Re: [PATCH 2/2] ll_union_merge(): pass name labels to ll_xdl_merge()

[Elijah Newren <newren@gmail.com>]

On Thu, Jun 10, 2021 at 6:00 AM Jeff King <peff@peff.net> wrote:
>
> Since cd1d61c44f (make union merge an xdl merge favor, 2010-03-01), we
> pass NULL to ll_xdl_merge() for the "name" labels of the ancestor, ours
> and theirs buffers. We usually use these for annotating conflict markers
> left in a file. For a union merge, these shouldn't matter; the point of
> it is that we'd never leave conflict markers in the first place.
>
> But there is one code path where we may dereference them: if the file
> contents appear to be binary, ll_binary_merge() will give up and pass
> them to warning() to generate a message for the user (that was true even
> when cd1d61c44f was written, though the warning was in ll_xdl_merge()
> back then).
>
> That can result in a segfault, though on many systems (including glibc),
> the printf routines will helpfully just say "(null)" instead. We can
> extend our binary-union test in t6406 to check stderr, which catches the
> problem on all systems.

Nice catch (as is your 1/2 as well).

> This also fixes a warning from "gcc -O3". Unlike lower optimization
> levels, it inlines enough to see that the NULL can make it to warning()
> and complains:
>
>   In function ‘ll_binary_merge’,
>       inlined from ‘ll_xdl_merge’ at ll-merge.c:115:10,
>       inlined from ‘ll_union_merge’ at ll-merge.c:151:9:
>   ll-merge.c:74:4: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
>      74 |    warning("Cannot merge binary files: %s (%s vs. %s)",
>         |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>      75 |     path, name1, name2);
>         |     ~~~~~~~~~~~~~~~~~~~

So the warning uses path as well as name1 and name2...

>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  ll-merge.c            | 2 +-
>  t/t6406-merge-attr.sh | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/ll-merge.c b/ll-merge.c
> index 145deb12fa..0ee34d8a01 100644
> --- a/ll-merge.c
> +++ b/ll-merge.c
> @@ -151,7 +151,7 @@ static int ll_union_merge(const struct ll_merge_driver *drv_unused,
>         o = *opts;
>         o.variant = XDL_MERGE_FAVOR_UNION;
>         return ll_xdl_merge(drv_unused, result, path_unused,

Should we also rename 'path_unused' to 'path', since it is actually used?

> -                           orig, NULL, src1, NULL, src2, NULL,
> +                           orig, orig_name, src1, name1, src2, name2,
>                             &o, marker_size);
>  }
>
> diff --git a/t/t6406-merge-attr.sh b/t/t6406-merge-attr.sh
> index c1c458d933..8494645837 100755
> --- a/t/t6406-merge-attr.sh
> +++ b/t/t6406-merge-attr.sh
> @@ -221,7 +221,8 @@ test_expect_success 'binary files with union attribute' '
>         printf "two\0" >bin.txt &&
>         git commit -am two &&
>
> -       test_must_fail git merge bin-main
> +       test_must_fail git merge bin-main 2>stderr &&
> +       grep -i "warning.*cannot merge.*HEAD vs. bin-main" stderr
>  '
>
>  test_done
> --
> 2.32.0.529.g079a794268

This patch has a minor textual conflict with my remerge-diff series,
but since I haven't submitted it yet, that's my problem rather than
yours...and it will be an easy fix anyway.

Anyway, good catches.  Other than maybe considering fixing the name of
'path_unused', this looks good to me.