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, 3 insertions, 0 deletions

diff --git a/Misc/NEWS b/Misc/NEWS
index 5ee7ea65ff..4e17a66bd9 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -320,6 +320,9 @@ Extension Modules
 Library
 -------
 
+- bpo-30048: Fixed ``Task.cancel()`` can be ignored when the task is
+  running coroutine and the coroutine returned without any more ``await``.
+
 - bpo-30298: Weaken the condition of deprecation warnings for inline modifiers.
   Now allowed several subsequential inline modifiers at the start of the
   pattern (e.g. ``'(?i)(?s)...'``).  In verbose mode whitespaces and comments