Client: Support running with no screens

[ChangeLog][QPA plugin] The QPA plugin now supports running with no screens
attached. This is handled by adding a fake screen when the last screen is
disconnected, similarly to what the other QPA plugins do.

Fixes: QTBUG-79111
Change-Id: I4a0e023ae784217dd030f0c62f12487fdff4825c
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/plugins/platforms/wayland/plugins/shellintegration/fullscreen-shell-v1/qwaylandfullscreenshellv1surface.cpp

@@ -50,10 +50,11 @@ QWaylandFullScreenShellV1Surface::QWaylandFullScreenShellV1Surface(QtWayland::zw
     , m_shell(shell)
     , m_window(window)
 {
-    auto screen = static_cast<QWaylandScreen *>(m_window->screen());
+    auto *screen = m_window->waylandScreen();
+    auto *output = screen ? screen->output() : nullptr;
     m_shell->present_surface(m_window->wlSurface(),
                              QtWayland::zwp_fullscreen_shell_v1::present_method_default,
-                             screen->output());
+                             output);
 }
 
 } // namespace QtWaylandClient

src/plugins/platforms/wayland/qwaylanddisplay.cpp

@@ -188,6 +188,18 @@ QWaylandDisplay::~QWaylandDisplay(void)
         wl_display_disconnect(mDisplay);
 }
 
+void QWaylandDisplay::ensureScreen()
+{
+    if (!mScreens.empty() || mPlaceholderScreen)
+        return; // There are real screens or we already have a fake one
+
+    qCInfo(lcQpaWayland) << "Creating a fake screen in order for Qt not to crash";
+
+    mPlaceholderScreen = new QPlatformPlaceholderScreen();
+    QWindowSystemInterface::handleScreenAdded(mPlaceholderScreen);
+    Q_ASSERT(!QGuiApplication::screens().empty());
+}
+
 void QWaylandDisplay::checkError() const
 {
     int ecode = wl_display_get_error(mDisplay);
@@ -253,8 +265,7 @@ void QWaylandDisplay::dispatchQueueWhile(wl_event_queue *queue, std::function<bo
 
 QWaylandScreen *QWaylandDisplay::screenForOutput(struct wl_output *output) const
 {
-    for (int i = 0; i < mScreens.size(); ++i) {
+    for (auto screen : qAsConst(mScreens)) {
-        QWaylandScreen *screen = static_cast<QWaylandScreen *>(mScreens.at(i));
         if (screen->output() == output)
             return screen;
     }
@@ -267,6 +278,11 @@ void QWaylandDisplay::handleScreenInitialized(QWaylandScreen *screen)
         return;
     mScreens.append(screen);
     QWindowSystemInterface::handleScreenAdded(screen);
+    if (mPlaceholderScreen) {
+        QWindowSystemInterface::handleScreenRemoved(mPlaceholderScreen);
+        // handleScreenRemoved deletes the platform screen
+        mPlaceholderScreen = nullptr;
+    }
 }
 
 void QWaylandDisplay::waitForScreens()
@@ -362,6 +378,8 @@ void QWaylandDisplay::registry_global_remove(uint32_t id)
                 for (QWaylandScreen *screen : qAsConst(mScreens)) {
                     if (screen->outputId() == id) {
                         mScreens.removeOne(screen);
+                        // If this is the last screen, we have to add a fake screen, or Qt will break.
+                        ensureScreen();
                         QWindowSystemInterface::handleScreenRemoved(screen);
                         break;
                     }

src/plugins/platforms/wayland/qwaylanddisplay_p.h

@@ -76,6 +76,7 @@ QT_BEGIN_NAMESPACE
 class QAbstractEventDispatcher;
 class QSocketNotifier;
 class QPlatformScreen;
+class QPlatformPlaceholderScreen;
 
 namespace QtWayland {
     class qt_surface_extension;
@@ -124,6 +125,8 @@ public:
 #endif
 
     QList<QWaylandScreen *> screens() const { return mScreens; }
+    QPlatformPlaceholderScreen *placeholderScreen() const { return mPlaceholderScreen; }
+    void ensureScreen();
 
     QWaylandScreen *screenForOutput(struct wl_output *output) const;
     void handleScreenInitialized(QWaylandScreen *screen);
@@ -228,6 +231,7 @@ private:
     QScopedPointer<QWaylandShm> mShm;
     QList<QWaylandScreen *> mWaitingScreens;
     QList<QWaylandScreen *> mScreens;
+    QPlatformPlaceholderScreen *mPlaceholderScreen = nullptr;
     QList<QWaylandInputDevice *> mInputDevices;
     QList<Listener> mRegistryListeners;
     QWaylandIntegration *mWaylandIntegration = nullptr;

src/plugins/platforms/wayland/qwaylandintegration.cpp

@@ -165,10 +165,10 @@ QPlatformWindow *QWaylandIntegration::createPlatformWindow(QWindow *window) cons
 
 #if QT_CONFIG(vulkan)
     if (window->surfaceType() == QSurface::VulkanSurface)
-        return new QWaylandVulkanWindow(window);
+        return new QWaylandVulkanWindow(window, mDisplay.data());
 #endif // QT_CONFIG(vulkan)
 
-    return new QWaylandShmWindow(window);
+    return new QWaylandShmWindow(window, mDisplay.data());
 }
 
 #if QT_CONFIG(opengl)
@@ -182,7 +182,7 @@ QPlatformOpenGLContext *QWaylandIntegration::createPlatformOpenGLContext(QOpenGL
 
 QPlatformBackingStore *QWaylandIntegration::createPlatformBackingStore(QWindow *window) const
 {
-    return new QWaylandShmBackingStore(window);
+    return new QWaylandShmBackingStore(window, mDisplay.data());
 }
 
 QAbstractEventDispatcher *QWaylandIntegration::createEventDispatcher() const
@@ -200,10 +200,8 @@ void QWaylandIntegration::initialize()
     QSocketNotifier *sn = new QSocketNotifier(fd, QSocketNotifier::Read, mDisplay.data());
     QObject::connect(sn, SIGNAL(activated(int)), mDisplay.data(), SLOT(flushRequests()));
 
-    if (mDisplay->screens().isEmpty()) {
+    // Qt does not support running with no screens
-        qWarning() << "Running on a compositor with no screens is not supported";
+    mDisplay->ensureScreen();
-        ::exit(EXIT_FAILURE);
-    }
 }
 
 QPlatformFontDatabase *QWaylandIntegration::fontDatabase() const

src/plugins/platforms/wayland/qwaylandscreen.cpp

@@ -165,11 +165,18 @@ QList<QPlatformScreen *> QWaylandScreen::virtualSiblings() const
 {
     QList<QPlatformScreen *> list;
     const QList<QWaylandScreen*> screens = mWaylandDisplay->screens();
-    list.reserve(screens.count());
+    auto *placeholder = mWaylandDisplay->placeholderScreen();
+
+    list.reserve(screens.count() + (placeholder ? 1 : 0));
+
     for (QWaylandScreen *screen : qAsConst(screens)) {
         if (screen->screen())
             list << screen;
     }
+
+    if (placeholder)
+        list << placeholder;
+
     return list;
 }
 
@@ -210,9 +217,11 @@ QPlatformCursor *QWaylandScreen::cursor() const
 }
 #endif // QT_CONFIG(cursor)
 
-QWaylandScreen * QWaylandScreen::waylandScreenFromWindow(QWindow *window)
+QWaylandScreen *QWaylandScreen::waylandScreenFromWindow(QWindow *window)
 {
     QPlatformScreen *platformScreen = QPlatformScreen::platformScreenForWindow(window);
+    if (platformScreen->isPlaceholder())
+        return nullptr;
     return static_cast<QWaylandScreen *>(platformScreen);
 }
 

src/plugins/platforms/wayland/qwaylandscreen_p.h

@@ -57,7 +57,6 @@
 #include <QtWaylandClient/private/qwayland-wayland.h>
 #include <QtWaylandClient/private/qwayland-xdg-output-unstable-v1.h>
 
-
 QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {

src/plugins/platforms/wayland/qwaylandshmbackingstore.cpp

@@ -151,9 +151,9 @@ QImage *QWaylandShmBuffer::imageInsideMargins(const QMargins &marginsIn)
 
 }
 
-QWaylandShmBackingStore::QWaylandShmBackingStore(QWindow *window)
+QWaylandShmBackingStore::QWaylandShmBackingStore(QWindow *window, QWaylandDisplay *display)
     : QPlatformBackingStore(window)
-    , mDisplay(QWaylandScreen::waylandScreenFromWindow(window)->display())
+    , mDisplay(display)
 {
 
 }

src/plugins/platforms/wayland/qwaylandshmbackingstore_p.h

@@ -88,7 +88,7 @@ private:
 class Q_WAYLAND_CLIENT_EXPORT QWaylandShmBackingStore : public QPlatformBackingStore
 {
 public:
-    QWaylandShmBackingStore(QWindow *window);
+    QWaylandShmBackingStore(QWindow *window, QWaylandDisplay *display);
     ~QWaylandShmBackingStore() override;
 
     QPaintDevice *paintDevice() override;

src/plugins/platforms/wayland/qwaylandshmwindow.cpp

@@ -49,8 +49,8 @@ QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {
 
-QWaylandShmWindow::QWaylandShmWindow(QWindow *window)
+QWaylandShmWindow::QWaylandShmWindow(QWindow *window, QWaylandDisplay *display)
-    : QWaylandWindow(window)
+    : QWaylandWindow(window, display)
 {
 }
 

src/plugins/platforms/wayland/qwaylandshmwindow_p.h

@@ -61,7 +61,7 @@ namespace QtWaylandClient {
 class Q_WAYLAND_CLIENT_EXPORT QWaylandShmWindow : public QWaylandWindow
 {
 public:
-    QWaylandShmWindow(QWindow *window);
+    QWaylandShmWindow(QWindow *window, QWaylandDisplay *display);
     ~QWaylandShmWindow() override;
 
     WindowType windowType() const override;

src/plugins/platforms/wayland/qwaylandsurface.cpp

@@ -72,8 +72,12 @@ QWaylandSurface *QWaylandSurface::fromWlSurface(::wl_surface *surface)
 
 void QWaylandSurface::handleScreenRemoved(QScreen *qScreen)
 {
-    auto *screen = static_cast<QWaylandScreen *>(qScreen->handle());
+    auto *platformScreen = qScreen->handle();
-    if (m_screens.removeOne(screen))
+    if (platformScreen->isPlaceholder())
+        return;
+
+    auto *waylandScreen = static_cast<QWaylandScreen *>(qScreen->handle());
+    if (m_screens.removeOne(waylandScreen))
         emit screensChanged();
 }
 

src/plugins/platforms/wayland/qwaylandvulkanwindow.cpp

@@ -43,8 +43,8 @@ QT_BEGIN_NAMESPACE
 
 namespace QtWaylandClient {
 
-QWaylandVulkanWindow::QWaylandVulkanWindow(QWindow *window)
+QWaylandVulkanWindow::QWaylandVulkanWindow(QWindow *window, QWaylandDisplay *display)
-    : QWaylandWindow(window)
+    : QWaylandWindow(window, display)
 {
 }
 

src/plugins/platforms/wayland/qwaylandvulkanwindow_p.h

@@ -50,7 +50,7 @@ namespace QtWaylandClient {
 class QWaylandVulkanWindow : public QWaylandWindow
 {
 public:
-    explicit QWaylandVulkanWindow(QWindow *window);
+    explicit QWaylandVulkanWindow(QWindow *window, QWaylandDisplay *display);
     ~QWaylandVulkanWindow() override;
 
     WindowType windowType() const override;

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -73,9 +73,9 @@ Q_LOGGING_CATEGORY(lcWaylandBackingstore, "qt.qpa.wayland.backingstore")
 
 QWaylandWindow *QWaylandWindow::mMouseGrab = nullptr;
 
-QWaylandWindow::QWaylandWindow(QWindow *window)
+QWaylandWindow::QWaylandWindow(QWindow *window, QWaylandDisplay *display)
     : QPlatformWindow(window)
-    , mDisplay(waylandScreen()->display())
+    , mDisplay(display)
     , mFrameQueue(mDisplay->createEventQueue())
     , mResizeAfterSwap(qEnvironmentVariableIsSet("QT_WAYLAND_RESIZE_AFTER_SWAP"))
 {
@@ -177,7 +177,7 @@ void QWaylandWindow::initWindow()
         }
     }
 
-    mScale = waylandScreen()->scale();
+    mScale = waylandScreen() ? waylandScreen()->scale() : 1; // fallback to 1 if we don't have a real screen
 
     // Enable high-dpi rendering. Scale() returns the screen scale factor and will
     // typically be integer 1 (normal-dpi) or 2 (high-dpi). Call set_buffer_scale()
@@ -402,14 +402,14 @@ void QWaylandWindow::closePopups(QWaylandWindow *parent)
     }
 }
 
-QWaylandScreen *QWaylandWindow::calculateScreenFromSurfaceEvents() const
+QPlatformScreen *QWaylandWindow::calculateScreenFromSurfaceEvents() const
 {
     if (mSurface) {
         if (auto *screen = mSurface->oldestEnteredScreen())
             return screen;
     }
 
-    return waylandScreen();
+    return QPlatformWindow::screen();
 }
 
 void QWaylandWindow::setVisible(bool visible)
@@ -690,7 +690,11 @@ QWaylandSubSurface *QWaylandWindow::subSurfaceWindow() const
 
 QWaylandScreen *QWaylandWindow::waylandScreen() const
 {
-    return static_cast<QWaylandScreen *>(QPlatformWindow::screen());
+    auto *platformScreen = QPlatformWindow::screen();
+    Q_ASSERT(platformScreen);
+    if (platformScreen->isPlaceholder())
+        return nullptr;
+    return static_cast<QWaylandScreen *>(platformScreen);
 }
 
 void QWaylandWindow::handleContentOrientationChange(Qt::ScreenOrientation orientation)
@@ -962,7 +966,7 @@ void QWaylandWindow::handleMouseEventWithDecoration(QWaylandInputDevice *inputDe
 
 void QWaylandWindow::handleScreensChanged()
 {
-    QWaylandScreen *newScreen = calculateScreenFromSurfaceEvents();
+    QPlatformScreen *newScreen = calculateScreenFromSurfaceEvents();
 
     if (newScreen == mLastReportedScreen)
         return;
@@ -970,7 +974,7 @@ void QWaylandWindow::handleScreensChanged()
     QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->QPlatformScreen::screen());
     mLastReportedScreen = newScreen;
 
-    int scale = newScreen->scale();
+    int scale = newScreen->isPlaceholder() ? 1 : static_cast<QWaylandScreen *>(newScreen)->scale();
     if (scale != mScale) {
         mScale = scale;
         if (mSurface && mDisplay->compositorVersion() >= 3)

src/plugins/platforms/wayland/qwaylandwindow_p.h

@@ -93,7 +93,7 @@ public:
         Vulkan
     };
 
-    QWaylandWindow(QWindow *window);
+    QWaylandWindow(QWindow *window, QWaylandDisplay *display);
     ~QWaylandWindow() override;
 
     virtual WindowType windowType() const = 0;
@@ -241,7 +241,7 @@ protected:
     bool mSentInitialResize = false;
     QPoint mOffset;
     int mScale = 1;
-    QWaylandScreen *mLastReportedScreen = nullptr;
+    QPlatformScreen *mLastReportedScreen = nullptr;
 
     QIcon mWindowIcon;
 
@@ -262,7 +262,7 @@ private:
     void reset(bool sendDestroyEvent = true);
     void sendExposeEvent(const QRect &rect);
     static void closePopups(QWaylandWindow *parent);
-    QWaylandScreen *calculateScreenFromSurfaceEvents() const;
+    QPlatformScreen *calculateScreenFromSurfaceEvents() const;
 
     void handleMouseEventWithDecoration(QWaylandInputDevice *inputDevice, const QWaylandPointerEvent &e);
     void handleScreensChanged();