Re: git difftool: No such file or directory

[David Aguilar <davvid@gmail.com>]

On Wed, Jun 30, 2021 at 5:38 PM Đoàn Trần Công Danh
<congdanhqx@gmail.com> wrote:
>
> On 2021-06-30 11:38:21+0200, Alan Blotz <work@blotz.org> wrote:
>
> > Thank you for filling out a Git bug report!
> > Please answer the following questions to help us understand your issue.
> >
> > What did you do before the bug happened? (Steps to reproduce your issue)
> >
> > mkdir broken-diff
> > cd broken-diff
> > git init
> > mkdir dir1
> > mkdir dir2
> > touch dir1/orig
> > cd dir2/
> > ln -s ../dir1/orig sym
> > cd ..
> > git add dir*
> > git ci -m "init"
> > git checkout -b b
> > git rm dir2/sym
> > git ci -m "remove"
> > git difftool -d master HEAD
> >
> > What did you expect to happen? (Expected behavior)
> >
> > git difftool shall compare both branches.
> >
> > What happened instead? (Actual behavior)
> >
> > git difftool prints an error:
> >
> > fatal: could not open '/tmp/git-difftool.l4UM7e/left/dir2/sym' for writing: No such file or directory
>
> It looks like this behaviour was there from the time difftool was
> re-written in C in 03831ef7b5, (difftool: implement the functionality
> in the builtin, 2017-01-19). The perl version didn't have this
> problem.
>
> The perl version create a file in place of that symlink and write the
> symlink's target into that file. The C version tries to write (and
> follow?) the symlink.
>
> This hack can fix the problem but I'm not sure it's correct:
> ----8<---
>
> diff --git a/builtin/difftool.c b/builtin/difftool.c
> index 2115e548a5..737ebb5b1a 100644
> --- a/builtin/difftool.c
> +++ b/builtin/difftool.c
> @@ -492,12 +492,14 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
>                 if (*entry->left) {
>                         add_path(&ldir, ldir_len, entry->path);
>                         ensure_leading_directories(ldir.buf);
> -                       write_file(ldir.buf, "%s", entry->left);
> +                       unlink(ldir.buf);
> +                       write_file_buf(ldir.buf, entry->left, strlen(entry->left));
>                 }
>                 if (*entry->right) {
>                         add_path(&rdir, rdir_len, entry->path);
>                         ensure_leading_directories(rdir.buf);
> -                       write_file(rdir.buf, "%s", entry->right);
> +                       unlink(rdir.buf);
> +                       write_file_buf(rdir.buf, entry->right, strlen(entry->right));
>                 }
>         }
> ---->8-----
>
> +Cc: Dscho, who wrote the C version.

Thank you for tracking this down!

I re-read the original perl version and this indeed looks like it
should do the trick since it's doing the same thing we did in perl.

I'm a little ashamed that we didn't have a test case to cover this use
case but the reproduction recipe you included looks like a great
start. We already have a few tests that use the SYMLINKS test prereq
so this would slot in well there.
-- 
David