All users of WorkerNodeHash take an AccessShareLock

The metadata cache simulates a SELECT on pg_dist_node. Now the locks it
takes also simulate that SELECT.

src/backend/distributed/master/master_metadata_utility.c

@@ -174,8 +174,6 @@ DistributedTableSize(Oid relationId, char *sizeQuery)
 
 	ErrorIfNotSuitableToGetSize(relationId);
 
-	LockRelationOid(DistNodeRelationId(), AccessShareLock);
-
 	workerNodeList = ActivePrimaryNodeList();
 
 	foreach(workerNodeCell, workerNodeList)

src/backend/distributed/utils/metadata_cache.c

@@ -43,6 +43,7 @@
 #include "nodes/makefuncs.h"
 #include "parser/parse_func.h"
 #include "parser/parse_type.h"
+#include "storage/lmgr.h"
 #include "utils/builtins.h"
 #include "utils/catcache.h"
 #include "utils/datum.h"
@@ -2272,6 +2273,12 @@ GetWorkerNodeHash(void)
 {
 	InitializeCaches(); /* ensure relevant callbacks are registered */
 
+	/*
+	 * Simulate a SELECT from pg_dist_node, ensure pg_dist_node doesn't change while our
+	 * caller is using WorkerNodeHash.
+	 */
+	LockRelationOid(DistNodeRelationId(), AccessShareLock);
+
 	/*
 	 * We might have some concurrent metadata changes. In order to get the changes,
 	 * we first need to accept the cache invalidation messages.

src/backend/distributed/utils/reference_table_utils.c

@@ -230,7 +230,6 @@ ReplicateShardToAllWorkers(ShardInterval *shardInterval)
 	List *workerNodeList = NULL;
 	ListCell *workerNodeCell = NULL;
 
-	LockRelationOid(DistNodeRelationId(), AccessShareLock);
 	workerNodeList = ActivePrimaryNodeList();
 
 	/*