MOD-16399 Skip the rebuild when CLUSTERSET carries an unchanged topology

src/cluster.c

@@ -1406,8 +1406,33 @@ static void SetClusterDataLongForm(RedisModuleString** argv, int argc){
     mr_dictEmpty(clusterCtx.nodesMsgIds, NULL);
 }
 
+static bool IsSameLongFormClusterSet(RedisModuleString** argv, int argc){
+    Cluster* current = clusterCtx.CurrCluster;
+    if (!current ||
+        !current->clusterSetCommand ||
+        current->clusterSetCommandSize != argc)
+        return false;
+
+    for (int i = 1; i < argc; ++i) {
+        /* MYID identifies the receiving shard and is not retained in clusterSetCommand. */
+        if (i == CLUSTERSET_MYID_LONG_FORM_INDEX)
+            continue;
+        size_t argLen;
+        const char* arg = RedisModule_StringPtrLen(argv[i], &argLen);
+        if (argLen != strlen(current->clusterSetCommand[i]) ||
+            memcmp(arg, current->clusterSetCommand[i], argLen) != 0)
+            return false;
+    }
+    return true;
+}
+
 static int MR_SetClusterData(RedisModuleString** argv, int argc){
     if (IsLongFormClusterSet(argc)) {
+        if (IsSameLongFormClusterSet(argv, argc)) {
+            RedisModule_Log(mr_staticCtx, "notice",
+                            "Skipping identical long-form cluster set");
+            return REDISMODULE_OK;
+        }
         SetClusterDataLongForm(argv, argc);
         return REDISMODULE_OK;
     } else if (IsShortFormClusterSet(argc)) {

tests/mr_test_module/pytests/test_network.py

@@ -211,7 +211,7 @@ def _handle_conn(self, sock, client_addr):
         conn = Connection(sock)
         self.new_conns.put(conn)
 
-    def _send_cluster_set(self):
+    def _send_cluster_set(self, mock_shard_id='2', first_arg='NO-USED'):
         # try to promote to internal connection
         promote_internal_client_if_supported(env=self.env)
         # IPv6 endpoints must be bracketed in host:port strings
@@ -220,7 +220,7 @@ def _send_cluster_set(self):
         # argv[6] => myId, argv[7] => "RANGES", argv[8] => numOfRanges, then repeating:
         # "SHARD" <id> "SLOTRANGE" <min> <max> "ADDR" <password@host:port> ["MASTER"]
         args = [
-            'NO-USED',  # [1]
+            first_arg,  # [1]
             'NO-USED',  # [2]
             'NO-USED',  # [3]
             'NO-USED',  # [4]
@@ -234,7 +234,7 @@ def _send_cluster_set(self):
             'ADDR', 'password@%s:6379' % endpoint_host,
             'MASTER',
             # Shard 2 (mock shard)
-            'SHARD', '2',
+            'SHARD', mock_shard_id,
             'SLOTRANGE', '8193', '16383',
             'ADDR', 'password@%s:%d' % (endpoint_host, self.port),
             'MASTER'
@@ -793,7 +793,30 @@ def testMassiveClusterSet(env, conn):
         with ShardMock(env, host) as shardMock:
             for i in range(1000):
                 conn = shardMock.GetConnection(sendHelloResponse=False)
-                shardMock._send_cluster_set()
+                # Keep exercising rebuilds now that identical updates are skipped.
+                shardMock._send_cluster_set(mock_shard_id=str(3 - (i % 2)))
+
+
+@MRTestDecorator(skipOnCluster=True)
+def testIdenticalLongFormClusterSetIsNoOp(env, conn):
+    for host in _get_hosts():
+        with ShardMock(env, host) as shardMock:
+            shardMock.GetConnection()
+            run_id = env.cmd('MRTESTS.INFOCLUSTER')[3]
+
+            shardMock._send_cluster_set()
+            env.assertEqual(env.cmd('MRTESTS.INFOCLUSTER')[3], run_id)
+
+            # RedisModuleString can carry embedded NUL bytes. Its explicit length
+            # must participate in the comparison so this safely rebuilds.
+            shardMock._send_cluster_set(first_arg=b'NO-USED\0changed')
+            env.assertNotEqual(env.cmd('MRTESTS.INFOCLUSTER')[3], run_id)
+            shardMock.GetConnection()
+
+            run_id = env.cmd('MRTESTS.INFOCLUSTER')[3]
+            shardMock._send_cluster_set(mock_shard_id='3')
+            env.assertNotEqual(env.cmd('MRTESTS.INFOCLUSTER')[3], run_id)
+            shardMock.GetConnection()
 
 @MRTestDecorator(skipOnCluster=True)
 def testMassiveClusterSetFromShard(env, conn):