Particles: fix handling of world-attached particles

Fix the cull callback to continue with normal culling. Also move
where the particle frame is added to the scene graph, to be sure
we don’t modify the scene during OSG traversals.

simgear/scene/model/particles.cxx

@@ -65,7 +65,7 @@ public:
 
     void updateParticleSystemsFromCullCallback(int currentFrameNumber, osg::NodeVisitor* nv);
 
-    void addParticleSystem(osgParticle::ParticleSystem* ps);
+    void addParticleSystem(osgParticle::ParticleSystem* ps, const osg::ref_ptr<osg::Group>& frame);
 
     void registerNewLocalParticleSystem(osg::Node* node, ParticleSystemRef ps);
     void registerNewWorldParticleSystem(osg::Node* node, ParticleSystemRef ps, osg::Group* frame);
@@ -90,6 +90,9 @@ public:
 
     ParticleSystemsWeakRefVec _systems;
     osg::ref_ptr<ParticlesGlobalManager::UpdaterCallback> _cullCallback;
+
+    using GroupRefVec = std::vector<osg::ref_ptr<osg::Group>>;
+    GroupRefVec _newWorldParticles;
 };
 
 /**
@@ -112,6 +115,11 @@ public:
                 _manager->updateParticleSystemsFromCullCallback(_frameNumber, nv);
             }
         }
+
+        // note, callback is responsible for scenegraph traversal so
+        // they must call traverse(node,nv) to ensure that the
+        // scene graph subtree (and associated callbacks) are traversed.
+        traverse(node, nv);
     }
 
     unsigned int _frameNumber = 0;
@@ -129,14 +137,7 @@ public:
     void operator()(osg::Node* node, osg::NodeVisitor* nv) override
     {
         auto d = ParticlesGlobalManager::instance()->d;
-        d->addParticleSystem(_system);
-
-        // for world-attached particle systems, attach the frame to the
-        // common root. This is the first safe place we can do this
-        if (_frame) {
-            d->internalGetCommonRoot()->addChild(_frame);
-        }
-
+        d->addParticleSystem(_system, _frame);
         node->removeUpdateCallback(this); // suicide
     }
 
@@ -151,10 +152,16 @@ ParticlesGlobalManager::ParticlesGlobalManagerPrivate::ParticlesGlobalManagerPri
     // this constructor might be called from an osgDB thread
 }
 
-void ParticlesGlobalManager::ParticlesGlobalManagerPrivate::addParticleSystem(osgParticle::ParticleSystem* ps)
+void ParticlesGlobalManager::ParticlesGlobalManagerPrivate::addParticleSystem(osgParticle::ParticleSystem* ps, const osg::ref_ptr<osg::Group>& frame)
 {
     std::lock_guard<std::mutex> g(_lock);
     _systems.push_back(ps);
+
+    // we're inside an update callback here, so better not modify the
+    // scene structure; defer it to later
+    if (frame.get()) {
+        _newWorldParticles.push_back(frame);
+    }
 }
 
 void ParticlesGlobalManager::ParticlesGlobalManagerPrivate::registerNewLocalParticleSystem(osg::Node* node, ParticleSystemRef ps)
@@ -182,16 +189,17 @@ void ParticlesGlobalManager::initFromMainThread()
     d->_commonRoot->addCullCallback(d->_cullCallback);
 }
 
-void ParticlesGlobalManager::setCurrentPosition(const SGGeod& pos)
-{
-    std::lock_guard<std::mutex> g(d->_lock);
-    d->_currentPosition = pos;
-}
-
-void ParticlesGlobalManager::setSimTime(double dt)
+void ParticlesGlobalManager::update(double dt, const SGGeod& pos)
 {
     std::lock_guard<std::mutex> g(d->_lock);
     d->_simulationDt = dt;
+    d->_currentPosition = pos;
+
+    for (auto f : d->_newWorldParticles) {
+        d->internalGetCommonRoot()->addChild(f);
+    }
+
+    d->_newWorldParticles.clear();
 }
 
 // this is called from the main thread, since it's an update callback

simgear/scene/model/particles.hxx

@@ -167,8 +167,10 @@ public:
     void setFrozen(bool e);
     bool isFrozen() const;
 
-    void setCurrentPosition(const SGGeod& pos);
-    void setSimTime(double dt);
+    /**
+        @brief update function: call from the main thread , outside of OSG calls.
+     */
+    void update(double dt, const SGGeod& pos);
 
     void setSwitchNode(const SGPropertyNode* n);