Fix ThreadSafeSingleton synchronization issue (#434)

* Fix ThreadSafeSingleton synchronization issue

* Update changelog

docs/main/changelog.rst

@@ -7,6 +7,12 @@ that were made in every particular version.
 From version 0.7.6 *Dependency Injector* framework strictly 
 follows `Semantic versioning`_
 
+Development version
+-------------------
+- Fix ``ThreadSafeSingleton`` synchronization issue.
+  See issue: `#433 <https://github.com/ets-labs/python-dependency-injector/issues/433>`_.
+  Thanks to `@garlandhu <https://github.com/garlandhu>`_ for reporting the issue.
+
 4.31.0
 ------
 - Implement providers' lazy initialization.

src/dependency_injector/providers.pyx

@@ -2819,17 +2819,14 @@ cdef class ThreadSafeSingleton(BaseSingleton):
         if instance is None:
             with self.__storage_lock:
                 if self.__storage is None:
-                    instance = __factory_call(self.__instantiator, args, kwargs)
-
-                    if __is_future_or_coroutine(instance):
+                    result = __factory_call(self.__instantiator, args, kwargs)
+                    if __is_future_or_coroutine(result):
                         future_result = asyncio.Future()
-                        instance = asyncio.ensure_future(instance)
-                        instance.add_done_callback(functools.partial(self._async_init_instance, future_result))
-                        self.__storage = future_result
-                        return future_result
-
-                    self.__storage = instance
-
+                        result = asyncio.ensure_future(result)
+                        result.add_done_callback(functools.partial(self._async_init_instance, future_result))
+                        result = future_result
+                    self.__storage = result
+                instance = self.__storage
         return instance