Re: [PATCH 4/5] add -p: avoid ambiguous signed/unsigned comparison

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Dscho

On 10/08/2022 16:02, Johannes Schindelin via GitGitGadget wrote:
> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> In the interactive `add` operation, users can choose to jump to specific
> hunks, and Git will present the hunk list in that case. To avoid showing
> too many lines at once, only a maximum of 21 hunks are shown, skipping
> the "mode change" pseudo hunk.
> 
> The comparison performed to skip the "mode change" pseudo hunk (if any)
> compares a signed integer `i` to the unsigned value `mode_change` (which
> can be 0 or 1 because it is a 1-bit type).
> 
> According to section 6.3.1.8 of the C99 standard (see e.g.
> https://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf), what should
> happen is an automatic conversion of the "lesser" type to the "greater"
> type, but since the types differ in signedness, it is ill-defined what
> is the correct "usual arithmetic conversion".
> 
> Which means that Visual C's behavior can (and does) differ from GCC's:
> When compiling Git using the latter, `add -p`'s `goto` command shows no
> hunks by default because it casts a negative start offset to a pretty
> large unsigned value, breaking the "goto hunk" test case in
> `t3701-add-interactive.sh`.
> 
> Let's avoid that by converting the unsigned bit explicitly to a signed
> integer.

Oh the joys of bit-fields!

> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>   add-patch.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/add-patch.c b/add-patch.c
> index 509ca04456b..3524555e2b0 100644
> --- a/add-patch.c
> +++ b/add-patch.c
> @@ -1547,7 +1547,7 @@ soft_increment:
>   			strbuf_remove(&s->answer, 0, 1);
>   			strbuf_trim(&s->answer);

Unrelated to this change but why don't we just call strbuf_reset() here?

>   			i = hunk_index - DISPLAY_HUNKS_LINES / 2;
> -			if (i < file_diff->mode_change)
> +			if (i < (int)file_diff->mode_change)

The change itself looks good

Best Wishes

Phillip

>   				i = file_diff->mode_change;
>   			while (s->answer.len == 0) {
>   				i = display_hunks(s, file_diff, i);