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

[Martin Ågren]

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 <bmw...@google.com>
Signed-off-by: Martin Ågren <martin.ag...@gmail.com>
---
Many thanks to Brandon for showing how this should have been done.

 builtin/grep.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/builtin/grep.c b/builtin/grep.c
index 2d65f27d0..5a6cfe6b4 100644
--- a/builtin/grep.c
+++ b/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;
-- 
2.15.0.415.gac1375d7e