bpo-30048: asyncio: fix Task.cancel() was ignored. (GH-1097)

when there are no more `await` or `yield (from)` before return in coroutine,
cancel was ignored.

example:

    async def coro():
        asyncio.Task.current_task().cancel()
        return 42
    ...
    res = await coro()  # should raise CancelledError

1 files changed, 6 insertions, 1 deletions

diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index 4fbcf3e..e453300 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -176,7 +176,12 @@ class Task(futures.Future):
             else:
                 result = coro.throw(exc)
         except StopIteration as exc:
-            self.set_result(exc.value)
+            if self._must_cancel:
+                # Task is cancelled right before coro stops.
+                self._must_cancel = False
+                self.set_exception(futures.CancelledError())
+            else:
+                self.set_result(exc.value)
         except futures.CancelledError:
             super().cancel()  # I.e., Future.cancel(self).
         except Exception as exc: