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>
Acked-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin Ågren

builtin/grep.c

@@ -431,7 +431,9 @@ static int grep_submodule(struct grep_opt *opt, struct repository *superproject,
 	 * store is no longer global and instead is a member of the repository
 	 * object.
 	 */
+	grep_read_lock();
 	add_to_alternates_memory(submodule.objectdir);
+	grep_read_unlock();
 
 	if (oid) {
 		struct object *object;