Offscreen QPA: implement a native interface

Many code paths simply expect to have a native interface
available, and won't check if a plugin is returning nullptr
for it. This leads to crashes or local workarounds
(e.g. 3197932e6fb03fb9c0ff669af858cb94e3836d79). Instead,
have offscreen implement a dummy native interface.

This requires shuffling some code for the X11 integration.

Pick-to: 5.15
Change-Id: I2bdceee379e4ded9b085ebbb4d03d1e074f60726
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/offscreen/qoffscreencommon.cpp

@@ -232,4 +232,6 @@ void QOffscreenBackingStore::clearHash()
 
 QHash<WId, QOffscreenBackingStore *> QOffscreenBackingStore::m_backingStoreForWinIdHash;
 
+QOffscreenPlatformNativeInterface::~QOffscreenPlatformNativeInterface() = default;
+
 QT_END_NAMESPACE

src/plugins/platforms/offscreen/qoffscreencommon.h

@@ -45,6 +45,7 @@
 #include <qpa/qplatformdrag.h>
 #endif
 #include <qpa/qplatformintegration.h>
+#include <qpa/qplatformnativeinterface.h>
 #include <qpa/qplatformscreen.h>
 #include <qpa/qplatformwindow.h>
 
@@ -116,6 +117,12 @@ private:
     static QHash<WId, QOffscreenBackingStore *> m_backingStoreForWinIdHash;
 };
 
+class QOffscreenPlatformNativeInterface : public QPlatformNativeInterface
+{
+public:
+    ~QOffscreenPlatformNativeInterface();
+};
+
 QT_END_NAMESPACE
 
 #endif

src/plugins/platforms/offscreen/qoffscreenintegration.cpp

@@ -244,6 +244,13 @@ QAbstractEventDispatcher *QOffscreenIntegration::createEventDispatcher() const
 #endif
 }
 
+QPlatformNativeInterface *QOffscreenIntegration::nativeInterface() const
+{
+    if (!m_nativeInterface)
+        m_nativeInterface.reset(new QOffscreenPlatformNativeInterface);
+    return m_nativeInterface.get();
+}
+
 static QString themeName() { return QStringLiteral("offscreen"); }
 
 QStringList QOffscreenIntegration::themeNames() const

src/plugins/platforms/offscreen/qoffscreenintegration.h

@@ -71,19 +71,22 @@ public:
     QPlatformFontDatabase *fontDatabase() const override;
     QAbstractEventDispatcher *createEventDispatcher() const override;
 
+    QPlatformNativeInterface *nativeInterface() const override;
+
     QStringList themeNames() const override;
     QPlatformTheme *createPlatformTheme(const QString &name) const override;
 
     static QOffscreenIntegration *createOffscreenIntegration(const QStringList& paramList);
 
     QList<QPlatformScreen *> screens() const;
-private:
+protected:
     QScopedPointer<QPlatformFontDatabase> m_fontDatabase;
 #if QT_CONFIG(draganddrop)
     QScopedPointer<QPlatformDrag> m_drag;
 #endif
     QScopedPointer<QPlatformInputContext> m_inputContext;
     QScopedPointer<QPlatformServices> m_services;
+    mutable QScopedPointer<QPlatformNativeInterface> m_nativeInterface;
     QList<QPlatformScreen *> m_screens;
     bool m_windowFrameMarginsEnabled = true;
 };

src/plugins/platforms/offscreen/qoffscreenintegration_x11.cpp

@@ -76,6 +76,8 @@ private:
     QOffscreenX11Connection *m_connection;
 };
 
+QOffscreenX11Integration::~QOffscreenX11Integration() = default;
+
 bool QOffscreenX11Integration::hasCapability(QPlatformIntegration::Capability cap) const
 {
     switch (cap) {
@@ -88,21 +90,27 @@ bool QOffscreenX11Integration::hasCapability(QPlatformIntegration::Capability ca
 
 QPlatformOpenGLContext *QOffscreenX11Integration::createPlatformOpenGLContext(QOpenGLContext *context) const
 {
-    if (!m_connection)
+    auto &connection = nativeInterface()->m_connection;
-        m_connection.reset(new QOffscreenX11Connection);
 
-    if (!m_connection->display())
+    if (!connection)
+        connection.reset(new QOffscreenX11Connection);
+
+    if (!connection->display())
         return nullptr;
 
-    return new QOffscreenX11GLXContext(m_connection->x11Info(), context);
+    return new QOffscreenX11GLXContext(connection->x11Info(), context);
 }
 
-QPlatformNativeInterface *QOffscreenX11Integration::nativeInterface() const
+QOffscreenX11PlatformNativeInterface *QOffscreenX11Integration::nativeInterface() const
 {
-   return const_cast<QOffscreenX11Integration *>(this);
+   if (!m_nativeInterface)
+       m_nativeInterface.reset(new QOffscreenX11PlatformNativeInterface);
+   return static_cast<QOffscreenX11PlatformNativeInterface *>(m_nativeInterface.data());
 }
 
-void *QOffscreenX11Integration::nativeResourceForScreen(const QByteArray &resource, QScreen *screen)
+QOffscreenX11PlatformNativeInterface::~QOffscreenX11PlatformNativeInterface() = default;
+
+void *QOffscreenX11PlatformNativeInterface::nativeResourceForScreen(const QByteArray &resource, QScreen *screen)
 {
     Q_UNUSED(screen);
     if (resource.toLower() == QByteArrayLiteral("display") ) {
@@ -114,7 +122,7 @@ void *QOffscreenX11Integration::nativeResourceForScreen(const QByteArray &resour
 }
 
 #ifndef QT_NO_OPENGL
-void *QOffscreenX11Integration::nativeResourceForContext(const QByteArray &resource, QOpenGLContext *context) {
+void *QOffscreenX11PlatformNativeInterface::nativeResourceForContext(const QByteArray &resource, QOpenGLContext *context) {
     if (resource.toLower() == QByteArrayLiteral("glxconfig") ) {
         if (context) {
             QOffscreenX11GLXContext *glxPlatformContext = static_cast<QOffscreenX11GLXContext *>(context->handle());

src/plugins/platforms/offscreen/qoffscreenintegration_x11.h

@@ -41,6 +41,7 @@
 #define QOFFSCREENINTEGRATION_X11_H
 
 #include "qoffscreenintegration.h"
+#include "qoffscreencommon.h"
 
 #include <qglobal.h>
 #include <qscopedpointer.h>
@@ -52,22 +53,27 @@ QT_BEGIN_NAMESPACE
 class QOffscreenX11Connection;
 class QOffscreenX11Info;
 
-class QOffscreenX11Integration : public QOffscreenIntegration,  public QPlatformNativeInterface
+class QOffscreenX11PlatformNativeInterface : public QOffscreenPlatformNativeInterface
 {
 public:
-    bool hasCapability(QPlatformIntegration::Capability cap) const override;
+    ~QOffscreenX11PlatformNativeInterface();
 
-    QPlatformOpenGLContext *createPlatformOpenGLContext(QOpenGLContext *context) const override;
-    QPlatformNativeInterface *nativeInterface()const override;
-
-    // QPlatformNativeInterface
     void *nativeResourceForScreen(const QByteArray &resource, QScreen *screen) override;
 #ifndef QT_NO_OPENGL
     void *nativeResourceForContext(const QByteArray &resource, QOpenGLContext *context) override;
 #endif
 
-private:
+    QScopedPointer<QOffscreenX11Connection> m_connection;
-    mutable QScopedPointer<QOffscreenX11Connection> m_connection;
+};
+
+class QOffscreenX11Integration : public QOffscreenIntegration
+{
+public:
+    ~QOffscreenX11Integration();
+    bool hasCapability(QPlatformIntegration::Capability cap) const override;
+
+    QPlatformOpenGLContext *createPlatformOpenGLContext(QOpenGLContext *context) const override;
+    QOffscreenX11PlatformNativeInterface *nativeInterface() const override;
 };
 
 class QOffscreenX11Connection {