Deadlock with QNetworkProxyFactory

Calling directly QNetworkProxyFactory::systemProxyForQuery() can deadlock with QNetworkAccessManager methods when multiple threads are used.

Thread 1: calling QNetworkProxyFactory::systemProxyForQuery()
Thread 2: calling QNetworkAccessManager::get()

With suitable timing, the two threads become deadlocked.

in QNetworkProxyFactory::systemProxyForQuery(), thread 1 gets QWindowsSystemProxy, and locks a mutex in it. Few steps further, thread 1 gets to QGlobalNetworkProxy::init(), which is protected by a mutex in QGlobalNetworkProxy.

Meanwhile in thread 2, the QNetworkAccessManager::get() proceeds to QGlobalNetworkProxy::proxyForQuery, which locks the mutex in QGlobalNetworkProxy. Few calls later, thread 2 ends up in QNetworkProxyFactory::systemProxyForQuery(). But the mutex there is already locked by thread1, which is now waiting on the QGlobalNetworkProxy mutex which was locked by thread 2.

Thread 1:
QtCore4.dll!QMutexPrivate::wait(int timeout=0xffffffff) Line 63 + 0x16 bytes C++
QtCore4.dll!QMutex::lock() Line 172 C++
QtCore4.dll!QMutexLocker::QMutexLocker(QMutex * m=0x01ba6b20) Line 103 C++
QtNetwork4.dll!QGlobalNetworkProxy::init() Line 260 C++
QtNetwork4.dll!QNetworkProxy::QNetworkProxy(QNetworkProxy::ProxyType type=HttpProxy, const QString & hostName="**.*.*.**", unsigned short port=0x1f90, const QString & user="", const QString & password="") Line 445 C++
QtNetwork4.dll!parseServerList(const QNetworkProxyQuery & query=

{...}, const QStringList & proxyList=[0x00000004]("**.*.*.*:8080","*.*.*.*:8080","*.*.*.*:8080","*.*.*.**:8080")) Line 223 + 0x5d bytes C++
> QtNetwork4.dll!QNetworkProxyFactory::systemProxyForQuery(const QNetworkProxyQuery & query={...}

) Line 394 + 0x10 bytes C++

Thread 2:
> QtCore4.dll!QMutexPrivate::wait(int timeout=0xffffffff) Line 63 + 0x16 bytes C++
QtCore4.dll!QMutex::lock() Line 209 C++
QtCore4.dll!QMutexLocker::QMutexLocker(QMutex * m=0x05d5be20) Line 103 C++
QtNetwork4.dll!QNetworkProxyFactory::systemProxyForQuery(const QNetworkProxyQuery & query=

{...}) Line 362 C++
QtNetwork4.dll!QSystemConfigurationProxyFactory::queryProxy(const QNetworkProxyQuery & query={...}

) Line 68 + 0x13 bytes C++
QtNetwork4.dll!QGlobalNetworkProxy::proxyForQuery(const QNetworkProxyQuery & query=

{...}) Line 318 + 0x14 bytes C++
QtNetwork4.dll!QNetworkProxyFactory::proxyForQuery(const QNetworkProxyQuery & query={...}

) Line 1303 + 0x16 bytes C++
QtNetwork4.dll!QNetworkAccessManagerPrivate::queryProxy(const QNetworkProxyQuery & query=

{...}) Line 1225 + 0xf bytes C++
QtNetwork4.dll!QNetworkAccessManager::createRequest(QNetworkAccessManager::Operation op=0x05d5e5f8, const QNetworkRequest & req={...}

, QIODevice * outgoingData=0x00000000) Line 1027 + 0x37 bytes C++
QtNetwork4.dll!QNetworkAccessManager::get(const QNetworkRequest & request=

{...}

) Line 688 + 0x14 bytes C++
QtDeclarative4.dll!QDeclarativeImageRequestHandler::event(QEvent * event=0x05d5c000) Line 289 C++