bpo-31033: Improve the traceback for cancelled asyncio tasks (GH-19951)
When an asyncio.Task is cancelled, the exception traceback now
starts with where the task was first interrupted. Previously,
the traceback only had "depth one."
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index 889f3e6..bed4da5 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -52,6 +52,8 @@
_loop = None
_source_traceback = None
_cancel_message = None
+ # A saved CancelledError for later chaining as an exception context.
+ _cancelled_exc = None
# This field is used for a dual purpose:
# - Its presence is a marker to declare that a class implements
@@ -124,6 +126,21 @@
raise RuntimeError("Future object is not initialized.")
return loop
+ def _make_cancelled_error(self):
+ """Create the CancelledError to raise if the Future is cancelled.
+
+ This should only be called once when handling a cancellation since
+ it erases the saved context exception value.
+ """
+ if self._cancel_message is None:
+ exc = exceptions.CancelledError()
+ else:
+ exc = exceptions.CancelledError(self._cancel_message)
+ exc.__context__ = self._cancelled_exc
+ # Remove the reference since we don't need this anymore.
+ self._cancelled_exc = None
+ return exc
+
def cancel(self, msg=None):
"""Cancel the future and schedule callbacks.
@@ -175,9 +192,8 @@
the future is done and has an exception set, this exception is raised.
"""
if self._state == _CANCELLED:
- raise exceptions.CancelledError(
- '' if self._cancel_message is None else self._cancel_message)
-
+ exc = self._make_cancelled_error()
+ raise exc
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Result is not ready.')
self.__log_traceback = False
@@ -194,8 +210,8 @@
InvalidStateError.
"""
if self._state == _CANCELLED:
- raise exceptions.CancelledError(
- '' if self._cancel_message is None else self._cancel_message)
+ exc = self._make_cancelled_error()
+ raise exc
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Exception is not set.')
self.__log_traceback = False
diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index a3a0a33..21b98b6 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -270,8 +270,7 @@
f'_step(): already done: {self!r}, {exc!r}')
if self._must_cancel:
if not isinstance(exc, exceptions.CancelledError):
- exc = exceptions.CancelledError(''
- if self._cancel_message is None else self._cancel_message)
+ exc = self._make_cancelled_error()
self._must_cancel = False
coro = self._coro
self._fut_waiter = None
@@ -293,11 +292,9 @@
else:
super().set_result(exc.value)
except exceptions.CancelledError as exc:
- if exc.args:
- cancel_msg = exc.args[0]
- else:
- cancel_msg = None
- super().cancel(msg=cancel_msg) # I.e., Future.cancel(self).
+ # Save the original exception so we can chain it later.
+ self._cancelled_exc = exc
+ super().cancel() # I.e., Future.cancel(self).
except (KeyboardInterrupt, SystemExit) as exc:
super().set_exception(exc)
raise
@@ -787,8 +784,7 @@
# Check if 'fut' is cancelled first, as
# 'fut.exception()' will *raise* a CancelledError
# instead of returning it.
- exc = exceptions.CancelledError(''
- if fut._cancel_message is None else fut._cancel_message)
+ exc = fut._make_cancelled_error()
outer.set_exception(exc)
return
else:
@@ -804,9 +800,12 @@
for fut in children:
if fut.cancelled():
- # Check if 'fut' is cancelled first, as
- # 'fut.exception()' will *raise* a CancelledError
- # instead of returning it.
+ # Check if 'fut' is cancelled first, as 'fut.exception()'
+ # will *raise* a CancelledError instead of returning it.
+ # Also, since we're adding the exception return value
+ # to 'results' instead of raising it, don't bother
+ # setting __context__. This also lets us preserve
+ # calling '_make_cancelled_error()' at most once.
res = exceptions.CancelledError(
'' if fut._cancel_message is None else
fut._cancel_message)
@@ -820,8 +819,7 @@
# If gather is being cancelled we must propagate the
# cancellation regardless of *return_exceptions* argument.
# See issue 32684.
- exc = exceptions.CancelledError(''
- if fut._cancel_message is None else fut._cancel_message)
+ exc = fut._make_cancelled_error()
outer.set_exception(exc)
else:
outer.set_result(results)
diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index 65bee52..63968e2 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -10,6 +10,7 @@
import re
import sys
import textwrap
+import traceback
import types
import unittest
import weakref
@@ -57,6 +58,22 @@
return 'coro=<%s() %s at %s>' % (qualname, state, src)
+def get_innermost_context(exc):
+ """
+ Return information about the innermost exception context in the chain.
+ """
+ depth = 0
+ while True:
+ context = exc.__context__
+ if context is None:
+ break
+
+ exc = context
+ depth += 1
+
+ return (type(exc), exc.args, depth)
+
+
class Dummy:
def __repr__(self):
@@ -111,9 +128,10 @@
self.assertEqual(t._cancel_message, None)
t.cancel('my message')
+ self.assertEqual(t._cancel_message, 'my message')
+
with self.assertRaises(asyncio.CancelledError):
self.loop.run_until_complete(t)
- self.assertEqual(t._cancel_message, 'my message')
def test_task_cancel_message_setter(self):
async def coro():
@@ -123,10 +141,8 @@
t._cancel_message = 'my new message'
self.assertEqual(t._cancel_message, 'my new message')
- # Also check that the value is used for cancel().
with self.assertRaises(asyncio.CancelledError):
self.loop.run_until_complete(t)
- self.assertEqual(t._cancel_message, 'my new message')
def test_task_del_collect(self):
class Evil:
@@ -548,8 +564,8 @@
def test_cancel_with_message_then_future_result(self):
# Test Future.result() after calling cancel() with a message.
cases = [
- ((), ('',)),
- ((None,), ('',)),
+ ((), ()),
+ ((None,), ()),
(('my message',), ('my message',)),
# Non-string values should roundtrip.
((5,), (5,)),
@@ -573,13 +589,17 @@
with self.assertRaises(asyncio.CancelledError) as cm:
loop.run_until_complete(task)
exc = cm.exception
- self.assertEqual(exc.args, expected_args)
+ self.assertEqual(exc.args, ())
+
+ actual = get_innermost_context(exc)
+ self.assertEqual(actual,
+ (asyncio.CancelledError, expected_args, 2))
def test_cancel_with_message_then_future_exception(self):
# Test Future.exception() after calling cancel() with a message.
cases = [
- ((), ('',)),
- ((None,), ('',)),
+ ((), ()),
+ ((None,), ()),
(('my message',), ('my message',)),
# Non-string values should roundtrip.
((5,), (5,)),
@@ -603,7 +623,11 @@
with self.assertRaises(asyncio.CancelledError) as cm:
loop.run_until_complete(task)
exc = cm.exception
- self.assertEqual(exc.args, expected_args)
+ self.assertEqual(exc.args, ())
+
+ actual = get_innermost_context(exc)
+ self.assertEqual(actual,
+ (asyncio.CancelledError, expected_args, 2))
def test_cancel_with_message_before_starting_task(self):
loop = asyncio.new_event_loop()
@@ -623,7 +647,11 @@
with self.assertRaises(asyncio.CancelledError) as cm:
loop.run_until_complete(task)
exc = cm.exception
- self.assertEqual(exc.args, ('my message',))
+ self.assertEqual(exc.args, ())
+
+ actual = get_innermost_context(exc)
+ self.assertEqual(actual,
+ (asyncio.CancelledError, ('my message',), 2))
def test_cancel_yield(self):
with self.assertWarns(DeprecationWarning):
@@ -805,6 +833,66 @@
self.assertTrue(nested_task.cancelled())
self.assertTrue(fut.cancelled())
+ def assert_text_contains(self, text, substr):
+ if substr not in text:
+ raise RuntimeError(f'text {substr!r} not found in:\n>>>{text}<<<')
+
+ def test_cancel_traceback_for_future_result(self):
+ # When calling Future.result() on a cancelled task, check that the
+ # line of code that was interrupted is included in the traceback.
+ loop = asyncio.new_event_loop()
+ self.set_event_loop(loop)
+
+ async def nested():
+ # This will get cancelled immediately.
+ await asyncio.sleep(10)
+
+ async def coro():
+ task = self.new_task(loop, nested())
+ await asyncio.sleep(0)
+ task.cancel()
+ await task # search target
+
+ task = self.new_task(loop, coro())
+ try:
+ loop.run_until_complete(task)
+ except asyncio.CancelledError:
+ tb = traceback.format_exc()
+ self.assert_text_contains(tb, "await asyncio.sleep(10)")
+ # The intermediate await should also be included.
+ self.assert_text_contains(tb, "await task # search target")
+ else:
+ self.fail('CancelledError did not occur')
+
+ def test_cancel_traceback_for_future_exception(self):
+ # When calling Future.exception() on a cancelled task, check that the
+ # line of code that was interrupted is included in the traceback.
+ loop = asyncio.new_event_loop()
+ self.set_event_loop(loop)
+
+ async def nested():
+ # This will get cancelled immediately.
+ await asyncio.sleep(10)
+
+ async def coro():
+ task = self.new_task(loop, nested())
+ await asyncio.sleep(0)
+ task.cancel()
+ done, pending = await asyncio.wait([task])
+ task.exception() # search target
+
+ task = self.new_task(loop, coro())
+ try:
+ loop.run_until_complete(task)
+ except asyncio.CancelledError:
+ tb = traceback.format_exc()
+ self.assert_text_contains(tb, "await asyncio.sleep(10)")
+ # The intermediate await should also be included.
+ self.assert_text_contains(tb,
+ "task.exception() # search target")
+ else:
+ self.fail('CancelledError did not occur')
+
def test_stop_while_run_in_complete(self):
def gen():
@@ -2391,15 +2479,14 @@
def test_cancel_gather_2(self):
cases = [
- ((), ('',)),
- ((None,), ('',)),
+ ((), ()),
+ ((None,), ()),
(('my message',), ('my message',)),
# Non-string values should roundtrip.
((5,), (5,)),
]
for cancel_args, expected_args in cases:
with self.subTest(cancel_args=cancel_args):
-
loop = asyncio.new_event_loop()
self.addCleanup(loop.close)
@@ -2417,15 +2504,20 @@
qwe = self.new_task(loop, test())
await asyncio.sleep(0.2)
qwe.cancel(*cancel_args)
- try:
- await qwe
- except asyncio.CancelledError as exc:
- self.assertEqual(exc.args, expected_args)
- else:
- self.fail('gather did not propagate the cancellation '
- 'request')
+ await qwe
- loop.run_until_complete(main())
+ 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))
+ else:
+ self.fail('gather did not propagate the cancellation '
+ 'request')
def test_exception_traceback(self):
# See http://bugs.python.org/issue28843