Review logical replication tablesync code

Most importantly, remove optimization in LogicalRepSyncTableStart that
skips the normal walrcv_startstreaming/endstreaming dance.  The
optimization is not critically important for production uses anyway,
since it only fires in cases with no activity, and saves an
uninteresting amount of work even then.  Critically, it obscures bugs by
hiding the interesting code path from test cases.

Also: in GetSubscriptionRelState, remove pointless relation open; access
pg_subscription_rel->srsubstate with GETSTRUCT as is typical rather than
SysCacheGetAttr; remove unused 'missing_ok' argument.
In wait_for_relation_state_change, use explicit catalog snapshot
invalidation rather than obscurely (and expensively) through
GetLatestSnapshot.
In various places: sprinkle comments more liberally and rewrite a number
of them.  Other cosmetic code improvements.

No backpatch, since no bug is being fixed here.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Petr Jelínek <petr.jelinek@2ndquadrant.com>
Discussion: https://postgr.es/m/20201010190637.GA5774@alvherre.pgsql

src/backend/catalog/pg_subscription.c

@@ -328,20 +328,16 @@ UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
 /*
  * Get state of subscription table.
  *
- * Returns SUBREL_STATE_UNKNOWN when not found and missing_ok is true.
+ * Returns SUBREL_STATE_UNKNOWN when the table is not in the subscription.
  */
 char
-GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn,
-						bool missing_ok)
+GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn)
 {
-	Relation	rel;
 	HeapTuple	tup;
 	char		substate;
 	bool		isnull;
 	Datum		d;
 
-	rel = table_open(SubscriptionRelRelationId, AccessShareLock);
-
 	/* Try finding the mapping. */
 	tup = SearchSysCache2(SUBSCRIPTIONRELMAP,
 						  ObjectIdGetDatum(relid),
@@ -349,22 +345,14 @@ GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn,
 
 	if (!HeapTupleIsValid(tup))
 	{
-		if (missing_ok)
-		{
-			table_close(rel, AccessShareLock);
 		*sublsn = InvalidXLogRecPtr;
 		return SUBREL_STATE_UNKNOWN;
 	}
 
-		elog(ERROR, "subscription table %u in subscription %u does not exist",
-			 relid, subid);
-	}
-
 	/* Get the state. */
-	d = SysCacheGetAttr(SUBSCRIPTIONRELMAP, tup,
-						Anum_pg_subscription_rel_srsubstate, &isnull);
-	Assert(!isnull);
-	substate = DatumGetChar(d);
+	substate = ((Form_pg_subscription_rel) GETSTRUCT(tup))->srsubstate;
+
+	/* Get the LSN */
 	d = SysCacheGetAttr(SUBSCRIPTIONRELMAP, tup,
 						Anum_pg_subscription_rel_srsublsn, &isnull);
 	if (isnull)
@@ -374,7 +362,6 @@ GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn,
 
 	/* Cleanup */
 	ReleaseSysCache(tup);
-	table_close(rel, AccessShareLock);
 
 	return substate;
 }

src/backend/replication/logical/relation.c

@@ -437,8 +437,7 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 	if (entry->state != SUBREL_STATE_READY)
 		entry->state = GetSubscriptionRelState(MySubscription->oid,
 											   entry->localreloid,
-											   &entry->statelsn,
-											   true);
+											   &entry->statelsn);
 
 	return entry;
 }

src/backend/replication/logical/worker.c

@@ -2060,6 +2060,7 @@ LogicalRepApplyLoop(XLogRecPtr last_received)
 {
 	TimestampTz last_recv_timestamp = GetCurrentTimestamp();
 	bool		ping_sent = false;
+	TimeLineID	tli;
 
 	/*
 	 * Init the ApplyMessageContext which we clean up after each replication
@@ -2201,12 +2202,7 @@ LogicalRepApplyLoop(XLogRecPtr last_received)
 
 		/* Check if we need to exit the streaming loop. */
 		if (endofstream)
-		{
-			TimeLineID	tli;
-
-			walrcv_endstreaming(wrconn, &tli);
 			break;
-		}
 
 		/*
 		 * Wait for more data or latch.  If we have unflushed transactions,
@@ -2283,6 +2279,9 @@ LogicalRepApplyLoop(XLogRecPtr last_received)
 			send_feedback(last_received, requestReply, requestReply);
 		}
 	}
+
+	/* All done */
+	walrcv_endstreaming(wrconn, &tli);
 }
 
 /*
@@ -3024,10 +3023,8 @@ ApplyWorkerMain(Datum main_arg)
 		/* This is table synchronization worker, call initial sync. */
 		syncslotname = LogicalRepSyncTableStart(&origin_startpos);
 
-		/* The slot name needs to be allocated in permanent memory context. */
-		oldctx = MemoryContextSwitchTo(ApplyContext);
-		myslotname = pstrdup(syncslotname);
-		MemoryContextSwitchTo(oldctx);
+		/* allocate slot name in long-lived context */
+		myslotname = MemoryContextStrdup(ApplyContext, syncslotname);
 
 		pfree(syncslotname);
 	}

src/include/catalog/pg_subscription_rel.h

@@ -80,8 +80,7 @@ extern void AddSubscriptionRelState(Oid subid, Oid relid, char state,
 									XLogRecPtr sublsn);
 extern void UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
 									   XLogRecPtr sublsn);
-extern char GetSubscriptionRelState(Oid subid, Oid relid,
-									XLogRecPtr *sublsn, bool missing_ok);
+extern char GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn);
 extern void RemoveSubscriptionRel(Oid subid, Oid relid);
 
 extern List *GetSubscriptionRelations(Oid subid);