如何使用 Qt Network 类防止关闭竞争条件

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【中文标题】如何使用 Qt Network 类防止关闭竞争条件【英文标题】:How to prevent shutdown race conditions with Qt Network classes 【发布时间】:2016-03-17 20:12:52 【问题描述】:

我们有一个 OS X C++ 应用程序,使用 Qt 5.5,它提供了一个简单的 HTTP 服务器接口。它本质上类似于 Qt (http://doc.qt.io/qt-5/qtnetwork-fortuneserver-example.html) 提供的 Fortune Server 示例,但我们看到的是应用程序在关闭时偶尔会出现段错误,并带有以下堆栈跟踪(线程 6 崩溃):

Thread 0:: Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib          0x00007fff8deb4fca __open + 10

Thread 1:: Dispatch queue: com.apple.libdispatch-manager
0   libsystem_kernel.dylib          0x00007fff8deb6232 kevent64 + 10
1   libdispatch.dylib               0x00007fff90f0426e _dispatch_mgr_thread + 52

Thread 2:
0   libsystem_kernel.dylib          0x00007fff8deb594a __workq_kernreturn + 10
1   libsystem_pthread.dylib         0x00007fff887833dd start_wqthread + 13

Thread 3:
0   libsystem_kernel.dylib          0x00007fff8deb594a __workq_kernreturn + 10
1   libsystem_pthread.dylib         0x00007fff887833dd start_wqthread + 13

Thread 4:
0   libsystem_kernel.dylib          0x00007fff8deb594a __workq_kernreturn + 10
1   libsystem_pthread.dylib         0x00007fff887833dd start_wqthread + 13

Thread 5:: Dispatch queue: com.apple.NSXPCConnection.m-user.com.apple.airportd
0   libsystem_platform.dylib        0x00007fff8923378d _os_lock_handoff_lock + 23
1   libobjc.A.dylib                 0x00007fff83258906 objc_object::sidetable_clearDeallocating() + 64
2   libobjc.A.dylib                 0x00007fff8323e651 objc_destructInstance + 145
3   libobjc.A.dylib                 0x00007fff8323e595 object_dispose + 22
4   com.apple.CoreFoundation        0x00007fff84eea448 -[__NSArrayM dealloc] + 376
5   libobjc.A.dylib                 0x00007fff8325889c objc_object::sidetable_release(bool) + 236
6   com.apple.Foundation            0x00007fff85747909 -[_NSXPCInterfaceMethodInfo dealloc] + 63
7   libobjc.A.dylib                 0x00007fff8325889c objc_object::sidetable_release(bool) + 236
8   com.apple.CoreFoundation        0x00007fff84ed5db0 CFRelease + 304
9   com.apple.CoreFoundation        0x00007fff84ee5b92 __CFBasicHashDrain + 498
10  com.apple.CoreFoundation        0x00007fff84ed5e8e CFRelease + 526
11  com.apple.Foundation            0x00007fff8578dd7a -[NSXPCInterface dealloc] + 28
12  libobjc.A.dylib                 0x00007fff8325889c objc_object::sidetable_release(bool) + 236
13  com.apple.Foundation            0x00007fff8578df0c -[NSXPCConnection dealloc] + 281
14  libobjc.A.dylib                 0x00007fff8325889c objc_object::sidetable_release(bool) + 236
15  libsystem_blocks.dylib          0x00007fff8d3166e5 _Block_release + 196
16  libdispatch.dylib               0x00007fff90effe73 _dispatch_client_callout + 8
17  libdispatch.dylib               0x00007fff90f035cd _dispatch_queue_drain + 1100
18  libdispatch.dylib               0x00007fff90f03030 _dispatch_queue_invoke + 202
19  libdispatch.dylib               0x00007fff90f02bef _dispatch_root_queue_drain + 463
20  libdispatch.dylib               0x00007fff90f02a1c _dispatch_worker_thread3 + 91
21  libsystem_pthread.dylib         0x00007fff88785a9d _pthread_wqthread + 729
22  libsystem_pthread.dylib         0x00007fff887833dd start_wqthread + 13

Thread 6 Crashed:: Qt bearer thread
0   org.qt-project.QtNetwork        0x0000000100a541cb QNetworkConfigurationManagerPrivate::~QNetworkConfigurationManagerPrivate() + 107
1   org.qt-project.QtNetwork        0x0000000100a5431e QNetworkConfigurationManagerPrivate::~QNetworkConfigurationManagerPrivate() + 14
2   org.qt-project.QtCore           0x0000000100d72427 QObject::event(QEvent*) + 823
3   org.qt-project.QtCore           0x0000000100d49588 QCoreApplication::notify(QObject*, QEvent*) + 104
4   org.qt-project.QtCore           0x0000000100d4a212 QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) + 1058
5   org.qt-project.QtCore           0x0000000100d997db QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 59
6   org.qt-project.QtCore           0x0000000100d46c1c QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 412
7   org.qt-project.QtCore           0x0000000100b9c07e QThread::exec() + 110
8   org.qt-project.QtCore           0x0000000100b9fc02 QThreadPrivate::start(void*) + 338
9   libsystem_pthread.dylib         0x00007fff8878605a _pthread_body + 131
10  libsystem_pthread.dylib         0x00007fff88785fd7 _pthread_start + 176
11  libsystem_pthread.dylib         0x00007fff887833ed thread_start + 13

如您所见,线程 0 已完成 - 我们在 main 之外。我确定在我们处理资源时我没有调用一些清理代码,但我不知道它是什么。

没有把我们所有的源代码放在这里,基本的调用链是:

class RestServer : public QObject 
RestServer::RestServer() 
    _tcpServer = new QTcpServer(this);


void RestServer::listen(quint16 port)

    if (!_tcpServer->listen(QHostAddress::LocalHost, port)) 
        LOG_ERROR("RestServer", "Failed to start server at: " << port);
        throw std::exception();
    
    _portNum = _tcpServer->serverPort();
    LOG_INFO("RestServer", "Server is listening at: " << _portNum);
    connect(_tcpServer, SIGNAL(newConnection()), this, SLOT(connectSocket()));

那么在我们的测试代码中,我们基本上是这样做的:

void RestAPIServer_test::responseCallback(QNetworkReply *reply)

  auto response = reply->readAll();
  _uri = response;

  reply->close();

  QCoreApplication::exit();


TEST_F(RestAPIServer_test, urlWithPercents)

  RestServer restServer();
  restServer.listen(0);
  quint16 port = restServer.serverPort();

  // "widget/foo bar.txt"
  QUrl serviceUrl(QString("http://localhost:%1/path/?path=widget%2Ffoo%20bar.txt").arg(port));

  QNetworkAccessManager networkManager(this);
  connect(&networkManager, SIGNAL(finished(QNetworkReply*)), this, SLOT(responseCallback(QNetworkReply*)));

  QNetworkRequest request(serviceUrl);
  request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded");
  networkManager.get(request);

  QCoreApplication::exec();

  ASSERT_EQ(QString("path=widget/foo bar.txt"), _uri);

【问题讨论】:

【参考方案1】:

使用父级在堆栈上实例化 QObject 是不安全的。看线

QNetworkAccessManager networkManager(this);

该对象将被销毁两次,这是不可能的,因此您会看到崩溃。把那行改成如下(当然变量networkManager的使用也要更新):

QNetworkAccessManager* networkManager=new QNetworkAccessManager(this);

【讨论】:

感谢您的提示。实际上,这个例子的前一个版本就是按照你说的做的,但是在这个例子中我把它改成了一个堆栈变量,看看问题是否是由于 networkManager 超出了这段代码的范围。在任何一种情况下,都会偶尔发生段错误。 如何在QCoreApplication 上调用quit() 来停止exec() 循环?我建议您在信号和quit() 之间插入一个代理插槽,您可以在其中尝试显式破坏您的服务器和网络访问管理器。然后,您可以直接或使用QTimer::singleShot() 将信号提升到quit() 插槽,以确保所有网络线程都正确完成。看看你会得到什么结果。 “使用父级实例化堆栈上的 QObject 是不安全的。” - 仅当父对象可能在堆栈上的对象之前被删除时才是正确的。否则,对象在销毁时会从父对象的子列表中删除,因此不会重复删除。 @EvgenyS。 : responseCallback 函数调用 QCoreApplication::quit。我在上面的示例中没有使用它,但我会编辑我的问题以包含它。 @EvgenyS.:根据我的编辑,您能解释一下“代理插槽”是什么意思吗?【参考方案2】:

问题似乎是由于 Qt 创建的一些与网络相关的线程在调用 QCoreApplication::quit() 后仍然运行了一段时间,可能是因为从注册的回调函数中调用了 quit()与 QTcpSocket。解决方案是在调用退出后休眠 1 秒:

void RestAPIServer_test::responseCallback(QNetworkReply *reply)

    auto response = reply->readAll();
    _uri = response;

    reply->close();

    QCoreApplication::exit();

    // This prevents a shutdown race condition where Qt service
    // threads continue to run after the call to exit().
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));

【讨论】:

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