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

From: Junio C Hamano <[email-masked]>
Date: Thu, 26 Jun 2025 07:33:21 -0700

Ayush Chandekar <[email-masked]> writes:

> 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
> `84c9dc2` (commit: allow core.commentChar=auto for character auto
> selection) but did not exhibt this issue at that time.

Use "git log -1 --format=reference", i.e.

84c9dc2c (commit: allow core.commentChar=auto for character auto
selection, 2014-05-17)

> The bug was introduced in commit `a6c2654` (rebase -m: fix --signoff
> with conflicts) where Git started writing conflict comments to the file
> at 'rebase_path_message()'.

Ditto.

> 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)

Looks quite straight-forward.  Nice.

Thanks.