sequences: Lock buffer before initializing page

anarazel · anarazel · commit 3f695b31179b · 2023-04-04T16:42:52.000-07:00
fill_seq_fork_with_data(), used to initialize a new sequence relation, only locked the buffer after calling PageInit(), even though PageInit() modifies page contents. This is unlikely to cause real-world issues, as the relation is exclusively locked at that point, and the buffer not yet marked dirty, so other processes should not access the buffer. This issue looks to have been present since the introduction of sequences in e8647c4. Given the low risk, it does not seem worth backpatching the fix. Discussion: https://postgr.es/m/20230404185501.wdkmo3k7bedlx6qk@awork3.anarazel.de
diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
@@ -377,7 +377,7 @@ fill_seq_fork_with_data(Relation rel, HeapTuple tuple, ForkNumber forkNum)
 
 	/* Initialize first page of relation with special magic number */
 
-	buf = ReadBufferExtended(rel, forkNum, P_NEW, RBM_NORMAL, NULL);
+	buf = ReadBufferExtended(rel, forkNum, P_NEW, RBM_ZERO_AND_LOCK, NULL);
 	Assert(BufferGetBlockNumber(buf) == 0);
 
 	page = BufferGetPage(buf);
@@ -388,8 +388,6 @@ fill_seq_fork_with_data(Relation rel, HeapTuple tuple, ForkNumber forkNum)
 
 	/* Now insert sequence tuple */
 
-	LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
-
 	/*
 	 * Since VACUUM does not process sequences, we have to force the tuple to
 	 * have xmin = FrozenTransactionId now.  Otherwise it would become
