QWaylandShmBackingStore: Don't clear new buffers in beginPaint

QFile::resize done upon creation of the buffer already ensures
the buffer is filled with zeroes, so there is no need to paint it
transparent if the buffer had just been created.

Change-Id: I73ab297f148987acf52a0e08c7dd1ca57f7255be
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/wayland/qwaylandshmbackingstore.cpp

@@ -71,6 +71,7 @@ QWaylandShmBuffer::QWaylandShmBuffer(QWaylandDisplay *display,
         file->open(fd, QIODevice::ReadWrite | QIODevice::Unbuffered, QFile::AutoCloseHandle);
         filePointer.reset(file);
     }
+    // NOTE beginPaint assumes a new buffer be all zeroes, which QFile::resize does.
     if (!filePointer->isOpen() || !filePointer->resize(alloc)) {
         qWarning("QWaylandShmBuffer: failed: %s", qUtf8Printable(filePointer->errorString()));
         return;
@@ -144,9 +145,9 @@ QWaylandShmBackingStore::QWaylandShmBackingStore(QWindow *window, QWaylandDispla
         // contents from the back buffer
         mBuffers.clear();
         mFrontBuffer = nullptr;
-        // ensureBackBuffer always resets mBackBuffer
+        // recreateBackBufferIfNeeded always resets mBackBuffer
         if (mRequestedSize.isValid() && waylandWindow())
-            ensureBackBuffer();
+            recreateBackBufferIfNeeded();
         qDeleteAll(copy);
     });
 }
@@ -181,12 +182,15 @@ void QWaylandShmBackingStore::beginPaint(const QRegion &region)
 {
     mPainting = true;
     waylandWindow()->setBackingStore(this);
-    ensureBackBuffer();
+    const bool bufferWasRecreated = recreateBackBufferIfNeeded();
 
     const QMargins margins = windowDecorationMargins();
     updateDirtyStates(region.translated(margins.left(), margins.top()));
 
-    if (mBackBuffer->image()->hasAlphaChannel()) {
+    // Although undocumented, QBackingStore::beginPaint expects the painted region
+    // to be cleared before use if the window has a surface format with an alpha.
+    // Fresh QWaylandShmBuffer are already cleared, so we don't need to clear those.
+    if (!bufferWasRecreated && mBackBuffer->image()->hasAlphaChannel()) {
         QPainter p(paintDevice());
         p.setCompositionMode(QPainter::CompositionMode_Source);
         const QColor blank = Qt::transparent;
@@ -238,8 +242,10 @@ void QWaylandShmBackingStore::resize(const QSize &size, const QRegion &)
     mRequestedSize = size;
 }
 
-QWaylandShmBuffer *QWaylandShmBackingStore::getBuffer(const QSize &size)
+QWaylandShmBuffer *QWaylandShmBackingStore::getBuffer(const QSize &size, bool &bufferWasRecreated)
 {
+    bufferWasRecreated = false;
+
     const auto copy = mBuffers; // remove when ported to vector<unique_ptr> + remove_if
     for (QWaylandShmBuffer *b : copy) {
         if (!b->busy()) {
@@ -258,14 +264,16 @@ QWaylandShmBuffer *QWaylandShmBackingStore::getBuffer(const QSize &size)
     if (mBuffers.size() < MAX_BUFFERS) {
         QImage::Format format = QPlatformScreen::platformScreenForWindow(window())->format();
         QWaylandShmBuffer *b = new QWaylandShmBuffer(mDisplay, size, format, waylandWindow()->scale());
+        bufferWasRecreated = true;
         mBuffers.push_front(b);
         return b;
     }
     return nullptr;
 }
 
-void QWaylandShmBackingStore::ensureBackBuffer()
+bool QWaylandShmBackingStore::recreateBackBufferIfNeeded()
 {
+    bool bufferWasRecreated = false;
     QMargins margins = windowDecorationMargins();
     qreal scale = waylandWindow()->scale();
     const QSize sizeWithMargins = (mRequestedSize + QSize(margins.left() + margins.right(), margins.top() + margins.bottom())) * scale;
@@ -278,12 +286,12 @@ void QWaylandShmBackingStore::ensureBackBuffer()
     // You can exercise the different codepaths with weston, switching between the gl and the
     // pixman renderer. With the gl renderer release events are sent early so we can effectively
     // run single buffered, while with the pixman renderer we have to use two.
-    QWaylandShmBuffer *buffer = getBuffer(sizeWithMargins);
+    QWaylandShmBuffer *buffer = getBuffer(sizeWithMargins, bufferWasRecreated);
     while (!buffer) {
         qCDebug(lcWaylandBackingstore, "QWaylandShmBackingStore: stalling waiting for a buffer to be released from the compositor...");
 
         mDisplay->blockingReadEvents();
-        buffer = getBuffer(sizeWithMargins);
+        buffer = getBuffer(sizeWithMargins, bufferWasRecreated);
     }
 
     qsizetype oldSizeInBytes = mBackBuffer ? mBackBuffer->image()->sizeInBytes() : 0;
@@ -325,6 +333,8 @@ void QWaylandShmBackingStore::ensureBackBuffer()
         windowDecoration()->update();
 
     buffer->dirtyRegion() = QRegion();
+
+    return bufferWasRecreated;
 }
 
 QImage *QWaylandShmBackingStore::entireSurface() const

src/plugins/platforms/wayland/qwaylandshmbackingstore_p.h

@@ -69,7 +69,7 @@ public:
     QMargins windowDecorationMargins() const;
     QImage *entireSurface() const;
     QImage *contentSurface() const;
-    void ensureBackBuffer();
+    bool recreateBackBufferIfNeeded();
 
     QWaylandWindow *waylandWindow() const;
     void iterateBuffer();
@@ -81,7 +81,7 @@ public:
 private:
     void updateDirtyStates(const QRegion &region);
     void updateDecorations();
-    QWaylandShmBuffer *getBuffer(const QSize &size);
+    QWaylandShmBuffer *getBuffer(const QSize &size, bool &bufferWasRecreated);
 
     QWaylandDisplay *mDisplay = nullptr;
     std::list<QWaylandShmBuffer *> mBuffers;

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -92,7 +92,7 @@ void QWaylandWindow::ensureSize()
 {
     if (mBackingStore) {
         setBackingStore(mBackingStore);
-        mBackingStore->ensureBackBuffer();
+        mBackingStore->recreateBackBufferIfNeeded();
     }
 }