Add thread-safe lock for _state and _rejection_handler0 modifications

fixes syrusakbary#87

promise/promise.py

@@ -1,7 +1,7 @@
 from collections import namedtuple
 from functools import partial, wraps
 from sys import version_info, exc_info
-from threading import RLock
+from threading import Lock
 from types import TracebackType
 from weakref import WeakKeyDictionary
 
@@ -43,6 +43,8 @@
 
 async_instance = Async()
 
+_state_lock = Lock()
+
 
 def get_default_scheduler():
     # type: () -> ImmediateScheduler
@@ -232,8 +234,9 @@ def _fulfill(self, value):
             err = make_self_resolution_error()
             # self._attach_extratrace(err)
             return self._reject(err)
-        self._state = STATE_FULFILLED
-        self._rejection_handler0 = value
+        with _state_lock:
+            self._state = STATE_FULFILLED
+            self._rejection_handler0 = value
 
         if self._length > 0:
             if self._is_async_guaranteed:
@@ -570,12 +573,14 @@ def _then(
     ):
         # type: (...) -> Promise[S]
         promise = self.__class__()  # type: Promise
-        target = self._target()
 
-        state = target._state
-        if state == STATE_PENDING:
-            target._add_callbacks(did_fulfill, did_reject, promise)
-        else:
+        with _state_lock:
+            target = self._target()
+            state = target._state
+            if state == STATE_PENDING:
+                target._add_callbacks(did_fulfill, did_reject, promise)
+
+        if state != STATE_PENDING:
             traceback = None
             if state == STATE_FULFILLED:
                 value = target._rejection_handler0