LanguageClient: do not send empty responses

Even if we do not have to provide any data just send a null as the
result so the server nows that the request did not fail.

Fixes: QTCREATORBUG-26116
Change-Id: I9b965389bb197cdd81d0d3ffbac05f289cabda40
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/languageserverprotocol/jsonrpcmessages.h

@@ -236,7 +236,7 @@ public:
 
     Utils::optional<Result> result() const
     {
-        const QJsonValue &result = m_jsonObject.value("result");
+        const QJsonValue &result = m_jsonObject.value(resultKey);
         if (result.isUndefined())
             return Utils::nullopt;
         return Utils::make_optional(Result(result));
@@ -259,6 +259,41 @@ public:
     { return JsonRpcMessage::isValid(errorMessage) && id().isValid(); }
 };
 
+template<typename ErrorDataType>
+class Response<std::nullptr_t, ErrorDataType> : public JsonRpcMessage
+{
+public:
+    explicit Response(const MessageId &id) { setId(id); }
+    using JsonRpcMessage::JsonRpcMessage;
+
+    MessageId id() const
+    { return MessageId(m_jsonObject.value(idKey)); }
+    void setId(MessageId id)
+    { this->m_jsonObject.insert(idKey, id); }
+
+    Utils::optional<std::nullptr_t> result() const
+    {
+        return m_jsonObject.value(resultKey).isNull() ? Utils::make_optional(nullptr)
+                                                      : Utils::nullopt;
+    }
+    void setResult(const std::nullptr_t &) { m_jsonObject.insert(resultKey, QJsonValue::Null); }
+    void clearResult() { m_jsonObject.remove(resultKey); }
+
+    using Error = ResponseError<ErrorDataType>;
+    Utils::optional<Error> error() const
+    {
+        const QJsonValue &val = m_jsonObject.value(errorKey);
+        return val.isUndefined() ? Utils::nullopt
+                                 : Utils::make_optional(fromJsonValue<Error>(val));
+    }
+    void setError(const Error &error)
+    { m_jsonObject.insert(errorKey, QJsonValue(error)); }
+    void clearError() { m_jsonObject.remove(errorKey); }
+
+    bool isValid(QString *errorMessage) const override
+    { return JsonRpcMessage::isValid(errorMessage) && id().isValid(); }
+};
+
 template <typename Result, typename ErrorDataType, typename Params>
 class Request : public Notification<Params>
 {

src/plugins/languageclient/client.cpp

@@ -1293,8 +1293,10 @@ void Client::handleMethod(const QString &method, const MessageId &id, const ICon
         response.setResult(result);
         sendContent(response);
     } else if (method == WorkDoneProgressCreateRequest::methodName) {
-        sendContent(WorkDoneProgressCreateRequest::Response(
+        WorkDoneProgressCreateRequest::Response response(
-            dynamic_cast<const WorkDoneProgressCreateRequest *>(content)->id()));
+            dynamic_cast<const WorkDoneProgressCreateRequest *>(content)->id());
+        response.setResult(nullptr);
+        sendContent(response);
     } else if (method == ProgressNotification::methodName) {
         if (Utils::optional<ProgressParams> params
             = dynamic_cast<const ProgressNotification *>(content)->params()) {