Replace RelationOpenSmgr() with RelationGetSmgr().

This is a back-patch of the v15-era commit f10f0ae42 into older
supported branches.  The idea is to design out bugs in which an
ill-timed relcache flush clears rel->rd_smgr partway through
some code sequence that wasn't expecting that.  We had another
report today of a corner case that reliably crashes v14 under
debug_discard_caches (nee CLOBBER_CACHE_ALWAYS), and therefore
would crash once in a blue moon in the field.  We're unlikely
to get rid of all such code paths unless we adopt the more
rigorous coding rules instituted by f10f0ae42.  Therefore,
even though this is a bit invasive, it's time to back-patch.
Some comfort can be taken in the fact that f10f0ae42 has been
in v15 for 16 months without problems.

I left the RelationOpenSmgr macro present in the back branches,
even though no core code should use it anymore, in order to not break
third-party extensions in minor releases.  Such extensions might opt
to start using RelationGetSmgr instead, to reduce their code
differential between v15 and earlier branches.  This carries a hazard
of failing to compile against headers from existing minor releases.
However, once compiled the extension should work fine even with such
releases, because RelationGetSmgr is a "static inline" function so
it creates no link-time dependency.  So depending on distribution
practices, that might be an OK tradeoff.

Per report from Spyridon Dimitrios Agathos.  Original patch
by Amul Sul.

Discussion: https://postgr.es/m/CAFM5RaqdgyusQvmWkyPYaWMwoK5gigdtW-7HcgHgOeAw7mqJ_Q@mail.gmail.com
Discussion: https://postgr.es/m/CANiYTQsU7yMFpQYnv=BrcRVqK_3U3mtAzAsJCaqtzsDHfsUbdQ@mail.gmail.com

contrib/amcheck/verify_nbtree.c

@@ -301,8 +301,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 	{
 		bool	heapkeyspace;
 
-		RelationOpenSmgr(indrel);
+		if (!smgrexists(RelationGetSmgr(indrel), MAIN_FORKNUM))
-		if (!smgrexists(indrel->rd_smgr, MAIN_FORKNUM))
 			ereport(ERROR,
 					(errcode(ERRCODE_INDEX_CORRUPTED),
 					 errmsg("index \"%s\" lacks a main relation fork",

contrib/bloom/blinsert.c

@@ -179,9 +179,9 @@ blbuildempty(Relation index)
 	 * this even when wal_level=minimal.
 	 */
 	PageSetChecksumInplace(metapage, BLOOM_METAPAGE_BLKNO);
-	smgrwrite(index->rd_smgr, INIT_FORKNUM, BLOOM_METAPAGE_BLKNO,
+	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, BLOOM_METAPAGE_BLKNO,
 			  (char *) metapage, true);
-	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+	log_newpage(&(RelationGetSmgr(index))->smgr_rnode.node, INIT_FORKNUM,
 				BLOOM_METAPAGE_BLKNO, metapage, true);
 
 	/*
@@ -189,7 +189,7 @@ blbuildempty(Relation index)
 	 * write did not go through shared_buffers and therefore a concurrent
 	 * checkpoint may have moved the redo pointer past our xlog record.
 	 */
-	smgrimmedsync(index->rd_smgr, INIT_FORKNUM);
+	smgrimmedsync(RelationGetSmgr(index), INIT_FORKNUM);
 }
 
 /*

contrib/pg_prewarm/autoprewarm.c

@@ -518,15 +518,13 @@ autoprewarm_database_main(Datum main_arg)
 			old_blk->filenode != blk->filenode ||
 			old_blk->forknum != blk->forknum)
 		{
-			RelationOpenSmgr(rel);
-
 			/*
 			 * smgrexists is not safe for illegal forknum, hence check whether
 			 * the passed forknum is valid before using it in smgrexists.
 			 */
 			if (blk->forknum > InvalidForkNumber &&
 				blk->forknum <= MAX_FORKNUM &&
-				smgrexists(rel->rd_smgr, blk->forknum))
+				smgrexists(RelationGetSmgr(rel), blk->forknum))
 				nblocks = RelationGetNumberOfBlocksInFork(rel, blk->forknum);
 			else
 				nblocks = 0;

contrib/pg_prewarm/pg_prewarm.c

@@ -109,8 +109,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		aclcheck_error(aclresult, get_relkind_objtype(rel->rd_rel->relkind), get_rel_name(relOid));
 
 	/* Check that the fork exists. */
-	RelationOpenSmgr(rel);
+	if (!smgrexists(RelationGetSmgr(rel), forkNumber))
-	if (!smgrexists(rel->rd_smgr, forkNumber))
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("fork \"%s\" does not exist for this relation",
@@ -178,7 +177,7 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		for (block = first_block; block <= last_block; ++block)
 		{
 			CHECK_FOR_INTERRUPTS();
-			smgrread(rel->rd_smgr, forkNumber, block, blockbuffer.data);
+			smgrread(RelationGetSmgr(rel), forkNumber, block, blockbuffer.data);
 			++blocks_done;
 		}
 	}

contrib/pg_visibility/pg_visibility.c

@@ -389,8 +389,8 @@ pg_truncate_visibility_map(PG_FUNCTION_ARGS)
 	/* Only some relkinds have a visibility map */
 	check_relation_relkind(rel);
 
-	RelationOpenSmgr(rel);
+	/* Forcibly reset cached file size */
-	rel->rd_smgr->smgr_vm_nblocks = InvalidBlockNumber;
+	RelationGetSmgr(rel)->smgr_vm_nblocks = InvalidBlockNumber;
 
 	visibilitymap_truncate(rel, 0);
 

src/backend/access/gist/gistbuild.c

@@ -514,7 +514,7 @@ gistBuildCallback(Relation index,
 	 */
 	if ((buildstate->bufferingMode == GIST_BUFFERING_AUTO &&
 		 buildstate->indtuples % BUFFERING_MODE_SWITCH_CHECK_STEP == 0 &&
-		 effective_cache_size < smgrnblocks(index->rd_smgr, MAIN_FORKNUM)) ||
+		 effective_cache_size < smgrnblocks(RelationGetSmgr(index), MAIN_FORKNUM)) ||
 		(buildstate->bufferingMode == GIST_BUFFERING_STATS &&
 		 buildstate->indtuples >= BUFFERING_MODE_TUPLE_SIZE_STATS_TARGET))
 	{

src/backend/access/hash/hashpage.c

@@ -1031,9 +1031,9 @@ _hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
 					zerobuf.data,
 					true);
 
-	RelationOpenSmgr(rel);
 	PageSetChecksumInplace(page, lastblock);
-	smgrextend(rel->rd_smgr, MAIN_FORKNUM, lastblock, zerobuf.data, false);
+	smgrextend(RelationGetSmgr(rel), MAIN_FORKNUM, lastblock, zerobuf.data,
+			   false);
 
 	return true;
 }

src/backend/access/heap/heapam.c

@@ -9098,8 +9098,7 @@ heap_sync(Relation rel)
 
 	/* main heap */
 	FlushRelationBuffers(rel);
-	/* FlushRelationBuffers will have opened rd_smgr */
+	smgrimmedsync(RelationGetSmgr(rel), MAIN_FORKNUM);
-	smgrimmedsync(rel->rd_smgr, MAIN_FORKNUM);
 
 	/* FSM is not critical, don't bother syncing it */
 
@@ -9110,7 +9109,7 @@ heap_sync(Relation rel)
 
 		toastrel = table_open(rel->rd_rel->reltoastrelid, AccessShareLock);
 		FlushRelationBuffers(toastrel);
-		smgrimmedsync(toastrel->rd_smgr, MAIN_FORKNUM);
+		smgrimmedsync(RelationGetSmgr(toastrel), MAIN_FORKNUM);
 		table_close(toastrel, AccessShareLock);
 	}
 }

src/backend/access/heap/heapam_handler.c

@@ -643,7 +643,6 @@ heapam_relation_copy_data(Relation rel, const RelFileNode *newrnode)
 	SMgrRelation dstrel;
 
 	dstrel = smgropen(*newrnode, rel->rd_backend);
-	RelationOpenSmgr(rel);
 
 	/*
 	 * Since we copy the file directly without looking at the shared buffers,
@@ -663,14 +662,14 @@ heapam_relation_copy_data(Relation rel, const RelFileNode *newrnode)
 	RelationCreateStorage(*newrnode, rel->rd_rel->relpersistence);
 
 	/* copy main fork */
-	RelationCopyStorage(rel->rd_smgr, dstrel, MAIN_FORKNUM,
+	RelationCopyStorage(RelationGetSmgr(rel), dstrel, MAIN_FORKNUM,
 						rel->rd_rel->relpersistence);
 
 	/* copy those extra forks that exist */
 	for (ForkNumber forkNum = MAIN_FORKNUM + 1;
 		 forkNum <= MAX_FORKNUM; forkNum++)
 	{
-		if (smgrexists(rel->rd_smgr, forkNum))
+		if (smgrexists(RelationGetSmgr(rel), forkNum))
 		{
 			smgrcreate(dstrel, forkNum, false);
 
@@ -682,7 +681,7 @@ heapam_relation_copy_data(Relation rel, const RelFileNode *newrnode)
 				(rel->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
 				 forkNum == INIT_FORKNUM))
 				log_smgrcreate(newrnode, forkNum);
-			RelationCopyStorage(rel->rd_smgr, dstrel, forkNum,
+			RelationCopyStorage(RelationGetSmgr(rel), dstrel, forkNum,
 								rel->rd_rel->relpersistence);
 		}
 	}
@@ -2045,18 +2044,23 @@ static uint64
 heapam_relation_size(Relation rel, ForkNumber forkNumber)
 {
 	uint64		nblocks = 0;
+	SMgrRelation reln;
 
-	/* Open it at the smgr level if not already done */
+	/*
-	RelationOpenSmgr(rel);
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
+	 */
+	reln = RelationGetSmgr(rel);
 
 	/* InvalidForkNumber indicates returning the size for all forks */
 	if (forkNumber == InvalidForkNumber)
 	{
 		for (int i = 0; i < MAX_FORKNUM; i++)
-			nblocks += smgrnblocks(rel->rd_smgr, i);
+			nblocks += smgrnblocks(reln, i);
 	}
 	else
-		nblocks = smgrnblocks(rel->rd_smgr, forkNumber);
+		nblocks = smgrnblocks(reln, forkNumber);
 
 	return nblocks * BLCKSZ;
 }

src/backend/access/heap/rewriteheap.c

@@ -336,11 +336,10 @@ end_heap_rewrite(RewriteState state)
 						state->rs_blockno,
 						state->rs_buffer,
 						true);
-		RelationOpenSmgr(state->rs_new_rel);
 
 		PageSetChecksumInplace(state->rs_buffer, state->rs_blockno);
 
-		smgrextend(state->rs_new_rel->rd_smgr, MAIN_FORKNUM, state->rs_blockno,
+		smgrextend(RelationGetSmgr(state->rs_new_rel), MAIN_FORKNUM, state->rs_blockno,
 				   (char *) state->rs_buffer, true);
 	}
 
@@ -708,11 +707,9 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 			 * fsync for this write; we'll do it ourselves in
 			 * end_heap_rewrite.
 			 */
-			RelationOpenSmgr(state->rs_new_rel);
-
 			PageSetChecksumInplace(page, state->rs_blockno);
 
-			smgrextend(state->rs_new_rel->rd_smgr, MAIN_FORKNUM,
+			smgrextend(RelationGetSmgr(state->rs_new_rel), MAIN_FORKNUM,
 					   state->rs_blockno, (char *) page, true);
 
 			state->rs_blockno++;

src/backend/access/heap/visibilitymap.c

@@ -452,13 +452,11 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
 	elog(DEBUG1, "vm_truncate %s %d", RelationGetRelationName(rel), nheapblocks);
 #endif
 
-	RelationOpenSmgr(rel);
-
 	/*
 	 * If no visibility map has been created yet for this relation, there's
 	 * nothing to truncate.
 	 */
-	if (!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+	if (!smgrexists(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM))
 		return;
 
 	/*
@@ -525,14 +523,14 @@ visibilitymap_truncate(Relation rel, BlockNumber nheapblocks)
 	else
 		newnblocks = truncBlock;
 
-	if (smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM) <= newnblocks)
+	if (smgrnblocks(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM) <= newnblocks)
 	{
 		/* nothing to do, the file was already smaller than requested size */
 		return;
 	}
 
 	/* Truncate the unused VM pages, and send smgr inval message */
-	smgrtruncate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, newnblocks);
+	smgrtruncate(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM, newnblocks);
 
 	/*
 	 * We might as well update the local smgr_vm_nblocks setting. smgrtruncate
@@ -554,31 +552,30 @@ static Buffer
 vm_readbuf(Relation rel, BlockNumber blkno, bool extend)
 {
 	Buffer		buf;
+	SMgrRelation reln;
 
 	/*
-	 * We might not have opened the relation at the smgr level yet, or we
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
-	 * might have been forced to close it by a sinval message.  The code below
+	 * gets closed.  It's safe as long as we only do smgr-level operations
-	 * won't necessarily notice relation extension immediately when extend =
+	 * between here and the last use of the pointer.
-	 * false, so we rely on sinval messages to ensure that our ideas about the
-	 * size of the map aren't too far out of date.
 	 */
-	RelationOpenSmgr(rel);
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * If we haven't cached the size of the visibility map fork yet, check it
 	 * first.
 	 */
-	if (rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber)
+	if (reln->smgr_vm_nblocks == InvalidBlockNumber)
 	{
-		if (smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+		if (smgrexists(reln, VISIBILITYMAP_FORKNUM))
-			rel->rd_smgr->smgr_vm_nblocks = smgrnblocks(rel->rd_smgr,
+			reln->smgr_vm_nblocks = smgrnblocks(reln,
-														VISIBILITYMAP_FORKNUM);
+												VISIBILITYMAP_FORKNUM);
 		else
-			rel->rd_smgr->smgr_vm_nblocks = 0;
+			reln->smgr_vm_nblocks = 0;
 	}
 
 	/* Handle requests beyond EOF */
-	if (blkno >= rel->rd_smgr->smgr_vm_nblocks)
+	if (blkno >= reln->smgr_vm_nblocks)
 	{
 		if (extend)
 			vm_extend(rel, blkno + 1);
@@ -626,6 +623,7 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 {
 	BlockNumber vm_nblocks_now;
 	PGAlignedBlock pg;
+	SMgrRelation reln;
 
 	PageInit((Page) pg.data, BLCKSZ, 0);
 
@@ -641,26 +639,30 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 */
 	LockRelationForExtension(rel, ExclusiveLock);
 
-	/* Might have to re-open if a cache flush happened */
+	/*
-	RelationOpenSmgr(rel);
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
+	 */
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * Create the file first if it doesn't exist.  If smgr_vm_nblocks is
 	 * positive then it must exist, no need for an smgrexists call.
 	 */
-	if ((rel->rd_smgr->smgr_vm_nblocks == 0 ||
+	if ((reln->smgr_vm_nblocks == 0 ||
-		 rel->rd_smgr->smgr_vm_nblocks == InvalidBlockNumber) &&
+		 reln->smgr_vm_nblocks == InvalidBlockNumber) &&
-		!smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM))
+		!smgrexists(reln, VISIBILITYMAP_FORKNUM))
-		smgrcreate(rel->rd_smgr, VISIBILITYMAP_FORKNUM, false);
+		smgrcreate(reln, VISIBILITYMAP_FORKNUM, false);
 
-	vm_nblocks_now = smgrnblocks(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
+	vm_nblocks_now = smgrnblocks(reln, VISIBILITYMAP_FORKNUM);
 
 	/* Now extend the file */
 	while (vm_nblocks_now < vm_nblocks)
 	{
 		PageSetChecksumInplace((Page) pg.data, vm_nblocks_now);
 
-		smgrextend(rel->rd_smgr, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
+		smgrextend(reln, VISIBILITYMAP_FORKNUM, vm_nblocks_now,
 				   pg.data, false);
 		vm_nblocks_now++;
 	}
@@ -672,10 +674,10 @@ vm_extend(Relation rel, BlockNumber vm_nblocks)
 	 * to keep checking for creation or extension of the file, which happens
 	 * infrequently.
 	 */
-	CacheInvalidateSmgr(rel->rd_smgr->smgr_rnode);
+	CacheInvalidateSmgr(reln->smgr_rnode);
 
 	/* Update local cache with the up-to-date size */
-	rel->rd_smgr->smgr_vm_nblocks = vm_nblocks_now;
+	reln->smgr_vm_nblocks = vm_nblocks_now;
 
 	UnlockRelationForExtension(rel, ExclusiveLock);
 }

src/backend/access/nbtree/nbtree.c

@@ -170,9 +170,9 @@ btbuildempty(Relation index)
 	 * this even when wal_level=minimal.
 	 */
 	PageSetChecksumInplace(metapage, BTREE_METAPAGE);
-	smgrwrite(index->rd_smgr, INIT_FORKNUM, BTREE_METAPAGE,
+	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, BTREE_METAPAGE,
 			  (char *) metapage, true);
-	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+	log_newpage(&RelationGetSmgr(index)->smgr_rnode.node, INIT_FORKNUM,
 				BTREE_METAPAGE, metapage, true);
 
 	/*
@@ -180,7 +180,7 @@ btbuildempty(Relation index)
 	 * write did not go through shared_buffers and therefore a concurrent
 	 * checkpoint may have moved the redo pointer past our xlog record.
 	 */
-	smgrimmedsync(index->rd_smgr, INIT_FORKNUM);
+	smgrimmedsync(RelationGetSmgr(index), INIT_FORKNUM);
 }
 
 /*

src/backend/access/nbtree/nbtsort.c

@@ -655,9 +655,6 @@ _bt_blnewpage(uint32 level)
 static void
 _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
 {
-	/* Ensure rd_smgr is open (could have been closed by relcache flush!) */
-	RelationOpenSmgr(wstate->index);
-
 	/* XLOG stuff */
 	if (wstate->btws_use_wal)
 	{
@@ -677,7 +674,7 @@ _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
 		if (!wstate->btws_zeropage)
 			wstate->btws_zeropage = (Page) palloc0(BLCKSZ);
 		/* don't set checksum for all-zero page */
-		smgrextend(wstate->index->rd_smgr, MAIN_FORKNUM,
+		smgrextend(RelationGetSmgr(wstate->index), MAIN_FORKNUM,
 				   wstate->btws_pages_written++,
 				   (char *) wstate->btws_zeropage,
 				   true);
@@ -692,14 +689,14 @@ _bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
 	if (blkno == wstate->btws_pages_written)
 	{
 		/* extending the file... */
-		smgrextend(wstate->index->rd_smgr, MAIN_FORKNUM, blkno,
+		smgrextend(RelationGetSmgr(wstate->index), MAIN_FORKNUM, blkno,
 				   (char *) page, true);
 		wstate->btws_pages_written++;
 	}
 	else
 	{
 		/* overwriting a block we zero-filled before */
-		smgrwrite(wstate->index->rd_smgr, MAIN_FORKNUM, blkno,
+		smgrwrite(RelationGetSmgr(wstate->index), MAIN_FORKNUM, blkno,
 				  (char *) page, true);
 	}
 
@@ -1305,10 +1302,7 @@ _bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
 	 * occurs.
 	 */
 	if (RelationNeedsWAL(wstate->index))
-	{
+		smgrimmedsync(RelationGetSmgr(wstate->index), MAIN_FORKNUM);
-		RelationOpenSmgr(wstate->index);
-		smgrimmedsync(wstate->index->rd_smgr, MAIN_FORKNUM);
-	}
 }
 
 /*

src/backend/access/spgist/spginsert.c

@@ -169,27 +169,27 @@ spgbuildempty(Relation index)
 	 * replayed.
 	 */
 	PageSetChecksumInplace(page, SPGIST_METAPAGE_BLKNO);
-	smgrwrite(index->rd_smgr, INIT_FORKNUM, SPGIST_METAPAGE_BLKNO,
+	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, SPGIST_METAPAGE_BLKNO,
 			  (char *) page, true);
-	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+	log_newpage(&(RelationGetSmgr(index))->smgr_rnode.node, INIT_FORKNUM,
 				SPGIST_METAPAGE_BLKNO, page, true);
 
 	/* Likewise for the root page. */
 	SpGistInitPage(page, SPGIST_LEAF);
 
 	PageSetChecksumInplace(page, SPGIST_ROOT_BLKNO);
-	smgrwrite(index->rd_smgr, INIT_FORKNUM, SPGIST_ROOT_BLKNO,
+	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, SPGIST_ROOT_BLKNO,
 			  (char *) page, true);
-	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+	log_newpage(&(RelationGetSmgr(index))->smgr_rnode.node, INIT_FORKNUM,
 				SPGIST_ROOT_BLKNO, page, true);
 
 	/* Likewise for the null-tuples root page. */
 	SpGistInitPage(page, SPGIST_LEAF | SPGIST_NULLS);
 
 	PageSetChecksumInplace(page, SPGIST_NULL_BLKNO);
-	smgrwrite(index->rd_smgr, INIT_FORKNUM, SPGIST_NULL_BLKNO,
+	smgrwrite(RelationGetSmgr(index), INIT_FORKNUM, SPGIST_NULL_BLKNO,
 			  (char *) page, true);
-	log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
+	log_newpage(&(RelationGetSmgr(index))->smgr_rnode.node, INIT_FORKNUM,
 				SPGIST_NULL_BLKNO, page, true);
 
 	/*
@@ -197,7 +197,7 @@ spgbuildempty(Relation index)
 	 * writes did not go through shared buffers and therefore a concurrent
 	 * checkpoint may have moved the redo pointer past our xlog record.
 	 */
-	smgrimmedsync(index->rd_smgr, INIT_FORKNUM);
+	smgrimmedsync(RelationGetSmgr(index), INIT_FORKNUM);
 }
 
 /*

src/backend/catalog/heap.c

@@ -415,8 +415,6 @@ heap_create(const char *relname,
 	 */
 	if (create_storage)
 	{
-		RelationOpenSmgr(rel);
-
 		switch (rel->rd_rel->relkind)
 		{
 			case RELKIND_VIEW:

src/backend/catalog/index.c

@@ -3009,10 +3009,9 @@ index_build(Relation heapRelation,
 	 * relfilenode won't change, and nothing needs to be done here.
 	 */
 	if (indexRelation->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
-		!smgrexists(indexRelation->rd_smgr, INIT_FORKNUM))
+		!smgrexists(RelationGetSmgr(indexRelation), INIT_FORKNUM))
 	{
-		RelationOpenSmgr(indexRelation);
+		smgrcreate(RelationGetSmgr(indexRelation), INIT_FORKNUM, false);
-		smgrcreate(indexRelation->rd_smgr, INIT_FORKNUM, false);
 		indexRelation->rd_indam->ambuildempty(indexRelation);
 	}
 

src/backend/catalog/storage.c

@@ -232,24 +232,24 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
 {
 	bool		fsm;
 	bool		vm;
-
+	SMgrRelation reln;
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(rel);
 
 	/*
-	 * Make sure smgr_targblock etc aren't pointing somewhere past new end
+	 * Make sure smgr_targblock etc aren't pointing somewhere past new end.
+	 * (Note: don't rely on this reln pointer below here.)
 	 */
-	rel->rd_smgr->smgr_targblock = InvalidBlockNumber;
+	reln = RelationGetSmgr(rel);
-	rel->rd_smgr->smgr_fsm_nblocks = InvalidBlockNumber;
+	reln->smgr_targblock = InvalidBlockNumber;
-	rel->rd_smgr->smgr_vm_nblocks = InvalidBlockNumber;
+	reln->smgr_fsm_nblocks = InvalidBlockNumber;
+	reln->smgr_vm_nblocks = InvalidBlockNumber;
 
 	/* Truncate the FSM first if it exists */
-	fsm = smgrexists(rel->rd_smgr, FSM_FORKNUM);
+	fsm = smgrexists(reln, FSM_FORKNUM);
 	if (fsm)
 		FreeSpaceMapTruncateRel(rel, nblocks);
 
 	/* Truncate the visibility map too if it exists. */
-	vm = smgrexists(rel->rd_smgr, VISIBILITYMAP_FORKNUM);
+	vm = smgrexists(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM);
 	if (vm)
 		visibilitymap_truncate(rel, nblocks);
 
@@ -312,7 +312,7 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
 	 * longer exist after truncation is complete, and then truncate the
 	 * corresponding files on disk.
 	 */
-	smgrtruncate(rel->rd_smgr, MAIN_FORKNUM, nblocks);
+	smgrtruncate(RelationGetSmgr(rel), MAIN_FORKNUM, nblocks);
 
 	/* We've done all the critical work, so checkpoints are OK now. */
 	MyProc->delayChkptEnd = false;
@@ -324,6 +324,12 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
  * Note that this requires that there is no dirty data in shared buffers. If
  * it's possible that there are, callers need to flush those using
  * e.g. FlushRelationBuffers(rel).
+ *
+ * Also note that this is frequently called via locutions such as
+ *		RelationCopyStorage(RelationGetSmgr(rel), ...);
+ * That's safe only because we perform only smgr and WAL operations here.
+ * If we invoked anything else, a relcache flush could cause our SMgrRelation
+ * argument to become a dangling pointer.
  */
 void
 RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
@@ -364,13 +370,23 @@ RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
 
 		if (!PageIsVerifiedExtended(page, blkno,
 									PIV_LOG_WARNING | PIV_REPORT_STAT))
+		{
+			/*
+			 * For paranoia's sake, capture the file path before invoking the
+			 * ereport machinery.  This guards against the possibility of a
+			 * relcache flush caused by, e.g., an errcontext callback.
+			 * (errcontext callbacks shouldn't be risking any such thing, but
+			 * people have been known to forget that rule.)
+			 */
+			char	   *relpath = relpathbackend(src->smgr_rnode.node,
+												 src->smgr_rnode.backend,
+												 forkNum);
+
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("invalid page in block %u of relation %s",
-							blkno,
+							blkno, relpath)));
-							relpathbackend(src->smgr_rnode.node,
+		}
-										   src->smgr_rnode.backend,
-										   forkNum))));
 
 		/*
 		 * WAL-log the copied page. Unfortunately we don't know what kind of a

src/backend/commands/tablecmds.c

@@ -13317,7 +13317,6 @@ index_copy_data(Relation rel, RelFileNode newrnode)
 	SMgrRelation dstrel;
 
 	dstrel = smgropen(newrnode, rel->rd_backend);
-	RelationOpenSmgr(rel);
 
 	/*
 	 * Since we copy the file directly without looking at the shared buffers,
@@ -13337,14 +13336,14 @@ index_copy_data(Relation rel, RelFileNode newrnode)
 	RelationCreateStorage(newrnode, rel->rd_rel->relpersistence);
 
 	/* copy main fork */
-	RelationCopyStorage(rel->rd_smgr, dstrel, MAIN_FORKNUM,
+	RelationCopyStorage(RelationGetSmgr(rel), dstrel, MAIN_FORKNUM,
 						rel->rd_rel->relpersistence);
 
 	/* copy those extra forks that exist */
 	for (ForkNumber forkNum = MAIN_FORKNUM + 1;
 		 forkNum <= MAX_FORKNUM; forkNum++)
 	{
-		if (smgrexists(rel->rd_smgr, forkNum))
+		if (smgrexists(RelationGetSmgr(rel), forkNum))
 		{
 			smgrcreate(dstrel, forkNum, false);
 
@@ -13356,7 +13355,7 @@ index_copy_data(Relation rel, RelFileNode newrnode)
 				(rel->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED &&
 				 forkNum == INIT_FORKNUM))
 				log_smgrcreate(&newrnode, forkNum);
-			RelationCopyStorage(rel->rd_smgr, dstrel, forkNum,
+			RelationCopyStorage(RelationGetSmgr(rel), dstrel, forkNum,
 								rel->rd_rel->relpersistence);
 		}
 	}

src/backend/storage/buffer/bufmgr.c

@@ -533,9 +533,6 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 	Assert(RelationIsValid(reln));
 	Assert(BlockNumberIsValid(blockNum));
 
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(reln);
-
 	if (RelationUsesLocalBuffers(reln))
 	{
 		/* see comments in ReadBufferExtended */
@@ -545,7 +542,7 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 					 errmsg("cannot access temporary tables of other sessions")));
 
 		/* pass it off to localbuf.c */
-		LocalPrefetchBuffer(reln->rd_smgr, forkNum, blockNum);
+		LocalPrefetchBuffer(RelationGetSmgr(reln), forkNum, blockNum);
 	}
 	else
 	{
@@ -555,7 +552,7 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 		int			buf_id;
 
 		/* create a tag so we can lookup the buffer */
-		INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode.node,
+		INIT_BUFFERTAG(newTag, RelationGetSmgr(reln)->smgr_rnode.node,
 					   forkNum, blockNum);
 
 		/* determine its hash code and partition lock ID */
@@ -569,7 +566,7 @@ PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
 
 		/* If not in buffers, initiate prefetch */
 		if (buf_id < 0)
-			smgrprefetch(reln->rd_smgr, forkNum, blockNum);
+			smgrprefetch(RelationGetSmgr(reln), forkNum, blockNum);
 
 		/*
 		 * If the block *is* in buffers, we do nothing.  This is not really
@@ -645,9 +642,6 @@ ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
 	bool		hit;
 	Buffer		buf;
 
-	/* Open it at the smgr level if not already done */
-	RelationOpenSmgr(reln);
-
 	/*
 	 * Reject attempts to read non-local temporary relations; we would be
 	 * likely to get wrong data since we have no visibility into the owning
@@ -663,7 +657,7 @@ ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
 	 * miss.
 	 */
 	pgstat_count_buffer_read(reln);
-	buf = ReadBuffer_common(reln->rd_smgr, reln->rd_rel->relpersistence,
+	buf = ReadBuffer_common(RelationGetSmgr(reln), reln->rd_rel->relpersistence,
 							forkNum, blockNum, mode, strategy, &hit);
 	if (hit)
 		pgstat_count_buffer_hit(reln);
@@ -2814,10 +2808,7 @@ RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
 		case RELKIND_SEQUENCE:
 		case RELKIND_INDEX:
 		case RELKIND_PARTITIONED_INDEX:
-			/* Open it at the smgr level if not already done */
+			return smgrnblocks(RelationGetSmgr(relation), forkNum);
-			RelationOpenSmgr(relation);
-
-			return smgrnblocks(relation->rd_smgr, forkNum);
 
 		case RELKIND_RELATION:
 		case RELKIND_TOASTVALUE:
@@ -3204,9 +3195,6 @@ FlushRelationBuffers(Relation rel)
 	int			i;
 	BufferDesc *bufHdr;
 
-	/* Open rel at the smgr level if not already done */
-	RelationOpenSmgr(rel);
-
 	if (RelationUsesLocalBuffers(rel))
 	{
 		for (i = 0; i < NLocBuffer; i++)
@@ -3231,7 +3219,7 @@ FlushRelationBuffers(Relation rel)
 
 				PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
-				smgrwrite(rel->rd_smgr,
+				smgrwrite(RelationGetSmgr(rel),
 						  bufHdr->tag.forkNum,
 						  bufHdr->tag.blockNum,
 						  localpage,
@@ -3272,7 +3260,7 @@ FlushRelationBuffers(Relation rel)
 		{
 			PinBuffer_Locked(bufHdr);
 			LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
-			FlushBuffer(bufHdr, rel->rd_smgr);
+			FlushBuffer(bufHdr, RelationGetSmgr(rel));
 			LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
 			UnpinBuffer(bufHdr, true);
 		}

src/backend/storage/freespace/freespace.c

@@ -263,13 +263,11 @@ FreeSpaceMapTruncateRel(Relation rel, BlockNumber nblocks)
 	uint16		first_removed_slot;
 	Buffer		buf;
 
-	RelationOpenSmgr(rel);
-
 	/*
 	 * If no FSM has been created yet for this relation, there's nothing to
 	 * truncate.
 	 */
-	if (!smgrexists(rel->rd_smgr, FSM_FORKNUM))
+	if (!smgrexists(RelationGetSmgr(rel), FSM_FORKNUM))
 		return;
 
 	/* Get the location in the FSM of the first removed heap block */
@@ -314,12 +312,12 @@ FreeSpaceMapTruncateRel(Relation rel, BlockNumber nblocks)
 	else
 	{
 		new_nfsmblocks = fsm_logical_to_physical(first_removed_address);
-		if (smgrnblocks(rel->rd_smgr, FSM_FORKNUM) <= new_nfsmblocks)
+		if (smgrnblocks(RelationGetSmgr(rel), FSM_FORKNUM) <= new_nfsmblocks)
 			return;				/* nothing to do; the FSM was already smaller */
 	}
 
 	/* Truncate the unused FSM pages, and send smgr inval message */
-	smgrtruncate(rel->rd_smgr, FSM_FORKNUM, new_nfsmblocks);
+	smgrtruncate(RelationGetSmgr(rel), FSM_FORKNUM, new_nfsmblocks);
 
 	/*
 	 * We might as well update the local smgr_fsm_nblocks setting.
@@ -546,8 +544,14 @@ fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
 {
 	BlockNumber blkno = fsm_logical_to_physical(addr);
 	Buffer		buf;
+	SMgrRelation reln;
 
-	RelationOpenSmgr(rel);
+	/*
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
+	 */
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * If we haven't cached the size of the FSM yet, check it first.  Also
@@ -555,18 +559,18 @@ fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
 	 * value might be stale.  (We send smgr inval messages on truncation, but
 	 * not on extension.)
 	 */
-	if (rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber ||
+	if (reln->smgr_fsm_nblocks == InvalidBlockNumber ||
-		blkno >= rel->rd_smgr->smgr_fsm_nblocks)
+		blkno >= reln->smgr_fsm_nblocks)
 	{
-		if (smgrexists(rel->rd_smgr, FSM_FORKNUM))
+		if (smgrexists(reln, FSM_FORKNUM))
-			rel->rd_smgr->smgr_fsm_nblocks = smgrnblocks(rel->rd_smgr,
+			reln->smgr_fsm_nblocks = smgrnblocks(reln,
-														 FSM_FORKNUM);
+												 FSM_FORKNUM);
 		else
-			rel->rd_smgr->smgr_fsm_nblocks = 0;
+			reln->smgr_fsm_nblocks = 0;
 	}
 
 	/* Handle requests beyond EOF */
-	if (blkno >= rel->rd_smgr->smgr_fsm_nblocks)
+	if (blkno >= reln->smgr_fsm_nblocks)
 	{
 		if (extend)
 			fsm_extend(rel, blkno + 1);
@@ -616,6 +620,7 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 {
 	BlockNumber fsm_nblocks_now;
 	PGAlignedBlock pg;
+	SMgrRelation reln;
 
 	PageInit((Page) pg.data, BLCKSZ, 0);
 
@@ -631,31 +636,35 @@ fsm_extend(Relation rel, BlockNumber fsm_nblocks)
 	 */
 	LockRelationForExtension(rel, ExclusiveLock);
 
-	/* Might have to re-open if a cache flush happened */
+	/*
-	RelationOpenSmgr(rel);
+	 * Caution: re-using this smgr pointer could fail if the relcache entry
+	 * gets closed.  It's safe as long as we only do smgr-level operations
+	 * between here and the last use of the pointer.
+	 */
+	reln = RelationGetSmgr(rel);
 
 	/*
 	 * Create the FSM file first if it doesn't exist.  If smgr_fsm_nblocks is
 	 * positive then it must exist, no need for an smgrexists call.
 	 */
-	if ((rel->rd_smgr->smgr_fsm_nblocks == 0 ||
+	if ((reln->smgr_fsm_nblocks == 0 ||
-		 rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber) &&
+		 reln->smgr_fsm_nblocks == InvalidBlockNumber) &&
-		!smgrexists(rel->rd_smgr, FSM_FORKNUM))
+		!smgrexists(reln, FSM_FORKNUM))
-		smgrcreate(rel->rd_smgr, FSM_FORKNUM, false);
+		smgrcreate(reln, FSM_FORKNUM, false);
 
-	fsm_nblocks_now = smgrnblocks(rel->rd_smgr, FSM_FORKNUM);
+	fsm_nblocks_now = smgrnblocks(reln, FSM_FORKNUM);
 
 	while (fsm_nblocks_now < fsm_nblocks)
 	{
 		PageSetChecksumInplace((Page) pg.data, fsm_nblocks_now);
 
-		smgrextend(rel->rd_smgr, FSM_FORKNUM, fsm_nblocks_now,
+		smgrextend(reln, FSM_FORKNUM, fsm_nblocks_now,
 				   pg.data, false);
 		fsm_nblocks_now++;
 	}
 
 	/* Update local cache with the up-to-date size */
-	rel->rd_smgr->smgr_fsm_nblocks = fsm_nblocks_now;
+	reln->smgr_fsm_nblocks = fsm_nblocks_now;
 
 	UnlockRelationForExtension(rel, ExclusiveLock);
 }

src/include/utils/rel.h

@@ -24,6 +24,7 @@
 #include "rewrite/prs2lock.h"
 #include "storage/block.h"
 #include "storage/relfilenode.h"
+#include "storage/smgr.h"
 #include "utils/relcache.h"
 #include "utils/reltrigger.h"
 
@@ -53,8 +54,7 @@ typedef LockInfoData *LockInfo;
 typedef struct RelationData
 {
 	RelFileNode rd_node;		/* relation physical identifier */
-	/* use "struct" here to avoid needing to include smgr.h: */
+	SMgrRelation rd_smgr;		/* cached file handle, or NULL */
-	struct SMgrRelationData *rd_smgr;	/* cached file handle, or NULL */
 	int			rd_refcnt;		/* reference count */
 	BackendId	rd_backend;		/* owning backend id, if temporary relation */
 	bool		rd_islocaltemp; /* rel is a temp rel of this session */
@@ -472,9 +472,33 @@ typedef struct ViewOptions
 	(RELKIND_HAS_STORAGE((relation)->rd_rel->relkind) && \
 	 ((relation)->rd_rel->relfilenode == InvalidOid))
 
+/*
+ * RelationGetSmgr
+ *		Returns smgr file handle for a relation, opening it if needed.
+ *
+ * Very little code is authorized to touch rel->rd_smgr directly.  Instead
+ * use this function to fetch its value.
+ *
+ * Note: since a relcache flush can cause the file handle to be closed again,
+ * it's unwise to hold onto the pointer returned by this function for any
+ * long period.  Recommended practice is to just re-execute RelationGetSmgr
+ * each time you need to access the SMgrRelation.  It's quite cheap in
+ * comparison to whatever an smgr function is going to do.
+ */
+static inline SMgrRelation
+RelationGetSmgr(Relation rel)
+{
+	if (unlikely(rel->rd_smgr == NULL))
+		smgrsetowner(&(rel->rd_smgr), smgropen(rel->rd_node, rel->rd_backend));
+	return rel->rd_smgr;
+}
+
 /*
  * RelationOpenSmgr
  *		Open the relation at the smgr level, if not already done.
+ *
+ * XXX this is now deprecated, and should not be used in new code.
+ * Instead, call RelationGetSmgr in place of fetching rd_smgr directly.
  */
 #define RelationOpenSmgr(relation) \
 	do { \
@@ -502,7 +526,8 @@ typedef struct ViewOptions
  *		Fetch relation's current insertion target block.
  *
  * Returns InvalidBlockNumber if there is no current target block.  Note
- * that the target block status is discarded on any smgr-level invalidation.
+ * that the target block status is discarded on any smgr-level invalidation,
+ * so there's no need to re-open the smgr handle if it's not currently open.
  */
 #define RelationGetTargetBlock(relation) \
 	( (relation)->rd_smgr != NULL ? (relation)->rd_smgr->smgr_targblock : InvalidBlockNumber )
@@ -513,8 +538,7 @@ typedef struct ViewOptions
  */
 #define RelationSetTargetBlock(relation, targblock) \
 	do { \
-		RelationOpenSmgr(relation); \
+		RelationGetSmgr(relation)->smgr_targblock = (targblock); \
-		(relation)->rd_smgr->smgr_targblock = (targblock); \
 	} while (0)
 
 /*