Memory leak in QAbstractSocketPrivate::readFromSocket() ?

[Alexey Volkov]

Hi!
I'm trying to find out what is causing a memory leak in my QTcpServer app. RSS is growing ~100MB for each 10GB of traffic. Have tested with Heaptrack in my local test setup, and it shows quite concerning stats: almost half of leak is contributed by QAbstractSocketPrivate::readFromSocket():

197.7kB (43.5%) leaked in QAbstractSocketPrivate::readFromSocket() in libQt5Network.so.5 and below.

The other major contributor is QIODevice::readAll() :
16.4kB (3.61%) leaked in QIODevice::readAll() in libQt5Core.so.5 and below.
(in 3 places, ~10%)

In all cases QRingBuffer::reserve() makes un-released allocation. By the time the trace was captured, all the sockets were closed. They should release all buffers but they do not.

Is there any good practice to avoid excess allocations in readAll() or to make sockets close all internal buffers on close?

Thanks is advance!

[Christian Ehrlicher]

Please provide a minimal, compilable example of your problem and also what exact Qt version you're using on which OS. I doubt there is a leak inside QAbstractSocket in current Qt6 or latest Qt5 versions but it might be.

[Alexey Volkov]

@Christian-Ehrlicher
It is Qt5.15.2 on Ubuntu 22.04
It is quite tricky to provide compilable example. It is QTcpServer "by the book" but the incoming connections are handled in separate threads.

[Axel Spoerl]

@Alexey-Volkov
Qt 5.15.2 is not supported any more.

It is quite tricky to provide compilable example.

That makes me confident that the leak is not in Qt.
Feel free to prove that I am wrong.

[Alexey Volkov]

@Axel-Spoerl
You got the trace on a screenshot. It is only Qt code. That does not exclude mis-use, of course. That's why I'm here

[Christian Ehrlicher]

@Alexey-Volkov said in Memory leak in QAbstractSocketPrivate::readFromSocket() ?:

Here is the code:
https://forge.bineon.team/onmydisk/onmydisk

This is neither minimal nor simple so if you really want to have someone looking into your code you should provide a reproducer.

[Axel Spoerl]

@Alexey-Volkov
The trace showing Qt code doesn’t mean anything.
The expectation that we should analyse a whole application project, while the leak can’t be isolated in a minimal reproducer, makes me even more confident ;-)

[Alexey Volkov]

Sure, I'm not expecting anyone to analyze my code. And not trying to find a bug in some else's code, I'm trying to solve the problem.
The leak trace from Heaptracker is quite interesting - it starts in main thread, goes through the main event loop, then reaches QAbstractSocketPrivate::readFromSocket() and ends up in QRingBuffer::reserve(). What on earth could lead to this, if the socket itself was not leaked? Could it be possible that it is "false positive" case, and the private object was not yet actually released?

[Axel Spoerl]

@Alexey-Volkov
When an abstract socket is deleted or goes out of scope, the private object is always released first (not only for QAbstractSocket, but for most Qt classes). A QObject::deleteLater can also appear like a memory leak, when the event loop in charge stops spinning to early. But that's all speculation and guesswork in the absence of a reproducer.

We're actually moving in circles: If you want to solve the problem, you have to spend time on it, dig to the bottom and isolate it into a minimal reproducer. If that's not possible, the bug is in your code. If you don't want to do it, the problem is obviously not serious enough.

[Alexey Volkov]

@Axel-Spoerl
Looks like it was actually the case. The thread was finished too early and deleteLater() did not do the job. I reworked it with explicit delete. Now in combination with malloc_trim(0) during the idle time I see some memory usage drops in docker stats.

The better approach could be deleting the socket on some delay after disconnected() signal, and finish the thread on socket's destroyed() signal. Will try.

[Alexey Volkov]

@Axel-Spoerl
Yes, the cause confirmed. This approach works! I see drops of RSS and docker memory statistics.
When the socket disconnects, first the socket should be deleted, then the thread can be stopped:

connect(socket, &QTcpSocket::disconnected,  socket, [this,  thread, socket]() {
        QTimer::singleShot(CLEANUP_INTERVAL, socket, [socket, thread]() {
            delete socket;
            thread->quit();
        });
    });

    connect(thread, &QThread::finished, thread, &QThread::deleteLater);

Thanks for advice!

[Axel Spoerl]

@Alexey-Volkov thanks for letting us know what it was. Glad that it works. Please don’t forget to mark the issue solved.