Subject: Re: [PATCH] rebase -i: improve error message when picking merge

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 4 Apr 2024 08:08:29 +0200

On Mon, Feb 26, 2024 at 10:58:07AM +0000, Phillip Wood via GitGitGadget wrote:
> From: Phillip Wood <[email-masked]>
> 
> The only todo commands that accept a merge commit are "merge" and
> "reset". All the other commands like "pick" or "reword" fail when they
> try to pick a a merge commit and print the message
> 
>     error: commit abc123 is a merge but no -m option was given.
> 
> followed by a hint about the command being rescheduled. This message is
> designed to help the user when they cherry-pick a merge and forget to
> pass "-m". For users who are rebasing the message is confusing as there
> is no way for rebase to cherry-pick the merge.
> 
> Improve the user experience by detecting the error when the todo list is
> parsed rather than waiting for the "pick" command to fail and print a
> message recommending the "merge" command instead. We recommend "merge"
> rather than "exec git cherry-pick -m ..." on the assumption that
> cherry-picking merges is relatively rare and it is more likely that the
> user chose "pick" by a mistake.
> 
> It would be possible to support cherry-picking merges by allowing the
> user to pass "-m" to "pick" commands but that adds complexity to do
> something that can already be achieved with
> 
>     exec git cherry-pick -m1 abc123

Okay. I think it's reasonable to abort early and give some advice here.
It's certainly a lot more user friendly than to abort during the rebase.
And if we ever gain the ability to e.g. `pick -m1` in the instruction
sheet directly we can adapt accordingly.

> The change is relatively straight forward but is complicated slightly as
> we now need to tell the parser if we're rebasing or not.
> 
> Reported-by: Stefan Haller <[email-masked]>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---
>     rebase -i: improve error message when picking merge
> 
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1672%2Fphillipwood%2Frebase-reject-merges-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1672/phillipwood/rebase-reject-merges-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/1672
> 
>  builtin/rebase.c              |  2 +-
>  rebase-interactive.c          |  7 ++---
>  sequencer.c                   | 49 ++++++++++++++++++++++++++++++-----
>  sequencer.h                   |  2 +-
>  t/t3404-rebase-interactive.sh | 33 +++++++++++++++++++++++
>  5 files changed, 81 insertions(+), 12 deletions(-)
> 
> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 5b086f651a6..a33e41c44da 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -297,7 +297,7 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags)
>  	else {
>  		discard_index(&the_index);
>  		if (todo_list_parse_insn_buffer(the_repository, todo_list.buf.buf,
> -						&todo_list))
> +						&todo_list, 1))

I think these booleans are somewhat hard to read. I may be
overengineering this, so please feel free to push back, but would it
make more sense to introduce a `unsigned flags` field and then have a
`PARSE_INSN_IS_REBASING` flag?

>  			BUG("unusable todo list");
>  
>  		ret = complete_action(the_repository, &replay, flags,
> diff --git a/rebase-interactive.c b/rebase-interactive.c
> index d9718409b3d..78d5ed1a41d 100644
> --- a/rebase-interactive.c
> +++ b/rebase-interactive.c
> @@ -114,7 +114,8 @@ int edit_todo_list(struct repository *r, struct todo_list *todo_list,
>  	 * it.  If there is an error, we do not return, because the user
>  	 * might want to fix it in the first place. */
>  	if (!initial)
> -		incorrect = todo_list_parse_insn_buffer(r, todo_list->buf.buf, todo_list) |
> +		incorrect = todo_list_parse_insn_buffer(r, todo_list->buf.buf,
> +							todo_list, 1) |
>  			file_exists(rebase_path_dropped());
>  
>  	if (todo_list_write_to_file(r, todo_list, todo_file, shortrevisions, shortonto,
> @@ -134,7 +135,7 @@ int edit_todo_list(struct repository *r, struct todo_list *todo_list,
>  	if (initial && new_todo->buf.len == 0)
>  		return -3;
>  
> -	if (todo_list_parse_insn_buffer(r, new_todo->buf.buf, new_todo)) {
> +	if (todo_list_parse_insn_buffer(r, new_todo->buf.buf, new_todo, 1)) {
>  		fprintf(stderr, _(edit_todo_list_advice));
>  		return -4;
>  	}
> @@ -234,7 +235,7 @@ int todo_list_check_against_backup(struct repository *r, struct todo_list *todo_
>  	int res = 0;
>  
>  	if (strbuf_read_file(&backup.buf, rebase_path_todo_backup(), 0) > 0) {
> -		todo_list_parse_insn_buffer(r, backup.buf.buf, &backup);
> +		todo_list_parse_insn_buffer(r, backup.buf.buf, &backup, 1);
>  		res = todo_list_check(&backup, todo_list);
>  	}
>  
> diff --git a/sequencer.c b/sequencer.c
> index 91de546b323..cf808c24d20 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -2550,8 +2550,37 @@ static int check_label_or_ref_arg(enum todo_command command, const char *arg)
>  	return 0;
>  }
>  
> +static int error_merge_commit(enum todo_command command)
> +{
> +	switch(command) {
> +	case TODO_PICK:
> +		return error(_("'%s' does not accept merge commits, "
> +			       "please use '%s'"),
> +			     todo_command_info[command].str, "merge -C");

I wonder how actionable these commands are. Can we give the full command
that the user can use instead, including the commit ID?

That raises another question though: how exactly is the user supposed to
perform the merge? Should they merge the merge commit, resulting in two
merge commits? Should they pick one of the sides, and if so, which one?
I guess the answer is "it depends", which makes it harder for us to come
up with actionable advice here.

> +	case TODO_REWORD:
> +		return error(_("'%s' does not accept merge commits, "
> +			       "please use '%s'"),
> +			     todo_command_info[command].str, "merge -c");

I was about to suggest that the above two cases should be merged. But
then I realized that it's "merge -c" here, but "merge -C" in the first
case.

Patrick

> +	case TODO_EDIT:
> +		return error(_("'%s' does not accept merge commits, "
> +			       "please use '%s' followed by '%s'"),
> +			     todo_command_info[command].str,
> +			     "merge -C", "break");
> +
> +	case TODO_FIXUP:
> +	case TODO_SQUASH:
> +		return error(_("cannot squash merge commit into another commit"));
> +
> +	default:
> +		BUG("unexpected todo_command");
> +	}
> +}
> +
>  static int parse_insn_line(struct repository *r, struct todo_item *item,
> -			   const char *buf, const char *bol, char *eol)
> +			   const char *buf, const char *bol, char *eol,
> +			   int rebasing)
>  {
>  	struct object_id commit_oid;
>  	char *end_of_object_name;
> @@ -2655,7 +2684,12 @@ static int parse_insn_line(struct repository *r, struct todo_item *item,
>  		return status;
>  
>  	item->commit = lookup_commit_reference(r, &commit_oid);
> -	return item->commit ? 0 : -1;
> +	if (!item->commit)
> +		return -1;
> +	if (rebasing && item->command != TODO_MERGE &&
> +	    item->commit->parents && item->commit->parents->next)
> +		return error_merge_commit(item->command);
> +	return 0;
>  }
>  
>  int sequencer_get_last_command(struct repository *r UNUSED, enum replay_action *action)
> @@ -2686,7 +2720,7 @@ int sequencer_get_last_command(struct repository *r UNUSED, enum replay_action *
>  }
>  
>  int todo_list_parse_insn_buffer(struct repository *r, char *buf,
> -				struct todo_list *todo_list)
> +				struct todo_list *todo_list, int rebasing)
>  {
>  	struct todo_item *item;
>  	char *p = buf, *next_p;
> @@ -2704,7 +2738,7 @@ int todo_list_parse_insn_buffer(struct repository *r, char *buf,
>  
>  		item = append_new_todo(todo_list);
>  		item->offset_in_buf = p - todo_list->buf.buf;
> -		if (parse_insn_line(r, item, buf, p, eol)) {
> +		if (parse_insn_line(r, item, buf, p, eol, rebasing)) {
>  			res = error(_("invalid line %d: %.*s"),
>  				i, (int)(eol - p), p);
>  			item->command = TODO_COMMENT + 1;
> @@ -2852,7 +2886,8 @@ static int read_populate_todo(struct repository *r,
>  	if (strbuf_read_file_or_whine(&todo_list->buf, todo_file) < 0)
>  		return -1;
>  
> -	res = todo_list_parse_insn_buffer(r, todo_list->buf.buf, todo_list);
> +	res = todo_list_parse_insn_buffer(r, todo_list->buf.buf, todo_list,
> +					  is_rebase_i(opts));
>  	if (res) {
>  		if (is_rebase_i(opts))
>  			return error(_("please fix this using "
> @@ -2882,7 +2917,7 @@ static int read_populate_todo(struct repository *r,
>  		struct todo_list done = TODO_LIST_INIT;
>  
>  		if (strbuf_read_file(&done.buf, rebase_path_done(), 0) > 0 &&
> -		    !todo_list_parse_insn_buffer(r, done.buf.buf, &done))
> +		    !todo_list_parse_insn_buffer(r, done.buf.buf, &done, 1))
>  			todo_list->done_nr = count_commands(&done);
>  		else
>  			todo_list->done_nr = 0;
> @@ -6286,7 +6321,7 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
>  	strbuf_release(&buf2);
>  	/* Nothing is done yet, and we're reparsing, so let's reset the count */
>  	new_todo.total_nr = 0;
> -	if (todo_list_parse_insn_buffer(r, new_todo.buf.buf, &new_todo) < 0)
> +	if (todo_list_parse_insn_buffer(r, new_todo.buf.buf, &new_todo, 1) < 0)
>  		BUG("invalid todo list after expanding IDs:\n%s",
>  		    new_todo.buf.buf);
>  
> diff --git a/sequencer.h b/sequencer.h
> index dcef7bb99c0..ed2c4b38514 100644
> --- a/sequencer.h
> +++ b/sequencer.h
> @@ -136,7 +136,7 @@ struct todo_list {
>  }
>  
>  int todo_list_parse_insn_buffer(struct repository *r, char *buf,
> -				struct todo_list *todo_list);
> +				struct todo_list *todo_list, int rebasing);
>  int todo_list_write_to_file(struct repository *r, struct todo_list *todo_list,
>  			    const char *file, const char *shortrevisions,
>  			    const char *shortonto, int num, unsigned flags);
> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
> index 64b641002e4..20b8589ad07 100755
> --- a/t/t3404-rebase-interactive.sh
> +++ b/t/t3404-rebase-interactive.sh
> @@ -2203,6 +2203,39 @@ test_expect_success 'bad labels and refs rejected when parsing todo list' '
>  	test_path_is_missing execed
>  '
>  
> +test_expect_success 'non-merge commands reject merge commits' '
> +	test_when_finished "test_might_fail git rebase --abort" &&
> +	git checkout E &&
> +	git merge I &&
> +	oid=$(git rev-parse HEAD) &&
> +	cat >todo <<-EOF &&
> +	pick $oid
> +	reword $oid
> +	edit $oid
> +	fixup $oid
> +	squash $oid
> +	EOF
> +	(
> +		set_replace_editor todo &&
> +		test_must_fail git rebase -i HEAD 2>actual
> +	) &&
> +	cat >expect <<-EOF &&
> +	error: ${SQ}pick${SQ} does not accept merge commits, please use ${SQ}merge -C${SQ}
> +	error: invalid line 1: pick $oid
> +	error: ${SQ}reword${SQ} does not accept merge commits, please use ${SQ}merge -c${SQ}
> +	error: invalid line 2: reword $oid
> +	error: ${SQ}edit${SQ} does not accept merge commits, please use ${SQ}merge -C${SQ} followed by ${SQ}break${SQ}
> +	error: invalid line 3: edit $oid
> +	error: cannot squash merge commit into another commit
> +	error: invalid line 4: fixup $oid
> +	error: cannot squash merge commit into another commit
> +	error: invalid line 5: squash $oid
> +	You can fix this with ${SQ}git rebase --edit-todo${SQ} and then run ${SQ}git rebase --continue${SQ}.
> +	Or you can abort the rebase with ${SQ}git rebase --abort${SQ}.
> +	EOF
> +	test_cmp expect actual
> +'
> +
>  # This must be the last test in this file
>  test_expect_success '$EDITOR and friends are unchanged' '
>  	test_editor_unchanged
> 
> base-commit: c875e0b8e036c12cfbf6531962108a063c7a821c
> -- 
> gitgitgadget
>