fix: preserve empty-method requests during decode

Author: he-yufeng

internal/jsonrpc2/messages.go

@@ -183,7 +183,11 @@ func DecodeMessage(data []byte) (Message, error) {
 	if err != nil {
 		return nil, err
 	}
-	if msg.Method != "" {
+	var fields map[string]json.RawMessage
+	if err := internaljson.Unmarshal(data, &fields); err != nil {
+		return nil, fmt.Errorf("unmarshaling jsonrpc message fields: %w", err)
+	}
+	if _, hasMethod := fields["method"]; hasMethod {
 		// has a method, must be a call
 		return &Request{
 			Method: msg.Method,

internal/jsonrpc2/wire_test.go

@@ -63,6 +63,23 @@ func TestWireMessage(t *testing.T) {
 	}
 }
 
+func TestDecodeMessageWithEmptyMethodIsRequest(t *testing.T) {
+	msg, err := jsonrpc2.DecodeMessage([]byte(`{"jsonrpc":"2.0","id":5,"method":"","params":{}}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	req, ok := msg.(*jsonrpc2.Request)
+	if !ok {
+		t.Fatalf("DecodeMessage returned %T, want *jsonrpc2.Request", msg)
+	}
+	if req.Method != "" {
+		t.Fatalf("Request.Method = %q, want empty string", req.Method)
+	}
+	if got := req.ID.Raw(); got != int64(5) {
+		t.Fatalf("Request.ID = %v, want 5", got)
+	}
+}
+
 func newNotification(method string, params any) jsonrpc2.Message {
 	msg, err := jsonrpc2.NewNotification(method, params)
 	if err != nil {