bpo-45390: Propagate CancelledError's message from cancelled task to its awaiter (GH-31383)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Doc/library/asyncio-task.rst

@@ -843,6 +843,9 @@ Task Object
       .. versionchanged:: 3.9
          Added the *msg* parameter.
 
+      .. versionchanged:: 3.11
+         The ``msg`` parameter is propagated from cancelled task to its awaiter.
+
       .. _asyncio_example_task_cancel:
 
       The following example illustrates how coroutines can intercept

Lib/asyncio/futures.py

@@ -132,6 +132,11 @@ class Future:
         This should only be called once when handling a cancellation since
         it erases the saved context exception value.
         """
+        if self._cancelled_exc is not None:
+            exc = self._cancelled_exc
+            self._cancelled_exc = None
+            return exc
+
         if self._cancel_message is None:
             exc = exceptions.CancelledError()
         else:

Lib/test/test_asyncio/test_tasks.py

@@ -124,9 +124,11 @@ class BaseTaskTests:
         t.cancel('my message')
         self.assertEqual(t._cancel_message, 'my message')
 
-        with self.assertRaises(asyncio.CancelledError):
+        with self.assertRaises(asyncio.CancelledError) as cm:
             self.loop.run_until_complete(t)
 
+        self.assertEqual('my message', cm.exception.args[0])
+
     def test_task_cancel_message_setter(self):
         async def coro():
             pass
@@ -135,9 +137,11 @@ class BaseTaskTests:
         t._cancel_message = 'my new message'
         self.assertEqual(t._cancel_message, 'my new message')
 
-        with self.assertRaises(asyncio.CancelledError):
+        with self.assertRaises(asyncio.CancelledError) as cm:
             self.loop.run_until_complete(t)
 
+        self.assertEqual('my new message', cm.exception.args[0])
+
     def test_task_del_collect(self):
         class Evil:
             def __del__(self):
@@ -590,11 +594,11 @@ class BaseTaskTests:
                 with self.assertRaises(asyncio.CancelledError) as cm:
                     loop.run_until_complete(task)
                 exc = cm.exception
-                self.assertEqual(exc.args, ())
+                self.assertEqual(exc.args, expected_args)
 
                 actual = get_innermost_context(exc)
                 self.assertEqual(actual,
-                    (asyncio.CancelledError, expected_args, 2))
+                    (asyncio.CancelledError, expected_args, 0))
 
     def test_cancel_with_message_then_future_exception(self):
         # Test Future.exception() after calling cancel() with a message.
@@ -624,11 +628,39 @@ class BaseTaskTests:
                 with self.assertRaises(asyncio.CancelledError) as cm:
                     loop.run_until_complete(task)
                 exc = cm.exception
-                self.assertEqual(exc.args, ())
+                self.assertEqual(exc.args, expected_args)
 
                 actual = get_innermost_context(exc)
                 self.assertEqual(actual,
-                    (asyncio.CancelledError, expected_args, 2))
+                    (asyncio.CancelledError, expected_args, 0))
+
+    def test_cancellation_exception_context(self):
+        loop = asyncio.new_event_loop()
+        self.set_event_loop(loop)
+        fut = loop.create_future()
+
+        async def sleep():
+            fut.set_result(None)
+            await asyncio.sleep(10)
+
+        async def coro():
+            inner_task = self.new_task(loop, sleep())
+            await fut
+            loop.call_soon(inner_task.cancel, 'msg')
+            try:
+                await inner_task
+            except asyncio.CancelledError as ex:
+                raise ValueError("cancelled") from ex
+
+        task = self.new_task(loop, coro())
+        with self.assertRaises(ValueError) as cm:
+            loop.run_until_complete(task)
+        exc = cm.exception
+        self.assertEqual(exc.args, ('cancelled',))
+
+        actual = get_innermost_context(exc)
+        self.assertEqual(actual,
+            (asyncio.CancelledError, ('msg',), 1))
 
     def test_cancel_with_message_before_starting_task(self):
         loop = asyncio.new_event_loop()
@@ -648,11 +680,11 @@ class BaseTaskTests:
         with self.assertRaises(asyncio.CancelledError) as cm:
             loop.run_until_complete(task)
         exc = cm.exception
-        self.assertEqual(exc.args, ())
+        self.assertEqual(exc.args, ('my message',))
 
         actual = get_innermost_context(exc)
         self.assertEqual(actual,
-            (asyncio.CancelledError, ('my message',), 2))
+            (asyncio.CancelledError, ('my message',), 0))
 
     def test_cancel_yield(self):
         async def task():
@@ -2296,15 +2328,17 @@ class BaseTaskTests:
                 try:
                     loop.run_until_complete(main())
                 except asyncio.CancelledError as exc:
-                    self.assertEqual(exc.args, ())
-                    exc_type, exc_args, depth = get_innermost_context(exc)
-                    self.assertEqual((exc_type, exc_args),
-                        (asyncio.CancelledError, expected_args))
-                    # The exact traceback seems to vary in CI.
-                    self.assertIn(depth, (2, 3))
+                    self.assertEqual(exc.args, expected_args)
+                    actual = get_innermost_context(exc)
+                    self.assertEqual(
+                        actual,
+                        (asyncio.CancelledError, expected_args, 0),
+                    )
                 else:
-                    self.fail('gather did not propagate the cancellation '
-                              'request')
+                    self.fail(
+                        'gather() does not propagate CancelledError '
+                        'raised by inner task to the gather() caller.'
+                    )
 
     def test_exception_traceback(self):
         # See http://bugs.python.org/issue28843

Misc/NEWS.d/next/Library/2022-02-17-11-00-16.bpo-45390.sVhG6M.rst

@@ -0,0 +1,2 @@
+Propagate :exc:`asyncio.CancelledError` message from inner task to outer
+awaiter.

Modules/_asynciomodule.c

@@ -77,7 +77,7 @@ typedef enum {
     int prefix##_blocking;                                                  \
     PyObject *dict;                                                         \
     PyObject *prefix##_weakreflist;                                         \
-    _PyErr_StackItem prefix##_cancelled_exc_state;
+    PyObject *prefix##_cancelled_exc;
 
 typedef struct {
     FutureObj_HEAD(fut)
@@ -496,7 +496,7 @@ future_init(FutureObj *fut, PyObject *loop)
     Py_CLEAR(fut->fut_exception);
     Py_CLEAR(fut->fut_source_tb);
     Py_CLEAR(fut->fut_cancel_msg);
-    _PyErr_ClearExcState(&fut->fut_cancelled_exc_state);
+    Py_CLEAR(fut->fut_cancelled_exc);
 
     fut->fut_state = STATE_PENDING;
     fut->fut_log_tb = 0;
@@ -612,25 +612,32 @@ future_set_exception(FutureObj *fut, PyObject *exc)
 }
 
 static PyObject *
-create_cancelled_error(PyObject *msg)
+create_cancelled_error(FutureObj *fut)
 {
     PyObject *exc;
+    if (fut->fut_cancelled_exc != NULL) {
+        /* transfer ownership */
+        exc = fut->fut_cancelled_exc;
+        fut->fut_cancelled_exc = NULL;
+        return exc;
+    }
+    PyObject *msg = fut->fut_cancel_msg;
     if (msg == NULL || msg == Py_None) {
         exc = PyObject_CallNoArgs(asyncio_CancelledError);
     } else {
         exc = PyObject_CallOneArg(asyncio_CancelledError, msg);
     }
+    PyException_SetContext(exc, fut->fut_cancelled_exc);
+    Py_CLEAR(fut->fut_cancelled_exc);
     return exc;
 }
 
 static void
 future_set_cancelled_error(FutureObj *fut)
 {
-    PyObject *exc = create_cancelled_error(fut->fut_cancel_msg);
+    PyObject *exc = create_cancelled_error(fut);
     PyErr_SetObject(asyncio_CancelledError, exc);
     Py_DECREF(exc);
-
-    _PyErr_ChainStackItem(&fut->fut_cancelled_exc_state);
 }
 
 static int
@@ -793,7 +800,7 @@ FutureObj_clear(FutureObj *fut)
     Py_CLEAR(fut->fut_exception);
     Py_CLEAR(fut->fut_source_tb);
     Py_CLEAR(fut->fut_cancel_msg);
-    _PyErr_ClearExcState(&fut->fut_cancelled_exc_state);
+    Py_CLEAR(fut->fut_cancelled_exc);
     Py_CLEAR(fut->dict);
     return 0;
 }
@@ -809,11 +816,8 @@ FutureObj_traverse(FutureObj *fut, visitproc visit, void *arg)
     Py_VISIT(fut->fut_exception);
     Py_VISIT(fut->fut_source_tb);
     Py_VISIT(fut->fut_cancel_msg);
+    Py_VISIT(fut->fut_cancelled_exc);
     Py_VISIT(fut->dict);
-
-    _PyErr_StackItem *exc_state = &fut->fut_cancelled_exc_state;
-    Py_VISIT(exc_state->exc_value);
-
     return 0;
 }
 
@@ -1369,15 +1373,7 @@ static PyObject *
 _asyncio_Future__make_cancelled_error_impl(FutureObj *self)
 /*[clinic end generated code: output=a5df276f6c1213de input=ac6effe4ba795ecc]*/
 {
-    PyObject *exc = create_cancelled_error(self->fut_cancel_msg);
-    _PyErr_StackItem *exc_state = &self->fut_cancelled_exc_state;
-
-    if (exc_state->exc_value) {
-        PyException_SetContext(exc, Py_NewRef(exc_state->exc_value));
-        _PyErr_ClearExcState(exc_state);
-    }
-
-    return exc;
+    return create_cancelled_error(self);
 }
 
 /*[clinic input]
@@ -2677,7 +2673,7 @@ task_step_impl(TaskObj *task, PyObject *exc)
 
         if (!exc) {
             /* exc was not a CancelledError */
-            exc = create_cancelled_error(task->task_cancel_msg);
+            exc = create_cancelled_error((FutureObj*)task);
 
             if (!exc) {
                 goto fail;
@@ -2751,8 +2747,8 @@ task_step_impl(TaskObj *task, PyObject *exc)
             Py_XDECREF(et);
 
             FutureObj *fut = (FutureObj*)task;
-            _PyErr_StackItem *exc_state = &fut->fut_cancelled_exc_state;
-            exc_state->exc_value = ev;
+            /* transfer ownership */
+            fut->fut_cancelled_exc = ev;
 
             return future_cancel(fut, NULL);
         }