Fix race condition on frame callback

If the frame listener callback is moved to a different queue while the
frame event is being processed, the event will not get delivered. (It
will not even show up in the WAYLAND_DEBUG output.) This will cause
waitForFrameSync() to hang until it times out.

To avoid this, perform the move just after the callback has been
created.

This exposed an issue with single-threaded rendering, where a new
update would be started from inside the callback, resetting
mWaitingForFrameCallback before waitForFrameSync could react to it.
This caused all rendering to freeze. To avoid that problem, do not
deliver update requests directly from the frame callback.

With the callback always on a separate queue, we then have to make
sure that queue is also dispatched during the main event loop,
otherwise the events may not be processed. To do this, we need a
mutex lock. But it turns out that we no longer need a global mutex
lock as long as the frame events are being dispatched on their own
queues, but can manage with per-window locks instead.

A final thing needed is to make sure the frame callback does not
request additional updates while we are already waiting for the main
thread to process the last one. This is to avoid flooding the main
event loop with events, when the main thread is processing them at
a slower pace than the frame rate.

Fixes: QTBUG-83263
Pick-to: 5.15
Done-with: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Change-Id: I0db9da64fc8ced147177391c2a7999c4cc7a0d58
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 0 insertions, 2 deletions

diff --git a/tests/auto/client/surface/tst_surface.cpp b/tests/auto/client/surface/tst_surface.cpp
index 289fbd62..c7b02e2c 100644
--- a/tests/auto/client/surface/tst_surface.cpp
+++ b/tests/auto/client/surface/tst_surface.cpp
@@ -64,7 +64,6 @@ void tst_surface::createDestroySurface()
 
 void tst_surface::waitForFrameCallbackRaster()
 {
-    QSKIP("TODO: This currently fails, needs a fix");
     class TestWindow : public QRasterWindow {
     public:
         explicit TestWindow() { resize(40, 40); }
@@ -100,7 +99,6 @@ void tst_surface::waitForFrameCallbackRaster()
 #if QT_CONFIG(opengl)
 void tst_surface::waitForFrameCallbackGl()
 {
-    QSKIP("TODO: This currently fails, needs a fix");
     class TestWindow : public QOpenGLWindow {
     public:
         explicit TestWindow()