bpo-33238: Add InvalidStateError to concurrent.futures. (GH-7056)

Future.set_result and Future.set_exception now raise InvalidStateError
if the futures are not pending or running. This mirrors the behavior
of asyncio.Future, and prevents AssertionErrors in asyncio.wrap_future
when set_result is called multiple times.

2 files changed, 9 insertions, 0 deletions

diff --git a/Lib/concurrent/futures/__init__.py b/Lib/concurrent/futures/__init__.py
index 8434fcf..d746aea 100644
--- a/Lib/concurrent/futures/__init__.py
+++ b/Lib/concurrent/futures/__init__.py
@@ -10,6 +10,7 @@ from concurrent.futures._base import (FIRST_COMPLETED,
                                       ALL_COMPLETED,
                                       CancelledError,
                                       TimeoutError,
+                                      InvalidStateError,
                                       BrokenExecutor,
                                       Future,
                                       Executor,
diff --git a/Lib/concurrent/futures/_base.py b/Lib/concurrent/futures/_base.py
index 4f22f7e..d4416c6 100644
--- a/Lib/concurrent/futures/_base.py
+++ b/Lib/concurrent/futures/_base.py
@@ -53,6 +53,10 @@ class TimeoutError(Error):
     """The operation exceeded the given deadline."""
     pass
 
+class InvalidStateError(Error):
+    """The operation is not allowed in this state."""
+    pass
+
 class _Waiter(object):
     """Provides the event that wait() and as_completed() block on."""
     def __init__(self):
@@ -513,6 +517,8 @@ class Future(object):
         Should only be used by Executor implementations and unit tests.
         """
         with self._condition:
+            if self._state in {CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED}:
+                raise InvalidStateError('{}: {!r}'.format(self._state, self))
             self._result = result
             self._state = FINISHED
             for waiter in self._waiters:
@@ -526,6 +532,8 @@ class Future(object):
         Should only be used by Executor implementations and unit tests.
         """
         with self._condition:
+            if self._state in {CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED}:
+                raise InvalidStateError('{}: {!r}'.format(self._state, self))
             self._exception = exception
             self._state = FINISHED
             for waiter in self._waiters: