QNetworkAccessManager not emitting readyRead or finished signal

GaryL

hi, can anyone help to debug on this QNetworkAccessManager wrapper? I use a python flask application to receive on http://localhost:8000/ and return 200, "hello world", but when I call session.get(QUrl{"http://localhost:8000/"}), it sucks on waiting for reply debug message and cannot go for no matter QNetworkReply::readyRead or QNetworkReply::finished signal, which cannot exit the loop.

.h:

class session : public QObject {
		Q_OBJECT
	public:
		session(void);

		response get(QUrl url, QUrlQuery query = QUrlQuery(),
					 std::map<QString, QVariant> headers = std::map<QString, QVariant>(), uint timeout_sec = 60);
		response post(QUrl url, JSON json = JSON(), std::map<QString, QVariant> headers = std::map<QString, QVariant>(),
					  uint timeout_sec = 60);
		response post(QUrl url, QUrlQuery data = QUrlQuery(),
					  std::map<QString, QVariant> headers = std::map<QString, QVariant>(), uint timeout_sec = 60);

	signals:
		void request_start_signal(void);

	public slots:
		void request_process_slot(void);

	private:
		QNetworkAccessManager manager;
		QByteArray request_data;
		MethodsEnum request_method;
		bool is_finished = true;
		bool is_running = false;
		QNetworkRequest* request = nullptr;
		QNetworkReply* reply = nullptr;

		response wait_for_reply(uint timeout_sec);
	};

.cpp

session::session(void) { connect(this, &session::request_start_signal, this, &session::request_process_slot); }

	void session::request_process_slot(void) {
		qDebug() << "request slot";
		this->is_running = true;
		switch (request_method) {
			case GET: this->reply = this->manager.get(*this->request); break;
			case POST: this->reply = this->manager.post(*this->request, this->request_data); break;
			default: break;
		}
		QEventLoop loop;
		connect(this->reply, &QNetworkReply::readyRead, &loop, &QEventLoop::quit);
		loop.exec();
		qDebug() << "request slot finished";
		this->is_finished = true;
		this->is_running = false;
	}

	response session::wait_for_reply(uint timeout_sec) {
		uint64_t start_time = QDateTime::currentMSecsSinceEpoch();
		while (!this->is_finished) { // just do blocking request is fine for this project's case
			qDebug() << "waiting for reply" << (QDateTime::currentMSecsSinceEpoch() - start_time) / 1000;
			if (timeout_sec > 0 && QDateTime::currentMSecsSinceEpoch() - start_time > timeout_sec * 1000) {
				qDebug() << "request timeout";
				this->is_finished = true;
				this->reply->disconnect();
				delete this->reply;
				return response();
			}
			QThread::msleep(100);
		}
		response res = response(this->request, this->reply);
		this->reply->disconnect();
		delete this->request;
		this->request = nullptr;
		delete this->reply;
		this->reply = nullptr;
		return res;
	}

	response session::get(QUrl url, QUrlQuery query, std::map<QString, QVariant> headers, uint timeout_sec) {
		qDebug() << "GET";
		while (!this->is_finished) {
			qDebug() << "waiting for prev request to finish";
			QThread::msleep(100);
		}
		url.setQuery(query.query());
		if (this->request != nullptr) {
			delete this->request;
		}
		this->request = new QNetworkRequest(url);
		qDebug() << 3;
		for (const auto& [key, value] : headers) {
			this->request->setRawHeader(key.toUtf8(), value.toByteArray());
		}
		this->request_method = GET;
		this->is_finished = false;
		emit this->request_start_signal();
		return this->wait_for_reply(timeout_sec);
	}

Christian Ehrlicher

@GaryL said in QNetworkAccessManager not emitting readyRead or finished signal:

QThread::msleep(100);

WTF?

Qt is async so post your request, return to the event loop and when the answer arrives Qt will call your slot.

J.Hilk

@GaryL said in QNetworkAccessManager not emitting readyRead or finished signal:

QThread::msleep(100)

no event handling, no signals, no slots being called

while (!this->is_finished)

same here

GaryL

@Christian-Ehrlicher @J-Hilk

Thanks for your help, but the problem is in function request_process_slot, not in function wait_for_reply.

I have used a QEventLoop loop to block the process, and connect it with connect(this->reply, &QNetworkReply::readyRead, &loop, &QEventLoop::quit);. but then not matter I use QNetworkReply::readyRead or QNetworkReply::finished as signal, it still sucks on this event loop.

I have connect(this, &session::request_start_signal, this, &session::request_process_slot); in the constructor, so request_process_slot can be called when emit this->request_start_signal(); in get function.

Christian Ehrlicher

@GaryL said in QNetworkAccessManager not emitting readyRead or finished signal:

I have used a QEventLoop loop to block the process

No!

Go back to the Qt event loop.

GaryL

@Christian-Ehrlicher umm... not really understand, you mean sth like this?

session::session(void) { connect(this, &session::request_start_signal, this, &session::request_process_slot); }

	void session::request_process_slot(void) {
		qDebug() << "request slot";
		this->is_running = true;
		switch (request_method) {
			case GET: this->reply = this->manager.get(*this->request); break;
			case POST: this->reply = this->manager.post(*this->request, this->request_data); break;
			default: break;
		}
	}

	response session::wait_for_reply(uint timeout_sec) {
		while (this->reply == nullptr) {
			QThread::msleep(10);
		}
		QEventLoop loop;
		connect(this->reply, &QNetworkReply::readyRead, &loop, &QEventLoop::quit);
		loop.exec();
		this->is_finished = true;
		response res = response(this->request, this->reply);
		this->reply->disconnect();
		delete this->request;
		this->request = nullptr;
		delete this->reply;
		this->reply = nullptr;
		return res;
	}

	response session::get(QUrl url, QUrlQuery query, std::map<QString, QVariant> headers, uint timeout_sec) {
		qDebug() << "GET";
		while (!this->is_finished) {
			qDebug() << "waiting for prev request to finish";
			QThread::msleep(100);
		}
		url.setQuery(query.query());
		if (this->request != nullptr) {
			delete this->request;
		}
		this->request = new QNetworkRequest(url);
		for (const auto& [key, value] : headers) {
			this->request->setRawHeader(key.toUtf8(), value.toByteArray());
		}
		this->request_method = GET;
		this->is_finished = false;
		emit this->request_start_signal();
		return this->wait_for_reply(timeout_sec);
	}

JonB

@GaryL
As the other two responders have said, you must not use either of

while (!this->is_finished)
QThread::msleep(100)

They go against how an event-driven framework like Qt works.

GaryL

@JonB I am doing is because I original use qDebug() to print the variables of this->reply inside the loop so to find out what is going on, but it turns out that all variables seems normal. On the other hand, the python flask script do have receive a request and returned 200, "Hello world", so I don't know what is actually going on...

JonB

@GaryL
At this point I would suggest you dump calling your code, especially wait_for_reply(), for a while. Just produce a minimal program without while, is_finished, msleep, QEventLoop etc. Just connect the reply's readyRead() to something which outputs whatever it receives, do your get() or post() and verify you get the returned result (without "waiting" for it).

J.Hilk

@GaryL said in QNetworkAccessManager not emitting readyRead or finished signal:

so I don't know what is actually going on...

again, you're blocking any and all event handling in your qt application. The OS doesn't care about that and forwards your requests, thats why you may see stuff on the other side.

Btw, the Pythonfyed c++ is hurting my soul :P, deeply

GaryL

@JonB @J-Hilk I have actually used another url which is https://accounts.spotify.com/api/token, this url can be successfully exceuted with the above code and get its return json, but the same code fails with http://localhost:8000/ or https://api.spotify.com/v1/me/player/currently-playing