Re: [PATCH v2 6/9] t4058: explore duplicate tree entry handling in a bit more detail

>
>
> On Fri, Dec 11 2020, Elijah Newren via GitGitGadget wrote:
>
> > While creating the last commit, I found a number of other cases where
> > git would segfault when faced with trees that have duplicate entries.
> > None of these segfaults are in the diffcore-rename code (they all occur
> > in cache-tree and unpack-trees).  Further, to my knowledge, no one has
> > ever been adversely affected by these bugs, and given that it has been
> > 15 years and folks have fixed a few other issues with historical
> > duplicate entries (as noted in the last commit), I am not sure we will
> > ever run into anyone having problems with these.  So I am not sure these
> > are worth fixing, but it doesn't hurt to at least document these
> > failures in the same test file that is concerned with duplicate tree
> > entries.
> > [...]
> > +test_expect_failure 'fast-forward from duplicate entries to non-duplicate' '
> > +     git merge update
> > +'
> > +
> >  test_done
>
> Per https://lore.kernel.org/git/87lf9b3mth.fsf@evledraar.gmail.com/
> isn't the noise of having a segfault from "git" worth fixing in itself
> though? I.e. something like this, so we at least se why it fails:
>
> diff --git a/t/t4058-diff-duplicates.sh b/t/t4058-diff-duplicates.sh
> index 54614b814db..ed91d9f7fe9 100755
> --- a/t/t4058-diff-duplicates.sh
> +++ b/t/t4058-diff-duplicates.sh
> @@ -182,8 +182,10 @@ test_expect_success 'switch to base branch and force status to be clean' '
>         test_must_be_empty actual
>  '
>
> -test_expect_failure 'fast-forward from duplicate entries to non-duplicate' '
> -       git merge update
> +test_expect_success 'fast-forward from duplicate entries to non-duplicate' '
> +       ! git merge update 2>err &&
> +       grep "^BUG: " err &&
> +       grep -F "should have entry at o->src_index->cache[1]" err
>  '
>
>  test_done
> diff --git a/unpack-trees.c b/unpack-trees.c
> index 8a1afbc1e49..230cb073fe1 100644
> --- a/unpack-trees.c
> +++ b/unpack-trees.c
> @@ -789,8 +789,11 @@ static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
>          */
>         for (i = 0; i < nr_entries; i++) {
>                 int new_ce_len, len, rc;
> +               int j = pos + i;
>
> -               src[0] = o->src_index->cache[pos + i];
> +               src[0] = o->src_index->cache[j];
> +               if (!src[0])
> +                       BUG("should have entry at o->src_index->cache[%d]", j);
>
>                 len = ce_namelen(src[0]);
>                 new_ce_len = cache_entry_size(len);
>