Subject: Re: [PATCH] add-patch: handle splitting hunks with diff.suppressBlankEmpty

From: Junio C Hamano <[email-masked]>
Date: Thu, 18 Jul 2024 09:29:35 -0700

"Phillip Wood via GitGitGadget" <[email-masked]> writes:

> From: Phillip Wood <[email-masked]>
>
> When "add -p" parses diffs, it looks for context lines starting with a
> single space. But when diff.suppressBlankEmpty is in effect, an empty
> context line will omit the space, giving us a true empty line. This
> confuses the parser, which is unable to split based on such a line.
>
> It's tempting to say that we should just make sure that we generate a
> diff without that option.  However, although we do not parse hunks that
> the user has manually edited with parse_diff() we do allow the user
> to split such hunks. As POSIX calls the decision of whether to print the
> space here "implementation-defined" we need to handle edited hunks where
> empty context lines omit the space.
>
> So let's handle both cases: a context line either starts with a space or
> consists of a totally empty line by normalizing the first character to a
> space when we parse them. Normalizing the first character rather than
> changing the code to check for a space or newline will hopefully future
> proof against introducing similar bugs if the code is changed.
>
> Reported-by: Ilya Tumaykin <[email-masked]>
> Helped-by: Jeff King <[email-masked]>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---

Well written.  Thanks for a pleasant read.

> @@ -953,7 +960,7 @@ static int split_hunk(struct add_p_state *s, struct file_diff *file_diff,
>  	context_line_count = 0;
>  
>  	while (splittable_into > 1) {
> -		ch = s->plain.buf[current];
> +		ch = normalize_marker(s->plain.buf + current);

I wondered if &s->plain.buf[current] is easier to grok, but what's
written already is good enough ;-)

There is another explicit mention of ' ' in merge_hunks().  Unless
we are normalizing the buffer here (which I do not think we do),
wouldn't we have to make sure that the code over there also knows
that an empty line in a patch is an unmodified empty line?

                if (plain[overlap_end] != ' ')
                        return error(_("expected context line "
                                       "#%d in\n%.*s"),