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If a process abandons a wait in LockBufferForCleanup (in practice, only happens if someone cancels a VACUUM) just before someone else sends it a signal indicating the buffer is available, it was possible for the wakeup to remain in the process' semaphore, causing misbehavior next time the process waited for an lmgr lock. Rather than try to prevent the race condition directly, it seems best to make the lock manager robust against leftover wakeups, by having it repeat waiting on the semaphore if the lock has not actually been granted or denied yet.