Make sure the frame callbacks are sent at the right time

When there is a rendering thread, like with QtQuick compositors,
there is a race condition where the frame callbacks are sent before
the last buffer the client sent is released.
Assume the following scenario:
attach(B1)/frame(F1)/commit()...frame started...attach(B2)/frame(F2)/
commit()...frame finished.
On frame finished the callback just installed is emitted before the
buffer B2 is being used and B1 released. That forces the client to
allocate a third buffer to draw the next frame.
Now, do not send out the frame callbacks until a new frame started.
The successive draw will release B1, use the new B2 and than send out F2.

Change-Id: I5743c7baf9fdd3cde28c5f594ff646c06abb74b7
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/src/compositor/wayland_wrapper/qwlsurface.cpp b/src/compositor/wayland_wrapper/qwlsurface.cpp
index a0e61bc22..cd5320677 100644
--- a/src/compositor/wayland_wrapper/qwlsurface.cpp
+++ b/src/compositor/wayland_wrapper/qwlsurface.cpp
@@ -78,6 +78,7 @@ public:
     FrameCallback(Surface *surf, wl_resource *res)
         : surface(surf)
         , resource(res)
+        , canSend(false)
     {
 #if WAYLAND_VERSION_MAJOR < 1 || (WAYLAND_VERSION_MAJOR == 1 && WAYLAND_VERSION_MINOR <= 2)
         res->data = this;
@@ -113,6 +114,7 @@ public:
     }
     Surface *surface;
     wl_resource *resource;
+    bool canSend;
 };
 
 Surface::Surface(struct wl_client *client, uint32_t id, Compositor *compositor)
@@ -269,9 +271,12 @@ GLuint Surface::textureId() const
 void Surface::sendFrameCallback()
 {
     uint time = m_compositor->currentTimeMsecs();
-    foreach (FrameCallback *callback, m_frameCallbacks)
-        callback->send(time);
-    m_frameCallbacks.clear();
+    foreach (FrameCallback *callback, m_frameCallbacks) {
+        if (callback->canSend) {
+            callback->send(time);
+            m_frameCallbacks.removeOne(callback);
+        }
+    }
 }
 
 void Surface::removeFrameCallback(FrameCallback *callback)
@@ -480,6 +485,12 @@ void Surface::surface_commit(Resource *)
     m_pendingFrameCallbacks.clear();
 }
 
+void Surface::frameStarted()
+{
+    foreach (FrameCallback *c, m_frameCallbacks)
+        c->canSend = true;
+}
+
 void Surface::setClassName(const QString &className)
 {
     if (m_className != className) {