Add QAbstractAspectPrivate::onEngineAboutToShutdown() virtual

Gives internal aspects a chance to unqueue any queued work that
could potentially deadlock during trying to exit the inner loop
in QAspectManager::exec().

Task-number: QTBUG-51421
Change-Id: I8f9892db2d8648c39092720f1a787c4db412d30d
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>

3 files changed, 22 insertions, 0 deletions

diff --git a/src/core/aspects/qabstractaspect.cpp b/src/core/aspects/qabstractaspect.cpp
index 309fa3faf..20a393738 100644
--- a/src/core/aspects/qabstractaspect.cpp
+++ b/src/core/aspects/qabstractaspect.cpp
@@ -68,6 +68,14 @@ QAbstractAspectPrivate *QAbstractAspectPrivate::get(QAbstractAspect *aspect)
 }
 
 /*!
+ *
+ * Called in the context of the main thread
+ */
+void QAbstractAspectPrivate::onEngineAboutToShutdown()
+{
+}
+
+/*!
     \class Qt3DCore::QAbstractAspect
     \inmodule Qt3DCore
     \brief QAbstractAspect is the base class for aspects that provide a vertical slice of behavior.
diff --git a/src/core/aspects/qabstractaspect_p.h b/src/core/aspects/qabstractaspect_p.h
index 5ce480288..5536f1e1d 100644
--- a/src/core/aspects/qabstractaspect_p.h
+++ b/src/core/aspects/qabstractaspect_p.h
@@ -93,6 +93,8 @@ public:
     void sceneNodeAdded(Qt3DCore::QSceneChangePtr &e) Q_DECL_OVERRIDE;
     void sceneNodeRemoved(Qt3DCore::QSceneChangePtr &e) Q_DECL_OVERRIDE;
 
+    virtual void onEngineAboutToShutdown();
+
     Q_DECLARE_PUBLIC(QAbstractAspect)
 
     QEntity *m_root;
diff --git a/src/core/aspects/qaspectmanager.cpp b/src/core/aspects/qaspectmanager.cpp
index 903d5b219..2f8b0473e 100644
--- a/src/core/aspects/qaspectmanager.cpp
+++ b/src/core/aspects/qaspectmanager.cpp
@@ -99,6 +99,18 @@ void QAspectManager::exitSimulationLoop()
 {
     qCDebug(Aspects) << Q_FUNC_INFO;
     m_runSimulationLoop.fetchAndStoreOrdered(0);
+
+    // Give any aspects a chance to unqueue any asynchronous work they
+    // may have scheduled that would otherwise potentially deadlock or
+    // cause races. For example, the QLogicAspect queues up a vector of
+    // QNodeIds to be processed by a callback on the main thread. However,
+    // if we don't unqueue this work and release its semaphore, the logic
+    // aspect would cause a deadlock when trying to exit the inner loop.
+    // This is because we call this function from the main thread and the
+    // logic aspect is waiting for the main thread to execute the
+    // QLogicComponent::onFrameUpdate() callback.
+    Q_FOREACH (QAbstractAspect *aspect, m_aspects)
+        aspect->d_func()->onEngineAboutToShutdown();
 }
 
 bool QAspectManager::isShuttingDown() const