I see this running vanilla single node racer with memory allocation fault injection.
This was triggered by a allocation failure in mdc_enqueue_finish(). But the issue is at the bottom of mdc_enqueue():
We should clean it_lock_handle and it_lock_mode as well.
More generally mdc_enqueue() should not have a *lockh parameter at all but to fix this we probably need split md_enqueue() into md_enqueue() and md_flock().