MirSurfaceItem: Remove an unnecessary and potentially infinite loop

(LP: #1477430)

We only need to "drop" or consume one frame to do the job of waking
the client up.
 Fixes: #1477430
Approved by: Gerry Boland, PS Jenkins bot

2 files changed, 7 insertions, 6 deletions

diff --git a/src/modules/Unity/Application/mirsurfaceitem.cpp b/src/modules/Unity/Application/mirsurfaceitem.cpp
index 53331e4..e8f1f2b 100644
--- a/src/modules/Unity/Application/mirsurfaceitem.cpp
+++ b/src/modules/Unity/Application/mirsurfaceitem.cpp
@@ -233,7 +233,7 @@ MirSurfaceItem::MirSurfaceItem(std::shared_ptr<mir::scene::Surface> surface,
     QQmlEngine::setObjectOwnership(this, QQmlEngine::CppOwnership);
 
     connect(&m_frameDropperTimer, &QTimer::timeout,
-            this, &MirSurfaceItem::dropPendingBuffers);
+            this, &MirSurfaceItem::dropPendingBuffer);
     // Rationale behind the frame dropper and its interval value:
     //
     // We want to give ample room for Qt scene graph to have a chance to fetch and render
@@ -735,22 +735,23 @@ void MirSurfaceItem::updateMirSurfaceFocus(bool focused)
     }
 }
 
-void MirSurfaceItem::dropPendingBuffers()
+void MirSurfaceItem::dropPendingBuffer()
 {
     QMutexLocker locker(&m_mutex);
 
     const void* const userId = (void*)123;  // TODO: Multimonitor support
 
-    while (m_surface->buffers_ready_for_compositor(userId) > 0) {
+    int framesPending = m_surface->buffers_ready_for_compositor(userId);
+    if (framesPending > 0) {
         // The line below looks like an innocent, effect-less, getter. But as this
         // method returns a unique_pointer, not holding its reference causes the
         // buffer to be destroyed/released straight away.
         for (auto const & item : m_surface->generate_renderables(userId))
             item->buffer();
-        qCDebug(QTMIR_SURFACES) << "MirSurfaceItem::dropPendingBuffers()"
+        qCDebug(QTMIR_SURFACES) << "MirSurfaceItem::dropPendingBuffer()"
             << "surface =" << this
             << "buffer dropped."
-            << m_surface->buffers_ready_for_compositor(userId)
+            << framesPending-1
             << "left.";
     }
 }
diff --git a/src/modules/Unity/Application/mirsurfaceitem.h b/src/modules/Unity/Application/mirsurfaceitem.h
index 952d3cd..24fb5ae 100644
--- a/src/modules/Unity/Application/mirsurfaceitem.h
+++ b/src/modules/Unity/Application/mirsurfaceitem.h
@@ -105,7 +105,7 @@ protected:
 
 private Q_SLOTS:
     void surfaceDamaged();
-    void dropPendingBuffers();
+    void dropPendingBuffer();
     void scheduleTextureUpdate();
 
     void scheduleMirSurfaceSizeUpdate();