gh-124309: Modernize the staggered_race implementation to support eager task factories

Lib/asyncio/base_events.py

@@ -1144,7 +1144,7 @@ async def create_connection(
  (functools.partial(self._connect_sock,
  exceptions, addrinfo, laddr_infos)
  for addrinfo in infos),
- happy_eyeballs_delay, loop=self)
+ happy_eyeballs_delay)
 
  if sock is None:
  exceptions = [exc for sub in exceptions for exc in sub]

Lib/asyncio/staggered.py

@@ -4,11 +4,12 @@
 
 import contextlib
 
-from . import events
-from . import exceptions as exceptions_mod
 from . import locks
 from . import tasks
+from . import taskgroups
 
+class _Done(Exception):
+ pass
 
 async def staggered_race(coro_fns, delay, *, loop=None):
  """Run coroutines with staggered start times and take the first to finish.
@@ -42,8 +43,6 @@ async def staggered_race(coro_fns, delay, *, loop=None):
  delay: amount of time, in seconds, between starting coroutines. If
  ``None``, the coroutines will run sequentially.
 
- loop: the event loop to use.
-
  Returns:
  tuple *(winner_result, winner_index, exceptions)* where
 
@@ -62,36 +61,20 @@ async def staggered_race(coro_fns, delay, *, loop=None):
 
  """
  # TODO: when we have aiter() and anext(), allow async iterables in coro_fns.
- loop = loop or events.get_running_loop()
- enum_coro_fns = enumerate(coro_fns)
  winner_result = None
  winner_index = None
  exceptions = []
- running_tasks = []
-
- async def run_one_coro(previous_failed) -> None:
- # Wait for the previous task to finish, or for delay seconds
- if previous_failed is not None:
- with contextlib.suppress(exceptions_mod.TimeoutError):
- # Use asyncio.wait_for() instead of asyncio.wait() here, so
- # that if we get cancelled at this point, Event.wait() is also
- # cancelled, otherwise there will be a "Task destroyed but it is
- # pending" later.
- await tasks.wait_for(previous_failed.wait(), delay)
- # Get the next coroutine to run
- try:
- this_index, coro_fn = next(enum_coro_fns)
- except StopIteration:
- return
- # Start task that will run the next coroutine
- this_failed = locks.Event()
- next_task = loop.create_task(run_one_coro(this_failed))
- running_tasks.append(next_task)
- assert len(running_tasks) == this_index + 2
- # Prepare place to put this coroutine's exceptions if not won
- exceptions.append(None)
- assert len(exceptions) == this_index + 1
 
+ if loop is not None:
+ import warnings
+ warnings._deprecated(
+ 'loop',
+ 'the {name!r} parameter is deprecated and slated for removal in '
+ 'Python {remove}; it does nothing since 3.14',
+ remove=(3, 16),
+ )
+
+ async def run_one_coro(this_index, coro_fn, this_failed):
  try:
  result = await coro_fn()
  except (SystemExit, KeyboardInterrupt):
@@ -105,34 +88,17 @@ async def run_one_coro(previous_failed) -> None:
  assert winner_index is None
  winner_index = this_index
  winner_result = result
- # Cancel all other tasks. We take care to not cancel the current
- # task as well. If we do so, then since there is no `await` after
- # here and CancelledError are usually thrown at one, we will
- # encounter a curious corner case where the current task will end
- # up as done() == True, cancelled() == False, exception() ==
- # asyncio.CancelledError. This behavior is specified in
- # https://bugs.python.org/issue30048
- for i, t in enumerate(running_tasks):
- if i != this_index:
- t.cancel()
-
- first_task = loop.create_task(run_one_coro(None))
- running_tasks.append(first_task)
+ raise _Done
+
  try:
- # Wait for a growing list of tasks to all finish: poor man's version of
- # curio's TaskGroup or trio's nursery
- done_count = 0
- while done_count != len(running_tasks):
- done, _ = await tasks.wait(running_tasks)
- done_count = len(done)
- # If run_one_coro raises an unhandled exception, it's probably a
- # programming error, and I want to see it.
- if __debug__:
- for d in done:
- if d.done() and not d.cancelled() and d.exception():
- raise d.exception()
- return winner_result, winner_index, exceptions
- finally:
- # Make sure no tasks are left running if we leave this function
- for t in running_tasks:
- t.cancel()
+ async with taskgroups.TaskGroup() as tg:
+ for this_index, coro_fn in enumerate(coro_fns):
+ this_failed = locks.Event()
+ exceptions.append(None)
+ tg.create_task(run_one_coro(this_index, coro_fn, this_failed))
+ with contextlib.suppress(TimeoutError):
+ await tasks.wait_for(this_failed.wait(), delay)
+ except* _Done:
+ pass
+
+ return winner_result, winner_index, exceptions

Lib/test/test_asyncio/test_eager_task_factory.py

@@ -213,6 +213,51 @@ async def run():
 
  self.run_coro(run())
 
+ # See GH-124309 for both of these
+ def test_staggered_race_with_eager_tasks(self):
+ async def fail():
+ await asyncio.sleep(0) # Dummy coroutine
+ raise ValueError("no good")
+
+ async def run():
+ winner, index, excs = await asyncio.staggered.staggered_race(
+ [
+ lambda: asyncio.sleep(2),
+ lambda: asyncio.sleep(1),
+ lambda: fail()
+ ],
+ delay=0.25
+ )
+ self.assertIsNone(winner)
+ self.assertEqual(index, 1)
+ self.assertIsNone(excs[index])
+ self.assertIsInstance(excs[0], asyncio.CancelledError)
+ self.assertIsInstance(excs[2], ValueError)
+
+ self.run_coro(run())
+
+ def test_staggered_race_with_eager_tasks_no_delay(self):
+ async def fail():
+ raise ValueError("no good")
+
+ async def run():
+ winner, index, excs = await asyncio.staggered.staggered_race(
+ [
+ lambda: fail(),
+ lambda: asyncio.sleep(1),
+ lambda: asyncio.sleep(0),
+ ],
+ delay=None
+ )
+ self.assertIsNone(winner)
+ self.assertEqual(index, 1)
+ self.assertIsNone(excs[index])
+ self.assertIsInstance(excs[0], ValueError)
+ self.assertEqual(len(excs), 2)
+
+ self.run_coro(run())
+
+
 
 class PyEagerTaskFactoryLoopTests(EagerTaskFactoryLoopTests, test_utils.TestCase):
  Task = tasks._PyTask

Lib/test/test_asyncio/test_staggered.py

@@ -82,16 +82,45 @@ async def test_none_successful(self):
  async def coro(index):
  raise ValueError(index)
 
+ for delay in [None, 0, 0.1, 1]:
+ with self.subTest(delay=delay):
+ winner, index, excs = await staggered_race(
+ [
+ lambda: coro(0),
+ lambda: coro(1),
+ ],
+ delay=delay,
+ )
+
+ self.assertIs(winner, None)
+ self.assertIs(index, None)
+ self.assertEqual(len(excs), 2)
+ self.assertIsInstance(excs[0], ValueError)
+ self.assertIsInstance(excs[1], ValueError)
+
+ async def test_long_delay_early_failure(self):
+ async def coro(index):
+ await asyncio.sleep(0) # Dummy coroutine for the 1 case
+ if index == 0:
+ await asyncio.sleep(0.1) # Dummy coroutine
+ raise ValueError(index)
+
+ return f'Res: {index}'
+
  winner, index, excs = await staggered_race(
  [
  lambda: coro(0),
  lambda: coro(1),
  ],
- delay=None,
+ delay=10,
  )
 
- self.assertIs(winner, None)
- self.assertIs(index, None)
+ self.assertEqual(winner, 'Res: 1')
+ self.assertEqual(index, 1)
  self.assertEqual(len(excs), 2)
  self.assertIsInstance(excs[0], ValueError)
- self.assertIsInstance(excs[1], ValueError)
+ self.assertIsNone(excs[1], None)
+
+
+if __name__ == "__main__":
+ unittest.main()

Misc/NEWS.d/next/Library/2024-09-23-18-18-23.gh-issue-124309.iFcarA.rst

@@ -0,0 +1,2 @@
+Modernize the :func:`!asyncio.staggered.staggered_task` function to now
+support :attr:`asyncio.eager_task_factory` and use :class:`asyncio.TaskGroup` internally.