Revert "Skip WAL for new relfilenodes, under wal_level=minimal."

This reverts commit cb2fd7eac285b1b0a24eeb2b8ed4456b66c5a09f.  Per
numerous buildfarm members, it was incompatible with parallel query, and
a test case assumed LP64.  Back-patch to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20200321224920.GB1763544@rfd.leadboat.com

10 files changed, 129 insertions, 136 deletions

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index 765329bbcd4..dd975b164c5 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -1004,44 +1004,23 @@ gistproperty(Oid index_oid, int attno,
 }
 
 /*
- * Some indexes are not WAL-logged, but we need LSNs to detect concurrent page
- * splits anyway. This function provides a fake sequence of LSNs for that
- * purpose.
+ * Temporary and unlogged GiST indexes are not WAL-logged, but we need LSNs
+ * to detect concurrent page splits anyway. This function provides a fake
+ * sequence of LSNs for that purpose.
  */
 XLogRecPtr
 gistGetFakeLSN(Relation rel)
 {
+	static XLogRecPtr counter = FirstNormalUnloggedLSN;
+
 	if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
 	{
 		/*
 		 * Temporary relations are only accessible in our session, so a simple
 		 * backend-local counter will do.
 		 */
-		static XLogRecPtr counter = FirstNormalUnloggedLSN;
-
 		return counter++;
 	}
-	else if (rel->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT)
-	{
-		/*
-		 * WAL-logging on this relation will start after commit, so its LSNs
-		 * must be distinct numbers smaller than the LSN at the next commit.
-		 * Emit a dummy WAL record if insert-LSN hasn't advanced after the
-		 * last call.
-		 */
-		static XLogRecPtr lastlsn = InvalidXLogRecPtr;
-		XLogRecPtr	currlsn = GetXLogInsertRecPtr();
-
-		/* Shouldn't be called for WAL-logging relations */
-		Assert(!RelationNeedsWAL(rel));
-
-		/* No need for an actual record if we already have a distinct LSN */
-		if (!XLogRecPtrIsInvalid(lastlsn) && lastlsn == currlsn)
-			currlsn = gistXLogAssignLSN();
-
-		lastlsn = currlsn;
-		return currlsn;
-	}
 	else
 	{
 		/*
diff --git a/src/backend/access/gist/gistxlog.c b/src/backend/access/gist/gistxlog.c
index b60dba052fa..d3f3a7b803f 100644
--- a/src/backend/access/gist/gistxlog.c
+++ b/src/backend/access/gist/gistxlog.c
@@ -449,9 +449,6 @@ gist_redo(XLogReaderState *record)
 		case XLOG_GIST_PAGE_DELETE:
 			gistRedoPageDelete(record);
 			break;
-		case XLOG_GIST_ASSIGN_LSN:
-			/* nop. See gistGetFakeLSN(). */
-			break;
 		default:
 			elog(PANIC, "gist_redo: unknown op code %u", info);
 	}
@@ -596,24 +593,6 @@ gistXLogPageDelete(Buffer buffer, FullTransactionId xid,
 }
 
 /*
- * Write an empty XLOG record to assign a distinct LSN.
- */
-XLogRecPtr
-gistXLogAssignLSN(void)
-{
-	int			dummy = 0;
-
-	/*
-	 * Records other than SWITCH_WAL must have content. We use an integer 0 to
-	 * follow the restriction.
-	 */
-	XLogBeginInsert();
-	XLogSetRecordFlags(XLOG_MARK_UNIMPORTANT);
-	XLogRegisterData((char *) &dummy, sizeof(dummy));
-	return XLogInsert(RM_GIST_ID, XLOG_GIST_ASSIGN_LSN);
-}
-
-/*
  * Write XLOG record about reuse of a deleted page.
  */
 void
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index a25d539ec47..29694b8aa4a 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -21,6 +21,7 @@
  *		heap_multi_insert - insert multiple tuples into a relation
  *		heap_delete		- delete a tuple from a relation
  *		heap_update		- replace a tuple in a relation with another tuple
+ *		heap_sync		- sync heap, for when no WAL has been written
  *
  * NOTES
  *	  This file contains the heap_ routines which implement
@@ -1938,7 +1939,7 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 	MarkBufferDirty(buffer);
 
 	/* XLOG stuff */
-	if (RelationNeedsWAL(relation))
+	if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
 	{
 		xl_heap_insert xlrec;
 		xl_heap_header xlhdr;
@@ -2121,7 +2122,7 @@ heap_multi_insert(Relation relation, TupleTableSlot **slots, int ntuples,
 	/* currently not needed (thus unsupported) for heap_multi_insert() */
 	AssertArg(!(options & HEAP_INSERT_NO_LOGICAL));
 
-	needwal = RelationNeedsWAL(relation);
+	needwal = !(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation);
 	saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
 												   HEAP_DEFAULT_FILLFACTOR);
 
@@ -8920,6 +8921,46 @@ heap2_redo(XLogReaderState *record)
 }
 
 /*
+ *	heap_sync		- sync a heap, for use when no WAL has been written
+ *
+ * This forces the heap contents (including TOAST heap if any) down to disk.
+ * If we skipped using WAL, and WAL is otherwise needed, we must force the
+ * relation down to disk before it's safe to commit the transaction.  This
+ * requires writing out any dirty buffers and then doing a forced fsync.
+ *
+ * Indexes are not touched.  (Currently, index operations associated with
+ * the commands that use this are WAL-logged and so do not need fsync.
+ * That behavior might change someday, but in any case it's likely that
+ * any fsync decisions required would be per-index and hence not appropriate
+ * to be done here.)
+ */
+void
+heap_sync(Relation rel)
+{
+	/* non-WAL-logged tables never need fsync */
+	if (!RelationNeedsWAL(rel))
+		return;
+
+	/* main heap */
+	FlushRelationBuffers(rel);
+	/* FlushRelationBuffers will have opened rd_smgr */
+	smgrimmedsync(rel->rd_smgr, MAIN_FORKNUM);
+
+	/* FSM is not critical, don't bother syncing it */
+
+	/* toast heap, if any */
+	if (OidIsValid(rel->rd_rel->reltoastrelid))
+	{
+		Relation	toastrel;
+
+		toastrel = table_open(rel->rd_rel->reltoastrelid, AccessShareLock);
+		FlushRelationBuffers(toastrel);
+		smgrimmedsync(toastrel->rd_smgr, MAIN_FORKNUM);
+		table_close(toastrel, AccessShareLock);
+	}
+}
+
+/*
  * Mask a heap page before performing consistency checks on it.
  */
 void
diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 56b35622f1a..ca52846b973 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -555,6 +555,17 @@ tuple_lock_retry:
 	return result;
 }
 
+static void
+heapam_finish_bulk_insert(Relation relation, int options)
+{
+	/*
+	 * If we skipped writing WAL, then we need to sync the heap (but not
+	 * indexes since those use WAL anyway / don't go through tableam)
+	 */
+	if (options & HEAP_INSERT_SKIP_WAL)
+		heap_sync(relation);
+}
+
 
 /* ------------------------------------------------------------------------
  * DDL related callbacks for heap AM.
@@ -687,6 +698,7 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 	IndexScanDesc indexScan;
 	TableScanDesc tableScan;
 	HeapScanDesc heapScan;
+	bool		use_wal;
 	bool		is_system_catalog;
 	Tuplesortstate *tuplesort;
 	TupleDesc	oldTupDesc = RelationGetDescr(OldHeap);
@@ -701,9 +713,12 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 	is_system_catalog = IsSystemRelation(OldHeap);
 
 	/*
-	 * Valid smgr_targblock implies something already wrote to the relation.
-	 * This may be harmless, but this function hasn't planned for it.
+	 * We need to log the copied data in WAL iff WAL archiving/streaming is
+	 * enabled AND it's a WAL-logged rel.
 	 */
+	use_wal = XLogIsNeeded() && RelationNeedsWAL(NewHeap);
+
+	/* use_wal off requires smgr_targblock be initially invalid */
 	Assert(RelationGetTargetBlock(NewHeap) == InvalidBlockNumber);
 
 	/* Preallocate values/isnull arrays */
@@ -713,7 +728,7 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 
 	/* Initialize the rewrite operation */
 	rwstate = begin_heap_rewrite(OldHeap, NewHeap, OldestXmin, *xid_cutoff,
-								 *multi_cutoff);
+								 *multi_cutoff, use_wal);
 
 
 	/* Set up sorting if wanted */
@@ -2510,6 +2525,7 @@ static const TableAmRoutine heapam_methods = {
 	.tuple_delete = heapam_tuple_delete,
 	.tuple_update = heapam_tuple_update,
 	.tuple_lock = heapam_tuple_lock,
+	.finish_bulk_insert = heapam_finish_bulk_insert,
 
 	.tuple_fetch_row_version = heapam_fetch_row_version,
 	.tuple_get_latest_tid = heap_get_latest_tid,
diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index 39e33763df4..9c29bc0e0f0 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -136,6 +136,7 @@ typedef struct RewriteStateData
 	Page		rs_buffer;		/* page currently being built */
 	BlockNumber rs_blockno;		/* block where page will go */
 	bool		rs_buffer_valid;	/* T if any tuples in buffer */
+	bool		rs_use_wal;		/* must we WAL-log inserts? */
 	bool		rs_logical_rewrite; /* do we need to do logical rewriting */
 	TransactionId rs_oldest_xmin;	/* oldest xmin used by caller to determine
 									 * tuple visibility */
@@ -229,13 +230,15 @@ static void logical_end_heap_rewrite(RewriteState state);
  * oldest_xmin	xid used by the caller to determine which tuples are dead
  * freeze_xid	xid before which tuples will be frozen
  * cutoff_multi	multixact before which multis will be removed
+ * use_wal		should the inserts to the new heap be WAL-logged?
  *
  * Returns an opaque RewriteState, allocated in current memory context,
  * to be used in subsequent calls to the other functions.
  */
 RewriteState
 begin_heap_rewrite(Relation old_heap, Relation new_heap, TransactionId oldest_xmin,
-				   TransactionId freeze_xid, MultiXactId cutoff_multi)
+				   TransactionId freeze_xid, MultiXactId cutoff_multi,
+				   bool use_wal)
 {
 	RewriteState state;
 	MemoryContext rw_cxt;
@@ -260,6 +263,7 @@ begin_heap_rewrite(Relation old_heap, Relation new_heap, TransactionId oldest_xm
 	/* new_heap needn't be empty, just locked */
 	state->rs_blockno = RelationGetNumberOfBlocks(new_heap);
 	state->rs_buffer_valid = false;
+	state->rs_use_wal = use_wal;
 	state->rs_oldest_xmin = oldest_xmin;
 	state->rs_freeze_xid = freeze_xid;
 	state->rs_cutoff_multi = cutoff_multi;
@@ -318,7 +322,7 @@ end_heap_rewrite(RewriteState state)
 	/* Write the last page, if any */
 	if (state->rs_buffer_valid)
 	{
-		if (RelationNeedsWAL(state->rs_new_rel))
+		if (state->rs_use_wal)
 			log_newpage(&state->rs_new_rel->rd_node,
 						MAIN_FORKNUM,
 						state->rs_blockno,
@@ -333,14 +337,18 @@ end_heap_rewrite(RewriteState state)
 	}
 
 	/*
-	 * When we WAL-logged rel pages, we must nonetheless fsync them.  The
+	 * If the rel is WAL-logged, must fsync before commit.  We use heap_sync
+	 * to ensure that the toast table gets fsync'd too.
+	 *
+	 * It's obvious that we must do this when not WAL-logging. It's less
+	 * obvious that we have to do it even if we did WAL-log the pages. The
 	 * reason is the same as in storage.c's RelationCopyStorage(): we're
 	 * writing data that's not in shared buffers, and so a CHECKPOINT
 	 * occurring during the rewriteheap operation won't have fsync'd data we
 	 * wrote before the checkpoint.
 	 */
 	if (RelationNeedsWAL(state->rs_new_rel))
-		smgrimmedsync(state->rs_new_rel->rd_smgr, MAIN_FORKNUM);
+		heap_sync(state->rs_new_rel);
 
 	logical_end_heap_rewrite(state);
 
@@ -638,6 +646,9 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 	{
 		int			options = HEAP_INSERT_SKIP_FSM;
 
+		if (!state->rs_use_wal)
+			options |= HEAP_INSERT_SKIP_WAL;
+
 		/*
 		 * While rewriting the heap for VACUUM FULL / CLUSTER, make sure data
 		 * for the TOAST table are not logically decoded.  The main heap is
@@ -676,7 +687,7 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
 			/* Doesn't fit, so write out the existing page */
 
 			/* XLOG stuff */
-			if (RelationNeedsWAL(state->rs_new_rel))
+			if (state->rs_use_wal)
 				log_newpage(&state->rs_new_rel->rd_node,
 							MAIN_FORKNUM,
 							state->rs_blockno,
diff --git a/src/backend/access/nbtree/nbtsort.c b/src/backend/access/nbtree/nbtsort.c
index 9111e2789c4..e66cd36dfae 100644
--- a/src/backend/access/nbtree/nbtsort.c
+++ b/src/backend/access/nbtree/nbtsort.c
@@ -31,6 +31,18 @@
  * them.  They will need to be re-read into shared buffers on first use after
  * the build finishes.
  *
+ * Since the index will never be used unless it is completely built,
+ * from a crash-recovery point of view there is no need to WAL-log the
+ * steps of the build.  After completing the index build, we can just sync
+ * the whole file to disk using smgrimmedsync() before exiting this module.
+ * This can be seen to be sufficient for crash recovery by considering that
+ * it's effectively equivalent to what would happen if a CHECKPOINT occurred
+ * just after the index build.  However, it is clearly not sufficient if the
+ * DBA is using the WAL log for PITR or replication purposes, since another
+ * machine would not be able to reconstruct the index from WAL.  Therefore,
+ * we log the completed index pages to WAL if and only if WAL archiving is
+ * active.
+ *
  * This code isn't concerned about the FSM at all. The caller is responsible
  * for initializing that.
  *
@@ -557,7 +569,12 @@ _bt_leafbuild(BTSpool *btspool, BTSpool *btspool2)
 	wstate.inskey = _bt_mkscankey(wstate.index, NULL);
 	/* _bt_mkscankey() won't set allequalimage without metapage */
 	wstate.inskey->allequalimage = _bt_allequalimage(wstate.index, true);
-	wstate.btws_use_wal = RelationNeedsWAL(wstate.index);
+
+	/*
+	 * We need to log index creation in WAL iff WAL archiving/streaming is
+	 * enabled UNLESS the index isn't WAL-logged anyway.
+	 */
+	wstate.btws_use_wal = XLogIsNeeded() && RelationNeedsWAL(wstate.index);
 
 	/* reserve the metapage */
 	wstate.btws_pages_alloced = BTREE_METAPAGE + 1;
@@ -1407,15 +1424,21 @@ _bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
 	_bt_uppershutdown(wstate, state);
 
 	/*
-	 * When we WAL-logged index pages, we must nonetheless fsync index files.
-	 * Since we're building outside shared buffers, a CHECKPOINT occurring
-	 * during the build has no way to flush the previously written data to
-	 * disk (indeed it won't know the index even exists).  A crash later on
-	 * would replay WAL from the checkpoint, therefore it wouldn't replay our
-	 * earlier WAL entries. If we do not fsync those pages here, they might
-	 * still not be on disk when the crash occurs.
+	 * If the index is WAL-logged, we must fsync it down to disk before it's
+	 * safe to commit the transaction.  (For a non-WAL-logged index we don't
+	 * care since the index will be uninteresting after a crash anyway.)
+	 *
+	 * It's obvious that we must do this when not WAL-logging the build. It's
+	 * less obvious that we have to do it even if we did WAL-log the index
+	 * pages.  The reason is that since we're building outside shared buffers,
+	 * a CHECKPOINT occurring during the build has no way to flush the
+	 * previously written data to disk (indeed it won't know the index even
+	 * exists).  A crash later on would replay WAL from the checkpoint,
+	 * therefore it wouldn't replay our earlier WAL entries. If we do not
+	 * fsync those pages here, they might still not be on disk when the crash
+	 * occurs.
 	 */
-	if (wstate->btws_use_wal)
+	if (RelationNeedsWAL(wstate->index))
 	{
 		RelationOpenSmgr(wstate->index);
 		smgrimmedsync(wstate->index->rd_smgr, MAIN_FORKNUM);
diff --git a/src/backend/access/rmgrdesc/gistdesc.c b/src/backend/access/rmgrdesc/gistdesc.c
index de309fb1227..3377367e125 100644
--- a/src/backend/access/rmgrdesc/gistdesc.c
+++ b/src/backend/access/rmgrdesc/gistdesc.c
@@ -80,9 +80,6 @@ gist_desc(StringInfo buf, XLogReaderState *record)
 		case XLOG_GIST_PAGE_DELETE:
 			out_gistxlogPageDelete(buf, (gistxlogPageDelete *) rec);
 			break;
-		case XLOG_GIST_ASSIGN_LSN:
-			/* No details to write out */
-			break;
 	}
 }
 
@@ -108,9 +105,6 @@ gist_identify(uint8 info)
 		case XLOG_GIST_PAGE_DELETE:
 			id = "PAGE_DELETE";
 			break;
-		case XLOG_GIST_ASSIGN_LSN:
-			id = "ASSIGN_LSN";
-			break;
 	}
 
 	return id;
diff --git a/src/backend/access/transam/README b/src/backend/access/transam/README
index eb9aac5fd39..b5a2cb2de83 100644
--- a/src/backend/access/transam/README
+++ b/src/backend/access/transam/README
@@ -717,38 +717,6 @@ then restart recovery.  This is part of the reason for not writing a WAL
 entry until we've successfully done the original action.
 
 
-Skipping WAL for New RelFileNode
---------------------------------
-
-Under wal_level=minimal, if a change modifies a relfilenode that ROLLBACK
-would unlink, in-tree access methods write no WAL for that change.  Code that
-writes WAL without calling RelationNeedsWAL() must check for this case.  This
-skipping is mandatory.  If a WAL-writing change preceded a WAL-skipping change
-for the same block, REDO could overwrite the WAL-skipping change.  If a
-WAL-writing change followed a WAL-skipping change for the same block, a
-related problem would arise.  When a WAL record contains no full-page image,
-REDO expects the page to match its contents from just before record insertion.
-A WAL-skipping change may not reach disk at all, violating REDO's expectation
-under full_page_writes=off.  For any access method, CommitTransaction() writes
-and fsyncs affected blocks before recording the commit.
-
-Prefer to do the same in future access methods.  However, two other approaches
-can work.  First, an access method can irreversibly transition a given fork
-from WAL-skipping to WAL-writing by calling FlushRelationBuffers() and
-smgrimmedsync().  Second, an access method can opt to write WAL
-unconditionally for permanent relations.  Under these approaches, the access
-method callbacks must not call functions that react to RelationNeedsWAL().
-
-This applies only to WAL records whose replay would modify bytes stored in the
-new relfilenode.  It does not apply to other records about the relfilenode,
-such as XLOG_SMGR_CREATE.  Because it operates at the level of individual
-relfilenodes, RelationNeedsWAL() can differ for tightly-coupled relations.
-Consider "CREATE TABLE t (); BEGIN; ALTER TABLE t ADD c text; ..." in which
-ALTER TABLE adds a TOAST relation.  The TOAST relation will skip WAL, while
-the table owning it will not.  ALTER TABLE SET TABLESPACE will cause a table
-to skip WAL, but that won't affect its indexes.
-
-
 Asynchronous Commit
 -------------------
 
@@ -852,12 +820,13 @@ Changes to a temp table are not WAL-logged, hence could reach disk in
 advance of T1's commit, but we don't care since temp table contents don't
 survive crashes anyway.
 
-Database writes that skip WAL for new relfilenodes are also safe.  In these
-cases it's entirely possible for the data to reach disk before T1's commit,
-because T1 will fsync it down to disk without any sort of interlock.  However,
-all these paths are designed to write data that no other transaction can see
-until after T1 commits.  The situation is thus not different from ordinary
-WAL-logged updates.
+Database writes made via any of the paths we have introduced to avoid WAL
+overhead for bulk updates are also safe.  In these cases it's entirely
+possible for the data to reach disk before T1's commit, because T1 will
+fsync it down to disk without any sort of interlock, as soon as it finishes
+the bulk update.  However, all these paths are designed to write data that
+no other transaction can see until after T1 commits.  The situation is thus
+not different from ordinary WAL-logged updates.
 
 Transaction Emulation during Recovery
 -------------------------------------
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index b6885b01bca..e3c60f23cd2 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2109,13 +2109,6 @@ CommitTransaction(void)
 	 */
 	PreCommit_on_commit_actions();
 
-	/*
-	 * Synchronize files that are created and not WAL-logged during this
-	 * transaction. This must happen before AtEOXact_RelationMap(), so that we
-	 * don't see committed-but-broken files after a crash.
-	 */
-	smgrDoPendingSyncs(true);
-
 	/* close large objects before lower-level cleanup */
 	AtEOXact_LargeObject(true);
 
@@ -2349,13 +2342,6 @@ PrepareTransaction(void)
 	 */
 	PreCommit_on_commit_actions();
 
-	/*
-	 * Synchronize files that are created and not WAL-logged during this
-	 * transaction. This must happen before EndPrepare(), so that we don't see
-	 * committed-but-broken files after a crash and COMMIT PREPARED.
-	 */
-	smgrDoPendingSyncs(true);
-
 	/* close large objects before lower-level cleanup */
 	AtEOXact_LargeObject(true);
 
@@ -2674,7 +2660,6 @@ AbortTransaction(void)
 	 */
 	AfterTriggerEndXact(false); /* 'false' means it's abort */
 	AtAbort_Portals();
-	smgrDoPendingSyncs(false);
 	AtEOXact_LargeObject(false);
 	AtAbort_Notify();
 	AtEOXact_RelationMap(false, is_parallel_worker);
diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 6cb143e161d..b217ffa52ff 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -549,8 +549,6 @@ typedef FakeRelCacheEntryData *FakeRelCacheEntry;
  * fields related to physical storage, like rd_rel, are initialized, so the
  * fake entry is only usable in low-level operations like ReadBuffer().
  *
- * This is also used for syncing WAL-skipped files.
- *
  * Caller must free the returned entry with FreeFakeRelcacheEntry().
  */
 Relation
@@ -559,20 +557,18 @@ CreateFakeRelcacheEntry(RelFileNode rnode)
 	FakeRelCacheEntry fakeentry;
 	Relation	rel;
 
+	Assert(InRecovery);
+
 	/* Allocate the Relation struct and all related space in one block. */
 	fakeentry = palloc0(sizeof(FakeRelCacheEntryData));
 	rel = (Relation) fakeentry;
 
 	rel->rd_rel = &fakeentry->pgc;
 	rel->rd_node = rnode;
-
-	/*
-	 * We will never be working with temp rels during recovery or while
-	 * syncing WAL-skipped files.
-	 */
+	/* We will never be working with temp rels during recovery */
 	rel->rd_backend = InvalidBackendId;
 
-	/* It must be a permanent table here */
+	/* It must be a permanent table if we're in recovery. */
 	rel->rd_rel->relpersistence = RELPERSISTENCE_PERMANENT;
 
 	/* We don't know the name of the relation; use relfilenode instead */
@@ -581,9 +577,9 @@ CreateFakeRelcacheEntry(RelFileNode rnode)
 	/*
 	 * We set up the lockRelId in case anything tries to lock the dummy
 	 * relation.  Note that this is fairly bogus since relNode may be
-	 * different from the relation's OID.  It shouldn't really matter though.
-	 * In recovery, we are running by ourselves and can't have any lock
-	 * conflicts.  While syncing, we already hold AccessExclusiveLock.
+	 * different from the relation's OID.  It shouldn't really matter though,
+	 * since we are presumably running by ourselves and can't have any lock
+	 * conflicts ...
 	 */
 	rel->rd_lockInfo.lockRelId.dbId = rnode.dbNode;
 	rel->rd_lockInfo.lockRelId.relId = rnode.relNode;