Merge pull request #521 from nathanielmanistaatgoogle/future

Change the interface of RPC Framework's Future interface.

6 files changed, 509 insertions, 219 deletions

diff --git a/src/python/src/_framework/face/_calls.py b/src/python/src/_framework/face/_calls.py
index 9128aef7c4..a7d8be5e43 100644
--- a/src/python/src/_framework/face/_calls.py
+++ b/src/python/src/_framework/face/_calls.py
@@ -29,6 +29,7 @@
 
 """Utility functions for invoking RPCs."""
 
+import sys
 import threading
 
 from _framework.base import interfaces as base_interfaces
@@ -79,20 +80,46 @@ def _stream_event_subscription(result_consumer, abortion_callback):
       _EventServicedIngestor(result_consumer, abortion_callback))
 
 
+# NOTE(nathaniel): This class has some extremely special semantics around
+# cancellation that allow it to be used by both "blocking" APIs and "futures"
+# APIs.
+#
+# Since futures.Future defines its own exception for cancellation, we want these
+# objects, when returned by methods of a returning-Futures-from-other-methods
+# object, to raise the same exception for cancellation. But that's weird in a
+# blocking API - why should this object, also returned by methods of blocking
+# APIs, raise exceptions from the "future" module? Should we do something like
+# have this class be parameterized by the type of exception that it raises in
+# cancellation circumstances?
+#
+# We don't have to take such a dramatic step: since blocking APIs define no
+# cancellation semantics whatsoever, there is no supported way for
+# blocking-API-users of these objects to cancel RPCs, and thus no supported way
+# for them to see an exception the type of which would be weird to them.
+#
+# Bonus: in both blocking and futures APIs, this object still properly raises
+# exceptions.CancellationError for any *server-side cancellation* of an RPC.
 class _OperationCancellableIterator(interfaces.CancellableIterator):
   """An interfaces.CancellableIterator for response-streaming operations."""
 
   def __init__(self, rendezvous, operation):
+    self._lock = threading.Lock()
     self._rendezvous = rendezvous
     self._operation = operation
+    self._cancelled = False
 
   def __iter__(self):
     return self
 
   def next(self):
+    with self._lock:
+      if self._cancelled:
+        raise future.CancelledError()
     return next(self._rendezvous)
 
   def cancel(self):
+    with self._lock:
+      self._cancelled = True
     self._operation.cancel()
     self._rendezvous.set_outcome(base_interfaces.Outcome.CANCELLED)
 
@@ -105,46 +132,126 @@ class _OperationFuture(future.Future):
     self._rendezvous = rendezvous
     self._operation = operation
 
-    self._outcome = None
+    self._cancelled = False
+    self._computed = False
+    self._payload = None
+    self._exception = None
+    self._traceback = None
     self._callbacks = []
 
   def cancel(self):
     """See future.Future.cancel for specification."""
     with self._condition:
-      if self._outcome is None:
+      if not self._cancelled and not self._computed:
         self._operation.cancel()
-        self._outcome = future.aborted()
+        self._cancelled = True
         self._condition.notify_all()
     return False
 
   def cancelled(self):
     """See future.Future.cancelled for specification."""
-    return False
+    with self._condition:
+      return self._cancelled
+
+  def running(self):
+    """See future.Future.running for specification."""
+    with self._condition:
+      return not self._cancelled and not self._computed
 
   def done(self):
     """See future.Future.done for specification."""
     with self._condition:
-      return (self._outcome is not None and
-              self._outcome.category is not future.ABORTED)
+      return self._cancelled or self._computed
+
+  def result(self, timeout=None):
+    """See future.Future.result for specification."""
+    with self._condition:
+      if self._cancelled:
+        raise future.CancelledError()
+      if self._computed:
+        if self._payload is None:
+          raise self._exception  # pylint: disable=raising-bad-type
+        else:
+          return self._payload
+
+      condition = threading.Condition()
+      def notify_condition(unused_future):
+        with condition:
+          condition.notify()
+      self._callbacks.append(notify_condition)
+
+    with condition:
+      condition.wait(timeout=timeout)
+
+    with self._condition:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        if self._payload is None:
+          raise self._exception  # pylint: disable=raising-bad-type
+        else:
+          return self._payload
+      else:
+        raise future.TimeoutError()
+
+  def exception(self, timeout=None):
+    """See future.Future.exception for specification."""
+    with self._condition:
+      if self._cancelled:
+        raise future.CancelledError()
+      if self._computed:
+        return self._exception
+
+      condition = threading.Condition()
+      def notify_condition(unused_future):
+        with condition:
+          condition.notify()
+      self._callbacks.append(notify_condition)
+
+    with condition:
+      condition.wait(timeout=timeout)
 
-  def outcome(self):
-    """See future.Future.outcome for specification."""
     with self._condition:
-      while self._outcome is None:
-        self._condition.wait()
-      return self._outcome
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._exception
+      else:
+        raise future.TimeoutError()
 
-  def add_done_callback(self, callback):
+  def traceback(self, timeout=None):
+    """See future.Future.traceback for specification."""
+    with self._condition:
+      if self._cancelled:
+        raise future.CancelledError()
+      if self._computed:
+        return self._traceback
+
+      condition = threading.Condition()
+      def notify_condition(unused_future):
+        with condition:
+          condition.notify()
+      self._callbacks.append(notify_condition)
+
+    with condition:
+      condition.wait(timeout=timeout)
+
+    with self._condition:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._traceback
+      else:
+        raise future.TimeoutError()
+
+  def add_done_callback(self, fn):
     """See future.Future.add_done_callback for specification."""
     with self._condition:
       if self._callbacks is not None:
-        self._callbacks.add(callback)
+        self._callbacks.add(fn)
         return
 
-      outcome = self._outcome
-
-    callable_util.call_logging_exceptions(
-        callback, _DONE_CALLBACK_LOG_MESSAGE, outcome)
+    callable_util.call_logging_exceptions(fn, _DONE_CALLBACK_LOG_MESSAGE, self)
 
   def on_operation_termination(self, operation_outcome):
     """Indicates to this object that the operation has terminated.
@@ -154,34 +261,42 @@ class _OperationFuture(future.Future):
         outcome of the operation.
     """
     with self._condition:
-      if (self._outcome is None and
-          operation_outcome is not base_interfaces.Outcome.COMPLETED):
-        self._outcome = future.raised(
-            _control.abortion_outcome_to_exception(operation_outcome))
-        self._condition.notify_all()
-
-      outcome = self._outcome
-      rendezvous = self._rendezvous
-      callbacks = list(self._callbacks)
-      self._callbacks = None
-
-    if outcome is None:
-      try:
-        return_value = next(rendezvous)
-      except Exception as e:  # pylint: disable=broad-except
-        outcome = future.raised(e)
+      cancelled = self._cancelled
+      if cancelled:
+        callbacks = list(self._callbacks)
+        self._callbacks = None
       else:
-        outcome = future.returned(return_value)
+        rendezvous = self._rendezvous
+
+    if not cancelled:
+      payload = None
+      exception = None
+      traceback = None
+      if operation_outcome == base_interfaces.Outcome.COMPLETED:
+        try:
+          payload = next(rendezvous)
+        except Exception as e:  # pylint: disable=broad-except
+          exception = e
+          traceback = sys.exc_info()[2]
+      else:
+        try:
+          # We raise and then immediately catch in order to create a traceback.
+          raise _control.abortion_outcome_to_exception(operation_outcome)
+        except Exception as e:  # pylint: disable=broad-except
+          exception = e
+          traceback = sys.exc_info()[2]
       with self._condition:
-        if self._outcome is None:
-          self._outcome = outcome
-          self._condition.notify_all()
-        else:
-          outcome = self._outcome
+        if not self._cancelled:
+          self._computed = True
+          self._payload = payload
+          self._exception = exception
+          self._traceback = traceback
+        callbacks = list(self._callbacks)
+        self._callbacks = None
 
     for callback in callbacks:
       callable_util.call_logging_exceptions(
-          callback, _DONE_CALLBACK_LOG_MESSAGE, outcome)
+          callback, _DONE_CALLBACK_LOG_MESSAGE, self)
 
 
 class _Call(interfaces.Call):
diff --git a/src/python/src/_framework/face/testing/future_invocation_asynchronous_event_service_test_case.py b/src/python/src/_framework/face/testing/future_invocation_asynchronous_event_service_test_case.py
index cf8b2eeb95..939b238b66 100644
--- a/src/python/src/_framework/face/testing/future_invocation_asynchronous_event_service_test_case.py
+++ b/src/python/src/_framework/face/testing/future_invocation_asynchronous_event_service_test_case.py
@@ -116,7 +116,7 @@ class FutureInvocationAsynchronousEventServiceTestCase(
 
         response_future = self.stub.future_value_in_value_out(
             name, request, _TIMEOUT)
-        response = response_future.outcome().return_value
+        response = response_future.result()
 
         test_messages.verify(request, response, self)
 
@@ -144,7 +144,7 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with request_iterator.pause():
           response_future = self.stub.future_stream_in_value_out(
               name, request_iterator, _TIMEOUT)
-        response = response_future.outcome().return_value
+        response = response_future.result()
 
         test_messages.verify(requests, response, self)
 
@@ -173,13 +173,13 @@ class FutureInvocationAsynchronousEventServiceTestCase(
 
         first_response_future = self.stub.future_value_in_value_out(
             name, first_request, _TIMEOUT)
-        first_response = first_response_future.outcome().return_value
+        first_response = first_response_future.result()
 
         test_messages.verify(first_request, first_response, self)
 
         second_response_future = self.stub.future_value_in_value_out(
             name, second_request, _TIMEOUT)
-        second_response = second_response_future.outcome().return_value
+        second_response = second_response_future.result()
 
         test_messages.verify(second_request, second_response, self)
 
@@ -192,10 +192,10 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.pause():
           response_future = self.stub.future_value_in_value_out(
               name, request, _TIMEOUT)
-          outcome = response_future.outcome()
-
-        self.assertIsInstance(
-            outcome.exception, exceptions.ExpirationError)
+          self.assertIsInstance(
+              response_future.exception(), exceptions.ExpirationError)
+          with self.assertRaises(exceptions.ExpirationError):
+            response_future.result()
 
   def testExpiredUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -203,11 +203,11 @@ class FutureInvocationAsynchronousEventServiceTestCase(
       for test_messages in test_messages_sequence:
         request = test_messages.request()
 
-        with self.control.pause(), self.assertRaises(
-            exceptions.ExpirationError):
+        with self.control.pause():
           response_iterator = self.stub.inline_value_in_stream_out(
               name, request, _TIMEOUT)
-          list(response_iterator)
+          with self.assertRaises(exceptions.ExpirationError):
+            list(response_iterator)
 
   def testExpiredStreamRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -218,10 +218,10 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.pause():
           response_future = self.stub.future_stream_in_value_out(
               name, iter(requests), _TIMEOUT)
-          outcome = response_future.outcome()
-
-        self.assertIsInstance(
-            outcome.exception, exceptions.ExpirationError)
+          self.assertIsInstance(
+              response_future.exception(), exceptions.ExpirationError)
+          with self.assertRaises(exceptions.ExpirationError):
+            response_future.result()
 
   def testExpiredStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -229,11 +229,11 @@ class FutureInvocationAsynchronousEventServiceTestCase(
       for test_messages in test_messages_sequence:
         requests = test_messages.requests()
 
-        with self.control.pause(), self.assertRaises(
-            exceptions.ExpirationError):
+        with self.control.pause():
           response_iterator = self.stub.inline_stream_in_stream_out(
               name, iter(requests), _TIMEOUT)
-          list(response_iterator)
+          with self.assertRaises(exceptions.ExpirationError):
+            list(response_iterator)
 
   def testFailedUnaryRequestUnaryResponse(self):
     for name, test_messages_sequence in (
@@ -244,13 +244,15 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.fail():
           response_future = self.stub.future_value_in_value_out(
               name, request, _TIMEOUT)
-          outcome = response_future.outcome()
 
-        # Because the servicer fails outside of the thread from which the
-        # servicer-side runtime called into it its failure is indistinguishable
-        # from simply not having called its response_callback before the
-        # expiration of the RPC.
-        self.assertIsInstance(outcome.exception, exceptions.ExpirationError)
+          # Because the servicer fails outside of the thread from which the
+          # servicer-side runtime called into it its failure is
+          # indistinguishable from simply not having called its
+          # response_callback before the expiration of the RPC.
+          self.assertIsInstance(
+              response_future.exception(), exceptions.ExpirationError)
+          with self.assertRaises(exceptions.ExpirationError):
+            response_future.result()
 
   def testFailedUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -276,13 +278,15 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.fail():
           response_future = self.stub.future_stream_in_value_out(
               name, iter(requests), _TIMEOUT)
-          outcome = response_future.outcome()
 
-        # Because the servicer fails outside of the thread from which the
-        # servicer-side runtime called into it its failure is indistinguishable
-        # from simply not having called its response_callback before the
-        # expiration of the RPC.
-        self.assertIsInstance(outcome.exception, exceptions.ExpirationError)
+          # Because the servicer fails outside of the thread from which the
+          # servicer-side runtime called into it its failure is
+          # indistinguishable from simply not having called its
+          # response_callback before the expiration of the RPC.
+          self.assertIsInstance(
+              response_future.exception(), exceptions.ExpirationError)
+          with self.assertRaises(exceptions.ExpirationError):
+            response_future.result()
 
   def testFailedStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -310,8 +314,8 @@ class FutureInvocationAsynchronousEventServiceTestCase(
             name, first_request, _TIMEOUT)
         second_response_future = self.stub.future_value_in_value_out(
             name, second_request, _TIMEOUT)
-        first_response = first_response_future.outcome().return_value
-        second_response = second_response_future.outcome().return_value
+        first_response = first_response_future.result()
+        second_response = second_response_future.result()
 
         test_messages.verify(first_request, first_response, self)
         test_messages.verify(second_request, second_response, self)
@@ -329,10 +333,10 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.pause():
           response_future = self.stub.future_value_in_value_out(
               name, request, _TIMEOUT)
-          cancelled = response_future.cancel()
+          cancel_method_return_value = response_future.cancel()
 
-        self.assertFalse(cancelled)
-        self.assertEqual(future.ABORTED, response_future.outcome().category)
+        self.assertFalse(cancel_method_return_value)
+        self.assertTrue(response_future.cancelled())
 
   def testCancelledUnaryRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -345,7 +349,7 @@ class FutureInvocationAsynchronousEventServiceTestCase(
               name, request, _TIMEOUT)
           response_iterator.cancel()
 
-        with self.assertRaises(exceptions.CancellationError):
+        with self.assertRaises(future.CancelledError):
           next(response_iterator)
 
   def testCancelledStreamRequestUnaryResponse(self):
@@ -357,10 +361,10 @@ class FutureInvocationAsynchronousEventServiceTestCase(
         with self.control.pause():
           response_future = self.stub.future_stream_in_value_out(
               name, iter(requests), _TIMEOUT)
-          cancelled = response_future.cancel()
+          cancel_method_return_value = response_future.cancel()
 
-        self.assertFalse(cancelled)
-        self.assertEqual(future.ABORTED, response_future.outcome().category)
+        self.assertFalse(cancel_method_return_value)
+        self.assertTrue(response_future.cancelled())
 
   def testCancelledStreamRequestStreamResponse(self):
     for name, test_messages_sequence in (
@@ -373,5 +377,5 @@ class FutureInvocationAsynchronousEventServiceTestCase(
               name, iter(requests), _TIMEOUT)
           response_iterator.cancel()
 
-        with self.assertRaises(exceptions.CancellationError):
+        with self.assertRaises(future.CancelledError):
           next(response_iterator)
diff --git a/src/python/src/_framework/foundation/_later_test.py b/src/python/src/_framework/foundation/_later_test.py
index fbd17a4ad9..50b67907db 100644
--- a/src/python/src/_framework/foundation/_later_test.py
+++ b/src/python/src/_framework/foundation/_later_test.py
@@ -33,7 +33,6 @@ import threading
 import time
 import unittest
 
-from _framework.foundation import future
 from _framework.foundation import later
 
 TICK = 0.1
@@ -44,10 +43,14 @@ class LaterTest(unittest.TestCase):
   def test_simple_delay(self):
     lock = threading.Lock()
     cell = [0]
-    def increment_cell():
+    return_value = object()
+
+    def computation():
       with lock:
         cell[0] += 1
-    computation_future = later.later(TICK * 2, increment_cell)
+      return return_value
+    computation_future = later.later(TICK * 2, computation)
+
     self.assertFalse(computation_future.done())
     self.assertFalse(computation_future.cancelled())
     time.sleep(TICK)
@@ -60,22 +63,21 @@ class LaterTest(unittest.TestCase):
     self.assertFalse(computation_future.cancelled())
     with lock:
       self.assertEqual(1, cell[0])
-    outcome = computation_future.outcome()
-    self.assertEqual(future.RETURNED, outcome.category)
+    self.assertEqual(return_value, computation_future.result())
 
   def test_callback(self):
     lock = threading.Lock()
     cell = [0]
     callback_called = [False]
-    outcome_passed_to_callback = [None]
-    def increment_cell():
+    future_passed_to_callback = [None]
+    def computation():
       with lock:
         cell[0] += 1
-    computation_future = later.later(TICK * 2, increment_cell)
+    computation_future = later.later(TICK * 2, computation)
     def callback(outcome):
       with lock:
         callback_called[0] = True
-        outcome_passed_to_callback[0] = outcome
+        future_passed_to_callback[0] = outcome
     computation_future.add_done_callback(callback)
     time.sleep(TICK)
     with lock:
@@ -83,63 +85,67 @@ class LaterTest(unittest.TestCase):
     time.sleep(TICK * 2)
     with lock:
       self.assertTrue(callback_called[0])
-      self.assertEqual(future.RETURNED, outcome_passed_to_callback[0].category)
+      self.assertTrue(future_passed_to_callback[0].done())
 
       callback_called[0] = False
-      outcome_passed_to_callback[0] = None
+      future_passed_to_callback[0] = None
 
     computation_future.add_done_callback(callback)
     with lock:
       self.assertTrue(callback_called[0])
-      self.assertEqual(future.RETURNED, outcome_passed_to_callback[0].category)
+      self.assertTrue(future_passed_to_callback[0].done())
 
   def test_cancel(self):
     lock = threading.Lock()
     cell = [0]
     callback_called = [False]
-    outcome_passed_to_callback = [None]
-    def increment_cell():
+    future_passed_to_callback = [None]
+    def computation():
       with lock:
         cell[0] += 1
-    computation_future = later.later(TICK * 2, increment_cell)
+    computation_future = later.later(TICK * 2, computation)
     def callback(outcome):
       with lock:
         callback_called[0] = True
-        outcome_passed_to_callback[0] = outcome
+        future_passed_to_callback[0] = outcome
     computation_future.add_done_callback(callback)
     time.sleep(TICK)
     with lock:
       self.assertFalse(callback_called[0])
     computation_future.cancel()
     self.assertTrue(computation_future.cancelled())
-    self.assertFalse(computation_future.done())
-    self.assertEqual(future.ABORTED, computation_future.outcome().category)
+    self.assertFalse(computation_future.running())
+    self.assertTrue(computation_future.done())
     with lock:
       self.assertTrue(callback_called[0])
-      self.assertEqual(future.ABORTED, outcome_passed_to_callback[0].category)
+      self.assertTrue(future_passed_to_callback[0].cancelled())
 
-  def test_outcome(self):
+  def test_result(self):
     lock = threading.Lock()
     cell = [0]
     callback_called = [False]
-    outcome_passed_to_callback = [None]
-    def increment_cell():
+    future_passed_to_callback_cell = [None]
+    return_value = object()
+
+    def computation():
       with lock:
         cell[0] += 1
-    computation_future = later.later(TICK * 2, increment_cell)
-    def callback(outcome):
+      return return_value
+    computation_future = later.later(TICK * 2, computation)
+
+    def callback(future_passed_to_callback):
       with lock:
         callback_called[0] = True
-        outcome_passed_to_callback[0] = outcome
+        future_passed_to_callback_cell[0] = future_passed_to_callback
     computation_future.add_done_callback(callback)
-    returned_outcome = computation_future.outcome()
-    self.assertEqual(future.RETURNED, returned_outcome.category)
+    returned_value = computation_future.result()
+    self.assertEqual(return_value, returned_value)
 
     # The callback may not yet have been called! Sleep a tick.
     time.sleep(TICK)
     with lock:
       self.assertTrue(callback_called[0])
-      self.assertEqual(future.RETURNED, outcome_passed_to_callback[0].category)
+      self.assertEqual(return_value, future_passed_to_callback_cell[0].result())
 
 if __name__ == '__main__':
   unittest.main()
diff --git a/src/python/src/_framework/foundation/_timer_future.py b/src/python/src/_framework/foundation/_timer_future.py
index 86bc073d56..4aa66991c5 100644
--- a/src/python/src/_framework/foundation/_timer_future.py
+++ b/src/python/src/_framework/foundation/_timer_future.py
@@ -29,6 +29,7 @@
 
 """Affords a Future implementation based on Python's threading.Timer."""
 
+import sys
 import threading
 import time
 
@@ -52,7 +53,9 @@ class TimerFuture(future.Future):
     self._computing = False
     self._computed = False
     self._cancelled = False
-    self._outcome = None
+    self._result = None
+    self._exception = None
+    self._traceback = None
     self._waiting = []
 
   def _compute(self):
@@ -70,19 +73,24 @@ class TimerFuture(future.Future):
         self._computing = True
 
     try:
-      returned_value = self._computation()
-      outcome = future.returned(returned_value)
+      return_value = self._computation()
+      exception = None
+      traceback = None
     except Exception as e:  # pylint: disable=broad-except
-      outcome = future.raised(e)
+      return_value = None
+      exception = e
+      traceback = sys.exc_info()[2]
 
     with self._lock:
       self._computing = False
       self._computed = True
-      self._outcome = outcome
+      self._return_value = return_value
+      self._exception = exception
+      self._traceback = traceback
       waiting = self._waiting
 
     for callback in waiting:
-      callback(outcome)
+      callback(self)
 
   def start(self):
     """Starts this Future.
@@ -104,13 +112,11 @@ class TimerFuture(future.Future):
       else:
         self._timer.cancel()
         self._cancelled = True
-        self._outcome = future.aborted()
-        outcome = self._outcome
         waiting = self._waiting
 
     for callback in waiting:
       try:
-        callback(outcome)
+        callback(self)
       except Exception:  # pylint: disable=broad-except
         pass
 
@@ -121,36 +127,102 @@ class TimerFuture(future.Future):
     with self._lock:
       return self._cancelled
 
+  def running(self):
+    """See future.Future.running for specification."""
+    with self._lock:
+      return not self._computed and not self._cancelled
+
   def done(self):
     """See future.Future.done for specification."""
     with self._lock:
-      return self._computed
+      return self._computed or self._cancelled
+
+  def result(self, timeout=None):
+    """See future.Future.result for specification."""
+    with self._lock:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        if self._exception is None:
+          return self._return_value
+        else:
+          raise self._exception  # pylint: disable=raising-bad-type
+
+      condition = threading.Condition()
+      def notify_condition(unused_future):
+        with condition:
+          condition.notify()
+      self._waiting.append(notify_condition)
+
+    with condition:
+      condition.wait(timeout=timeout)
+
+    with self._lock:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        if self._exception is None:
+          return self._return_value
+        else:
+          raise self._exception  # pylint: disable=raising-bad-type
+      else:
+        raise future.TimeoutError()
+
+  def exception(self, timeout=None):
+    """See future.Future.exception for specification."""
+    with self._lock:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._exception
+
+      condition = threading.Condition()
+      def notify_condition(unused_future):
+        with condition:
+          condition.notify()
+      self._waiting.append(notify_condition)
+
+    with condition:
+      condition.wait(timeout=timeout)
+
+    with self._lock:
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._exception
+      else:
+        raise future.TimeoutError()
 
-  def outcome(self):
-    """See future.Future.outcome for specification."""
+  def traceback(self, timeout=None):
+    """See future.Future.traceback for specification."""
     with self._lock:
-      if self._computed or self._cancelled:
-        return self._outcome
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._traceback
 
       condition = threading.Condition()
-      def notify_condition(unused_outcome):
+      def notify_condition(unused_future):
         with condition:
           condition.notify()
       self._waiting.append(notify_condition)
 
     with condition:
-      condition.wait()
+      condition.wait(timeout=timeout)
 
     with self._lock:
-      return self._outcome
+      if self._cancelled:
+        raise future.CancelledError()
+      elif self._computed:
+        return self._traceback
+      else:
+        raise future.TimeoutError()
 
-  def add_done_callback(self, callback):
+  def add_done_callback(self, fn):
     """See future.Future.add_done_callback for specification."""
     with self._lock:
       if not self._computed and not self._cancelled:
-        self._waiting.append(callback)
+        self._waiting.append(fn)
         return
-      else:
-        outcome = self._outcome
 
-    callback(outcome)
+    fn(self)
diff --git a/src/python/src/_framework/foundation/callable_util.py b/src/python/src/_framework/foundation/callable_util.py
index 1f7546cb76..32b0751a01 100644
--- a/src/python/src/_framework/foundation/callable_util.py
+++ b/src/python/src/_framework/foundation/callable_util.py
@@ -29,18 +29,47 @@
 
 """Utilities for working with callables."""
 
+import abc
+import collections
+import enum
 import functools
 import logging
 
-from _framework.foundation import future
+
+class Outcome(object):
+  """A sum type describing the outcome of some call.
+
+  Attributes:
+    kind: One of Kind.RETURNED or Kind.RAISED respectively indicating that the
+      call returned a value or raised an exception.
+    return_value: The value returned by the call. Must be present if kind is
+      Kind.RETURNED.
+    exception: The exception raised by the call. Must be present if kind is
+      Kind.RAISED.
+  """
+  __metaclass__ = abc.ABCMeta
+
+  @enum.unique
+  class Kind(enum.Enum):
+    """Identifies the general kind of the outcome of some call."""
+
+    RETURNED = object()
+    RAISED = object()
+
+
+class _EasyOutcome(
+    collections.namedtuple(
+        '_EasyOutcome', ['kind', 'return_value', 'exception']),
+    Outcome):
+  """A trivial implementation of Outcome."""
 
 
 def _call_logging_exceptions(behavior, message, *args, **kwargs):
   try:
-    return future.returned(behavior(*args, **kwargs))
+    return _EasyOutcome(Outcome.Kind.RETURNED, behavior(*args, **kwargs), None)
   except Exception as e:  # pylint: disable=broad-except
     logging.exception(message)
-    return future.raised(e)
+    return _EasyOutcome(Outcome.Kind.RAISED, None, e)
 
 
 def with_exceptions_logged(behavior, message):
@@ -72,7 +101,7 @@ def call_logging_exceptions(behavior, message, *args, **kwargs):
     **kwargs: Keyword arguments to pass to the given behavior.
 
   Returns:
-    A future.Outcome describing whether the given behavior returned a value or
-      raised an exception.
+    An Outcome describing whether the given behavior returned a value or raised
+      an exception.
   """
   return _call_logging_exceptions(behavior, message, *args, **kwargs)
diff --git a/src/python/src/_framework/foundation/future.py b/src/python/src/_framework/foundation/future.py
index f00c503257..bfc16fc1ea 100644
--- a/src/python/src/_framework/foundation/future.py
+++ b/src/python/src/_framework/foundation/future.py
@@ -27,146 +27,210 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""The Future interface missing from Python's standard library.
+"""A Future interface.
 
-Python's concurrent.futures library defines a Future class very much like the
-Future defined here, but since that class is concrete and without construction
-semantics it is only available within the concurrent.futures library itself.
-The Future class defined here is an entirely abstract interface that anyone may
+Python doesn't have a Future interface in its standard library. In the absence
+of such a standard, three separate, incompatible implementations
+(concurrent.futures.Future, ndb.Future, and asyncio.Future) have appeared. This
+interface attempts to be as compatible as possible with
+concurrent.futures.Future. From ndb.Future it adopts a traceback-object accessor
+method.
+
+Unlike the concrete and implemented Future classes listed above, the Future
+class defined in this module is an entirely abstract interface that anyone may
 implement and use.
+
+The one known incompatibility between this interface and the interface of
+concurrent.futures.Future is that this interface defines its own CancelledError
+and TimeoutError exceptions rather than raising the implementation-private
+concurrent.futures._base.CancelledError and the
+built-in-but-only-in-3.3-and-later TimeoutError.
 """
 
 import abc
-import collections
-
-RETURNED = object()
-RAISED = object()
-ABORTED = object()
-
 
-class Outcome(object):
-  """A sum type describing the outcome of some computation.
-
-  Attributes:
-    category: One of RETURNED, RAISED, or ABORTED, respectively indicating
-      that the computation returned a value, raised an exception, or was
-      aborted.
-    return_value: The value returned by the computation. Must be present if
-      category is RETURNED.
-    exception: The exception raised by the computation. Must be present if
-      category is RAISED.
-  """
-  __metaclass__ = abc.ABCMeta
 
+class TimeoutError(Exception):
+  """Indicates that a particular call timed out."""
 
-class _EasyOutcome(
-    collections.namedtuple('_EasyOutcome',
-                           ['category', 'return_value', 'exception']),
-    Outcome):
-  """A trivial implementation of Outcome."""
 
-# All Outcomes describing abortion are indistinguishable so there might as well
-# be only one.
-_ABORTED_OUTCOME = _EasyOutcome(ABORTED, None, None)
+class CancelledError(Exception):
+  """Indicates that the computation underlying a Future was cancelled."""
 
 
-def aborted():
-  """Returns an Outcome indicating that a computation was aborted.
+class Future(object):
+  """A representation of a computation in another control flow.
 
-  Returns:
-    An Outcome indicating that a computation was aborted.
+  Computations represented by a Future may be yet to be begun, may be ongoing,
+  or may have already completed.
   """
-  return _ABORTED_OUTCOME
-
-
-def raised(exception):
-  """Returns an Outcome indicating that a computation raised an exception.
-
-  Args:
-    exception: The exception raised by the computation.
+  __metaclass__ = abc.ABCMeta
 
-  Returns:
-    An Outcome indicating that a computation raised the given exception.
-  """
-  return _EasyOutcome(RAISED, None, exception)
+  # NOTE(nathaniel): This isn't the return type that I would want to have if it
+  # were up to me. Were this interface being written from scratch, the return
+  # type of this method would probably be a sum type like:
+  #
+  # NOT_COMMENCED
+  # COMMENCED_AND_NOT_COMPLETED
+  # PARTIAL_RESULT<Partial_Result_Type>
+  # COMPLETED<Result_Type>
+  # UNCANCELLABLE
+  # NOT_IMMEDIATELY_DETERMINABLE
+  @abc.abstractmethod
+  def cancel(self):
+    """Attempts to cancel the computation.
 
+    This method does not block.
 
-def returned(value):
-  """Returns an Outcome indicating that a computation returned a value.
+    Returns:
+      True if the computation has not yet begun, will not be allowed to take
+        place, and determination of both was possible without blocking. False
+        under all other circumstances including but not limited to the
+        computation's already having begun, the computation's already having
+        finished, and the computation's having been scheduled for execution on a
+        remote system for which a determination of whether or not it commenced
+        before being cancelled cannot be made without blocking.
+    """
+    raise NotImplementedError()
 
-  Args:
-    value: The value returned by the computation.
+  # NOTE(nathaniel): Here too this isn't the return type that I'd want this
+  # method to have if it were up to me. I think I'd go with another sum type
+  # like:
+  #
+  # NOT_CANCELLED (this object's cancel method hasn't been called)
+  # NOT_COMMENCED
+  # COMMENCED_AND_NOT_COMPLETED
+  # PARTIAL_RESULT<Partial_Result_Type>
+  # COMPLETED<Result_Type>
+  # UNCANCELLABLE
+  # NOT_IMMEDIATELY_DETERMINABLE
+  #
+  # Notice how giving the cancel method the right semantics obviates most
+  # reasons for this method to exist.
+  @abc.abstractmethod
+  def cancelled(self):
+    """Describes whether the computation was cancelled.
 
-  Returns:
-    An Outcome indicating that a computation returned the given value.
-  """
-  return _EasyOutcome(RETURNED, value, None)
+    This method does not block.
 
+    Returns:
+      True if the computation was cancelled any time before its result became
+        immediately available. False under all other circumstances including but
+        not limited to this object's cancel method not having been called and
+        the computation's result having become immediately available.
+    """
+    raise NotImplementedError()
 
-class Future(object):
-  """A representation of a computation happening in another control flow.
+  @abc.abstractmethod
+  def running(self):
+    """Describes whether the computation is taking place.
 
-  Computations represented by a Future may have already completed, may be
-  ongoing, or may be yet to be begun.
+    This method does not block.
 
-  Computations represented by a Future are considered uninterruptable; once
-  started they will be allowed to terminate either by returning or raising
-  an exception.
-  """
-  __metaclass__ = abc.ABCMeta
+    Returns:
+      True if the computation is scheduled to take place in the future or is
+        taking place now, or False if the computation took place in the past or
+        was cancelled.
+    """
+    raise NotImplementedError()
 
+  # NOTE(nathaniel): These aren't quite the semantics I'd like here either. I
+  # would rather this only returned True in cases in which the underlying
+  # computation completed successfully. A computation's having been cancelled
+  # conflicts with considering that computation "done".
   @abc.abstractmethod
-  def cancel(self):
-    """Attempts to cancel the computation.
+  def done(self):
+    """Describes whether the computation has taken place.
+
+    This method does not block.
 
     Returns:
-      True if the computation will not be allowed to take place or False if
-        the computation has already taken place or is currently taking place.
+      True if the computation is known to have either completed or have been
+        unscheduled or interrupted. False if the computation may possibly be
+        executing or scheduled to execute later.
     """
     raise NotImplementedError()
 
   @abc.abstractmethod
-  def cancelled(self):
-    """Describes whether the computation was cancelled.
+  def result(self, timeout=None):
+    """Accesses the outcome of the computation or raises its exception.
+
+    This method may return immediately or may block.
+
+    Args:
+      timeout: The length of time in seconds to wait for the computation to
+        finish or be cancelled, or None if this method should block until the
+        computation has finished or is cancelled no matter how long that takes.
 
     Returns:
-      True if the computation was cancelled and did not take place or False
-        if the computation took place, is taking place, or is scheduled to
-        take place in the future.
+      The return value of the computation.
+
+    Raises:
+      TimeoutError: If a timeout value is passed and the computation does not
+        terminate within the allotted time.
+      CancelledError: If the computation was cancelled.
+      Exception: If the computation raised an exception, this call will raise
+        the same exception.
     """
     raise NotImplementedError()
 
   @abc.abstractmethod
-  def done(self):
-    """Describes whether the computation has taken place.
+  def exception(self, timeout=None):
+    """Return the exception raised by the computation.
+
+    This method may return immediately or may block.
+
+    Args:
+      timeout: The length of time in seconds to wait for the computation to
+        terminate or be cancelled, or None if this method should block until
+        the computation is terminated or is cancelled no matter how long that
+        takes.
 
     Returns:
-      True if the computation took place; False otherwise.
+      The exception raised by the computation, or None if the computation did
+        not raise an exception.
+
+    Raises:
+      TimeoutError: If a timeout value is passed and the computation does not
+        terminate within the allotted time.
+      CancelledError: If the computation was cancelled.
     """
     raise NotImplementedError()
 
   @abc.abstractmethod
-  def outcome(self):
-    """Accesses the outcome of the computation.
+  def traceback(self, timeout=None):
+    """Access the traceback of the exception raised by the computation.
 
-    If the computation has not yet completed, this method blocks until it has.
+    This method may return immediately or may block.
+
+    Args:
+      timeout: The length of time in seconds to wait for the computation to
+        terminate or be cancelled, or None if this method should block until
+        the computation is terminated or is cancelled no matter how long that
+        takes.
 
     Returns:
-      An Outcome describing the outcome of the computation.
+      The traceback of the exception raised by the computation, or None if the
+        computation did not raise an exception.
+
+    Raises:
+      TimeoutError: If a timeout value is passed and the computation does not
+        terminate within the allotted time.
+      CancelledError: If the computation was cancelled.
     """
     raise NotImplementedError()
 
   @abc.abstractmethod
-  def add_done_callback(self, callback):
+  def add_done_callback(self, fn):
     """Adds a function to be called at completion of the computation.
 
-    The callback will be passed an Outcome object describing the outcome of
+    The callback will be passed this Future object describing the outcome of
     the computation.
 
     If the computation has already completed, the callback will be called
     immediately.
 
     Args:
-      callback: A callable taking an Outcome as its single parameter.
+      fn: A callable taking a this Future object as its single parameter.
     """
     raise NotImplementedError()