Avoid to re-establish replication if node is already myself primary i…

…n CLUSTER REPLICATE (valkey-io#884)

If n is already myself primary, there is no need to re-establish the
replication connection.

In the past we allow a replica node to reconnect with its primary via
this CLUSTER REPLICATE command, it will use psync. But since valkey-io#885, we
will assume that a full sync is needed in this case, so if we don't do
this, the replica will always use full sync.

Signed-off-by: Binbin <binloveplay1314@qq.com>
Co-authored-by: Ping Xie <pingxie@google.com>

src/cluster_legacy.c

@@ -6603,6 +6603,13 @@ int clusterCommandSpecial(client *c) {
             return 1;
         }
 
+        /* If `n` is already my primary, there is no need to re-establish the
+         * replication connection. */
+        if (myself->replicaof == n) {
+            addReply(c, shared.ok);
+            return 1;
+        }
+
         /* Set the primary.
          * If the instance is a primary, it is an empty primary.
          * If the instance is a replica, it had a totally different replication history.