python · miss-islington · Oct 3, 2018 · Oct 2, 2018
@@ -590,6 +590,42 @@ def task():
         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():

diff --git a/Misc/NEWS.d/next/Library/2018-10-02-19-36-34.bpo-34872.yWZRhI.rst b/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
@@ -2662,14 +2662,19 @@ task_step_impl(TaskObj *task, PyObject *exc)
 
             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;
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix self-cancellation in C implementation of asyncio.Task