bpo-34872: Fix self-cancellation in C implementation of asyncio.Task (GH-9679)

The C implementation of asyncio.Task currently fails to perform the
cancellation cleanup correctly in the following scenario.

    async def task1():
        async def task2():
            await task3     # task3 is never cancelled

        asyncio.current_task().cancel()
        await asyncio.create_task(task2())

The actuall error is a hardcoded call to `future_cancel()` instead of
calling the `cancel()` method of a future-like object.

Thanks to Vladimir Matveev for noticing the code discrepancy and to
Yury Selivanov for coming up with a pathological scenario.

Lib/test/test_asyncio/test_tasks.py

@@ -622,6 +622,42 @@ class BaseTaskTests:
         self.assertFalse(t._must_cancel)  # White-box test.
         self.assertFalse(t.cancel())
 
+    def test_cancel_awaited_task(self):
+        # This tests for a relatively rare condition when
+        # a task cancellation is requested for a task which is not
+        # currently blocked, such as a task cancelling itself.
+        # In this situation we must ensure that whatever next future
+        # or task the cancelled task blocks on is cancelled correctly
+        # as well.  See also bpo-34872.
+        loop = asyncio.new_event_loop()
+        self.addCleanup(lambda: loop.close())
+
+        task = nested_task = None
+        fut = self.new_future(loop)
+
+        async def nested():
+            await fut
+
+        async def coro():
+            nonlocal nested_task
+            # Create a sub-task and wait for it to run.
+            nested_task = self.new_task(loop, nested())
+            await asyncio.sleep(0)
+
+            # Request the current task to be cancelled.
+            task.cancel()
+            # Block on the nested task, which should be immediately
+            # cancelled.
+            await nested_task
+
+        task = self.new_task(loop, coro())
+        with self.assertRaises(asyncio.CancelledError):
+            loop.run_until_complete(task)
+
+        self.assertTrue(task.cancelled())
+        self.assertTrue(nested_task.cancelled())
+        self.assertTrue(fut.cancelled())
+
     def test_stop_while_run_in_complete(self):
 
         def gen():

Misc/NEWS.d/next/Library/2018-10-02-19-36-34.bpo-34872.yWZRhI.rst

@@ -0,0 +1 @@
+Fix self-cancellation in C implementation of asyncio.Task

Modules/_asynciomodule.c

@@ -2713,14 +2713,19 @@ set_exception:
 
         if (task->task_must_cancel) {
             PyObject *r;
-            r = future_cancel(fut);
+            int is_true;
+            r = _PyObject_CallMethodId(fut, &PyId_cancel, NULL);
             if (r == NULL) {
                 return NULL;
             }
-            if (r == Py_True) {
+            is_true = PyObject_IsTrue(r);
+            Py_DECREF(r);
+            if (is_true < 0) {
+                return NULL;
+            }
+            else if (is_true) {
                 task->task_must_cancel = 0;
             }
-            Py_DECREF(r);
         }
 
         Py_RETURN_NONE;