Re: new segfault in master (6a6c0f10a70a6eb1)

[Duy Nguyen <pclouds@gmail.com>]

On Sun, May 12, 2019 at 6:02 AM Jeff King <peff@peff.net> wrote:
>
> On Sat, May 11, 2019 at 06:31:20PM -0400, Jeff King wrote:
>
> > On Sat, May 11, 2019 at 08:57:11PM +0000, Eric Wong wrote:
> >
> > > This test-tool submodule segfault seems new.  Noticed it while
> > > checking dmesg for other things.
> >
> > Yeah, I hadn't seen it before. It's almost certainly the expect_failure
> > added in 2b1257e463 (t/helper: add test-submodule-nested-repo-config,
> > 2018-10-25), since otherwise we'd be complaining of a test failure.
> >
> > I know we don't expect this to do the right thing yet, but it seems like
> > there's still a bug, since the test seems to think we should output a
> > nice message (and it's possible that the segfault can be triggered from
> > non-test-tool code, too).
> >
> > +cc the author.
>
> Actually, the plot thickens. That test _used to_ correctly expect
> failure (well, sort of -- it greps for the string with %s, which is
> wrong!). But then more recently in d9b8b8f896 (submodule-config.c: use
> repo_get_oid for reading .gitmodules, 2019-04-16), it started actually
> doing the lookup in the correct repo. And that started the segfault,
> because nobody has actually loaded the index for the submodule.
>
> I don't think this can be triggered outside of test-tool. There are
> four ways to get to config_from_gitmodules():
>
>   - via repo_read_gitmodules(), which explicitly loads the index
>
>   - via print_config_from_gitmodules(). This is called from
>     submodule--helper, but only with the_repository as the argument (and
>     I _think_ that the_repository->index is never NULL, because we point
>     it at the_index).
>
>   - via fetch_config_from_gitmodules(), which always passes
>     the_repository
>
>   - via update_clone_config_from_gitmodules(), likewise
>
> But regardless, I think it makes sense to load the index on demand when
> we need it here, which makes Antonio's original test pass (like the
> patch below).
>
> The segfault ultimately comes from repo_get_oid(); we feed it
> ":.gitmodules" and it blindly looks at repo->index. It's probably worth
> it being a bit more defensive and just returning "no such entry" if
> there's no index to look at (it could also load on demand, I guess, but
> it seems like too low a level to be making that kind of decision).
>
> I'm out of time for now, but I'll look into cleaning this up and writing
> a real commit message later.
>
> ---
> diff --git a/submodule-config.c b/submodule-config.c
> index 4264ee216f..ad2444bcec 100644
> --- a/submodule-config.c
> +++ b/submodule-config.c
> @@ -630,7 +630,8 @@ static void config_from_gitmodules(config_fn_t fn, struct repository *repo, void
>                 file = repo_worktree_path(repo, GITMODULES_FILE);
>                 if (file_exists(file)) {
>                         config_source.file = file;
> -               } else if (repo_get_oid(repo, GITMODULES_INDEX, &oid) >= 0 ||
> +               } else if ((repo_read_index(repo) >= 0 &&
> +                           repo_get_oid(repo, GITMODULES_INDEX, &oid) >= 0) ||
>                            repo_get_oid(repo, GITMODULES_HEAD, &oid) >= 0) {
>                         config_source.blob = oidstr = xstrdup(oid_to_hex(&oid));
>                         if (repo != the_repository)
> diff --git a/t/t7411-submodule-config.sh b/t/t7411-submodule-config.sh
> index fcc0fb82d8..ad28e93880 100755
> --- a/t/t7411-submodule-config.sh
> +++ b/t/t7411-submodule-config.sh
> @@ -243,18 +243,14 @@ test_expect_success 'reading nested submodules config' '
>         )
>  '
>
> -# When this test eventually passes, before turning it into
> -# test_expect_success, remember to replace the test_i18ngrep below with
> -# a "test_must_be_empty warning" to be sure that the warning is actually
> -# removed from the code.
> -test_expect_failure 'reading nested submodules config when .gitmodules is not in the working tree' '
> +test_expect_success 'reading nested submodules config when .gitmodules is not in the working tree' '

I did miss this test. Yeah your fix makes sense.

>         test_when_finished "git -C super/submodule checkout .gitmodules" &&
>         (cd super &&
>                 echo "./nested_submodule" >expect &&
>                 rm submodule/.gitmodules &&
>                 test-tool submodule-nested-repo-config \
>                         submodule submodule.nested_submodule.url >actual 2>warning &&
> -               test_i18ngrep "nested submodules without %s in the working tree are not supported yet" warning &&
> +               test_must_be_empty warning &&
>                 test_cmp expect actual
>         )
>  '



-- 
Duy