Avoid using a fake relcache entry to own an SmgrRelation.

If an error occurs before we close the fake relcache entry, the the
fake relcache entry will be destroyed by the SmgrRelation will
survive until end of transaction. Its smgr_owner pointer ends up
pointing to already-freed memory.

The original reason for using a fake relcache entry here was to try
to avoid reusing an SMgrRelation across a relevant invalidation. To
avoid that problem, just call smgropen() again each time we need a
reference to it. Hopefully someday we will come up with a more
elegant approach, but accessing uninitialized memory is bad so let's
do this for now.

Dilip Kumar, reviewed by Andres Freund and Tom Lane. Report by
Justin Pryzby.

Discussion: http://postgr.es/m/20220802175043.GA13682@telsasoft.com
Discussion: http://postgr.es/m/CAFiTN-vSFeE6_W9z698XNtFROOA_nSqUXWqLcG0emob_kJ+dEQ@mail.gmail.com

src/backend/commands/dbcommands.c

@@ -258,8 +258,8 @@ ScanSourceDatabasePgClass(Oid tbid, Oid dbid, char *srcpath)
 	Page		page;
 	List	   *rlocatorlist = NIL;
 	LockRelId	relid;
-	Relation	rel;
 	Snapshot	snapshot;
+	SMgrRelation	smgr;
 	BufferAccessStrategy bstrategy;
 
 	/* Get pg_class relfilenumber. */
@@ -276,16 +276,9 @@ ScanSourceDatabasePgClass(Oid tbid, Oid dbid, char *srcpath)
 	rlocator.dbOid = dbid;
 	rlocator.relNumber = relfilenumber;
 
-	/*
+	smgr = smgropen(rlocator, InvalidBackendId);
-	 * We can't use a real relcache entry for a relation in some other
+	nblocks = smgrnblocks(smgr, MAIN_FORKNUM);
-	 * database, but since we're only going to access the fields related to
+	smgrclose(smgr);
-	 * physical storage, a fake one is good enough. If we didn't do this and
-	 * used the smgr layer directly, we would have to worry about
-	 * invalidations.
-	 */
-	rel = CreateFakeRelcacheEntry(rlocator);
-	nblocks = smgrnblocks(RelationGetSmgr(rel), MAIN_FORKNUM);
-	FreeFakeRelcacheEntry(rel);
 
 	/* Use a buffer access strategy since this is a bulk read operation. */
 	bstrategy = GetAccessStrategy(BAS_BULKREAD);

src/backend/storage/buffer/bufmgr.c

@@ -487,9 +487,9 @@ static void FindAndDropRelationBuffers(RelFileLocator rlocator,
 									   ForkNumber forkNum,
 									   BlockNumber nForkBlock,
 									   BlockNumber firstDelBlock);
-static void RelationCopyStorageUsingBuffer(Relation src, Relation dst,
+static void RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
-										   ForkNumber forkNum,
+										   RelFileLocator dstlocator,
-										   bool isunlogged);
+										   ForkNumber forkNum, bool permanent);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
 static int	rlocator_comparator(const void *p1, const void *p2);
@@ -3701,8 +3701,9 @@ FlushRelationsAllBuffers(SMgrRelation *smgrs, int nrels)
  * --------------------------------------------------------------------
  */
 static void
-RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
+RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
-							   bool permanent)
+							   RelFileLocator dstlocator,
+							   ForkNumber forkNum, bool permanent)
 {
 	Buffer		srcBuf;
 	Buffer		dstBuf;
@@ -3722,7 +3723,8 @@ RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
 	use_wal = XLogIsNeeded() && (permanent || forkNum == INIT_FORKNUM);
 
 	/* Get number of blocks in the source relation. */
-	nblocks = smgrnblocks(RelationGetSmgr(src), forkNum);
+	nblocks = smgrnblocks(smgropen(srclocator, InvalidBackendId),
+						  forkNum);
 
 	/* Nothing to copy; just return. */
 	if (nblocks == 0)
@@ -3738,14 +3740,14 @@ RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
 		CHECK_FOR_INTERRUPTS();
 
 		/* Read block from source relation. */
-		srcBuf = ReadBufferWithoutRelcache(src->rd_locator, forkNum, blkno,
+		srcBuf = ReadBufferWithoutRelcache(srclocator, forkNum, blkno,
 										   RBM_NORMAL, bstrategy_src,
 										   permanent);
 		LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
 		srcPage = BufferGetPage(srcBuf);
 
 		/* Use P_NEW to extend the destination relation. */
-		dstBuf = ReadBufferWithoutRelcache(dst->rd_locator, forkNum, P_NEW,
+		dstBuf = ReadBufferWithoutRelcache(dstlocator, forkNum, P_NEW,
 										   RBM_NORMAL, bstrategy_dst,
 										   permanent);
 		LockBuffer(dstBuf, BUFFER_LOCK_EXCLUSIVE);
@@ -3783,24 +3785,13 @@ void
 CreateAndCopyRelationData(RelFileLocator src_rlocator,
 						  RelFileLocator dst_rlocator, bool permanent)
 {
-	Relation	src_rel;
+	RelFileLocatorBackend rlocator;
-	Relation	dst_rel;
 	char		relpersistence;
 
 	/* Set the relpersistence. */
 	relpersistence = permanent ?
 		RELPERSISTENCE_PERMANENT : RELPERSISTENCE_UNLOGGED;
 
-	/*
-	 * We can't use a real relcache entry for a relation in some other
-	 * database, but since we're only going to access the fields related to
-	 * physical storage, a fake one is good enough. If we didn't do this and
-	 * used the smgr layer directly, we would have to worry about
-	 * invalidations.
-	 */
-	src_rel = CreateFakeRelcacheEntry(src_rlocator);
-	dst_rel = CreateFakeRelcacheEntry(dst_rlocator);
-
 	/*
 	 * Create and copy all forks of the relation.  During create database we
 	 * have a separate cleanup mechanism which deletes complete database
@@ -3810,15 +3801,16 @@ CreateAndCopyRelationData(RelFileLocator src_rlocator,
 	RelationCreateStorage(dst_rlocator, relpersistence, false);
 
 	/* copy main fork. */
-	RelationCopyStorageUsingBuffer(src_rel, dst_rel, MAIN_FORKNUM, permanent);
+	RelationCopyStorageUsingBuffer(src_rlocator, dst_rlocator, MAIN_FORKNUM,
+								   permanent);
 
 	/* copy those extra forks that exist */
 	for (ForkNumber forkNum = MAIN_FORKNUM + 1;
 		 forkNum <= MAX_FORKNUM; forkNum++)
 	{
-		if (smgrexists(RelationGetSmgr(src_rel), forkNum))
+		if (smgrexists(smgropen(src_rlocator, InvalidBackendId), forkNum))
 		{
-			smgrcreate(RelationGetSmgr(dst_rel), forkNum, false);
+			smgrcreate(smgropen(dst_rlocator, InvalidBackendId), forkNum, false);
 
 			/*
 			 * WAL log creation if the relation is persistent, or this is the
@@ -3828,14 +3820,19 @@ CreateAndCopyRelationData(RelFileLocator src_rlocator,
 				log_smgrcreate(&dst_rlocator, forkNum);
 
 			/* Copy a fork's data, block by block. */
-			RelationCopyStorageUsingBuffer(src_rel, dst_rel, forkNum,
+			RelationCopyStorageUsingBuffer(src_rlocator, dst_rlocator, forkNum,
 										   permanent);
 		}
 	}
 
-	/* Release fake relcache entries. */
+	/* close source and destination smgr if exists. */
-	FreeFakeRelcacheEntry(src_rel);
+	rlocator.backend = InvalidBackendId;
-	FreeFakeRelcacheEntry(dst_rel);
+
+	rlocator.locator = src_rlocator;
+	smgrcloserellocator(rlocator);
+
+	rlocator.locator = dst_rlocator;
+	smgrcloserellocator(rlocator);
 }
 
 /* ---------------------------------------------------------------------