Merge pull request #24574 from gnossen/single_threaded_future_backport

[Backport] Implement grpc.Future interface in SingleThreadedRendezvous

src/python/grpcio/grpc/_channel.py

@@ -442,15 +442,112 @@ class _Rendezvous(grpc.RpcError, grpc.RpcContext):
                 self._state.condition.notify_all()
 
 
-class _SingleThreadedRendezvous(_Rendezvous, grpc.Call):  # pylint: disable=too-many-ancestors
+class _SingleThreadedRendezvous(_Rendezvous, grpc.Call, grpc.Future):  # pylint: disable=too-many-ancestors
     """An RPC iterator operating entirely on a single thread.
 
     The __next__ method of _SingleThreadedRendezvous does not depend on the
     existence of any other thread, including the "channel spin thread".
     However, this means that its interface is entirely synchronous. So this
-    class cannot fulfill the grpc.Future interface.
+    class cannot completely fulfill the grpc.Future interface. The result,
+    exception, and traceback methods will never block and will instead raise
+    an exception if calling the method would result in blocking.
+
+    This means that these methods are safe to call from add_done_callback
+    handlers.
     """
 
+    def _is_complete(self):
+        return self._state.code is not None
+
+    def cancelled(self):
+        with self._state.condition:
+            return self._state.cancelled
+
+    def running(self):
+        with self._state.condition:
+            return self._state.code is None
+
+    def done(self):
+        with self._state.condition:
+            return self._state.code is not None
+
+    def result(self, timeout=None):
+        """Returns the result of the computation or raises its exception.
+
+        This method will never block. Instead, it will raise an exception
+        if calling this method would otherwise result in blocking.
+
+        Since this method will never block, any `timeout` argument passed will
+        be ignored.
+        """
+        del timeout
+        with self._state.condition:
+            if not self._is_complete():
+                raise grpc.experimental.UsageError(
+                    "_SingleThreadedRendezvous only supports result() when the RPC is complete."
+                )
+            if self._state.code is grpc.StatusCode.OK:
+                return self._state.response
+            elif self._state.cancelled:
+                raise grpc.FutureCancelledError()
+            else:
+                raise self
+
+    def exception(self, timeout=None):
+        """Return the exception raised by the computation.
+
+        This method will never block. Instead, it will raise an exception
+        if calling this method would otherwise result in blocking.
+
+        Since this method will never block, any `timeout` argument passed will
+        be ignored.
+        """
+        del timeout
+        with self._state.condition:
+            if not self._is_complete():
+                raise grpc.experimental.UsageError(
+                    "_SingleThreadedRendezvous only supports exception() when the RPC is complete."
+                )
+            if self._state.code is grpc.StatusCode.OK:
+                return None
+            elif self._state.cancelled:
+                raise grpc.FutureCancelledError()
+            else:
+                return self
+
+    def traceback(self, timeout=None):
+        """Access the traceback of the exception raised by the computation.
+
+        This method will never block. Instead, it will raise an exception
+        if calling this method would otherwise result in blocking.
+
+        Since this method will never block, any `timeout` argument passed will
+        be ignored.
+        """
+        del timeout
+        with self._state.condition:
+            if not self._is_complete():
+                raise grpc.experimental.UsageError(
+                    "_SingleThreadedRendezvous only supports traceback() when the RPC is complete."
+                )
+            if self._state.code is grpc.StatusCode.OK:
+                return None
+            elif self._state.cancelled:
+                raise grpc.FutureCancelledError()
+            else:
+                try:
+                    raise self
+                except grpc.RpcError:
+                    return sys.exc_info()[2]
+
+    def add_done_callback(self, fn):
+        with self._state.condition:
+            if self._state.code is None:
+                self._state.callbacks.append(functools.partial(fn, self))
+                return
+
+        fn(self)
+
     def initial_metadata(self):
         """See grpc.Call.initial_metadata"""
         with self._state.condition:

src/python/grpcio_tests/tests/unit/_interceptor_test.py

@@ -547,10 +547,6 @@ class InterceptorTest(unittest.TestCase):
             's1:intercept_service', 's2:intercept_service'
         ])
 
-    # NOTE: The single-threaded unary-stream path does not support the
-    # grpc.Future interface, so this test does not apply.
-    @unittest.skipIf(os.getenv("GRPC_SINGLE_THREADED_UNARY_STREAM"),
-                     "Not supported.")
     def testInterceptedUnaryRequestStreamResponseWithError(self):
         request = _EXCEPTION_REQUEST