Re: [PATCH 2/3] index-pack: drop type_cas mutex

> The type_cas lock lost all of its callers in f08cbf60fe (index-pack:
> make quantum of work smaller, 2020-09-08), so we can safely delete it.
> The compiler didn't alert us that the variable became unused, because we
> still call pthread_mutex_init() and pthread_mutex_destroy() on it.
> 
> It's worth considering also whether that commit was in error to remove
> the use of the lock. Why don't we need it now, if we did before, as
> described in ab791dd138 (index-pack: fix race condition with duplicate
> bases, 2014-08-29)? I think the answer is that we now look at and assign
> the child_obj->real_type field in the main thread while holding the
> work_lock(). So we don't have to worry about racing with the worker
> threads.