fix asynchrounous loading of Chunks

Mutex is only necessary for modifying the QCache object.
When spanning Mutex also over parsing section, only one Chunk will be
loaded and all others are blocked.

chunkcache.cpp

@@ -59,9 +59,9 @@ QString ChunkCache::getPath() {
 
 Chunk *ChunkCache::fetch(int x, int z) {
   ChunkID id(x, z);
-  mutex.lock();
-  Chunk *chunk = cache[id];
-  mutex.unlock();
+  //mutex.lock();
+  Chunk *chunk = cache[id];   // const operation
+  //mutex.unlock();
   if (chunk != NULL) {
     if (chunk->loaded)
       return chunk;
@@ -72,7 +72,7 @@ Chunk *ChunkCache::fetch(int x, int z) {
   connect(chunk, SIGNAL(structureFound(QSharedPointer<GeneratedStructure>)),
           this,  SLOT  (routeStructure(QSharedPointer<GeneratedStructure>)));
   mutex.lock();
-  cache.insert(id, chunk);
+  cache.insert(id, chunk);    // non-const operation !
   mutex.unlock();
   ChunkLoader *loader = new ChunkLoader(path, x, z, cache, &mutex);
   connect(loader, SIGNAL(loaded(int, int)),

chunkloader.cpp

@@ -13,6 +13,7 @@ ChunkLoader::~ChunkLoader() {
 }
 
 void ChunkLoader::run() {
+  // get coordinates of Region file
   int rx = x >> 5;
   int rz = z >> 5;
 
@@ -42,13 +43,17 @@ void ChunkLoader::run() {
     emit loaded(x, z);
     return;
   }
-  NBT nbt(raw);
+  // get existing Chunk entry from Cache
   ChunkID id(x, z);
-  mutex->lock();
-  Chunk *chunk = cache[id];
-  if (chunk)
+  //mutex->lock();
+  Chunk *chunk = cache[id];   // const operation
+  //mutex->unlock();
+  // parse Chunk data
+  // Chunk will be flagged "loaded" in a thread save way
+  if (chunk) {
+    NBT nbt(raw);
     chunk->load(nbt);
-  mutex->unlock();
+  }
   f.unmap(raw);
   f.close();