[Interest] Trouble with Android JNI and QEventLoop

[Fabrice Mousset | GEOCEPT GmbH]

Thanks for your reply!

The thread ID is the same, all messages are from the same process (Activity and Service are on different processes).
I forgot to say that I have already tried QMutex to lock the access, but that is not useful here because if I set it up as "NonRecursive", it will dead lock!

Regards

Fabrice

Von: Marc Van Daele <marc.van.daele90 at gmail.com> 
Gesendet: Montag, 13. Juli 2020 11:36
An: Fabrice Mousset | GEOCEPT GmbH <fabrice.mousset at geocept.com>
Cc: Qt Interest <interest at qt-project.org>
Betreff: Re: [Interest] Trouble with Android JNI and QEventLoop

Can you also print the thread-pointer/id next to the name?  
Maybe (just guessing) one QtMainLoopThread is from the main app and the other one is from the service?
You could add a QMutexLocker (https://doc.qt.io/qt-5/qmutexlocker.html) to ensure that the calls are executed sequentially.

Kind Regards,

Marc

On Mon, 13 Jul 2020 at 09:25, Fabrice Mousset | GEOCEPT GmbH <mailto:fabrice.mousset at geocept.com> wrote:
Hi all,

First, I tried to send this mail to Android mailing list, but got an error message as reply, so I try here. Sorry if I am wrong

I have a random issue with one of my Android service I've build with Qt 5.12.9.
I have centralized JNI interface in one C++ class, which is a singleton.
My service is single threaded, so there should not be a threading issue... I guess!

So here my issue: I have a function which will populate a list. This function could be called at service begin or while receiving a specific Broadcast message.
Sometimes, it happens that this function is called twice before first call is finished.

I have at some traces to follow what's happening, something like:
void MyService::updateList(int calledFrom) {
    qDebug() << "Update List start #"<< calledFrom << "@" << QThread::currentThread()->objectName();
    ...
    qDebug() << "Update List end #"<< calledFrom << "@" << QThread::currentThread()->objectName();}
}

And, on LogCat I can see:
Update List start # 0 @ "QtMainLoopThread"
Update List start # 1 @ "QtMainLoopThread"
Update List end # 1 @ "QtMainLoopThread"
Update List end # 0 @ "QtMainLoopThread"


Here is the way I handle JNI calls:
void JniHandler:: stateChanged(JNIEnv *, jobject, jint newState) {
    auto* pInstance = JniHandler::instance();
    if(pInstance)
    {
        QMetaObject::invokeMethod(pInstance, [pInstance , newState] {
            ...
            });
     }
}

I tried to change QMetaObject::invokeMethod call to add ' Qt::QueuedConnection', but didn't change anything.
I also change to use QTimer::singleShot():
    if(pInstance)
    {
        QEventLoop myLoop;
        Q_UNUSED(myLoop)
        QTimer::singleShot(0, pInstance, [pInstance , newState] {
            ...
            });
     }

But still have the same issue.

How is this possible?
What I am doing wrong?

Best regards

Fabrice Mousset

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