep_empty = !unset;
 	opts->type = REBASE_MERGE;
 	return 0;
 }
@@ -471,7 +473,7 @@ int cmd_rebase__interactive(int argc, const char **argv, const char *prefix)
 		OPT_NEGBIT(0, "ff", &opts.flags, N_("allow fast-forward"),
 			   REBASE_FORCE),
 		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
-			N_("(DEPRECATED) keep empty commits"),
+			N_("keep commits which start empty"),
 			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN,
 			parse_opt_keep_empty },
 		OPT_BOOL_F(0, "allow-empty-message", &opts.allow_empty_message,
@@ -1162,6 +1164,7 @@ 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, "autosquash", opts->autosquash ? "t" : "");
 	add_var(&script_snippet, "gpg_sign_opt", opts->gpg_sign_opt);
 	add_var(&script_snippet, "cmd", opts->cmd);
@@ -1547,7 +1550,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			       N_("how to handle commits that become empty"),
 			       PARSE_OPT_NONEG, parse_opt_empty),
 		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
-			N_("(DEPRECATED) keep empty commits"),
+			N_("keep commits which start empty"),
 			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN,
 			parse_opt_keep_empty },
 		OPT_BOOL(0, "autosquash", &options.autosquash,

sequencer.c

@@ -4591,6 +4591,7 @@ 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 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;
@@ -4645,6 +4646,8 @@ 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 && !keep_empty)
+			continue;
 
 		strbuf_reset(&oneline);
 		pretty_print_commit(pp, commit, &oneline);
@@ -4822,6 +4825,7 @@ 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;
 	const char *insn = flags & TODO_LIST_ABBREVIATE_CMDS ? "p" : "pick";
 	int rebase_merges = flags & TODO_LIST_REBASE_MERGES;
 
@@ -4861,6 +4865,8 @@ int sequencer_make_script(struct repository *r, struct strbuf *out, int argc,
 
 		if (!is_empty && (commit->object.flags & PATCHSAME))
 			continue;
+		if (is_empty && !keep_empty)
+			continue;
 		strbuf_addf(out, "%s %s ", insn,
 			    oid_to_hex(&commit->object.oid));
 		pretty_print_commit(&pp, commit, out);

sequencer.h

@@ -134,7 +134,7 @@ 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) */ /* No longer used */
+#define TODO_LIST_KEEP_EMPTY (1U << 0)
 #define TODO_LIST_SHORTEN_IDS (1U << 1)
 #define TODO_LIST_ABBREVIATE_CMDS (1U << 2)
 #define TODO_LIST_REBASE_MERGES (1U << 3)

t/t3421-rebase-topology-linear.sh

@@ -220,14 +220,13 @@ test_have_prereq !REBASE_P || test_run_rebase failure -p
 test_run_rebase () {
 	result=$1
 	shift
-	test_expect_$result "rebase $* --keep-empty" "
+	test_expect_$result "rebase $* --no-keep-empty drops begin-empty commits" "
 		reset_rebase &&
-		git rebase $* --keep-empty c l &&
-		test_cmp_rev c HEAD~3 &&
-		test_linear_range 'd k l' c..
+		git rebase $* --no-keep-empty c l &&
+		test_cmp_rev c HEAD~2 &&
+		test_linear_range 'd l' c..
 	"
 }
-test_run_rebase success --apply
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
@@ -242,7 +241,6 @@ test_run_rebase () {
 		test_linear_range 'd k l' j..
 	"
 }
-test_run_rebase success --apply
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p

t/t3424-rebase-empty.sh

@@ -123,6 +123,42 @@ test_expect_success 'rebase --interactive uses default of --empty=ask' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rebase --merge --empty=drop --keep-empty' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=drop --keep-empty upstream &&
+
+	test_write_lines D C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --merge --empty=drop --no-keep-empty' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=drop --no-keep-empty upstream &&
+
+	test_write_lines C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'rebase --merge --empty=keep --keep-empty' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=keep --keep-empty 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=keep --no-keep-empty' '
+	git checkout -B testing localmods &&
+	git rebase --merge --empty=keep --no-keep-empty upstream &&
+
+	test_write_lines C2 C B A >expect &&
+	git log --format=%s >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'rebase --merge does not leave state laying around' '
 	git checkout -B testing localmods~2 &&
 	git rebase --merge upstream &&