Fix CLUSTER REPLICAS time complexity, should be O(N) (redis#12477)

We iterate over all replicas to get the result, the time complexity
should be O(N), like CLUSTER NODES complexity is O(N).

Author: enjoy-binbin

src/cluster.c

@@ -6294,6 +6294,7 @@ NULL
     } else if ((!strcasecmp(c->argv[1]->ptr,"slaves") ||
                 !strcasecmp(c->argv[1]->ptr,"replicas")) && c->argc == 3) {
         /* CLUSTER SLAVES <NODE ID> */
+        /* CLUSTER REPLICAS <NODE ID> */
         clusterNode *n = clusterLookupNode(c->argv[2]->ptr, sdslen(c->argv[2]->ptr));
         int j;
 

src/commands.def

@@ -964,14 +964,14 @@ struct COMMAND_STRUCT CLUSTER_Subcommands[] = {
 {MAKE_CMD("myid","Returns the ID of a node.","O(1)","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_MYID_History,0,CLUSTER_MYID_Tips,0,clusterCommand,2,CMD_STALE,0,CLUSTER_MYID_Keyspecs,0,NULL,0)},
 {MAKE_CMD("myshardid","Returns the shard ID of a node.","O(1)","7.2.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_MYSHARDID_History,0,CLUSTER_MYSHARDID_Tips,1,clusterCommand,2,CMD_STALE,0,CLUSTER_MYSHARDID_Keyspecs,0,NULL,0)},
 {MAKE_CMD("nodes","Returns the cluster configuration for a node.","O(N) where N is the total number of Cluster nodes","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_NODES_History,0,CLUSTER_NODES_Tips,1,clusterCommand,2,CMD_STALE,0,CLUSTER_NODES_Keyspecs,0,NULL,0)},
-{MAKE_CMD("replicas","Lists the replica nodes of a master node.","O(1)","5.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_REPLICAS_History,0,CLUSTER_REPLICAS_Tips,1,clusterCommand,3,CMD_ADMIN|CMD_STALE,0,CLUSTER_REPLICAS_Keyspecs,0,NULL,1),.args=CLUSTER_REPLICAS_Args},
+{MAKE_CMD("replicas","Lists the replica nodes of a master node.","O(N) where N is the number of replicas.","5.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_REPLICAS_History,0,CLUSTER_REPLICAS_Tips,1,clusterCommand,3,CMD_ADMIN|CMD_STALE,0,CLUSTER_REPLICAS_Keyspecs,0,NULL,1),.args=CLUSTER_REPLICAS_Args},
 {MAKE_CMD("replicate","Configure a node as replica of a master node.","O(1)","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_REPLICATE_History,0,CLUSTER_REPLICATE_Tips,0,clusterCommand,3,CMD_NO_ASYNC_LOADING|CMD_ADMIN|CMD_STALE,0,CLUSTER_REPLICATE_Keyspecs,0,NULL,1),.args=CLUSTER_REPLICATE_Args},
 {MAKE_CMD("reset","Resets a node.","O(N) where N is the number of known nodes. The command may execute a FLUSHALL as a side effect.","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_RESET_History,0,CLUSTER_RESET_Tips,0,clusterCommand,-2,CMD_ADMIN|CMD_STALE|CMD_NOSCRIPT,0,CLUSTER_RESET_Keyspecs,0,NULL,1),.args=CLUSTER_RESET_Args},
 {MAKE_CMD("saveconfig","Forces a node to save the cluster configuration to disk.","O(1)","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SAVECONFIG_History,0,CLUSTER_SAVECONFIG_Tips,0,clusterCommand,2,CMD_NO_ASYNC_LOADING|CMD_ADMIN|CMD_STALE,0,CLUSTER_SAVECONFIG_Keyspecs,0,NULL,0)},
 {MAKE_CMD("set-config-epoch","Sets the configuration epoch for a new node.","O(1)","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SET_CONFIG_EPOCH_History,0,CLUSTER_SET_CONFIG_EPOCH_Tips,0,clusterCommand,3,CMD_NO_ASYNC_LOADING|CMD_ADMIN|CMD_STALE,0,CLUSTER_SET_CONFIG_EPOCH_Keyspecs,0,NULL,1),.args=CLUSTER_SET_CONFIG_EPOCH_Args},
 {MAKE_CMD("setslot","Binds a hash slot to a node.","O(1)","3.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SETSLOT_History,0,CLUSTER_SETSLOT_Tips,0,clusterCommand,-4,CMD_NO_ASYNC_LOADING|CMD_ADMIN|CMD_STALE,0,CLUSTER_SETSLOT_Keyspecs,0,NULL,2),.args=CLUSTER_SETSLOT_Args},
 {MAKE_CMD("shards","Returns the mapping of cluster slots to shards.","O(N) where N is the total number of cluster nodes","7.0.0",CMD_DOC_NONE,NULL,NULL,"cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SHARDS_History,0,CLUSTER_SHARDS_Tips,1,clusterCommand,2,CMD_LOADING|CMD_STALE,0,CLUSTER_SHARDS_Keyspecs,0,NULL,0)},
-{MAKE_CMD("slaves","Lists the replica nodes of a master node.","O(1)","3.0.0",CMD_DOC_DEPRECATED,"`CLUSTER REPLICAS`","5.0.0","cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SLAVES_History,0,CLUSTER_SLAVES_Tips,1,clusterCommand,3,CMD_ADMIN|CMD_STALE,0,CLUSTER_SLAVES_Keyspecs,0,NULL,1),.args=CLUSTER_SLAVES_Args},
+{MAKE_CMD("slaves","Lists the replica nodes of a master node.","O(N) where N is the number of replicas.","3.0.0",CMD_DOC_DEPRECATED,"`CLUSTER REPLICAS`","5.0.0","cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SLAVES_History,0,CLUSTER_SLAVES_Tips,1,clusterCommand,3,CMD_ADMIN|CMD_STALE,0,CLUSTER_SLAVES_Keyspecs,0,NULL,1),.args=CLUSTER_SLAVES_Args},
 {MAKE_CMD("slots","Returns the mapping of cluster slots to nodes.","O(N) where N is the total number of Cluster nodes","3.0.0",CMD_DOC_DEPRECATED,"`CLUSTER SHARDS`","7.0.0","cluster",COMMAND_GROUP_CLUSTER,CLUSTER_SLOTS_History,2,CLUSTER_SLOTS_Tips,1,clusterCommand,2,CMD_LOADING|CMD_STALE,0,CLUSTER_SLOTS_Keyspecs,0,NULL,0)},
 {0}
 };

src/commands/cluster-replicas.json

@@ -1,7 +1,7 @@
 {
     "REPLICAS": {
         "summary": "Lists the replica nodes of a master node.",
-        "complexity": "O(1)",
+        "complexity": "O(N) where N is the number of replicas.",
         "group": "cluster",
         "since": "5.0.0",
         "arity": 3,

src/commands/cluster-slaves.json

@@ -1,7 +1,7 @@
 {
     "SLAVES": {
         "summary": "Lists the replica nodes of a master node.",
-        "complexity": "O(1)",
+        "complexity": "O(N) where N is the number of replicas.",
         "group": "cluster",
         "since": "3.0.0",
         "arity": 3,