Client: Fix 100ms freeze when applications do not swap after deliverUpdateRequest

[ChangeLog][QPA plugin] Fixed a 100 ms freeze that would occur if applications
did not draw after receiving a deliverUpdateRequest().

QtQuick does this at the start of animations. This should get rid of those
backingstore warnings (and also remove a 100ms freeze before animations start
in those instances).

Fixes: QTBUG-76813
Change-Id: Id366bf4a14f402fa44530ae46e7b66d9988c14f6
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
Reviewed-by: John Brooks <john.brooks@qt.io>

2 files changed, 9 insertions, 38 deletions

diff --git a/src/client/qwaylandwindow.cpp b/src/client/qwaylandwindow.cpp
index ae26ba049..8d34afd1f 100644
--- a/src/client/qwaylandwindow.cpp
+++ b/src/client/qwaylandwindow.cpp
@@ -1105,25 +1105,6 @@ QVariant QWaylandWindow::property(const QString &name, const QVariant &defaultVa
 
 void QWaylandWindow::timerEvent(QTimerEvent *event)
 {
-    if (event->timerId() == mFallbackUpdateTimerId) {
-        killTimer(mFallbackUpdateTimerId);
-        mFallbackUpdateTimerId = -1;
-        qCDebug(lcWaylandBackingstore) << "mFallbackUpdateTimer timed out";
-
-        if (!isExposed()) {
-            qCDebug(lcWaylandBackingstore) << "Fallback update timer: Window not exposed,"
-                                           << "not delivering update request.";
-            return;
-        }
-
-        if (mWaitingForUpdate && hasPendingUpdateRequest() && !mWaitingForFrameCallback) {
-            qCWarning(lcWaylandBackingstore) << "Delivering update request through fallback timer,"
-                                             << "may not be in sync with display";
-            deliverUpdateRequest();
-        }
-    }
-
-
     if (mFrameCallbackTimerId.testAndSetOrdered(event->timerId(), -1)) {
         killTimer(event->timerId());
         qCDebug(lcWaylandBackingstore) << "Didn't receive frame callback in time, window should now be inexposed";
@@ -1135,6 +1116,7 @@ void QWaylandWindow::timerEvent(QTimerEvent *event)
 
 void QWaylandWindow::requestUpdate()
 {
+    qCDebug(lcWaylandBackingstore) << "requestUpdate";
     Q_ASSERT(hasPendingUpdateRequest()); // should be set by QPA
 
     // If we have a frame callback all is good and will be taken care of there
@@ -1142,20 +1124,17 @@ void QWaylandWindow::requestUpdate()
         return;
 
     // If we've already called deliverUpdateRequest(), but haven't seen any attach+commit/swap yet
-    if (mWaitingForUpdate) {
-        // Ideally, we should just have returned here, but we're not guaranteed that the client
-        // will actually update, so start this timer to deliver another request update after a while
-        // *IF* the client doesn't update.
-        int fallbackTimeout = 100;
-        mFallbackUpdateTimerId = startTimer(fallbackTimeout);
-        return;
-    }
+    // This is a somewhat redundant behavior and might indicate a bug in the calling code, so log
+    // here so we can get this information when debugging update/frame callback issues.
+    // Continue as nothing happened, though.
+    if (mWaitingForUpdate)
+        qCDebug(lcWaylandBackingstore) << "requestUpdate called twice without committing anything";
 
     // Some applications (such as Qt Quick) depend on updates being delivered asynchronously,
     // so use invokeMethod to delay the delivery a bit.
     QMetaObject::invokeMethod(this, [this] {
         // Things might have changed in the meantime
-        if (hasPendingUpdateRequest() && !mWaitingForUpdate && !mWaitingForFrameCallback)
+        if (hasPendingUpdateRequest() && !mWaitingForFrameCallback)
             deliverUpdateRequest();
     }, Qt::QueuedConnection);
 }
@@ -1165,6 +1144,7 @@ void QWaylandWindow::requestUpdate()
 // Can be called from the render thread (without locking anything) so make sure to not make races in this method.
 void QWaylandWindow::handleUpdate()
 {
+    qCDebug(lcWaylandBackingstore) << "handleUpdate" << QThread::currentThread();
     // TODO: Should sync subsurfaces avoid requesting frame callbacks?
     QReadLocker lock(&mSurfaceLock);
     if (!isInitialized())
@@ -1175,15 +1155,6 @@ void QWaylandWindow::handleUpdate()
         mFrameCallback = nullptr;
     }
 
-    if (mFallbackUpdateTimerId != -1) {
-        // Ideally, we would stop the fallback timer here, but since we're on another thread,
-        // it's not allowed. Instead we set mFallbackUpdateTimer to -1 here, so we'll just
-        // ignore it if it times out before it's cleaned up by the invokeMethod call.
-        int id = mFallbackUpdateTimerId;
-        mFallbackUpdateTimerId = -1;
-        QMetaObject::invokeMethod(this, [this, id] { killTimer(id); }, Qt::QueuedConnection);
-    }
-
     mFrameCallback = frame();
     wl_callback_add_listener(mFrameCallback, &QWaylandWindow::callbackListener, this);
     mWaitingForFrameCallback = true;
@@ -1203,6 +1174,7 @@ void QWaylandWindow::handleUpdate()
 
 void QWaylandWindow::deliverUpdateRequest()
 {
+    qCDebug(lcWaylandBackingstore) << "deliverUpdateRequest";
     mWaitingForUpdate = true;
     QPlatformWindow::deliverUpdateRequest();
 }
diff --git a/src/client/qwaylandwindow_p.h b/src/client/qwaylandwindow_p.h
index b03d92e56..e4a1124e7 100644
--- a/src/client/qwaylandwindow_p.h
+++ b/src/client/qwaylandwindow_p.h
@@ -232,7 +232,6 @@ protected:
 
     // True when we have called deliverRequestUpdate, but the client has not yet attached a new buffer
     bool mWaitingForUpdate = false;
-    int mFallbackUpdateTimerId = -1; // Started when waiting for app to commit
 
     QMutex mResizeLock;
     bool mWaitingToApplyConfigure = false;