Re: [PATCH v4 03/19] rebase (interactive-backend): make --keep-empty the default

> From: Elijah Newren <newren@gmail.com>
> 
> Different rebase backends have different treatment for commits which
> start empty (i.e. have no changes relative to their parent), and the
> --keep-empty option was added at some point to allow adjusting behavior
> for the interactive backend.

> The handling of commits which start empty
> is actually quite similar to commit b00bf1c9a8dd (git-rebase: make
> --allow-empty-message the default, 2018-06-27), which pointed out that
> the behavior for various backends is often more happenstance than
> design.  The specific change made in that commit is actually quite
> relevant as well and much of the logic there directly applies here.
> 
> It makes a lot of sense in 'git commit' to error out on the creation of
> empty commits, unless an override flag is provided.  However, once
> someone determines that there is a rare case that merits using the
> manual override to create such a commit, it is somewhere between
> annoying and harmful to have to take extra steps to keep such
> intentional commits around.  Granted, empty commits are quite rare,
> which is why handling of them doesn't get considered much and folks tend
> to defer to existing (accidental) behavior and assume there was a reason
> for it, leading them to just add flags (--keep-empty in this case) that
> allow them to override the bad defaults.  Fix the interactive backend so
> that --keep-empty is the default, much like we did with
> --allow-empty-message.  The am backend should also be fixed to have
> --keep-empty semantics for commits that start empty, but that is not
> included in this patch other than a testcase documenting the failure.

> Note that there was one test in t3421 which appears to have been written
> expecting --keep-empty to not be the default as correct behavior.  This
> test was introduced in commit 00b8be5a4d38 ("add tests for rebasing of
> empty commits", 2013-06-06), which was part of a series focusing on
> rebase topology and which had an interesting original cover letter at
> https://lore.kernel.org/git/1347949878-12578-1-git-send-email-martinvonz@gmail.com/
> which noted
>      Your input especially appreciated on whether you agree with the
>      intent of the test cases.
> and then went into a long example about how one of the many tests added
> had several questions about whether it was correct.  As such, I believe
> most the tests in that series were about testing rebase topology with as
> many different flags as possible and were not trying to state in general
> how those flags should behave otherwise.
> 
> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>   Documentation/git-rebase.txt      | 24 ++++++-----
>   builtin/rebase.c                  | 28 ++++++++----
>   rebase-interactive.c              |  7 +--
>   rebase-interactive.h              |  2 +-
>   sequencer.c                       | 20 +++------
>   sequencer.h                       |  2 +-
>   t/t3421-rebase-topology-linear.sh | 16 +++----
>   t/t3424-rebase-empty.sh           | 72 +++++++++++++++++++++++++++++++
>   t/t3427-rebase-subtree.sh         | 12 +++---
>   9 files changed, 128 insertions(+), 55 deletions(-)
>   create mode 100755 t/t3424-rebase-empty.sh
> 
> diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
> index c83be7ffc2..1d19542d79 100644
> --- a/Documentation/git-rebase.txt
> +++ b/Documentation/git-rebase.txt
> @@ -259,10 +259,13 @@ See also INCOMPATIBLE OPTIONS below.
>   	unchanged as a result.
>   
>   --keep-empty::
> -	Keep the commits that do not change anything from its
> -	parents in the result.
> +	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.

> -See also INCOMPATIBLE OPTIONS below.
> +See also BEHAVIORAL DIFFERENCES and INCOMPATIBLE OPTIONS below.
>   
>   --allow-empty-message::
>   	No-op.  Rebasing commits with an empty message used to fail
> @@ -577,15 +580,14 @@ 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 am backend unfortunately drops intentionally empty commits, i.e.
> +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 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.
> +The interactive backend keeps intentionally empty commits.
> +Unfortunately, it always halts whenever it runs across a commit that
> +becomes empty, even when the rebase is not explicitly interactive.
>   
>   Directory rename detection
>   ~~~~~~~~~~~~~~~~~~~~~~~~~~
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index faa4e0d406..537b3241ce 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -77,7 +77,6 @@ struct rebase_options {
>   	const char *action;
>   	int signoff;
>   	int allow_rerere_autoupdate;
> -	int keep_empty;
>   	int autosquash;
>   	char *gpg_sign_opt;
>   	int autostash;
> @@ -375,7 +374,6 @@ 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 |= 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;
> @@ -439,6 +437,17 @@ 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_ARG(arg);
> +
> +	opts->type = REBASE_INTERACTIVE;
> +	return 0;
> +}

>   static const char * const builtin_rebase_interactive_usage[] = {
>   	N_("git rebase--interactive [<options>]"),
>   	NULL
> @@ -452,7 +461,10 @@ 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_HIDDEN,
> +			parse_opt_keep_empty },
>   		OPT_BOOL_F(0, "allow-empty-message", &opts.allow_empty_message,
>   			   N_("allow commits with empty messages"),
>   			   PARSE_OPT_HIDDEN),
> @@ -1145,7 +1157,6 @@ 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);
> @@ -1483,8 +1494,10 @@ 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")),
> +		{ OPTION_CALLBACK, 'k', "keep-empty", &options, NULL,
> +			N_("(DEPRECATED) keep empty commits"),
> +			PARSE_OPT_NOARG | PARSE_OPT_HIDDEN,
> +			parse_opt_keep_empty },
>   		OPT_BOOL(0, "autosquash", &options.autosquash,
>   			 N_("move commits that begin with "
>   			    "squash!/fixup! under -i")),
> @@ -1747,9 +1760,6 @@ 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 (gpg_sign) {
>   		free(options.gpg_sign_opt);
>   		options.gpg_sign_opt = xstrfmt("-S%s", gpg_sign);
> diff --git a/rebase-interactive.c b/rebase-interactive.c
> index aa18ae82b7..7e7b4061bf 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(int command_count,
>   		      const char *shortrevisions, const char *shortonto,
>   		      struct strbuf *buf)
>   {
> @@ -80,11 +80,6 @@ void append_todo_help(unsigned keep_empty, int command_count,
>   			"the rebase will be aborted.\n\n");
>   
>   	strbuf_add_commented_lines(buf, msg, strlen(msg));
> -
> -	if (!keep_empty) {
> -		msg = _("Note that empty commits are commented out");
> -		strbuf_add_commented_lines(buf, msg, strlen(msg));
> -	}
>   }
>   
>   int edit_todo_list(struct repository *r, struct todo_list *todo_list,
> diff --git a/rebase-interactive.h b/rebase-interactive.h
> index 44dbb06311..05354ca341 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(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 b9dbf1adb0..c21fc202b1 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -1489,7 +1489,7 @@ static int allow_empty(struct repository *r,
>   		       struct replay_opts *opts,
>   		       struct commit *commit)
>   {
> -	int index_unchanged, empty_commit;
> +	int index_unchanged, originally_empty;
>   
>   	/*
>   	 * Three cases:
> @@ -1513,10 +1513,10 @@ static int allow_empty(struct repository *r,
>   	if (opts->keep_redundant_commits)
>   		return 1;
>   
> -	empty_commit = is_original_commit_empty(commit);
> -	if (empty_commit < 0)
> -		return empty_commit;
> -	if (!empty_commit)
> +	originally_empty = is_original_commit_empty(commit);
> +	if (originally_empty < 0)
> +		return originally_empty;
> +	if (!originally_empty)
>   		return 0;
>   	else
>   		return 1;
> @@ -4566,7 +4566,6 @@ 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;
> @@ -4629,8 +4628,6 @@ 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)
> -				strbuf_addf(&buf, "%c ", comment_line_char);
>   			strbuf_addf(&buf, "%s %s %s", cmd_pick,
>   				    oid_to_hex(&commit->object.oid),
>   				    oneline.buf);
> @@ -4797,7 +4794,6 @@ 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;
>   
> @@ -4833,12 +4829,10 @@ 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)
> -			strbuf_addf(out, "%c ", comment_line_char);
>   		strbuf_addf(out, "%s %s ", insn,
>   			    oid_to_hex(&commit->object.oid));
>   		pretty_print_commit(&pp, commit, out);
> @@ -4975,7 +4969,7 @@ 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(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 9f9ae291e3..c165e0ff25 100644
> --- a/sequencer.h
> +++ b/sequencer.h
> @@ -132,7 +132,7 @@ 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)
> diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
> index 325072b0a3..57334dca7e 100755
> --- a/t/t3421-rebase-topology-linear.sh
> +++ b/t/t3421-rebase-topology-linear.sh
> @@ -205,17 +205,17 @@ test_expect_success 'setup of linear history for empty commit tests' '
>   test_run_rebase () {
>   	result=$1
>   	shift
> -	test_expect_$result "rebase $* drops empty commit" "
> +	test_expect_$result "rebase $* keeps begin-empty commits" "
>   		reset_rebase &&
> -		git rebase $* c l &&
> -		test_cmp_rev c HEAD~2 &&
> -		test_linear_range 'd l' c..
> +		git rebase $* j l &&
> +		test_cmp_rev c HEAD~4 &&
> +		test_linear_range 'j d k l' c..
>   	"
>   }
> -test_run_rebase success ''
> +test_run_rebase failure ''
>   test_run_rebase success -m
>   test_run_rebase success -i
> -test_have_prereq !REBASE_P || test_run_rebase success -p
> +test_have_prereq !REBASE_P || test_run_rebase failure -p
>   
>   test_run_rebase () {
>   	result=$1
> @@ -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..22d97e143b
> --- /dev/null
> +++ b/t/t3424-rebase-empty.sh
> @@ -0,0 +1,72 @@
> +#!/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_failure 'rebase (am-backend) with a variety of empty commits' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout -B testing localmods &&
> +	# rebase (--am) should not drop commits that start empty
> +	git rebase upstream &&
> +
> +	test_write_lines D C B A >expect &&
> +	git log --format=%s >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_failure 'rebase --merge with a variety of empty commits' '
> +	test_when_finished "git rebase --abort" &&
> +	git checkout -B testing localmods &&
> +	# rebase --merge should not halt on the commit that becomes empty
> +	git rebase --merge upstream &&
> +
> +	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 with a variety of empty commits' '
> +	git checkout -B testing localmods &&
> +	test_must_fail git rebase --interactive upstream &&
> +
> +	git rebase --skip &&
> +
> +	test_write_lines D 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..8dceef61cf 100755
> --- a/t/t3427-rebase-subtree.sh
> +++ b/t/t3427-rebase-subtree.sh
> @@ -85,23 +85,23 @@ test_expect_failure REBASE_P 'Rebase -Xsubtree --keep-empty --preserve-merges --
>   	verbose test "$(commit_message HEAD)" = "Empty commit"
>   '
>   
> -test_expect_success 'Rebase -Xsubtree --keep-empty --onto commit' '
> +test_expect_success 'Rebase -Xsubtree --onto commit' '
>   	reset_rebase &&
>   	git checkout -b rebase-onto to-rebase &&
> -	test_must_fail git rebase -Xsubtree=files_subtree --keep-empty --onto files-master master &&
> +	test_must_fail git rebase -Xsubtree=files_subtree --onto files-master master &&
>   	: first pick results in no changes &&
> -	git rebase --continue &&
> +	git rebase --skip &&
>   	verbose test "$(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 --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 --rebase-merges --onto files-master --root &&
>   	: first pick results in no changes &&
> -	git rebase --continue &&
> +	git rebase --skip &&
>   	verbose test "$(commit_message HEAD~2)" = "master4" &&
>   	verbose test "$(commit_message HEAD~)" = "files_subtree/master5" &&
>   	verbose test "$(commit_message HEAD)" = "Empty commit"
>