Fix data-corruption hazard in WAL-logged CREATE DATABASE.

tglsfdc · tglsfdc · commit 692df425b688 · 2022-08-06T11:50:34.000-04:00
RelationCopyStorageUsingBuffer thought it could skip copying empty pages, but of course that does not work at all, because subsequent blocks will be out of place. Also fix it to acquire share lock on the source buffer. It *might* be safe to not do that, but it's not very certain, and I don't think this code deserves any benefit of the doubt. Dilip Kumar, per complaint from me Discussion: https://postgr.es/m/3679800.1659654066@sss.pgh.pa.us
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
@@ -3741,23 +3741,19 @@ RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
 		srcBuf = ReadBufferWithoutRelcache(src->rd_locator, forkNum, blkno,
 										   RBM_NORMAL, bstrategy_src,
 										   permanent);
+		LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
 		srcPage = BufferGetPage(srcBuf);
-		if (PageIsNew(srcPage) || PageIsEmpty(srcPage))
-		{
-			ReleaseBuffer(srcBuf);
-			continue;
-		}
 
 		/* Use P_NEW to extend the destination relation. */
 		dstBuf = ReadBufferWithoutRelcache(dst->rd_locator, forkNum, P_NEW,
 										   RBM_NORMAL, bstrategy_dst,
 										   permanent);
 		LockBuffer(dstBuf, BUFFER_LOCK_EXCLUSIVE);
+		dstPage = BufferGetPage(dstBuf);
 
 		START_CRIT_SECTION();
 
 		/* Copy page data from the source to the destination. */
-		dstPage = BufferGetPage(dstBuf);
 		memcpy(dstPage, srcPage, BLCKSZ);
 		MarkBufferDirty(dstBuf);
 
@@ -3768,7 +3764,7 @@ RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
 		END_CRIT_SECTION();
 
 		UnlockReleaseBuffer(dstBuf);
-		ReleaseBuffer(srcBuf);
+		UnlockReleaseBuffer(srcBuf);
 	}
 }
 
