Move the wayland socket polling to a separate event thread

New event threads is introduced which calls poll() on the wayland fd,
instead of relying on the event dispatcher by using the QSocketNotifier.
This allows to call in the proper order the wl_display_prepare_read(),
poll() and wl_display_read_events() functions.

One thread is responsible for the default queue; when needed, it emit
a signal so that the main thread can dispatch the queue. Another thread
is responsible for the dedicated queue for frame callbacks; this thread
will dispatch events on the thread itself.

QWaylandWindow is updated to, instead of each window's dedicated event
queue, use this queue for frame callbacks.

Co-authored-by: Ratchanan Srirattanamet <ratchanan@ubports.com>
Task-number: QTBUG-66075
Change-Id: Ibb33ad7f4193b866d1b8d7a0405a94d59dcad5eb
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/plugins/platforms/wayland/qwaylanddisplay.cpp

@@ -90,10 +90,201 @@
 
 #include <tuple> // for std::tie
 
+static void checkWaylandError(struct wl_display *display)
+{
+    int ecode = wl_display_get_error(display);
+    if ((ecode == EPIPE || ecode == ECONNRESET)) {
+        // special case this to provide a nicer error
+        qWarning("The Wayland connection broke. Did the Wayland compositor die?");
+    } else {
+        qWarning("The Wayland connection experienced a fatal error: %s", strerror(ecode));
+    }
+    _exit(1);
+}
+
 QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {
 
+class EventThread : public QThread
+{
+    Q_OBJECT
+public:
+    enum OperatingMode {
+        EmitToDispatch, // Emit the signal, allow dispatching in a differnt thread.
+        SelfDispatch, // Dispatch the events inside this thread.
+    };
+
+    EventThread(struct wl_display * wl, struct wl_event_queue * ev_queue,
+                OperatingMode mode)
+        : m_fd(wl_display_get_fd(wl))
+        , m_pipefd{ -1, -1 }
+        , m_wldisplay(wl)
+        , m_wlevqueue(ev_queue)
+        , m_mode(mode)
+        , m_reading(true)
+        , m_quitting(false)
+    {
+        setObjectName(QStringLiteral("WaylandEventThread"));
+    }
+
+    void readAndDispatchEvents()
+    {
+        /*
+         * Dispatch pending events and flush the requests at least once. If the event thread
+         * is not reading, try to call _prepare_read() to allow the event thread to poll().
+         * If that fails, re-try dispatch & flush again until _prepare_read() is successful.
+         *
+         * This allow any call to readAndDispatchEvents() to start event thread's polling,
+         * not only the one issued from event thread's waitForReading(), which means functions
+         * called from dispatch_pending() can safely spin an event loop.
+         */
+        for (;;) {
+            if (dispatchQueuePending() < 0) {
+                checkWaylandError(m_wldisplay);
+                return;
+            }
+
+            wl_display_flush(m_wldisplay);
+
+            // We have to check if event thread is reading every time we dispatch
+            // something, as that may recursively call this function.
+            if (m_reading.loadAcquire())
+                break;
+
+            if (prepareReadQueue() == 0) {
+                QMutexLocker l(&m_mutex);
+                m_reading.storeRelease(true);
+                m_cond.wakeOne();
+                break;
+            }
+        }
+    }
+
+    void stop()
+    {
+        // We have to both write to the pipe and set the flag, as the thread may be
+        // either in the poll() or waiting for _prepare_read().
+        if (m_pipefd[1] != -1 && write(m_pipefd[1], "\0", 1) == -1)
+            qWarning("Failed to write to the pipe: %s.", strerror(errno));
+
+        {
+            QMutexLocker l(&m_mutex);
+            m_quitting = true;
+            m_cond.wakeOne();
+        }
+
+        wait();
+    }
+
+Q_SIGNALS:
+    void needReadAndDispatch();
+
+protected:
+    void run() override
+    {
+        // we use this pipe to make the loop exit otherwise if we simply used a flag on the loop condition, if stop() gets
+        // called while poll() is blocking the thread will never quit since there are no wayland messages coming anymore.
+        struct Pipe
+        {
+            Pipe(int *fds)
+                : fds(fds)
+            {
+                if (qt_safe_pipe(fds) != 0)
+                    qWarning("Pipe creation failed. Quitting may hang.");
+            }
+            ~Pipe()
+            {
+                if (fds[0] != -1) {
+                    close(fds[0]);
+                    close(fds[1]);
+                }
+            }
+
+            int *fds;
+        } pipe(m_pipefd);
+
+        // Make the main thread call wl_prepare_read(), dispatch the pending messages and flush the
+        // outbound ones. Wait until it's done before proceeding, unless we're told to quit.
+        while (waitForReading()) {
+            pollfd fds[2] = { { m_fd, POLLIN, 0 }, { m_pipefd[0], POLLIN, 0 } };
+            poll(fds, 2, -1);
+
+            if (fds[1].revents & POLLIN) {
+                // we don't really care to read the byte that was written here since we're closing down
+                wl_display_cancel_read(m_wldisplay);
+                break;
+            }
+
+            if (fds[0].revents & POLLIN)
+                wl_display_read_events(m_wldisplay);
+                // The polll was succesfull and the event thread did the wl_display_read_events(). On the next iteration of the loop
+                // the event sent to the main thread will cause it to dispatch the messages just read, unless the loop exits in which
+                // case we don't care anymore about them.
+            else
+                wl_display_cancel_read(m_wldisplay);
+        }
+    }
+
+private:
+    bool waitForReading()
+    {
+        Q_ASSERT(QThread::currentThread() == this);
+
+        m_reading.storeRelease(false);
+
+        if (m_mode == SelfDispatch) {
+            readAndDispatchEvents();
+        } else {
+            Q_EMIT needReadAndDispatch();
+
+            QMutexLocker lock(&m_mutex);
+            // m_reading might be set from our emit or some other invocation of
+            // readAndDispatchEvents().
+            while (!m_reading.loadRelaxed() && !m_quitting)
+                m_cond.wait(&m_mutex);
+        }
+
+        return !m_quitting;
+    }
+
+    int dispatchQueuePending()
+    {
+        if (m_wlevqueue)
+            return wl_display_dispatch_queue_pending(m_wldisplay, m_wlevqueue);
+        else
+            return wl_display_dispatch_pending(m_wldisplay);
+    }
+
+    int prepareReadQueue()
+    {
+        if (m_wlevqueue)
+            return wl_display_prepare_read_queue(m_wldisplay, m_wlevqueue);
+        else
+            return wl_display_prepare_read(m_wldisplay);
+    }
+
+    int m_fd;
+    int m_pipefd[2];
+    wl_display *m_wldisplay;
+    wl_event_queue *m_wlevqueue;
+    OperatingMode m_mode;
+
+    /* Concurrency note when operating in EmitToDispatch mode:
+     * m_reading is set to false inside event thread's waitForReading(), and is
+     * set to true inside main thread's readAndDispatchEvents().
+     * The lock is not taken when setting m_reading to false, as the main thread
+     * is not actively waiting for it to turn false. However, the lock is taken
+     * inside readAndDispatchEvents() before setting m_reading to true,
+     * as the event thread is actively waiting for it under the wait condition.
+     */
+
+    QAtomicInteger<bool> m_reading;
+    bool m_quitting;
+    QMutex m_mutex;
+    QWaitCondition m_cond;
+};
+
 Q_LOGGING_CATEGORY(lcQpaWayland, "qt.qpa.wayland"); // for general (uncategorized) Wayland platform logging
 
 struct wl_surface *QWaylandDisplay::createSurface(void *handle)
@@ -167,6 +358,12 @@ QWaylandDisplay::QWaylandDisplay(QWaylandIntegration *waylandIntegration)
 
 QWaylandDisplay::~QWaylandDisplay(void)
 {
+    if (m_eventThread)
+        m_eventThread->stop();
+
+    if (m_frameEventQueueThread)
+        m_frameEventQueueThread->stop();
+
     if (mSyncCallback)
         wl_callback_destroy(mSyncCallback);
 
@@ -211,100 +408,34 @@ void QWaylandDisplay::ensureScreen()
     Q_ASSERT(!QGuiApplication::screens().empty());
 }
 
-void QWaylandDisplay::checkError() const
-{
-    int ecode = wl_display_get_error(mDisplay);
-    if ((ecode == EPIPE || ecode == ECONNRESET)) {
-        // special case this to provide a nicer error
-        qWarning("The Wayland connection broke. Did the Wayland compositor die?");
-    } else {
-        qWarning("The Wayland connection experienced a fatal error: %s", strerror(ecode));
-    }
-    _exit(1);
-}
-
+// Called in main thread, either from queued signal or directly.
 void QWaylandDisplay::flushRequests()
 {
-    if (wl_display_prepare_read(mDisplay) == 0) {
-        wl_display_read_events(mDisplay);
-    }
+    m_eventThread->readAndDispatchEvents();
+}
 
-    if (wl_display_dispatch_pending(mDisplay) < 0)
-        checkError();
+// We have to wait until we have an eventDispatcher before creating the eventThread,
+// otherwise forceRoundTrip() may block inside _events_read() because eventThread is
+// polling.
+void QWaylandDisplay::initEventThread()
+{
+    m_eventThread.reset(
+            new EventThread(mDisplay, /* default queue */ nullptr, EventThread::EmitToDispatch));
+    connect(m_eventThread.get(), &EventThread::needReadAndDispatch, this,
+            &QWaylandDisplay::flushRequests, Qt::QueuedConnection);
+    m_eventThread->start();
 
-    {
-        QReadLocker locker(&m_frameQueueLock);
-        for (const FrameQueue &q : mExternalQueues) {
-            QMutexLocker locker(q.mutex);
-            while (wl_display_prepare_read_queue(mDisplay, q.queue) != 0)
-                wl_display_dispatch_queue_pending(mDisplay, q.queue);
-            wl_display_read_events(mDisplay);
-            wl_display_dispatch_queue_pending(mDisplay, q.queue);
-        }
-    }
-
-    wl_display_flush(mDisplay);
+    // wl_display_disconnect() free this.
+    m_frameEventQueue = wl_display_create_queue(mDisplay);
+    m_frameEventQueueThread.reset(
+            new EventThread(mDisplay, m_frameEventQueue, EventThread::SelfDispatch));
+    m_frameEventQueueThread->start();
 }
 
 void QWaylandDisplay::blockingReadEvents()
 {
     if (wl_display_dispatch(mDisplay) < 0)
-        checkError();
-}
-
-void QWaylandDisplay::destroyFrameQueue(const QWaylandDisplay::FrameQueue &q)
-{
-    QWriteLocker locker(&m_frameQueueLock);
-    auto it = std::find_if(mExternalQueues.begin(),
-                           mExternalQueues.end(),
-                           [&q] (const QWaylandDisplay::FrameQueue &other){ return other.queue == q.queue; });
-    Q_ASSERT(it != mExternalQueues.end());
-    mExternalQueues.erase(it);
-    if (q.queue != nullptr)
-        wl_event_queue_destroy(q.queue);
-    delete q.mutex;
-}
-
-QWaylandDisplay::FrameQueue QWaylandDisplay::createFrameQueue()
-{
-    QWriteLocker locker(&m_frameQueueLock);
-    FrameQueue q{createEventQueue()};
-    mExternalQueues.append(q);
-    return q;
-}
-
-wl_event_queue *QWaylandDisplay::createEventQueue()
-{
-    return wl_display_create_queue(mDisplay);
-}
-
-void QWaylandDisplay::dispatchQueueWhile(wl_event_queue *queue, std::function<bool ()> condition, int timeout)
-{
-    if (!condition())
-        return;
-
-    QElapsedTimer timer;
-    timer.start();
-    struct pollfd pFd = qt_make_pollfd(wl_display_get_fd(mDisplay), POLLIN);
-    while (timeout == -1 || timer.elapsed() < timeout) {
-        while (wl_display_prepare_read_queue(mDisplay, queue) != 0)
-            wl_display_dispatch_queue_pending(mDisplay, queue);
-
-        wl_display_flush(mDisplay);
-
-        const int remaining = qMax(timeout - timer.elapsed(), 0ll);
-        const int pollTimeout = timeout == -1 ? -1 : remaining;
-        if (qt_poll_msecs(&pFd, 1, pollTimeout) > 0)
-            wl_display_read_events(mDisplay);
-        else
-            wl_display_cancel_read(mDisplay);
-
-        if (wl_display_dispatch_queue_pending(mDisplay, queue) < 0)
-            checkError();
-
-        if (!condition())
-            break;
-    }
+        checkWaylandError(mDisplay);
 }
 
 void QWaylandDisplay::checkTextInputProtocol()
@@ -748,4 +879,6 @@ QWaylandCursorTheme *QWaylandDisplay::loadCursorTheme(const QString &name, int p
 
 } // namespace QtWaylandClient
 
+#include "qwaylanddisplay.moc"
+
 QT_END_NAMESPACE

src/plugins/platforms/wayland/qwaylanddisplay_p.h

@@ -111,6 +111,7 @@ class QWaylandSurface;
 class QWaylandShellIntegration;
 class QWaylandCursor;
 class QWaylandCursorTheme;
+class EventThread;
 
 typedef void (*RegistryListener)(void *data,
                                  struct wl_registry *registry,
@@ -122,12 +123,6 @@ class Q_WAYLAND_CLIENT_EXPORT QWaylandDisplay : public QObject, public QtWayland
     Q_OBJECT
 
 public:
-    struct FrameQueue {
-        FrameQueue(wl_event_queue *q = nullptr) : queue(q), mutex(new QMutex) {}
-        wl_event_queue *queue;
-        QMutex *mutex;
-    };
-
     QWaylandDisplay(QWaylandIntegration *waylandIntegration);
     ~QWaylandDisplay(void) override;
 
@@ -215,20 +210,16 @@ public:
     void handleKeyboardFocusChanged(QWaylandInputDevice *inputDevice);
     void handleWindowDestroyed(QWaylandWindow *window);
 
-    wl_event_queue *createEventQueue();
-    FrameQueue createFrameQueue();
-    void destroyFrameQueue(const FrameQueue &q);
-    void dispatchQueueWhile(wl_event_queue *queue, std::function<bool()> condition, int timeout = -1);
+    wl_event_queue *frameEventQueue() { return m_frameEventQueue; };
 
     bool isKeyboardAvailable() const;
 
+    void initEventThread();
 public slots:
     void blockingReadEvents();
     void flushRequests();
 
 private:
-    void checkError() const;
-
     void handleWaylandSync();
     void requestWaylandSync();
 
@@ -246,6 +237,9 @@ private:
     };
 
     struct wl_display *mDisplay = nullptr;
+    QScopedPointer<EventThread> m_eventThread;
+    wl_event_queue *m_frameEventQueue = nullptr;
+    QScopedPointer<EventThread> m_frameEventQueueThread;
     QtWayland::wl_compositor mCompositor;
     QScopedPointer<QWaylandShm> mShm;
     QList<QWaylandScreen *> mWaitingScreens;
@@ -298,10 +292,8 @@ private:
     QPointer<QWaylandWindow> mLastInputWindow;
     QPointer<QWaylandWindow> mLastKeyboardFocus;
     QList<QWaylandWindow *> mActiveWindows;
-    QList<FrameQueue> mExternalQueues;
     struct wl_callback *mSyncCallback = nullptr;
     static const wl_callback_listener syncCallbackListener;
-    QReadWriteLock m_frameQueueLock;
 
     bool mClientSideInputContextRequested = !QPlatformInputContextFactory::requested().isNull();
 

src/plugins/platforms/wayland/qwaylandintegration.cpp

@@ -208,11 +208,9 @@ void QWaylandIntegration::initializePlatform()
 
 void QWaylandIntegration::initialize()
 {
-    int fd = wl_display_get_fd(mDisplay->wl_display());
-    QSocketNotifier *sn = new QSocketNotifier(fd, QSocketNotifier::Read, mDisplay.data());
-    QObject::connect(sn, SIGNAL(activated(QSocketDescriptor)), mDisplay.data(), SLOT(flushRequests()));
+    mDisplay->initEventThread();
 
-    // Call this after eventDispatcher is connected with QSocketNotifier for QWaylandDisplay::forceRoundTrip()
+    // Call this after initializing event thread for QWaylandDisplay::forceRoundTrip()
     initializePlatform();
 
     // But the aboutToBlock() and awake() should be connected after initializePlatform().

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -76,7 +76,6 @@ QWaylandWindow *QWaylandWindow::mMouseGrab = nullptr;
 QWaylandWindow::QWaylandWindow(QWindow *window, QWaylandDisplay *display)
     : QPlatformWindow(window)
     , mDisplay(display)
-    , mFrameQueue(mDisplay->createFrameQueue())
     , mResizeAfterSwap(qEnvironmentVariableIsSet("QT_WAYLAND_RESIZE_AFTER_SWAP"))
 {
     {
@@ -95,8 +94,6 @@ QWaylandWindow::QWaylandWindow(QWindow *window, QWaylandDisplay *display)
 
 QWaylandWindow::~QWaylandWindow()
 {
-    mDisplay->destroyFrameQueue(mFrameQueue);
-
     delete mWindowDecoration;
 
     if (mSurface)
@@ -673,6 +670,8 @@ const wl_callback_listener QWaylandWindow::callbackListener = {
 
 void QWaylandWindow::handleFrameCallback()
 {
+    QMutexLocker locker(&mFrameSyncMutex);
+
     mWaitingForFrameCallback = false;
     mFrameCallbackElapsedTimer.invalidate();
 
@@ -683,6 +682,8 @@ void QWaylandWindow::handleFrameCallback()
         mWaitingForUpdateDelivery = true;
         QMetaObject::invokeMethod(this, &QWaylandWindow::doHandleFrameCallback, Qt::QueuedConnection);
     }
+
+    mFrameSyncWait.notify_all();
 }
 
 void QWaylandWindow::doHandleFrameCallback()
@@ -699,8 +700,10 @@ void QWaylandWindow::doHandleFrameCallback()
 
 bool QWaylandWindow::waitForFrameSync(int timeout)
 {
-    QMutexLocker locker(mFrameQueue.mutex);
-    mDisplay->dispatchQueueWhile(mFrameQueue.queue, [&]() { return mWaitingForFrameCallback; }, timeout);
+    QMutexLocker locker(&mFrameSyncMutex);
+
+    QDeadlineTimer deadline(timeout);
+    while (mWaitingForFrameCallback && mFrameSyncWait.wait(&mFrameSyncMutex, deadline)) { }
 
     if (mWaitingForFrameCallback) {
         qCDebug(lcWaylandBackingstore) << "Didn't receive frame callback in time, window should now be inexposed";
@@ -1340,8 +1343,11 @@ void QWaylandWindow::requestUpdate()
     Q_ASSERT(hasPendingUpdateRequest()); // should be set by QPA
 
     // If we have a frame callback all is good and will be taken care of there
-    if (mWaitingForFrameCallback)
-        return;
+    {
+        QMutexLocker locker(&mFrameSyncMutex);
+        if (mWaitingForFrameCallback)
+            return;
+    }
 
     // If we've already called deliverUpdateRequest(), but haven't seen any attach+commit/swap yet
     // This is a somewhat redundant behavior and might indicate a bug in the calling code, so log
@@ -1354,7 +1360,12 @@ void QWaylandWindow::requestUpdate()
     // so use invokeMethod to delay the delivery a bit.
     QMetaObject::invokeMethod(this, [this] {
         // Things might have changed in the meantime
-        if (hasPendingUpdateRequest() && !mWaitingForFrameCallback)
+        {
+            QMutexLocker locker(&mFrameSyncMutex);
+            if (mWaitingForFrameCallback)
+                return;
+        }
+        if (hasPendingUpdateRequest())
             deliverUpdateRequest();
     }, Qt::QueuedConnection);
 }
@@ -1374,9 +1385,10 @@ void QWaylandWindow::handleUpdate()
     if (!mSurface)
         return;
 
-    QMutexLocker locker(mFrameQueue.mutex);
+    QMutexLocker locker(&mFrameSyncMutex);
+
     struct ::wl_surface *wrappedSurface = reinterpret_cast<struct ::wl_surface *>(wl_proxy_create_wrapper(mSurface->object()));
-    wl_proxy_set_queue(reinterpret_cast<wl_proxy *>(wrappedSurface), mFrameQueue.queue);
+    wl_proxy_set_queue(reinterpret_cast<wl_proxy *>(wrappedSurface), mDisplay->frameEventQueue());
     mFrameCallback = wl_surface_frame(wrappedSurface);
     wl_proxy_wrapper_destroy(wrappedSurface);
     wl_callback_add_listener(mFrameCallback, &QWaylandWindow::callbackListener, this);
@@ -1386,6 +1398,8 @@ void QWaylandWindow::handleUpdate()
     // Start a timer for handling the case when the compositor stops sending frame callbacks.
     if (mFrameCallbackTimeout > 0) {
         QMetaObject::invokeMethod(this, [this] {
+            QMutexLocker locker(&mFrameSyncMutex);
+
             if (mWaitingForFrameCallback) {
                 if (mFrameCallbackCheckIntervalTimerId < 0)
                     mFrameCallbackCheckIntervalTimerId = startTimer(mFrameCallbackTimeout);

src/plugins/platforms/wayland/qwaylandwindow_p.h

@@ -274,7 +274,7 @@ protected:
     int mFrameCallbackCheckIntervalTimerId = -1;
     QElapsedTimer mFrameCallbackElapsedTimer;
     struct ::wl_callback *mFrameCallback = nullptr;
-    QWaylandDisplay::FrameQueue mFrameQueue;
+    QMutex mFrameSyncMutex;
     QWaitCondition mFrameSyncWait;
 
     // True when we have called deliverRequestUpdate, but the client has not yet attached a new buffer

tests/auto/wayland/CMakeLists.txt

@@ -6,6 +6,7 @@ add_subdirectory(client)
 add_subdirectory(datadevicev1)
 add_subdirectory(fullscreenshellv1)
 add_subdirectory(iviapplication)
+add_subdirectory(multithreaded)
 add_subdirectory(nooutput)
 add_subdirectory(output)
 add_subdirectory(primaryselectionv1)

tests/auto/wayland/multithreaded/CMakeLists.txt

@@ -0,0 +1,12 @@
+# Generated from multithreaded.pro.
+
+#####################################################################
+## tst_multithreaded Test:
+#####################################################################
+
+qt_internal_add_test(tst_multithreaded
+    SOURCES
+        tst_multithreaded.cpp
+    PUBLIC_LIBRARIES
+        SharedClientTest
+)

tests/auto/wayland/multithreaded/tst_multithreaded.cpp

@@ -0,0 +1,165 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Copyright (C) 2020 UBports Foundataion.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <unistd.h>
+#include <poll.h>
+
+#include <wayland-client-core.h>
+
+#include <QtGui/QScreen>
+#include <QAbstractEventDispatcher>
+#include <qpa/qplatformnativeinterface.h>
+
+#include "mockcompositor.h"
+
+using namespace MockCompositor;
+
+/*
+ * This class simulate a thread from another library which use poll() to wait
+ * for data from Wayland compositor.
+ */
+
+class ExternalWaylandReaderThread : public QThread
+{
+public:
+    ExternalWaylandReaderThread(struct wl_display *disp)
+        : QThread()
+        , m_disp(disp)
+    {
+        setObjectName(QStringLiteral("ExternalWaylandReader"));
+    }
+
+    ~ExternalWaylandReaderThread()
+    {
+        if (m_pipefd[1] != -1 && write(m_pipefd[1], "q", 1) == -1)
+            qWarning("Failed to write to the pipe: %s.", strerror(errno));
+
+        wait();
+    }
+
+protected:
+    void run() override
+    {
+        // we use this pipe to make the loop exit otherwise if we simply used a flag on the loop condition, if stop() gets
+        // called while poll() is blocking the thread will never quit since there are no wayland messages coming anymore.
+        struct Pipe
+        {
+            Pipe(int *fds)
+                : fds(fds)
+            {
+                if (::pipe(fds) != 0)
+                    qWarning("Pipe creation failed. Quitting may hang.");
+            }
+            ~Pipe()
+            {
+                if (fds[0] != -1) {
+                    close(fds[0]);
+                    close(fds[1]);
+                }
+            }
+
+            int *fds;
+        } pipe(m_pipefd);
+
+        struct wl_event_queue *a_queue = wl_display_create_queue(m_disp);
+        struct pollfd fds[2] = { { wl_display_get_fd(m_disp), POLLIN, 0 },
+                                 { m_pipefd[0], POLLIN, 0 } };
+
+        while (true) {
+            // No wl_proxy is assigned to this queue, thus guaranteed to be always empty.
+            Q_ASSERT(wl_display_prepare_read_queue(m_disp, a_queue) == 0);
+            wl_display_flush(m_disp);
+
+            // Wakeup every 10 seconds so that if Qt blocks in _read_events(),
+            // it won't last forever.
+            poll(fds, /* nfds */ 2, 10000);
+
+            if (fds[0].revents & POLLIN) {
+                wl_display_read_events(m_disp);
+            } else {
+                wl_display_cancel_read(m_disp);
+            }
+
+            if (fds[1].revents & POLLIN) {
+                char pipeIn;
+                read(m_pipefd[0], &pipeIn, 1);
+                if (pipeIn == 'q')
+                    break;
+            }
+        }
+
+        wl_event_queue_destroy(a_queue);
+    }
+
+private:
+    struct wl_display *m_disp;
+    int m_pipefd[2] = { -1, -1 };
+};
+
+class tst_multithreaded : public QObject, private DefaultCompositor
+{
+    Q_OBJECT
+private slots:
+    void initTestCase()
+    {
+        m_config.autoConfigure = true;
+        m_config.autoEnter = false;
+    }
+    void init()
+    {
+        // a test case is given new simulated thread.
+        QPlatformNativeInterface *native = QGuiApplication::platformNativeInterface();
+        struct wl_display *wl_dpy =
+                (struct wl_display *)native->nativeResourceForWindow("display", NULL);
+
+        m_extThread.reset(new ExternalWaylandReaderThread(wl_dpy));
+        m_extThread->start();
+    }
+    void cleanup()
+    {
+        QTRY_VERIFY2(isClean(), qPrintable(dirtyMessage()));
+    }
+
+    void mainThreadIsNotBlocked();
+
+public:
+    QScopedPointer<ExternalWaylandReaderThread> m_extThread;
+};
+
+void tst_multithreaded::mainThreadIsNotBlocked()
+{
+    QElapsedTimer timer;
+    timer.start();
+
+    QTest::qWait(100);
+    QVERIFY(timer.elapsed() < 200);
+}
+
+QCOMPOSITOR_TEST_MAIN(tst_multithreaded)
+#include "tst_multithreaded.moc"