Check for relation length overrun soon enough.

tglsfdc · tglsfdc · commit 04118de78fe9 · 2021-09-09T11:45:48.000-04:00
We don't allow relations to exceed 2^32-1 blocks, because block numbers are 32 bits and the last possible block number is reserved to mean InvalidBlockNumber. There is a check for this in mdextend, but that's really way too late, because the smgr API requires us to create a buffer for the block-to-be-added, and we do not want to have any buffer with blocknum InvalidBlockNumber. (Such a case can trigger assertions in bufmgr.c, plus I think it might confuse ReadBuffer's logic for data-past-EOF later on.) So put the check into ReadBuffer. Per report from Christoph Berg. It's been like this forever, so back-patch to all supported branches. Discussion: https://postgr.es/m/YTn1iTkUYBZfcODk@msg.credativ.de
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
@@ -739,7 +739,16 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 
 	/* Substitute proper block number if caller asked for P_NEW */
 	if (isExtend)
+	{
 		blockNum = smgrnblocks(smgr, forkNum);
+		/* Fail if relation is already at maximum possible length */
+		if (blockNum == P_NEW)
+			ereport(ERROR,
+					(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+					 errmsg("cannot extend relation %s beyond %u blocks",
+							relpath(smgr->smgr_rnode, forkNum),
+							P_NEW)));
+	}
 
 	if (isLocalBuf)
 	{
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
@@ -437,7 +437,8 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	/*
 	 * If a relation manages to grow to 2^32-1 blocks, refuse to extend it any
 	 * more --- we mustn't create a block whose number actually is
-	 * InvalidBlockNumber.
+	 * InvalidBlockNumber.  (Note that this failure should be unreachable
+	 * because of upstream checks in bufmgr.c.)
 	 */
 	if (blocknum == InvalidBlockNumber)
 		ereport(ERROR,
