only return locked FontStack objects

include/mbgl/util/exclusive.hpp

@@ -0,0 +1,30 @@
+#ifndef MBGL_UTIL_EXCLUSIVE
+#define MBGL_UTIL_EXCLUSIVE
+
+#include <memory>
+#include <mutex>
+
+
+namespace mbgl {
+namespace util {
+
+template <class T>
+class exclusive {
+public:
+    inline exclusive(T* val, std::unique_ptr<std::lock_guard<std::mutex>> mtx) : ptr(val), lock(std::move(mtx)) {}
+
+    inline T* operator->() { return ptr; }
+    inline const T* operator->() const { return ptr; }
+    inline T* operator*() { return ptr; }
+    inline const T* operator*() const { return ptr; }
+
+private:
+    T *ptr;
+    std::unique_ptr<std::lock_guard<std::mutex>> lock;
+};
+
+
+} // end namespace util
+} // end namespace mbgl
+
+#endif