SharedObject::onRender deadlock fix

Call gl::globalRelease() for paused surfaces, this fixes a very common deadlock on mac.

But for PC, by inspection, a race condition could occur over the _syncRequested boolean, between the main and qml rendering thread.
To fix this, we split render and renderSync into separate messages.

libraries/qml/src/qml/impl/RenderEventHandler.cpp

@@ -31,6 +31,10 @@ bool RenderEventHandler::event(QEvent* e) {
             onRender();
             return true;
 
+        case OffscreenEvent::RenderSync:
+            onRenderSync();
+            return true;
+
         case OffscreenEvent::Initialize:
             onInitalize();
             return true;
@@ -106,6 +110,14 @@ void RenderEventHandler::resize() {
 }
 
 void RenderEventHandler::onRender() {
+    qmlRender(false);
+}
+
+void RenderEventHandler::onRenderSync() {
+    qmlRender(true);
+}
+
+void RenderEventHandler::qmlRender(bool sceneGraphSync) {
     if (_shared->isQuit()) {
         return;
     }
@@ -117,7 +129,8 @@ void RenderEventHandler::onRender() {
     PROFILE_RANGE(render_qml_gl, __FUNCTION__);
 
     gl::globalLock();
-    if (!_shared->preRender()) {
+    if (!_shared->preRender(sceneGraphSync)) {
+        gl::globalRelease();
         return;
     }
 

libraries/qml/src/qml/impl/RenderEventHandler.h

@@ -25,6 +25,7 @@ public:
     enum Type {
         Initialize = QEvent::User + 1,
         Render,
+        RenderSync,
         Quit
     };
 
@@ -45,6 +46,8 @@ private:
     void onInitalize();
     void resize();
     void onRender();
+    void onRenderSync();
+    void qmlRender(bool sceneGraphSync);
     void onQuit();
 
     SharedObject* const _shared;
@@ -59,4 +62,4 @@ private:
 
 }}}  // namespace hifi::qml::impl
 
-#endif
+#endif

libraries/qml/src/qml/impl/SharedObject.cpp

@@ -344,17 +344,17 @@ void SharedObject::setSize(const QSize& size) {
 #endif
 }
 
-bool SharedObject::preRender() {
+bool SharedObject::preRender(bool sceneGraphSync) {
 #ifndef DISABLE_QML
     QMutexLocker lock(&_mutex);
     if (_paused) {
-        if (_syncRequested) {
+        if (sceneGraphSync) {
             wake();
         }
         return false;
     }
 
-    if (_syncRequested) {
+    if (sceneGraphSync) {
         bool syncResult = true;
         if (!nsightActive()) {
             PROFILE_RANGE(render_qml_gl, "sync")
@@ -364,7 +364,6 @@ bool SharedObject::preRender() {
         if (!syncResult) {
             return false;
         }
-        _syncRequested = false;
     }
 #endif
 
@@ -475,9 +474,10 @@ void SharedObject::onRender() {
         lock.unlock();
         _renderControl->polishItems();
         lock.relock();
-        QCoreApplication::postEvent(_renderObject, new OffscreenEvent(OffscreenEvent::Render));
+        QCoreApplication::postEvent(_renderObject, new OffscreenEvent(OffscreenEvent::RenderSync));
         // sync and render request, main and render threads must be synchronized
         wait();
+        _syncRequested = false;
     } else {
         QCoreApplication::postEvent(_renderObject, new OffscreenEvent(OffscreenEvent::Render));
     }

libraries/qml/src/qml/impl/SharedObject.h

@@ -71,7 +71,7 @@ public:
 private:
     bool event(QEvent* e) override;
 
-    bool preRender();
+    bool preRender(bool sceneGraphSync);
     void shutdownRendering(OffscreenGLCanvas& canvas, const QSize& size);
     // Called by the render event handler, from the render thread
     void initializeRenderControl(QOpenGLContext* context);