v2-0002-Avoid-setting-the-fake-relcache-entry-as-smgr-own.patch

From 85d17fb7c91fdb6b40f09d00e9a5606ba2c90e57 Mon Sep 17 00:00:00 2001
From: Dilip Kumar <dilip.kumar@enterprisedb.com>
Date: Fri, 5 Aug 2022 10:59:18 +0530
Subject: [PATCH v2 2/3] Avoid setting the fake relcache entry as smgr owner
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During CREATE DATABASE, we are not connected to the source and the
destination DB so in order to operate on the storage we are using
FakeRelCacheEntry and by using that we are calling RelationGetSmgr().

So the problem is that this function will set the temporary
FakeRelCacheEntry as an owner of the smgr.  Now if there is any
error before we close the FakeRelCacheEntry then the memory of the
fake relcache entry will be released at the transaction abort but
the smgr will survive the transaction.  So now smgr is pointing
to some already release memory and it will have random behavior
when we try to access the smgr next time.

For fixing the issue instead of using the FakeRelCacheEntry, directly
call the smgropen() but do not keep the reference to the smgr.
So every time call smgropen() whenever we need it.  This is required to
ensure that we do not access the smgr pointer which might have already
been closed during interrupt processing.
---
 src/backend/commands/dbcommands.c   | 12 +----------
 src/backend/storage/buffer/bufmgr.c | 43 +++++++++++++------------------------
 2 files changed, 16 insertions(+), 39 deletions(-)

diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 9f990a8..88d4fe1 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -258,7 +258,6 @@ ScanSourceDatabasePgClass(Oid tbid, Oid dbid, char *srcpath)
 	Page		page;
 	List	   *rlocatorlist = NIL;
 	LockRelId	relid;
-	Relation	rel;
 	Snapshot	snapshot;
 	BufferAccessStrategy bstrategy;
 
@@ -276,16 +275,7 @@ ScanSourceDatabasePgClass(Oid tbid, Oid dbid, char *srcpath)
 	rlocator.dbOid = dbid;
 	rlocator.relNumber = relfilenumber;
 
-	/*
-	 * 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.
-	 */
-	rel = CreateFakeRelcacheEntry(rlocator);
-	nblocks = smgrnblocks(RelationGetSmgr(rel), MAIN_FORKNUM);
-	FreeFakeRelcacheEntry(rel);
+	nblocks = smgrnblocks(smgropen(rlocator, InvalidBackendId), MAIN_FORKNUM);
 
 	/* Use a buffer access strategy since this is a bulk read operation. */
 	bstrategy = GetAccessStrategy(BAS_BULKREAD);
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 7f992c3..b488306 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/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,
-										   ForkNumber forkNum,
-										   bool isunlogged);
+static void RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
+										   RelFileLocator dstlocator,
+										   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,
-							   bool permanent)
+RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
+							   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)
@@ -3738,7 +3740,7 @@ 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);
 
@@ -3746,7 +3748,7 @@ RelationCopyStorageUsingBuffer(Relation src, Relation dst, ForkNumber forkNum,
 		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);
@@ -3784,8 +3786,6 @@ void
 CreateAndCopyRelationData(RelFileLocator src_rlocator,
 						  RelFileLocator dst_rlocator, bool permanent)
 {
-	Relation	src_rel;
-	Relation	dst_rel;
 	char		relpersistence;
 
 	/* Set the relpersistence. */
@@ -3793,16 +3793,6 @@ CreateAndCopyRelationData(RelFileLocator src_rlocator,
 		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
 	 * directory.  Therefore, each individual relation doesn't need to be
@@ -3811,15 +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
@@ -3829,14 +3820,10 @@ 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. */
-	FreeFakeRelcacheEntry(src_rel);
-	FreeFakeRelcacheEntry(dst_rel);
 }
 
 /* ---------------------------------------------------------------------
-- 
1.8.3.1