Add native interface for X11 application, exposing display and connection

The major use-case of the now private QX11Info from Qt X11 Extras was
getting hold of the Xlib display and XCB connection, for example in KDE:

  https://lxr.kde.org/search?%21v=kf5-qt5&_filestring=&_string=QX11Info

A new native interface for QGuiApplication has now been added that
exposes these two properties, e.g.:

  if (auto *x11App = app.nativeInterface<QX11Application>())
      qDebug() << x11App->display() << x11App->connection();

To avoid type clashes one of the enum values of QXcbNativeInterface's
ResourceType had to be renamed.

Pick-to: 6.2
Task-number: QTBUG-93633
Change-Id: I2e366a2bb88bd3965ac6172ad000ae32209f43e7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/gui/CMakeLists.txt

@@ -87,7 +87,7 @@ qt_internal_add_module(Gui
         kernel/qevent.cpp kernel/qevent.h kernel/qevent_p.h
         kernel/qgenericplugin.cpp kernel/qgenericplugin.h
         kernel/qgenericpluginfactory.cpp kernel/qgenericpluginfactory.h
-        kernel/qguiapplication.cpp kernel/qguiapplication.h kernel/qguiapplication_p.h
+        kernel/qguiapplication.cpp kernel/qguiapplication.h kernel/qguiapplication_p.h kernel/qguiapplication_platform.h
         kernel/qguivariant.cpp
         kernel/qhighdpiscaling.cpp kernel/qhighdpiscaling_p.h
         kernel/qinputdevice.cpp kernel/qinputdevice.h kernel/qinputdevice_p.h

src/gui/configure.cmake

@@ -870,7 +870,7 @@ qt_feature("tuiotouch" PRIVATE
     PURPOSE "Provides the TuioTouch input plugin."
     CONDITION QT_FEATURE_network AND QT_FEATURE_udpsocket
 )
-qt_feature("xcb" PRIVATE
+qt_feature("xcb" PUBLIC
     SECTION "Platform plugins"
     LABEL "XCB"
     AUTODETECT NOT APPLE

src/gui/kernel/qguiapplication.cpp

@@ -4208,8 +4208,22 @@ QInputDeviceManager *QGuiApplicationPrivate::inputDeviceManager()
     return m_inputDeviceManager;
 }
 
+/*!
+    \fn template <typename QNativeInterface> QNativeInterface *QGuiApplication::nativeInterface() const
+
+    Returns a native interface of the given type for the application.
+
+    This function provides access to platform specific functionality
+    of QGuiApplication, as defined in the QNativeInterface namespace:
+
+    \annotatedlist native-interfaces-qguiapplication
+
+    If the requested interface is not available a \nullptr is returned.
+ */
+
 void *QGuiApplication::resolveInterface(const char *name, int revision) const
 {
+    using namespace QNativeInterface;
     using namespace QNativeInterface::Private;
 
     auto *platformIntegration = QGuiApplicationPrivate::platformIntegration();
@@ -4218,6 +4232,9 @@ void *QGuiApplication::resolveInterface(const char *name, int revision) const
 #if defined(Q_OS_WIN)
     QT_NATIVE_INTERFACE_RETURN_IF(QWindowsApplication, platformIntegration);
 #endif
+#if QT_CONFIG(xcb)
+    QT_NATIVE_INTERFACE_RETURN_IF(QX11Application, platformNativeInterface());
+#endif
 
     return QCoreApplication::resolveInterface(name, revision);
 }

src/gui/kernel/qguiapplication.h

@@ -219,4 +219,6 @@ private:
 
 QT_END_NAMESPACE
 
+#include <QtGui/qguiapplication_platform.h>
+
 #endif // QGUIAPPLICATION_H

src/gui/kernel/qguiapplication_platform.h

@@ -0,0 +1,71 @@
+/****************************************************************************
+**
+** Copyright (C) 2021 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 QGUIAPPLICATION_PLATFORM_H
+#define QGUIAPPLICATION_PLATFORM_H
+
+#include <QtGui/qtguiglobal.h>
+
+#include <QtCore/qnativeinterface.h>
+#include <QtGui/qguiapplication.h>
+
+#if QT_CONFIG(xcb)
+typedef struct _XDisplay Display;
+struct xcb_connection_t;
+#endif
+
+QT_BEGIN_NAMESPACE
+
+namespace QNativeInterface
+{
+
+#if QT_CONFIG(xcb) || defined(Q_CLANG_QDOC)
+struct Q_GUI_EXPORT QX11Application
+{
+    QT_DECLARE_NATIVE_INTERFACE(QX11Application, 1, QGuiApplication)
+    virtual Display *display() const = 0;
+    virtual xcb_connection_t *connection() const = 0;
+};
+#endif
+
+} // QNativeInterface
+
+QT_END_NAMESPACE
+
+#endif // QGUIAPPLICATION_PLATFORM_H

src/gui/platform/unix/qunixnativeinterface.cpp

@@ -179,6 +179,36 @@ QT_DEFINE_PRIVATE_NATIVE_INTERFACE(QXcbScreen);
 
 QT_DEFINE_PRIVATE_NATIVE_INTERFACE(QXcbWindow);
 
+/*!
+    \class QNativeInterface::QX11Application
+    \since 6.0
+    \brief Native interface to an X11 application.
+
+    Accessed through QGuiApplication::nativeInterface().
+
+    \inmodule QtGui
+    \ingroup native-interfaces
+    \ingroup native-interfaces-qguiapplication
+*/
+
+/*!
+    \fn Display *QNativeInterface::QX11Application::display() const
+
+    \return the X display of the application, for use with Xlib.
+
+    \sa connection()
+*/
+
+/*!
+    \fn xcb_connection_t *QNativeInterface::QX11Application::connection() const
+
+    \return the X connection of the application, for use with XCB.
+
+    \sa display()
+*/
+
+QT_DEFINE_NATIVE_INTERFACE(QX11Application);
+
 #endif // QT_CONFIG(xcb)
 
 #if QT_CONFIG(vsp2)

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -118,7 +118,7 @@ void *QXcbNativeInterface::nativeResourceForIntegration(const QByteArray &resour
     case RootWindow:
         result = rootWindow();
         break;
-    case Display:
+    case XDisplay:
         result = display();
         break;
     case AtspiBus:
@@ -155,7 +155,7 @@ void *QXcbNativeInterface::nativeResourceForScreen(const QByteArray &resourceStr
 
     const QXcbScreen *xcbScreen = static_cast<QXcbScreen *>(screen->handle());
     switch (resourceType(lowerCaseResource)) {
-    case Display:
+    case XDisplay:
 #if QT_CONFIG(xcb_xlib)
         result = xcbScreen->connection()->xlib_display();
 #endif
@@ -203,7 +203,7 @@ void *QXcbNativeInterface::nativeResourceForWindow(const QByteArray &resourceStr
         return result;
 
     switch (resourceType(lowerCaseResource)) {
-    case Display:
+    case XDisplay:
         result = displayForWindow(window);
         break;
     case Connection:
@@ -365,18 +365,17 @@ void *QXcbNativeInterface::rootWindow()
     return nullptr;
 }
 
-void *QXcbNativeInterface::display()
+Display *QXcbNativeInterface::display() const
 {
 #if QT_CONFIG(xcb_xlib)
     QXcbIntegration *integration = QXcbIntegration::instance();
-    QXcbConnection *connection = integration->connection();
-    if (connection)
-        return connection->xlib_display();
+    if (QXcbConnection *connection = integration->connection())
+        return reinterpret_cast<Display *>(connection->xlib_display());
 #endif
     return nullptr;
 }
 
-void *QXcbNativeInterface::connection()
+xcb_connection_t *QXcbNativeInterface::connection() const
 {
     QXcbIntegration *integration = QXcbIntegration::instance();
     return integration->connection()->xcb_connection();

src/plugins/platforms/xcb/qxcbnativeinterface.h

@@ -45,6 +45,8 @@
 
 #include <QtCore/QRect>
 
+#include <QtGui/qguiapplication.h>
+
 #include "qxcbexport.h"
 #include "qxcbconnection.h"
 
@@ -54,11 +56,12 @@ class QXcbScreen;
 class QXcbNativeInterfaceHandler;
 
 class Q_XCB_EXPORT QXcbNativeInterface : public QPlatformNativeInterface
+    , public QNativeInterface::QX11Application
 {
     Q_OBJECT
 public:
     enum ResourceType {
-        Display,
+        XDisplay,
         Connection,
         Screen,
         AppTime,
@@ -109,9 +112,11 @@ public:
     void *startupId();
     void *x11Screen();
     void *rootWindow();
-    void *display();
+
+    Display *display() const override;
+    xcb_connection_t *connection() const override;
+
     void *atspiBus();
-    void *connection();
     static void setStartupId(const char *);
     static void setAppTime(QScreen *screen, xcb_timestamp_t time);
     static void setAppUserTime(QScreen *screen, xcb_timestamp_t time);