commit a67bd53d3f80ac9c518b5426aa35459bd373b2b3
author Elvis Pranskevichus <elvis@magic.io> Wed Oct 03 11:49:00 2018 -0400
committer Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Wed Oct 03 08:49:00 2018 -0700
tree abe411bae972f961a257f94d19e7ba4e2d03a071
parent 063755c20184e80f587d522600536d1ba70a5f7e

[3.7] bpo-34872: Fix self-cancellation in C implementation of asyncio.Task (GH-9679) (GH-9691)



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..
(cherry picked from commit 548ce9dedd2e90945970671d441436a6a91608ab)

Co-authored-by: Elvis Pranskevichus <elvis@magic.io>


https://bugs.python.org/issue34872

Modules/_asynciomodule.c

diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 0c161fb..9f7500a 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -2662,14 +2662,19 @@
 
             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;