Re: [PATCH v2 7/8] submodule-config: pass repo upon blob config read

[Jonathan Tan <jonathantanmy@google.com>]

> /*
>  * NEEDSWORK: repo_read_gitmodules() might call
>  * add_to_alternates_memory() via config_from_gitmodules(). This
>  * operation causes a race condition with concurrent object readings
>  * performed by the worker threads. That's why we need obj_read_lock()
>  * here. It should be removed once it's no longer necessary to add the
>  * subrepo's odbs to the in-memory alternates list.
>  */
> obj_read_lock();
> repo_read_gitmodules(subrepo, 0);
> 
> Back when I wrote this comment, my conclusion was that the alternates
> mechanics were the only thread-unsafe object-reading operations in
> repo_read_gitmodules()'s call chains. So once the add-to-alternates
> mechanics were gone, we could also remove the lock.
> 
> But with further inspection now, I see that this is not really the
> case. For example, we have a few global variables in packfile.c
> collecting some statistics (pack_mmap_calls, pack_open_windows, etc.)
> which are updated on obj readings from both the_repository *and*
> submodules. So I no longer think its safe to remove the
> obj_read_lock() protection here, as the NEEDSWORK comment suggests,
> even if we are not using the alternates list anymore.
> 
> Do you want to remove this comment in your patchset? I can also send a
> follow-up patch explaining this situation and removing the comment
> (but not the locking), if you prefer.

I think you can make the patch yourself - the comment change you
describe seems unrelated to this patch set.

> Ok. Like in grep_submodule(), this should no longer add the submodule
> ODB to the alternates list, so this call is now mostly used as a
> fallback and also for testing.
> 
> To see if we are indeed testing this add-to-alternates case, I
> reverted the change that made the code read from the submodule instead
> of the_repository:
> 
> diff --git a/config.c b/config.c
> index a85c12e6cc..cd37a9dcd9 100644
> --- a/config.c
> +++ b/config.c
> @@ -1805,7 +1805,7 @@ int git_config_from_blob_oid(config_fn_t fn,
>         unsigned long size;
>         int ret;
> 
> -       buf = repo_read_object_file(repo, oid, &type, &size);
> +       buf = read_object_file(oid, &type, &size);
> 
> Then, I ran t7814-grep-recurse-submodules.sh , where you've added the
> GIT_TEST_FATAL_REGISTER_SUBMODULE_ODB=1 envvar. This correctly
> produced the following error:
> 
> BUG: submodule.c:205: register_all_submodule_odb_as_alternates() called
> [...]
> not ok 23 - grep --recurse-submodules with submodules without
> .gitmodules in the working tree
> 
> Nice! So the change made by this patch is covered by test 23. I think
> it would be nice to mention that in this patch's message.

Thanks for checking this. I'll mention this in the commit message.