Make pg_replication_origin_drop safe against concurrent drops.

Currently, we get the origin id from the name and then drop the origin by
taking ExclusiveLock on ReplicationOriginRelationId. So, two concurrent
sessions can get the id from the name at the same time and then when they
try to drop the origin, one of the sessions will get the either
"tuple concurrently deleted" or "cache lookup failed for replication
origin ..".

To prevent this race condition we do the entire operation under lock. This
obviates the need for replorigin_drop() API and we have removed it so if
any extension authors are using it they need to instead use
replorigin_drop_by_name. See it's usage in pg_replication_origin_drop().

Author: Peter Smith
Reviewed-by: Amit Kapila, Euler Taveira, Petr Jelinek, and Alvaro
Herrera
Discussion: https://www.postgresql.org/message-id/CAHut%2BPuW8DWV5fskkMWWMqzt-x7RPcNQOtJQBp6SdwyRghCk7A%40mail.gmail.com

src/backend/commands/subscriptioncmds.c

@@ -926,7 +926,6 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	ListCell   *lc;
 	char		originname[NAMEDATALEN];
 	char	   *err = NULL;
-	RepOriginId originid;
 	WalReceiverConn *wrconn = NULL;
 	StringInfoData cmd;
 	Form_pg_subscription form;
@@ -1050,9 +1049,7 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 
 	/* Remove the origin tracking if exists. */
 	snprintf(originname, sizeof(originname), "pg_%u", subid);
-	originid = replorigin_by_name(originname, true);
-	if (originid != InvalidRepOriginId)
-		replorigin_drop(originid, false);
+	replorigin_drop_by_name(originname, true, false);
 
 	/*
 	 * If there is no slot associated with the subscription, we can finish

src/backend/replication/logical/origin.c

@@ -322,27 +322,15 @@ replorigin_create(char *roname)
 	return roident;
 }
 
-
 /*
- * Drop replication origin.
- *
- * Needs to be called in a transaction.
+ * Helper function to drop a replication origin.
  */
-void
-replorigin_drop(RepOriginId roident, bool nowait)
+static void
+replorigin_drop_guts(Relation rel, RepOriginId roident, bool nowait)
 {
 	HeapTuple	tuple;
-	Relation	rel;
 	int			i;
 
-	Assert(IsTransactionState());
-
-	/*
-	 * To interlock against concurrent drops, we hold ExclusiveLock on
-	 * pg_replication_origin throughout this function.
-	 */
-	rel = table_open(ReplicationOriginRelationId, ExclusiveLock);
-
 	/*
 	 * First, clean up the slot state info, if there is any matching slot.
 	 */
@@ -415,11 +403,40 @@ restart:
 	ReleaseSysCache(tuple);
 
 	CommandCounterIncrement();
-
-	/* now release lock again */
-	table_close(rel, ExclusiveLock);
 }
 
+/*
+ * Drop replication origin (by name).
+ *
+ * Needs to be called in a transaction.
+ */
+void
+replorigin_drop_by_name(char *name, bool missing_ok, bool nowait)
+{
+	RepOriginId roident;
+	Relation	rel;
+
+	Assert(IsTransactionState());
+
+	/*
+	 * To interlock against concurrent drops, we hold ExclusiveLock on
+	 * pg_replication_origin till xact commit.
+	 *
+	 * XXX We can optimize this by acquiring the lock on a specific origin by
+	 * using LockSharedObject if required. However, for that, we first to
+	 * acquire a lock on ReplicationOriginRelationId, get the origin_id, lock
+	 * the specific origin and then re-check if the origin still exists.
+	 */
+	rel = table_open(ReplicationOriginRelationId, ExclusiveLock);
+
+	roident = replorigin_by_name(name, missing_ok);
+
+	if (OidIsValid(roident))
+		replorigin_drop_guts(rel, roident, nowait);
+
+	/* We keep the lock on pg_replication_origin until commit */
+	table_close(rel, NoLock);
+}
 
 /*
  * Lookup replication origin via its oid and return the name.
@@ -1256,16 +1273,12 @@ Datum
 pg_replication_origin_drop(PG_FUNCTION_ARGS)
 {
 	char	   *name;
-	RepOriginId roident;
 
 	replorigin_check_prerequisites(false, false);
 
 	name = text_to_cstring((text *) DatumGetPointer(PG_GETARG_DATUM(0)));
 
-	roident = replorigin_by_name(name, false);
-	Assert(OidIsValid(roident));
-
-	replorigin_drop(roident, true);
+	replorigin_drop_by_name(name, false, true);
 
 	pfree(name);
 

src/include/replication/origin.h

@@ -40,7 +40,7 @@ extern PGDLLIMPORT TimestampTz replorigin_session_origin_timestamp;
 /* API for querying & manipulating replication origins */
 extern RepOriginId replorigin_by_name(char *name, bool missing_ok);
 extern RepOriginId replorigin_create(char *name);
-extern void replorigin_drop(RepOriginId roident, bool nowait);
+extern void replorigin_drop_by_name(char *name, bool missing_ok, bool nowait);
 extern bool replorigin_by_oid(RepOriginId roident, bool missing_ok,
 							  char **roname);