pd-ctl: print warn information when set max-replica to less than current

tools/pd-ctl/pdctl/command/config_command.go

@@ -373,6 +373,9 @@
 	if err != nil {
 		val = value
 	}
+	if key == "max-replicas" {
+		checkMaxReplicas(cmd, val)
+	}
 	data[key] = val
 	reqData, err := json.Marshal(data)
 	if err != nil {
@@ -386,6 +389,30 @@
 	return nil
 }
 
+func checkMaxReplicas(cmd *cobra.Command, value any) {
+	newReplica, ok := value.(float64)
+	if !ok {
+		// If the type is not float64, it will be handled elsewhere
+		return
+	}
+	header := buildHeader(cmd)
+	r, err := doRequest(cmd, replicatePrefix, http.MethodGet, header)
+	if err != nil {
+		cmd.Printf("Failed to get config when checking config: %s\n", err)
+		return
+	}
+	oldConfig := make(map[string]any)
+	err = json.Unmarshal([]byte(r), &oldConfig)
+	if err != nil {
+		cmd.Printf("Failed to unmarshal config when checking config: %s\n", err)
+		return
+	}
+	oldReplica, ok := oldConfig["max-replicas"].(float64)
+	if ok && newReplica < oldReplica {
+		cmd.Printf("Setting max-replica to %v which is less than the current replicas (%v). This may pose a risk. Please confirm the setting.\n", newReplica, oldReplica)
+	}
+}
+
 func setConfigCommandFunc(cmd *cobra.Command, args []string) {
 	if len(args) != 2 {
 		cmd.Println(cmd.UsageString())

tools/pd-ctl/tests/config/config_test.go

@@ -1136,6 +1136,7 @@ func (suite *configTestSuite) checkUpdateDefaultReplicaConfig(cluster *pdTests.T
 	output, err := tests.ExecuteCommand(cmd, "-u", pdAddr, "config", "set", "max-replicas", "2")
 	re.NoError(err)
 	re.Contains(string(output), "Success!")
+	re.Contains(string(output), "which is less than the current replicas")
 	checkMaxReplicas(2)
 	output, err = tests.ExecuteCommand(cmd, "-u", pdAddr, "config", "set", "location-labels", "zone,host")
 	re.NoError(err)
@@ -1156,6 +1157,7 @@ func (suite *configTestSuite) checkUpdateDefaultReplicaConfig(cluster *pdTests.T
 	output, err = tests.ExecuteCommand(cmd, "-u", pdAddr, "config", "set", "max-replicas", "3")
 	re.NoError(err)
 	re.Contains(string(output), "Success!")
+	re.NotContains(string(output), "which is less than the current replicas")
 	checkMaxReplicas(3)
 	checkRuleCount(3)