[3.6] bpo-34872: Fix self-cancellation in C implementation of asyncio.Task (GH-9679) (GH-9690)

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)


https://bugs.python.org/issue34872

3 files changed, 45 insertions, 3 deletions

diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index e8ec09e..084846b 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -621,6 +621,42 @@ 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():
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
new file mode 100644
index 0000000..cd02710
--- /dev/null
+++ 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
diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 6ad3d72..cbd6fe3 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -2121,14 +2121,19 @@ 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;