Bug 597336, part 3: Generalize the not-using-X-compositing check when creating shadow-layer backing surfaces. r=karl

gfx/layers/ipc/ShadowLayerUtilsX11.cpp

@@ -49,6 +49,15 @@
 namespace mozilla {
 namespace layers {
 
+// Return true if we're likely compositing using X and so should use
+// Xlib surfaces in shadow layers.
+static PRBool
+UsingXCompositing()
+{
+  return (gfxASurface::SurfaceTypeXlib ==
+          gfxPlatform::GetPlatform()->ScreenReferenceSurface()->GetType());
+}
+
 // LookReturn a pointer to |aFormat| that lives in the Xrender library.
 // All code using render formats assumes it doesn't need to copy.
 static XRenderPictFormat*
@@ -94,13 +103,14 @@ ShadowLayerForwarder::PlatformAllocDoubleBuffer(const gfxIntSize& aSize,
                                                 SurfaceDescriptor* aFrontBuffer,
                                                 SurfaceDescriptor* aBackBuffer)
 {
-  gfxPlatform* platform = gfxPlatform::GetPlatform();
-#ifdef MOZ_WIDGET_QT
-  // If optimized platform surface is Image, then it is better to continue with Shmem
-  if (platform->ScreenReferenceSurface()->GetType() != gfxASurface::SurfaceTypeXlib)
+  if (!UsingXCompositing()) {
+    // If we're not using X compositing, we're probably compositing on
+    // the client side, in which case X surfaces would just slow
+    // things down.  Use Shmem instead.
     return PR_FALSE;
-#endif
+  }
 
+  gfxPlatform* platform = gfxPlatform::GetPlatform();
   nsRefPtr<gfxASurface> front = platform->CreateOffscreenSurface(aSize, aContent);
   nsRefPtr<gfxASurface> back = platform->CreateOffscreenSurface(aSize, aContent);
   if (!front || !back ||
@@ -142,13 +152,26 @@ ShadowLayerForwarder::PlatformDestroySharedSurface(SurfaceDescriptor* aSurface)
 /*static*/ void
 ShadowLayerForwarder::PlatformSyncBeforeUpdate()
 {
-  XSync(DefaultXDisplay(), False);
+  if (UsingXCompositing()) {
+    // If we're using X surfaces, then we need to finish all pending
+    // operations on the back buffers before handing them to the
+    // parent, otherwise the surface might be used by the parent's
+    // Display in between two operations queued by our Display.
+    XSync(DefaultXDisplay(), False);
+  }
 }
 
 /*static*/ void
 ShadowLayerManager::PlatformSyncBeforeReplyUpdate()
 {
-  XSync(DefaultXDisplay(), False);
+  if (UsingXCompositing()) {
+    // If we're using X surfaces, we need to finish all pending
+    // operations on the *front buffers* before handing them back to
+    // the child, even though they will be read operations.
+    // Otherwise, the child might start scribbling on new back buffers
+    // that are still participating in requests as old front buffers.
+    XSync(DefaultXDisplay(), False);
+  }
 }
 
 PRBool