Qt 5.6 alpha QtWebEngine. How work with QWebEngineUrlRequestJob?

Question

I have inherited from QWebEngineUrlSchemeHandler. Redefined method requestStarted(QWebEngineUrlRequestJob *request)

After I try to call QWebEngineUrlRequestJob::reply

for example

void CustomUrlSchemeHandler::requestStarted(QWebEngineUrlRequestJob *request)
{
    qDebug() << "CustomUrlSchemeHandler::requestStarted -->>> " << request->requestUrl();

    QMimeDatabase HelpViewer;
    QMimeType mt = HelpViewer.mimeTypeForUrl(request->requestUrl());
    const QString mimeType = mt.name();
    QByteArray arr = QString::fromStdString("<html><body>Hello world</body></html>").toUtf8();
    QBuffer *buffer = new QBuffer(&arr, this);
    buffer->open(QIODevice::ReadOnly);
    request->reply(mimeType.toLatin1(), buffer);

    return;
}

But the program crashes. see log:

    [1126/113403:FATAL:weak_ptr.cc(26)] Check failed: sequence_checker_.CalledOnValidSequencedThread(). WeakPtrs must be checked on the same sequenced thread.
Backtrace:
    base::debug::StackTrace::StackTrace [0x11A78691+33] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\debug\stack_trace_win.cc:205)
    logging::LogMessage::~LogMessage [0x119CB2AF+63] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\logging.cc:544)
    base::internal::WeakReference::Flag::IsValid [0x119FB75A+234] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\memory\weak_ptr.cc:28)
    base::internal::WeakReference::is_valid [0x119FB7B2+50] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\memory\weak_ptr.cc:43)
    base::WeakPtr<base::ObserverListBase<content::ServiceWorkerContextObserver> >::get [0x109EF57F+31] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\memory\weak_ptr.h:204)
    base::internal::InvokeHelper<1,void,base::internal::RunnableAdapter<void (__thiscall content::DownloadResourceHandler::*)(void)>,base::internal::TypeList<base::WeakPtr<content::DownloadResourceHandler> const &> >::MakeItSo [0x107B8ADB+11] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\bind_internal.h:300)
    base::internal::Invoker<base::IndexSequence<0>,base::internal::BindState<base::internal::RunnableAdapter<void (__thiscall content::DownloadResourceHandler::*)(void)>,void __cdecl(content::DownloadResourceHandler *),base::internal::TypeList<base::WeakPtr<c [0x107BAD0A+58] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\bind_internal.h:346)
    base::Callback<net::URLRequestContext * __cdecl(void)>::Run [0x1096CACF+47] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\callback.h:396)
    base::debug::TaskAnnotator::RunTask [0x11A9580D+541] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\debug\task_annotator.cc:64)
    base::MessageLoop::RunTask [0x119DAAF8+456] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_loop.cc:475)
    base::MessageLoop::DeferOrRunPendingTask [0x119D9204+52] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_loop.cc:485)
    base::MessageLoop::DoWork [0x119D974D+221] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_loop.cc:594)
    base::MessagePumpForIO::DoRunLoop [0x11A980A2+50] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_pump_win.cc:524)
    base::MessagePumpWin::RunWithDispatcher [0x11A99B42+130] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_pump_win.cc:51)
    base::MessagePumpWin::Run [0x11A99AAC+28] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_pump_win.cc:58)
    base::MessageLoop::RunHandler [0x119DA907+247] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_loop.cc:438)
    base::RunLoop::Run [0x11A0A2E6+70] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\run_loop.cc:56)
    base::MessageLoop::Run [0x119DA7BD+237] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\message_loop\message_loop.cc:288)
    base::Thread::Run [0x11A20AF6+22] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\threading\thread.cc:199)
    content::BrowserThreadImpl::IOThreadRun [0x10603B74+52] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\content\browser\browser_thread_impl.cc:212)
    content::BrowserThreadImpl::Run [0x1060498B+235] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\content\browser\browser_thread_impl.cc:246)
    base::Thread::ThreadMain [0x11A215F9+745] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\threading\thread.cc:248)
    base::`anonymous namespace'::ThreadFunc [0x11A283B6+262] (e:\lib\qt\qt_git\qt5\qtwebengine\src\3rdparty\chromium\base\threading\platform_thread_win.cc:84)
    BaseThreadInitThunk [0x75D57C04+36]
    RtlInitializeExceptionChain [0x77E4AD1F+143]
    RtlInitializeExceptionChain [0x77E4ACEA+90]
    (No symbol) [0x00000000]

As far as I know, the Qt 5.6 beta is still not released... Did you mean alpha? However, since you are using a "BETA", you shouldn't be suprised if the program crashes. Create a bug report at https://bugreports.qt.io — Felix, Nov 26 '15 at 18:22
I build Qt from commit f128a9a7e1009d9ee11665dce08d5bfafa91ef66; it's latest commit for 5.6 at this moment — Yury Bely, Nov 27 '15 at 07:58

score 2 · Answer 1 · answered Jun 11 '16 at 07:18

The reason of the crash is not obvious, because Qt documentation is not clear on the details how QWebEngineUrlSchemeHandler should be implemented.

Your program crashes because QWebEngineUrlRequestJob::reply() uses the buffer parameter later, out of the caller's scope, probably from a different thread. The local variable QByteArray arr which you declared on the stack (and used as backing storage for the buffer) will be out of scope by then and QWebEngine will be accessing an invalid pointer.

Another thing is, reply() does not take ownership of the passed buffer object and never deletes it. This introduces a memory leak. You cannot simply delete it right after calling reply() for the same reason as above. To ensure the buffer object is deleted when no longer needed, connect the request's destroyed() signal to the buffer's deleteLater() slot.

Here is a complete working and leak-free example:

void CustomUrlSchemeHandler::requestStarted(QWebEngineUrlRequestJob *request)
{
    QBuffer *buffer = new QBuffer;
    connect(request, SIGNAL(destroyed()), buffer, SLOT(deleteLater()));

    buffer->open(QIODevice::WriteOnly);
    buffer->write("<html><body>Hello world!</body></html>");
    buffer->close();

    request->reply("text/html", buffer);
}

Note: You don't need to use a QByteArray at all. The default constructor for QBuffer allocates its own storage.

score 1 · Answer 2 · edited May 16 '16 at 09:23

1

One reason for the crash is that arr goes out of scope.
You have to change

QByteArray arr = QString::fromStdString("<html><body>Hello world</body></html>").toUtf8();
QBuffer *buffer = new QBuffer(&arr, this);

to

QByteArray *pArr = new QByteArray(QString::fromStdString("<html><body>Hello world</body></html>").toUtf8());
QBuffer *buffer = new QBuffer(pArr, this);

edited May 16 '16 at 09:23

Tunaki

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answered May 16 '16 at 08:08

Thomas Aho

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Qt 5.6 alpha QtWebEngine. How work with QWebEngineUrlRequestJob?

2 Answers2