Re: [PATCH v2] grep: take the read-lock when adding a submodule

> With --recurse-submodules, we add each submodule that we encounter to
> the list of alternate object databases. With threading, our changes to
> the list are not protected against races. Indeed, ThreadSanitizer
> reports a race when we call `add_to_alternates_memory()` around the same
> time that another thread is reading in the list through
> `read_sha1_file()`.
> 
> Take the grep read-lock while adding the submodule. The lock is used to
> serialize uses of non-thread-safe parts of Git's API, including
> `read_sha1_file()`.
> 
> Helped-by: Brandon Williams <bmwill@google.com>
> Signed-off-by: Martin Ågren <martin.agren@gmail.com>