Fix race condition when rendering the UI

This fix depends on a fix in ign-rendering module, because it
depends on the new Camera::SwapFromThread function
Without it, compilation will fail

Affects gazebosim/gz-rendering#304

Signed-off-by: Matias N. Goldberg <dark_sylinc@yahoo.com.ar>

src/gui/plugins/scene3d/Scene3D.cc

@@ -76,6 +76,7 @@
 std::condition_variable g_renderCv;
 
 Q_DECLARE_METATYPE(std::string)
+Q_DECLARE_METATYPE(ignition::gazebo::RenderSync*)
 
 namespace ignition
 {
@@ -447,6 +448,45 @@ using namespace gazebo;
 
 QList<QThread *> RenderWindowItemPrivate::threads;
 
+/////////////////////////////////////////////////
+void RenderSync::requestQtThreadToBlock(std::unique_lock<std::mutex> &lock)
+{
+  this->renderStallState = RenderStallState::WorkerThreadRequested;
+  this->cv.wait(lock, [this]
+  { return this->renderStallState == RenderStallState::QtThreadBlocked; });
+}
+
+/////////////////////////////////////////////////
+void RenderSync::releaseQtThreadFromBlock(std::unique_lock<std::mutex> &lock)
+{
+  this->renderStallState = RenderStallState::Unblocked;
+  lock.unlock();
+  this->cv.notify_one();
+}
+
+/////////////////////////////////////////////////
+void RenderSync::waitForWorkerThread()
+{
+  {
+    std::unique_lock<std::mutex> lock(this->mutex);
+    if(this->renderStallState == RenderStallState::WorkerThreadRequested)
+    {
+      // Worker thread asked us to wait!
+      this->renderStallState = RenderStallState::QtThreadBlocked;
+      lock.unlock();
+      // Wake up worker thread
+      this->cv.notify_one();
+    }
+  }
+
+  {
+    // Wait until we're clear to go
+    std::unique_lock<std::mutex> lock(this->mutex);
+    this->cv.wait(lock, [this]
+    { return this->renderStallState == RenderStallState::Unblocked; });
+  }
+}
+
 /////////////////////////////////////////////////
 IgnRenderer::IgnRenderer()
   : dataPtr(new IgnRendererPrivate)
@@ -472,7 +512,7 @@ RenderUtil *IgnRenderer::RenderUtil() const
 }
 
 /////////////////////////////////////////////////
-void IgnRenderer::Render()
+void IgnRenderer::Render(RenderSync *renderSync)
 {
   rendering::ScenePtr scene = this->dataPtr->renderUtil.Scene();
   if (!scene)
@@ -488,6 +528,8 @@ void IgnRenderer::Render()
   IGN_PROFILE("IgnRenderer::Render");
   if (this->textureDirty)
   {
+    std::unique_lock<std::mutex> lock(renderSync->mutex);
+    renderSync->requestQtThreadToBlock(lock);
     this->dataPtr->camera->SetImageWidth(this->textureSize.width());
     this->dataPtr->camera->SetImageHeight(this->textureSize.height());
     this->dataPtr->camera->SetAspectRatio(this->textureSize.width() /
@@ -498,6 +540,7 @@ void IgnRenderer::Render()
       this->dataPtr->camera->PreRender();
     }
     this->textureDirty = false;
+    renderSync->releaseQtThreadFromBlock(lock);
   }
 
   // texture id could change so get the value in every render update
@@ -855,6 +898,8 @@ void IgnRenderer::Render()
   // only has an effect in video recording lockstep mode
   // this notifes ECM to continue updating the scene
   g_renderCv.notify_one();
+
+  this->dataPtr->camera->SwapFromThread();
 }
 
 /////////////////////////////////////////////////
@@ -2168,10 +2213,11 @@ RenderThread::RenderThread()
 {
   RenderWindowItemPrivate::threads << this;
   qRegisterMetaType<std::string>();
+  qRegisterMetaType<RenderSync*>("RenderSync*");
 }
 
 /////////////////////////////////////////////////
-void RenderThread::RenderNext()
+void RenderThread::RenderNext(RenderSync *renderSync)
 {
   this->context->makeCurrent(this->surface);
 
@@ -2188,7 +2234,7 @@ void RenderThread::RenderNext()
     return;
   }
 
-  this->ignRenderer.Render();
+  this->ignRenderer.Render(renderSync);
 
   emit TextureReady(this->ignRenderer.textureId, this->ignRenderer.textureSize);
 }
@@ -2251,7 +2297,7 @@ TextureNode::~TextureNode()
 }
 
 /////////////////////////////////////////////////
-void TextureNode::NewTexture(int _id, const QSize &_size)
+void TextureNode::NewTexture(uint _id, const QSize &_size)
 {
   this->mutex.lock();
   this->id = _id;
@@ -2266,8 +2312,9 @@ void TextureNode::NewTexture(int _id, const QSize &_size)
 /////////////////////////////////////////////////
 void TextureNode::PrepareNode()
 {
+  renderSync.waitForWorkerThread();
   this->mutex.lock();
-  int newId = this->id;
+  uint newId = this->id;
   QSize sz = this->size;
   this->id = 0;
   this->mutex.unlock();
@@ -2299,7 +2346,7 @@ void TextureNode::PrepareNode()
 
     // This will notify the rendering thread that the texture is now being
     // rendered and it can start rendering to the other one.
-    emit TextureInUse();
+    emit TextureInUse(&this->renderSync);
   }
 }
 
@@ -2442,7 +2489,8 @@ QSGNode *RenderWindowItem::updatePaintNode(QSGNode *_node,
 
     // Get the production of FBO textures started..
     QMetaObject::invokeMethod(this->dataPtr->renderThread, "RenderNext",
-        Qt::QueuedConnection);
+                              Qt::QueuedConnection,
+                              Q_ARG( RenderSync*, &node->renderSync ));
   }
 
   node->setRect(this->boundingRect());

src/gui/plugins/scene3d/Scene3D.hh

@@ -166,6 +166,45 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     private: std::unique_ptr<Scene3DPrivate> dataPtr;
   };
 
+  class RenderSync
+  {
+    /// \brief Cond. variable to synchronize rendering on specific events
+    /// (e.g. texture resize) or for debugging (e.g. keep
+    /// all API calls sequential)
+    public: std::mutex mutex;
+
+    /// \brief Cond. variable to synchronize rendering on specific events
+    /// (e.g. texture resize) or for debugging (e.g. keep
+    /// all API calls sequential)
+    public: std::condition_variable cv;
+
+    public: enum class RenderStallState
+            {
+              /// All threads will continue as normal
+              Unblocked,
+              /// Worker thread requested Qt thread to be blocked; and is
+              /// waiting for Qt thread to see this
+              WorkerThreadRequested,
+              /// Qt thread saw WorkerThreadRequested, set the variable to
+              /// QtThreadBlocked; and is now waiting for worker thread to do
+              /// its thing and set it back to Unblocked so that Qt can resume
+              QtThreadBlocked
+            };
+
+    /// \brief See TextureNode::RenderSync::RenderStallState
+    public: RenderStallState renderStallState =
+        RenderStallState::Unblocked /*GUARDED_BY(sharedRenderMutex)*/;
+
+    /// \brief Must be called from worker thread when we want to block
+    /// \param[in] lock Acquired lock. Must be based on this->mutex
+    public: void requestQtThreadToBlock(std::unique_lock<std::mutex> &lock);
+    /// \brief Must be called from worker thread when we are done
+    /// \param[in] lock Acquired lock. Must be based on this->mutex
+    public: void releaseQtThreadFromBlock(std::unique_lock<std::mutex> &lock);
+    /// \brief Must be called from Qt thread periodically
+    public: void waitForWorkerThread();
+  };
+
   /// \brief Ign-rendering renderer.
   /// All ign-rendering calls should be performed inside this class as it makes
   /// sure that opengl calls in the underlying render engine do not interfere
@@ -183,7 +222,7 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     public: ~IgnRenderer() override;
 
     ///  \brief Main render function
-    public: void Render();
+    public: void Render(RenderSync *renderSync);
 
     /// \brief Initialize the render engine
     public: void Initialize();
@@ -450,7 +489,10 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
         bool _waitForTarget);
 
     /// \brief Render texture id
-    public: GLuint textureId = 0u;
+    /// Values is constantly constantly cycled/swapped/changed
+    /// from a worker thread
+    /// Don't read this directly
+    public: GLuint textureId;
 
     /// \brief Initial Camera pose
     public: math::Pose3d cameraPose = math::Pose3d(0, 0, 2, 0, 0.4, 0);
@@ -484,7 +526,7 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     public: RenderThread();
 
     /// \brief Render the next frame
-    public slots: void RenderNext();
+    public slots: void RenderNext(RenderSync *renderSync);
 
     /// \brief Shutdown the thread and the render engine
     public slots: void ShutDown();
@@ -496,7 +538,7 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     /// to be displayed
     /// \param[in] _id GLuid of the opengl texture
     /// \param[in] _size Size of the texture
-    signals: void TextureReady(int _id, const QSize &_size);
+    signals: void TextureReady(uint _id, const QSize &_size);
 
     /// \brief Offscreen surface to render to
     public: QOffscreenSurface *surface = nullptr;
@@ -730,29 +772,32 @@ inline namespace IGNITION_GAZEBO_VERSION_NAMESPACE {
     ///  store the texture id and size and schedule an update on the window.
     /// \param[in] _id OpenGL render texture Id
     /// \param[in] _size Texture size
-    public slots: void NewTexture(int _id, const QSize &_size);
+    public slots: void NewTexture(uint _id, const QSize &_size);
 
     /// \brief Before the scene graph starts to render, we update to the
     /// pending texture
     public slots: void PrepareNode();
 
     /// \brief Signal emitted when the texture is being rendered and renderer
     /// can start rendering next frame
-    signals: void TextureInUse();
+    signals: void TextureInUse(RenderSync *renderSync);
 
     /// \brief Signal emitted when a new texture is ready to trigger window
     /// update
     signals: void PendingNewTexture();
 
     /// \brief OpenGL texture id
-    public: int id = 0;
+    public: uint id = 0;
 
     /// \brief Texture size
     public: QSize size = QSize(0, 0);
 
     /// \brief Mutex to protect the texture variables
     public: QMutex mutex;
 
+    /// \brief See RenderSync
+    public: RenderSync renderSync;
+
     /// \brief Qt's scene graph texture
     public: QSGTexture *texture = nullptr;