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.

1 files changed, 8 insertions, 3 deletions

diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 6bf0fd6..eb503fb 100644
--- a/Modules/_asynciomodule.c
+++ b/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;