bpo-38248: Fix inconsistent immediate asyncio.Task cancellation (GH-16330)

Lib/asyncio/tasks.py

@@ -284,7 +284,7 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
             if self._must_cancel:
                 # Task is cancelled right before coro stops.
                 self._must_cancel = False
-                super().set_exception(exceptions.CancelledError())
+                super().cancel()
             else:
                 super().set_result(exc.value)
         except exceptions.CancelledError:

Lib/test/test_asyncio/test_tasks.py

@@ -604,9 +604,11 @@ class BaseTaskTests:
             return 12
 
         t = self.new_task(loop, task())
+        self.assertFalse(t.cancelled())
         self.assertRaises(
             asyncio.CancelledError, loop.run_until_complete, t)
         self.assertTrue(t.done())
+        self.assertTrue(t.cancelled())
         self.assertFalse(t._must_cancel)  # White-box test.
         self.assertFalse(t.cancel())
 
@@ -621,9 +623,11 @@ class BaseTaskTests:
             return 12
 
         t = self.new_task(loop, task())
+        self.assertFalse(t.cancelled())
         self.assertRaises(
             asyncio.CancelledError, loop.run_until_complete, t)
         self.assertTrue(t.done())
+        self.assertTrue(t.cancelled())
         self.assertFalse(t._must_cancel)  # White-box test.
         self.assertFalse(t.cancel())
 

Misc/NEWS.d/next/Library/2019-09-22-13-05-36.bpo-38248.Yo3N_1.rst

@@ -0,0 +1 @@
+asyncio: Fix inconsistent immediate Task cancellation

Modules/_asynciomodule.c

@@ -2628,18 +2628,19 @@ task_step_impl(TaskObj *task, PyObject *exc)
         if (_PyGen_FetchStopIterationValue(&o) == 0) {
             /* The error is StopIteration and that means that
                the underlying coroutine has resolved */
+
+            PyObject *res;
             if (task->task_must_cancel) {
                 // Task is cancelled right before coro stops.
-                Py_DECREF(o);
                 task->task_must_cancel = 0;
-                et = asyncio_CancelledError;
-                Py_INCREF(et);
-                ev = NULL;
-                tb = NULL;
-                goto set_exception;
+                res = future_cancel((FutureObj*)task);
             }
-            PyObject *res = future_set_result((FutureObj*)task, o);
+            else {
+                res = future_set_result((FutureObj*)task, o);
+            }
+
             Py_DECREF(o);
+
             if (res == NULL) {
                 return NULL;
             }