Android dns resolve crash

[wantfat]

Hi, I had a problem, When android phone network is shutdown, my app crash and get the follow result

1 dlfree 0xb6dcbf4c
2 free 0xb6db2258
3 __res_ndestroy 0xb6dba4da
4 __res_vinit 0xb6dba578
5 QHostInfoAgent::fromName(QString const&) 0xa2aadd7e
6 QHostInfoRunnable::run() 0xa2aa5516
7 QThreadPoolThread::run() 0xa2e6e05e
8 QThreadPrivate::start(void *) 0xa2e6c400
9 __pthread_start(void *) 0xb6db659c
10 __start_thread 0xb6db44c4
11 ??

This is my source code.

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "QAbstractEventDispatcher"
#include "QThread"
#include "QTimer"
#include "QNetworkAccessManager"
#include "QNetworkReply"
#include "QNetworkReply"

int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

QGuiApplication app(argc, argv);

QQmlApplicationEngine engine;
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                 &app, [url](QObject *obj, const QUrl &objUrl) {
    if (!obj && url == objUrl)
        QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);

for (int i = 0; i < 10; i++) {
    QThread *thr = new QThread();
    thr->start();

    while (!QAbstractEventDispatcher::instance(thr)) {
        thr->wait(1);
    }

    QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thr), [=](){
        QTimer *t = new QTimer();
        t->setSingleShot(true);
        t->start(1);

        QNetworkAccessManager *mgr = new QNetworkAccessManager();
        QObject::connect(t, &QTimer::timeout, [=](){
            QNetworkRequest request;
            request.setUrl(QString("http://xyz%1.cn").arg(i));

            QNetworkReply *reply = mgr->get(request);
            QObject::connect(reply, &QNetworkReply::finished, [=](){
                reply->deleteLater();
                t->start();
            });
        });
    });
}

return app.exec();

}

Thanks for any helps.

[wantfat]

It seem that crash occur only multiple thread call QNetworkAccessManager.get() and the url is difference.

[jsulm]

@wantfat said in Android dns resolve crash:

QNetworkReply *reply = mgr->get(request);

Did you check that reply is a valid pointer?

[wantfat]

@jsulm Yes, I tried it just now, but the same thing happened. I changed the code to

QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thr), ={
QTimer *t = new QTimer();
t->setSingleShot(true);
t->start(1);

        QObject::connect(t, &QTimer::timeout, [=](){
            QHostInfo::lookupHost(QString("xyz%1.cn").arg(i), [=](const QHostInfo &info){
                qDebug() << info.errorString();
                QMetaObject::invokeMethod(t, "start");
            });
        });
    });

It also crash in the same place.

[jsulm]

@wantfat said in Android dns resolve crash:

I tried it just now, but the same thing happened

How? Please show the code.

"It also crash in the same place." - which place exactly?

[wantfat]

@jsulm This is the code, I check reply valid
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "QAbstractEventDispatcher"
#include "QThread"
#include "QTimer"
#include "QNetworkAccessManager"
#include "QNetworkReply"
#include "QNetworkReply"
#include "QHostInfo"

int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

QGuiApplication app(argc, argv);

QQmlApplicationEngine engine;
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                 &app, [url](QObject *obj, const QUrl &objUrl) {
    if (!obj && url == objUrl)
        QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);

for (int i = 0; i < 10; i++) {
    QThread *thr = new QThread();
    thr->start();

    while (!QAbstractEventDispatcher::instance(thr)) {
        thr->wait(1);
    }

    QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thr), [=](){
        QTimer *t = new QTimer();
        t->setSingleShot(true);
        t->start(1);

        QNetworkAccessManager *mgr = new QNetworkAccessManager();
        QObject::connect(t, &QTimer::timeout, [=](){
            QNetworkRequest request;
            request.setUrl(QString("http://xyz%1.cn").arg(i));

            QNetworkReply *reply = mgr->get(request);
            if (reply == nullptr) {
                qDebug() << "reply is null";
                return;
            }

            QObject::connect(reply, &QNetworkReply::finished, [=](){
                reply->deleteLater();
                t->start();
            });
        });
    });
}

return app.exec();

}

When it crash, The stack is as follows

1 dlfree 0xb6dcbf4c
2 free 0xb6db2258
3 __res_ndestroy 0xb6dba4da
4 __res_vinit 0xb6dba578
5 QHostInfoAgent::fromName(QString const&) 0xa2aadd7e
6 QHostInfoRunnable::run() 0xa2aa5516
7 QThreadPoolThread::run() 0xa2e6e05e
8 QThreadPrivate::start(void *) 0xa2e6c400
9 __pthread_start(void *) 0xb6db659c
10 __start_thread 0xb6db44c4
11 ??

[SGaist]

Hi,

Why are you creating 10 threads making get requests almost as fast as possible all the time your application is running ?

On a side note, you are leaking QNetworkAccessManager and QTimer objects.

[wantfat]

@sgaist If I had two threads, It would still crash, just for a long time. the request is serial execution .
thank you.

[wantfat]

I run on another phone (Android 9, armv7) , The code work well. The mobile phone that had abnormalities is Android 5.1 armv7. I am not sure whether it depend on mobile phone .

[J.Hilk]

@wantfat you do realize that

QThread *thr = new QThread();
    thr->start();

    while (!QAbstractEventDispatcher::instance(thr)) {
        thr->wait(1);
    }

is doing nothing but delay forcefully delay the loop by 1 millisecond?
Also you don't need a QNetworkAccessManager instance for each get request. QNetworkAccessManager can handle 5 get requests in parallel, IIRC, and the rest is automatically queued

[wantfat]

@j-hilk After call QThread::start the eventDispatcher of thread is null, so must take a sleep wait for eventDispatcher be create, QNetworkAccessManager only one instance for per thread .

[J.Hilk]

@wantfat said in Android dns resolve crash:

, QNetworkAccessManager only one instance for per thread

no, from your code, you create 10 QNetworkAccessManager instances and all live inside the main(Gui) thread

[wantfat]

@j-hilk QMetaObject::invokeMethod(QAbstractEventDispatcher::instance(thr), ={

});
This code make sure lambda run on thr.

[J.Hilk]

@wantfat have you checked that ? by combining invokeMethod with a lambada the object reference pointer becomes a context pointer. I see no reason, why it should be called from the Thread instance, or I personally don't know, never tried threading in this unusual way.

[wantfat]

@j-hilk Yes, I have checked it, this is detail info, https://stackoverflow.com/a/21653558

[wantfat]

The test code work well on Qt 5.12.3, The problematic version is Qt 5.12.4 . I compared two versions of the relevant code

If method __res_init resolve fail dlsym be call, and it return a valid pointer, But it is not a thread safe function. Maybe so.

[wantfat]

I solved this problem by export thread safe __res_init

#if (QT_VERSION == QT_VERSION_CHECK(5,12,4))

#include "resolv.h"
#include "QMutex"

static QMutex mut;

int __res_init(void) {
QMutexLocker lck(&mut);
return res_init();
}

#endif

Thanks every one.