Release lock after encountering bogs row in vac_truncate_clog()

anarazel · anarazel · commit 83ecfa9fad11 · 2023-07-13T13:03:28.000-07:00
When vac_truncate_clog() encounters bogus datfrozenxid / datminmxid values, it returns early. Unfortunately, until now, it did not release WrapLimitsVacuumLock. If the backend later tries to acquire WrapLimitsVacuumLock, the session / autovacuum worker hangs in an uncancellable way. Similarly, other sessions will hang waiting for the lock. However, if the backend holding the lock exited or errored out for some reason, the lock was released. The bug was introduced as a side effect of 566372b. It is interesting that there are no production reports of this problem. That is likely due to a mix of bugs leading to bogus values having gotten less common, process exit releasing locks and instances of hangs being hard to debug for "normal" users. Discussion: https://postgr.es/m/20230621221208.vhsqgduwfpzwxnpg@awork3.anarazel.de
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
@@ -1893,12 +1893,16 @@ vac_truncate_clog(TransactionId frozenXID,
 		ereport(WARNING,
 				(errmsg("some databases have not been vacuumed in over 2 billion transactions"),
 				 errdetail("You might have already suffered transaction-wraparound data loss.")));
+		LWLockRelease(WrapLimitsVacuumLock);
 		return;
 	}
 
 	/* chicken out if data is bogus in any other way */
 	if (bogus)
+	{
+		LWLockRelease(WrapLimitsVacuumLock);
 		return;
+	}
 
 	/*
 	 * Advance the oldest value for commit timestamps before truncating, so
