xcb: call processXcbEvents() on every event loop iteration

It is necessary for QTBUG-69687. The original code processes the xcb
event queue only when new events have arrived, but if we want to do an
event filtering that buffers some events and processes them later based
on set/unset flags (e.g. QEventLoop::ExcludeUserInputEvents), we need
to call processXcbEvents() on every event loop iteration, not only when
new events have arrived from X server.

The required functionality is implemented by having custom event dispatchers,
instead of using the generic ones from QtGenericUnixDispatcher::
createUnixEventDispatcher() / eventdispatcher_support-private. This also
enables for further customizations, as might be necessary by QTBUG-70095.

Task-number: QTBUG-69687
Change-Id: I1f8b2400d26cccf17279d57bb4b678e40c615f33
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbconnection.h

@@ -495,12 +495,11 @@ public:
     QXcbGlIntegration *glIntegration() const;
 
     void flush() { xcb_flush(m_connection); }
+    void processXcbEvents();
 
 protected:
     bool event(QEvent *e) override;
 
-    void processXcbEvents();
-
 private:
     void initializeAllAtoms();
     void initializeShm();

src/plugins/platforms/xcb/qxcbeventdispatcher.cpp

@@ -0,0 +1,161 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** 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 Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** 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-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#include "qxcbeventdispatcher.h"
+#include "qxcbconnection.h"
+
+#include <QtCore/QCoreApplication>
+
+#include <qpa/qwindowsysteminterface.h>
+
+QT_BEGIN_NAMESPACE
+
+QXcbUnixEventDispatcher::QXcbUnixEventDispatcher(QXcbConnection *connection, QObject *parent)
+    : QEventDispatcherUNIX(parent)
+    , m_connection(connection)
+{
+}
+
+QXcbUnixEventDispatcher::~QXcbUnixEventDispatcher()
+{
+}
+
+bool QXcbUnixEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)
+{
+    const bool didSendEvents = QEventDispatcherUNIX::processEvents(flags);
+    m_connection->processXcbEvents();
+    // The following line should not be necessary after QTBUG-70095
+    return QWindowSystemInterface::sendWindowSystemEvents(flags) || didSendEvents;
+}
+
+bool QXcbUnixEventDispatcher::hasPendingEvents()
+{
+    extern uint qGlobalPostedEventsCount();
+    return qGlobalPostedEventsCount() || QWindowSystemInterface::windowSystemEventsQueued();
+}
+
+void QXcbUnixEventDispatcher::flush()
+{
+    if (qApp)
+        qApp->sendPostedEvents();
+}
+
+#if QT_CONFIG(glib)
+struct XcbEventSource
+{
+    GSource source;
+    QXcbGlibEventDispatcher *dispatcher;
+    QXcbGlibEventDispatcherPrivate *dispatcher_p;
+    QXcbConnection *connection = nullptr;
+};
+
+static gboolean xcbSourcePrepare(GSource *source, gint *timeout)
+{
+    Q_UNUSED(timeout)
+    auto xcbEventSource = reinterpret_cast<XcbEventSource *>(source);
+    return xcbEventSource->dispatcher_p->wakeUpCalled;
+}
+
+static gboolean xcbSourceCheck(GSource *source)
+{
+    return xcbSourcePrepare(source, nullptr);
+}
+
+static gboolean xcbSourceDispatch(GSource *source, GSourceFunc, gpointer)
+{
+    auto xcbEventSource = reinterpret_cast<XcbEventSource *>(source);
+    xcbEventSource->connection->processXcbEvents();
+    // The following line should not be necessary after QTBUG-70095
+    QWindowSystemInterface::sendWindowSystemEvents(xcbEventSource->dispatcher->flags());
+    return true;
+}
+
+QXcbGlibEventDispatcher::QXcbGlibEventDispatcher(QXcbConnection *connection, QObject *parent)
+    : QEventDispatcherGlib(*new QXcbGlibEventDispatcherPrivate(), parent)
+{
+    Q_D(QXcbGlibEventDispatcher);
+
+    m_xcbEventSourceFuncs.prepare = xcbSourcePrepare;
+    m_xcbEventSourceFuncs.check = xcbSourceCheck;
+    m_xcbEventSourceFuncs.dispatch = xcbSourceDispatch;
+    m_xcbEventSourceFuncs.finalize = nullptr;
+
+    m_xcbEventSource = reinterpret_cast<XcbEventSource *>(
+                g_source_new(&m_xcbEventSourceFuncs, sizeof(XcbEventSource)));
+
+    m_xcbEventSource->dispatcher = this;
+    m_xcbEventSource->dispatcher_p = d_func();
+    m_xcbEventSource->connection = connection;
+
+    g_source_set_can_recurse(&m_xcbEventSource->source, true);
+    g_source_attach(&m_xcbEventSource->source, d->mainContext);
+}
+
+QXcbGlibEventDispatcherPrivate::QXcbGlibEventDispatcherPrivate()
+{
+}
+
+QXcbGlibEventDispatcher::~QXcbGlibEventDispatcher()
+{
+    g_source_destroy(&m_xcbEventSource->source);
+    g_source_unref(&m_xcbEventSource->source);
+}
+
+bool QXcbGlibEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)
+{
+    m_flags = flags;
+    return QEventDispatcherGlib::processEvents(m_flags);
+}
+
+#endif // QT_CONFIG(glib)
+
+QAbstractEventDispatcher *QXcbEventDispatcher::createEventDispatcher(QXcbConnection *connection)
+{
+#if QT_CONFIG(glib)
+    if (qEnvironmentVariableIsEmpty("QT_NO_GLIB") && QEventDispatcherGlib::versionSupported()) {
+        qCDebug(lcQpaXcb, "using glib dispatcher");
+        return new QXcbGlibEventDispatcher(connection);
+    } else
+#endif
+    {
+        qCDebug(lcQpaXcb, "using unix dispatcher");
+        return new QXcbUnixEventDispatcher(connection);
+    }
+}
+
+QT_END_NAMESPACE

src/plugins/platforms/xcb/qxcbeventdispatcher.h

@@ -0,0 +1,117 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** 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 Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** 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-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#ifndef QXCBEVENTDISPATCHER_H
+#define QXCBEVENTDISPATCHER_H
+
+#include <QtCore/QObject>
+#include <QtCore/QEventLoop>
+
+#include <QtCore/private/qeventdispatcher_unix_p.h>
+#if QT_CONFIG(glib)
+#include <QtCore/private/qeventdispatcher_glib_p.h>
+#include <glib.h>
+#endif
+
+QT_BEGIN_NAMESPACE
+
+class QXcbConnection;
+
+class QXcbUnixEventDispatcher : public QEventDispatcherUNIX
+{
+    Q_OBJECT
+public:
+    explicit QXcbUnixEventDispatcher(QXcbConnection *connection, QObject *parent = nullptr);
+    ~QXcbUnixEventDispatcher();
+    bool processEvents(QEventLoop::ProcessEventsFlags flags) override;
+
+    // Maybe some user code depends on deprecated QUnixEventDispatcherQPA::
+    // hasPendingEvents() / flush() implementation, so keep it around until
+    // Qt 6. These methods are deprecated in QAbstractEventDispatcher.
+    bool hasPendingEvents() override; // ### Qt 6 remove
+    void flush() override;            // ### Qt 6 remove
+
+private:
+    QXcbConnection *m_connection;
+};
+
+#if QT_CONFIG(glib)
+
+struct XcbEventSource;
+class QXcbGlibEventDispatcherPrivate;
+
+class QXcbGlibEventDispatcher : public QEventDispatcherGlib
+{
+    Q_OBJECT
+    Q_DECLARE_PRIVATE(QXcbGlibEventDispatcher)
+
+public:
+    explicit QXcbGlibEventDispatcher(QXcbConnection *connection, QObject *parent = nullptr);
+    ~QXcbGlibEventDispatcher();
+
+    bool processEvents(QEventLoop::ProcessEventsFlags flags) override;
+    QEventLoop::ProcessEventsFlags flags() const { return m_flags; }
+
+private:
+    XcbEventSource *m_xcbEventSource;
+    GSourceFuncs m_xcbEventSourceFuncs;
+    QEventLoop::ProcessEventsFlags m_flags;
+};
+
+class QXcbGlibEventDispatcherPrivate : public QEventDispatcherGlibPrivate
+{
+    Q_DECLARE_PUBLIC(QXcbGlibEventDispatcher)
+
+public:
+    QXcbGlibEventDispatcherPrivate();
+};
+
+#endif // QT_CONFIG(glib)
+
+class QXcbEventDispatcher
+{
+public:
+    static QAbstractEventDispatcher *createEventDispatcher(QXcbConnection *connection);
+
+private:
+    QXcbConnection *m_connection;
+};
+
+QT_END_NAMESPACE
+
+#endif // QXCBEVENTDISPATCHER_H

src/plugins/platforms/xcb/qxcbeventqueue.cpp

@@ -76,7 +76,6 @@ QT_BEGIN_NAMESPACE
 QXcbEventQueue::QXcbEventQueue(QXcbConnection *connection)
     : m_connection(connection)
 {
-    connect(this, &QXcbEventQueue::eventsPending, m_connection, &QXcbConnection::processXcbEvents, Qt::QueuedConnection);
     connect(this, &QXcbEventQueue::finished, m_connection, &QXcbConnection::processXcbEvents);
 
     // Lets init the list with one node, so we don't have to check for
@@ -105,8 +104,7 @@ QXcbEventQueue::~QXcbEventQueue()
 
 void QXcbEventQueue::registerEventDispatcher(QAbstractEventDispatcher *dispatcher)
 {
-    // Flush the xcb connection before the event dispatcher is going to block.
-    connect(dispatcher, &QAbstractEventDispatcher::aboutToBlock, m_connection, &QXcbConnection::flush);
+    connect(this, &QXcbEventQueue::eventsPending, dispatcher, &QAbstractEventDispatcher::wakeUp, Qt::QueuedConnection);
 }
 
 xcb_generic_event_t *QXcbEventQueue::takeFirst()

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -47,6 +47,7 @@
 #include "qxcbnativeinterface.h"
 #include "qxcbclipboard.h"
 #include "qxcbeventqueue.h"
+#include "qxcbeventdispatcher.h"
 #if QT_CONFIG(draganddrop)
 #include "qxcbdrag.h"
 #endif
@@ -58,7 +59,6 @@
 
 #include <xcb/xcb.h>
 
-#include <QtEventDispatcherSupport/private/qgenericunixeventdispatcher_p.h>
 #include <QtFontDatabaseSupport/private/qgenericunixfontdatabase_p.h>
 #include <QtServiceSupport/private/qgenericunixservices_p.h>
 
@@ -343,7 +343,7 @@ bool QXcbIntegration::hasCapability(QPlatformIntegration::Capability cap) const
 
 QAbstractEventDispatcher *QXcbIntegration::createEventDispatcher() const
 {
-    QAbstractEventDispatcher *dispatcher = createUnixEventDispatcher();
+    QAbstractEventDispatcher *dispatcher = QXcbEventDispatcher::createEventDispatcher(defaultConnection());
     for (int i = 0; i < m_connections.size(); i++)
         m_connections[i]->eventQueue()->registerEventDispatcher(dispatcher);
     return dispatcher;

src/plugins/platforms/xcb/qxcbintegration.h

@@ -53,7 +53,6 @@ QT_BEGIN_NAMESPACE
 class QXcbConnection;
 class QAbstractEventDispatcher;
 class QXcbNativeInterface;
-class QXcbScreen;
 
 class Q_XCB_EXPORT QXcbIntegration : public QPlatformIntegration
 {

src/plugins/platforms/xcb/xcb_qpa_lib.pro

@@ -5,7 +5,7 @@ DEFINES += QT_NO_FOREACH
 QT += \
     core-private gui-private \
     service_support-private theme_support-private \
-    eventdispatcher_support-private fontdatabase_support-private \
+    fontdatabase_support-private \
     edid_support-private
 
 qtHaveModule(linuxaccessibility_support-private): \
@@ -13,6 +13,8 @@ qtHaveModule(linuxaccessibility_support-private): \
 
 qtConfig(vulkan): QT += vulkan_support-private
 
+qtConfig(glib) : QMAKE_USE_PRIVATE += glib
+
 SOURCES = \
         qxcbclipboard.cpp \
         qxcbconnection.cpp \
@@ -28,7 +30,8 @@ SOURCES = \
         qxcbimage.cpp \
         qxcbxsettings.cpp \
         qxcbsystemtraytracker.cpp \
-        qxcbeventqueue.cpp
+        qxcbeventqueue.cpp \
+        qxcbeventdispatcher.cpp
 
 HEADERS = \
         qxcbclipboard.h \
@@ -47,7 +50,8 @@ HEADERS = \
         qxcbxsettings.h \
         qxcbsystemtraytracker.h \
         qxcbxkbcommon.h \
-        qxcbeventqueue.h
+        qxcbeventqueue.h \
+        qxcbeventdispatcher.h
 
 qtConfig(draganddrop) {
     SOURCES += qxcbdrag.cpp