rpc: fix unmarshaling of null result in CallContext (#26723)

The change fixes unmarshaling of JSON null results into json.RawMessage.

---------
Co-authored-by: Jason Yuan <jason.yuan@curvegrid.com>
Co-authored-by: Jason Yuan <jason.yuan869@gmail.com>

rpc/client.go

@@ -345,7 +345,10 @@ func (c *Client) CallContext(ctx context.Context, result interface{}, method str
 	case len(resp.Result) == 0:
 		return ErrNoResult
 	default:
-		return json.Unmarshal(resp.Result, &result)
+		if result == nil {
+			return nil
+		}
+		return json.Unmarshal(resp.Result, result)
 	}
 }
 

rpc/client_test.go

@@ -69,6 +69,26 @@ func TestClientResponseType(t *testing.T) {
 	}
 }
 
+// This test checks calling a method that returns 'null'.
+func TestClientNullResponse(t *testing.T) {
+	server := newTestServer()
+	defer server.Stop()
+
+	client := DialInProc(server)
+	defer client.Close()
+
+	var result json.RawMessage
+	if err := client.Call(&result, "test_null"); err != nil {
+		t.Fatal(err)
+	}
+	if result == nil {
+		t.Fatal("Expected non-nil result")
+	}
+	if !reflect.DeepEqual(result, json.RawMessage("null")) {
+		t.Errorf("Expected null, got %s", result)
+	}
+}
+
 // This test checks that server-returned errors with code and data come out of Client.Call.
 func TestClientErrorData(t *testing.T) {
 	server := newTestServer()

rpc/server_test.go

@@ -45,7 +45,7 @@ func TestServerRegisterName(t *testing.T) {
 		t.Fatalf("Expected service calc to be registered")
 	}
 
-	wantCallbacks := 12
+	wantCallbacks := 13
 	if len(svc.callbacks) != wantCallbacks {
 		t.Errorf("Expected %d callbacks for service 'service', got %d", wantCallbacks, len(svc.callbacks))
 	}

rpc/testservice_test.go

@@ -78,6 +78,10 @@ func (o *MarshalErrObj) MarshalText() ([]byte, error) {
 
 func (s *testService) NoArgsRets() {}
 
+func (s *testService) Null() any {
+	return nil
+}
+
 func (s *testService) Echo(str string, i int, args *echoArgs) echoResult {
 	return echoResult{str, i, args}
 }