Only call initial_setup_default on the main thread, and document that shader duplication can crash if done from a different thread. Removed the material mutex from the prop cache as it's not needed anymore.

material_cache/prop_material_cache.cpp

@@ -282,6 +282,8 @@ void PropMaterialCache::refresh_rects() {
 }
 
 void PropMaterialCache::initial_setup_default() {
+	//Note: call only on the main thread! Shader->duplicate() can crash if done from an another thread!
+
 	PropCache *pc = PropCache::get_singleton();
 
 	pc->ensure_materials_loaded();

prop_instance_merger.cpp

@@ -533,6 +533,21 @@ void PropInstanceMerger::_build() {
 
 	Ref<PropMaterialCache> cache = PropCache::get_singleton()->material_cache_get(_prop_data);
 
+	if (!cache->get_initialized()) {
+		//lock it!
+		cache->mutex_lock();
+
+		//check again, this thread might have gotten here after an another one already did the initialization!
+		//this check might not be needed here
+		if (!cache->get_initialized()) {
+			//this will set up materials, and settings
+			//needs to be called from the main thread!
+			cache->initial_setup_default();
+		}
+
+		cache->mutex_unlock();
+	}
+
 	_job->set_material_cache(cache);
 
 	prop_preprocess(Transform(), _prop_data);
@@ -588,7 +603,6 @@ void PropInstanceMerger::_prop_preprocess(Transform transform, const Ref<PropDat
 		Ref<PropDataTiledWall> tiled_wall_data = e;
 
 		if (tiled_wall_data.is_valid()) {
-
 			_job->add_tiled_wall(tiled_wall_data, t);
 
 			if (tiled_wall_data->get_collision()) {

prop_instance_prop_job.cpp

@@ -267,7 +267,9 @@ void PropInstancePropJob::phase_setup_cache() {
 			//check again, this thread might have gotten here after an another one already did the initialization!
 			if (!_material_cache->get_initialized()) {
 				//this will set up materials, and settings
-				_material_cache->initial_setup_default();
+				//Can only be called from the main thread!
+				//Merger calls this
+				//_material_cache->initial_setup_default();
 
 				_material_cache->prop_add_textures(_prop_instace->get_prop_data());
 

singleton/prop_cache.cpp

@@ -170,13 +170,9 @@ void PropCache::materials_load() {
 }
 
 void PropCache::ensure_materials_loaded() {
-	_material_mutex.lock();
-
 	if (_materials.size() != _material_paths.size()) {
 		materials_load();
 	}
-
-	_material_mutex.unlock();
 }
 
 Vector<Variant> PropCache::materials_get() {

singleton/prop_cache.h

@@ -122,8 +122,6 @@ protected:
 	Mutex _tiled_wall_material_cache_mutex;
 	Mutex _custom_keyed_material_cache_mutex;
 
-	Mutex _material_mutex;
-
 #ifdef TEXTURE_PACKER_PRESENT
 	int _texture_flags;
 	int _max_atlas_size;

tiled_wall/tiled_wall.cpp

@@ -155,6 +155,7 @@ void TiledWall::refresh() {
 
 		//An anouther thread could have initialized it before wo got the mutex!
 		if (!_cache->get_initialized()) {
+			//can only be called from the main thread!
 			_cache->initial_setup_default();
 
 			_data->setup_cache(_cache);