Subject: Re: [PATCH] scalar: avoid segfault in reconfigure --all

From: Patrick Steinhardt <[email-masked]>
Date: Thu, 2 May 2024 08:46:53 +0200

On Tue, Apr 30, 2024 at 04:58:52PM +0000, Derrick Stolee via GitGitGadget wrote:
> From: Derrick Stolee <[email-masked]>
> 
> During the latest v2.45.0 update, 'scalar reconfigure --all' started to
> segfault on my machine. Breaking it down via the debugger, it was
> faulting on a NULL reference to the_hash_algo, which is a macro pointing
> to the_repository->hash_algo.
> 
> This NULL reference appears to be due to the way the loop is abusing the
> the_repository pointer, pointing it to a local repository struct after
> discovering that the current directory is a valid Git repository. This
> repo-swapping bit was in the original implementation from 4582676075
> (scalar: teach 'reconfigure' to optionally handle all registered
> enlistments, 2021-12-03), but only recently started segfaulting while
> trying to parse the HEAD reference. This also only happens on the
> _second_ repository in the list, so does not reproduce if there is only
> one registered repo.

Interesting. This also has some overlap with my patch series that aims
to drop the default-SHA1 fallback that we have in place for
`the_repository` [1].

> Add a test to t9210-scalar.sh to test 'scalar reconfigure --all' with
> multiple registered repos, as a precaution. Unfortunately, I was unable
> to reproduce the segfault using this test, so there is some coverage
> left to be desired. What exactly causes my setup to hit this bug but not
> this test structure? Unclear.

One way to trigger _a_ BUG is to use an "includeIf.onbranch:foobar.path"
config. This will cause Git to try and look up the "HEAD" reference, but
because we have a partially-configured repository, only, that will crash
with:

    BUG: refs.c:2123: reference backend is unknown

Whether that bug is the one that you have seen I cannot tell. It at
least does not sound like it.

    test_expect_success 'scalar reconfigure --all with includeIf.onbranch' '
        repos="two three four" &&
        for num in $repos
        do
            git init $num/src &&
            scalar register $num/src &&
            git -C $num/src config includeif."onbranch:foo".path something &&
            git -C $num/src config core.preloadIndex false || return 1
        done &&

        scalar reconfigure --all &&

        for num in $repos
        do
            test true = "$(git -C $num/src config core.preloadIndex)" || return 1
        done
    '

Another issue, which is likely the one you report here, is if you have a
repository with detached "HEAD". In that case we use `get_oid_hex()` to
parse "HEAD", which will implicitly use `the_hash_algo`. But because you
unset it in the second round this will fail with a segfault when you try
to access it:

    ./test-lib.sh: line 1069: 583995 Segmentation fault      (core dumped) scalar reconfigure --all

This is the following testcase:

    test_expect_success 'scalar reconfigure --all with detached HEADs' '
        repos="two three four" &&
        for num in $repos
        do
            rm -rf $num/src &&
            git init $num/src &&
            scalar register $num/src &&
            git -C $num/src config core.preloadIndex false &&
            test_commit -C $num/src initial &&
            git -C $num/src switch --detach HEAD || return 1
        done &&

        scalar reconfigure --all &&

        for num in $repos
        do
            test true = "$(git -C $num/src config core.preloadIndex)" || return 1
        done
    '

This issue should be fixed by my patch series in [1] because we start to
use `get_oid_hex_any()` to parse detached HEADs.

Anyway, your fix is indeed effective because with `repo_init()` we now
properly configure the repository.

[1]: https://lore.kernel.org/git/[email-masked]/

Patrick