Fix race condition and bad memory access highlighting render tiles

Is was possible that interface will be refreshed at thesame time
as render engine will start freeing render parts.

Not sure if we can get away without RW mutex here, seems we need
one way of synchronization or another..

source/blender/render/intern/include/render_types.h

@@ -194,6 +194,7 @@ struct Render
 	struct Object *camera_override;
 	unsigned int lay, layer_override;
 	
+	ThreadRWMutex partsmutex;
 	ListBase parts;
 	
 	/* render engine */

source/blender/render/intern/source/external_engine.c

@@ -375,9 +375,12 @@ void RE_engine_get_current_tiles(Render *re, int *total_tiles_r, rcti **tiles_r)
 	rcti *tiles = NULL;
 	int allocation_size = 0, allocation_step = BLENDER_MAX_THREADS;
 
+	BLI_rw_mutex_lock(&re->partsmutex, THREAD_LOCK_READ);
+
 	if (re->engine && (re->engine->flag & RE_ENGINE_HIGHLIGHT_TILES) == 0) {
 		*total_tiles_r = 0;
 		*tiles_r = NULL;
+		BLI_rw_mutex_unlock(&re->partsmutex);
 		return;
 	}
 
@@ -404,7 +407,7 @@ void RE_engine_get_current_tiles(Render *re, int *total_tiles_r, rcti **tiles_r)
 			total_tiles++;
 		}
 	}
-
+	BLI_rw_mutex_unlock(&re->partsmutex);
 	*total_tiles_r = total_tiles;
 	*tiles_r = tiles;
 }
@@ -478,6 +481,8 @@ bool RE_bake_engine(
 	engine->tile_y = 0;
 	engine->flag &= ~RE_ENGINE_RENDERING;
 
+	BLI_rw_mutex_lock(&re->partsmutex, THREAD_LOCK_WRITE);
+
 	/* re->engine becomes zero if user changed active render engine during render */
 	if (!persistent_data || !re->engine) {
 		RE_engine_free(engine);
@@ -485,6 +490,7 @@ bool RE_bake_engine(
 	}
 
 	RE_parts_free(re);
+	BLI_rw_mutex_unlock(&re->partsmutex);
 
 	if (BKE_reports_contain(re->reports, RPT_ERROR))
 		G.is_break = true;
@@ -663,6 +669,8 @@ int RE_engine_render(Render *re, int do_all)
 
 	render_result_free_list(&engine->fullresult, engine->fullresult.first);
 
+	BLI_rw_mutex_lock(&re->partsmutex, THREAD_LOCK_WRITE);
+
 	/* re->engine becomes zero if user changed active render engine during render */
 	if (!persistent_data || !re->engine) {
 		RE_engine_free(engine);
@@ -682,6 +690,7 @@ int RE_engine_render(Render *re, int do_all)
 	}
 
 	RE_parts_free(re);
+	BLI_rw_mutex_unlock(&re->partsmutex);
 
 	if (BKE_reports_contain(re->reports, RPT_ERROR))
 		G.is_break = true;

source/blender/render/intern/source/pipeline.c

@@ -385,6 +385,7 @@ Render *RE_NewRender(const char *name)
 		BLI_addtail(&RenderGlobal.renderlist, re);
 		BLI_strncpy(re->name, name, RE_MAXNAME);
 		BLI_rw_mutex_init(&re->resultmutex);
+		BLI_rw_mutex_init(&re->partsmutex);
 		re->eval_ctx = MEM_callocN(sizeof(EvaluationContext), "re->eval_ctx");
 		re->eval_ctx->mode = DAG_EVAL_RENDER;
 	}
@@ -423,6 +424,7 @@ void RE_FreeRender(Render *re)
 		RE_engine_free(re->engine);
 
 	BLI_rw_mutex_end(&re->resultmutex);
+	BLI_rw_mutex_end(&re->partsmutex);
 
 	BLI_freelistN(&re->r.layers);
 	
@@ -1268,8 +1270,9 @@ static void threaded_tile_processor(Render *re)
 
 	/* unset threadsafety */
 	g_break = 0;
-	
+	BLI_rw_mutex_lock(&re->partsmutex, THREAD_LOCK_WRITE);
 	RE_parts_free(re);
+	BLI_rw_mutex_unlock(&re->partsmutex);
 	re->viewplane = viewplane; /* restore viewplane, modified by pano render */
 }