Re: [GSOC PATCH v2] commit: avoid scanning trailing comments when 'core.commentChar' is "auto"

> When core.commentChar is set to "auto", Git selects a comment character
> by scanning the commit message contents and avoiding any character
> already present in the message.
> 
> If the message still contains old conflict comments (starting with a
> comment character), Git assumes that character is in use and chooses a
> different one. As a result, those existing comment lines are no longer
> recognized as comments and end up being included in the final commit
> message.
> 
> To avoid this, skip scanning the trailing comment block when selecting
> the comment character. This allows Git to safely reuse the original
> character when appropriate, keeping the commit message clean and free of
> leftover conflict information.

> Background:
> 
> The "auto" value for core.commentchar was introduced in the commit
> 84c9dc2c5a (commit: allow core.commentChar=auto for character auto
> selection, 2014-05-17) but did not exhibt this issue at that time.
> 
> The bug was introduced in commit a6c2654f83 (rebase -m: fix --signoff
> with conflicts, 2024-04-18) where Git started writing conflict comments
> to the file at 'rebase_path_message()'.
> 
> Mentored-by: Christian Couder <christian.couder@gmail.com>
> Mentored-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
> Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
> ---
> 
> Thanks to Christian for mentoring, and to Kristopher and Junio for their reviews!
> 
>   builtin/commit.c           |  6 +++++-
>   t/t3418-rebase-continue.sh | 14 ++++++++++++++
>   2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/builtin/commit.c b/builtin/commit.c
> index fba0dded64..63e7158e98 100644
> --- a/builtin/commit.c
> +++ b/builtin/commit.c
> @@ -688,6 +688,10 @@ static void adjust_comment_line_char(const struct strbuf *sb)
>   	char candidates[] = "#;@!$%^&|:";
>   	char *candidate;
>   	const char *p;
> +	size_t cutoff;
> +
> +	/* Ignore comment chars in trailing comments (e.g., Conflicts:) */
> +	cutoff = sb->len - ignored_log_message_bytes(sb->buf, sb->len);

>   
>   	if (!memchr(sb->buf, candidates[0], sb->len)) {
>   		free(comment_line_str_to_free);
> @@ -700,7 +704,7 @@ static void adjust_comment_line_char(const struct strbuf *sb)
>   	candidate = strchr(candidates, *p);
>   	if (candidate)
>   		*candidate = ' ';
> -	for (p = sb->buf; *p; p++) {
> +	for (p = sb->buf; p + 1 < sb->buf + cutoff; p++) {
>   		if ((p[0] == '\n' || p[0] == '\r') && p[1]) {
>   			candidate = strchr(candidates, p[1]);
>   			if (candidate)
> diff --git a/t/t3418-rebase-continue.sh b/t/t3418-rebase-continue.sh
> index 127216f722..ccfe77af6c 100755
> --- a/t/t3418-rebase-continue.sh
> +++ b/t/t3418-rebase-continue.sh
> @@ -328,6 +328,20 @@ test_expect_success 'there is no --no-reschedule-failed-exec in an ongoing rebas
>   	test_expect_code 129 git rebase --edit-todo --no-reschedule-failed-exec
>   '
>   
> +test_expect_success 'no change in comment character due to conflicts markers with core.commentChar=auto' '
> +	test_commit base file &&

> +	git checkout -b branch-a &&
> +	test_commit A file &&
> +	git checkout -b branch-b base &&
> +	test_commit B file &&
> +	test_must_fail git rebase branch-a &&
> +	printf "B\nA\n" >file &&
> +	git add file &&
> +	GIT_EDITOR="cat >actual" git -c core.commentChar=auto rebase --continue &&
> +	# Check that "#" is still the comment character.
> +	test_grep "^# Changes to be committed:$" actual

> +'
> +
>   test_orig_head_helper () {
>   	test_when_finished 'git rebase --abort &&
>   		git checkout topic &&