Qt Network seems to persistently use memory

[LorenDB]

Upon further analysis, I found that OPENSSL_LH_insert is not the source of the memory creep; again, the memory creep is not actually caused by a leak according to Heaptrack.

[LorenDB]

Valgrind does not give any errors. Output is linked here if anyone is interested.

Edit: full output from running Valgrind with valgrind -s --leak-check=full is here.

[SGaist]

I am seeing Wayland related stuff there, did you try to run your application as console only ? It's all the more things removed that may have an influence.

[LorenDB]

Do you mind explaining what you mean by "console only"? Does that mean removing all GUI code and testing the application or something else?

[JoeCFD]

@LorenDB Change to use X instead of Wayland. There is a setting for switch on the login screen

[SGaist]

@LorenDB yes that's that.

[jeremy_k]

@LorenDB said in Qt Network seems to persistently use memory:

I have a program that is riddled with calls that pretty much look like this:

[...]
while (!rep->isFinished())
    qApp->processEvents();

Is there a reason this is avoiding the event loop by busy waiting? DeferredDelete events have some restrictions which this code appears to be tripping over.

#include <QCoreApplication>
#include <QObject>
#include <QDebug>
#include <QTimer>

void tryWithExec()
{
    QObject *obj = new QObject;
    obj->setObjectName("timeout slot");
    QObject::connect(obj, &QObject::destroyed, [](QObject *object) { qDebug() << object << "destroyed"; QCoreApplication::quit(); } );
    obj->deleteLater();
    qDebug() << "QCoreApplication::processEvents()";
    QCoreApplication::instance()->processEvents();
    qDebug() << "returning to event loop";
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QObject *obj = new QObject;
    obj->setObjectName("pre-exec");
    QObject::connect(obj, &QObject::destroyed, [](QObject *object) { qDebug() << object<< "destroyed";} );
    QTimer timer;
    timer.setInterval(1000);
    timer.setSingleShot(true);
    QObject::connect(&timer, &QTimer::timeout, &tryWithExec);
    timer.start();
    obj->deleteLater();
    qDebug() << "QCoreApplication::processEvents()";
    a.processEvents();
    qDebug() << "QCoreApplication::exec()";
    return a.exec();
}

Using Qt 5.15.2, the output is:

QCoreApplication::processEvents()
QCoreApplication::exec()
QObject(0x7fd68440b590, name = "pre-exec") destroyed
QCoreApplication::processEvents()
returning to event loop
QObject(0x7fd686006a70, name = "timeout slot") destroyed

Note that deferred deletion is not happening in response to QCoreApplication::processEvents().

[LorenDB]

Wow, I have all sorts of advice! Let me sort through this:

@SGaist I already tested with a console application that only does the internet check calls (no other network activity is occurring, at least not anything that I wrote into the test app). The memory creep is still there.

@JoeCFD I'd rather not log into X for various reasons (for example, I'm on KDE Plasma, which for some reason has terrible X support for graphical effects anymore). In fact, I don't think that the fault rests with Wayland, since I've experienced memory creep on Windows as well and since my console test app (see above) also experienced memory creep.

@jeremy_k Hmm, that's a good point. In fact, your suggestion triggered a memory that I had of seeing something similar in Qt Assistant. I looked it up and found that you can call sendPostedEvents() to do such things as execute DeferredDelete events. I will try executing that as well and get back with the results (preliminary testing looks promising). In answer to your question, yes, I have a reason for busy waiting: I specifically want to make sure that network requests execute in a specific order (I'm modifying time records on Clockify, if you want to know, and requesting a stop and then a start can sometimes result in requests finishing in the wrong order which leads to wacky behavior) and decided to add synchronous execution to my network requests that needed it. (I do have the ability to run async requests, however.)

Thanks to all involved for the input so far.

[JoeCFD]

https://forum.qt.io/topic/128729/real-confusion-about-when-to-delete-qnetworkreply-object/10
Check this one out and you may be able to get some help.
In your code,
if auto rep = m_manager.head(req); finishes immediately, the following connect is useless since finished has been sent out.

[LorenDB]

@JoeCFD said in Qt Network seems to persistently use memory:

https://forum.qt.io/topic/128729/real-confusion-about-when-to-delete-qnetworkreply-object/10
Check this one out and you may be able to get some help.
In your code,
if auto rep = m_manager.head(req); finishes immediately, the following connect is useless since finished has been sent out.

I tried that without success. I then tried calling setAutoDeleteReplies(true) on my QNetworkAccessManager, also without success.

Another thing that I should point out that I am sure that the problem lies at least partly with Qt Network because when conditions are such that my status requests to Clockify return large(ish) amounts of data, memory creep is larger than when there is little to no data returned. Before anybody asks, no, I am not permanently storing all of this data unless the JSON library I'm using is caching things that I'm parsing behind the scenes.

[JoeCFD]

You may need to make debug Qt build manually and use valgrind to find where the problem is. Standard Qt installation is a release build.
Also try a lower version 5.13 or 5.12 to see if the same problem exists.<===this can be done quickly on Linux

[jeremy_k]

@LorenDB said in Qt Network seems to persistently use memory:

I specifically want to make sure that network requests execute in a specific order

Chain the sending of the next request to the completion of the current reply. Eg:

void Manager::onReplyFinished(QNetworkReply *reply)
{
    if (!this->m_requestQueue.isEmpty()) {
        this->get(this->m_requestsQueue.takeFirst());
    }
    // process the current reply
}

[LorenDB]

@jeremy_k the problem is that that approach won't work for my whole application. Some requests return data that needs to be processed by the caller (e.g. the caller is asking if a certain condition is met, and that condition is determined by the state of an online resource).

[jeremy_k]

@LorenDB said in Qt Network seems to persistently use memory:

@jeremy_k the problem is that that approach won't work for my whole application. Some requests return data that needs to be processed by the caller (e.g. the caller is asking if a certain condition is met, and that condition is determined by the state of an online resource).

So have the caller connect to QNetworkReply::finished(), or use QNetworkRequest::originatingObject() to indicate which object should be notified.

[LorenDB]

@jeremy_k said in Qt Network seems to persistently use memory:

So have the caller connect to QNetworkReply::finished(), or use QNetworkRequest::originatingObject() to indicate which object should be notified.

That won't really work in my situation. I'm processing data that depends on the value of this reply, and I need to temporarily hang in order to finish. If I try to do a slot-based callback system, my code will quickly turn into an unmanageable jungle (oh wait, it already is... /s).

[JonB]

@LorenDB
Then if you have to block because of the existing code, can you not use QEventLoop::exec() here?

[LorenDB]

At one point, I was using a QEventLoop, but I decided that calling processEvents would have the same effect. Am I wrong?

[JonB]

@LorenDB
Using a local QEventLoop loop, calling loop.exec() and exiting with loop.quit() on your reply finished signal is the standard way of forcing a sync point. I don't know why you'd use processEvents() here. Your case is discussed in, say, https://stackoverflow.com/questions/29449561/qeventloop-proper-usage. Note the comments/replies do warn you it is evil :) That's all I know.

[jeremy_k]

@LorenDB said in Qt Network seems to persistently use memory:

@jeremy_k said in Qt Network seems to persistently use memory:

So have the caller connect to QNetworkReply::finished(), or use QNetworkRequest::originatingObject() to indicate which object should be notified.

That won't really work in my situation. I'm processing data that depends on the value of this reply, and I need to temporarily hang in order to finish. If I try to do a slot-based callback system, my code will quickly turn into an unmanageable jungle (oh wait, it already is... /s).

It sounds like you are trying to dig your way out of a hole by using a more complicated shovel.

[LorenDB]

@JonB @jeremy_k OK, point taken. I will switch back to QEventLoop. However, the memory usage problem will still persist with QEventLoop, if memory serves me correctly (no pun intended).