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.

Author: elprans

Lib/test/test_asyncio/test_tasks.py

@@ -622,6 +622,42 @@ 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():

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 @@ 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;