"""Support for running coroutines in parallel with staggered start times."""
__all__ = 'staggered_race',
import contextlib
from . import events
from . import exceptions as exceptions_mod
from . import locks
from . import tasks
async def staggered_race(coro_fns, delay, *, loop=None):
"""Run coroutines with staggered start times and take the first to finish.
This method takes an iterable of coroutine functions. The first one is
started immediately. From then on, whenever the immediately preceding one
fails (raises an exception), or when *delay* seconds has passed, the next
coroutine is started. This continues until one of the coroutines complete
successfully, in which case all others are cancelled, or until all
coroutines fail.
The coroutines provided should be well-behaved in the following way:
* They should only ``return`` if completed successfully.
* They should always raise an exception if they did not complete
successfully. In particular, if they handle cancellation, they should
probably reraise, like this::
try:
# do work
except asyncio.CancelledError:
# undo partially completed work
raise
Args:
coro_fns: an iterable of coroutine functions, i.e. callables that
return a coroutine object when called. Use ``functools.partial`` or
lambdas to pass arguments.
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
- *winner_result*: the result of the winning coroutine, or ``None``
if no coroutines won.
- *winner_index*: the index of the winning coroutine in
``coro_fns``, or ``None`` if no coroutines won. If the winning
coroutine may return None on success, *winner_index* can be used
to definitively determine whether any coroutine won.
- *exceptions*: list of exceptions returned by the coroutines.
``len(exceptions)`` is equal to the number of coroutines actually
started, and the order is the same as in ``coro_fns``. The winning
coroutine's entry is ``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
unhandled_exceptions = []
exceptions = []
running_tasks = set()
on_completed_fut = None
def task_done(task):
running_tasks.discard(task)
if (
on_completed_fut is not None
and not on_completed_fut.done()
and not running_tasks
):
on_completed_fut.set_result(None)
if task.cancelled():
return
exc = task.exception()
if exc is None:
return
unhandled_exceptions.append(exc)
async def run_one_coro(ok_to_start, previous_failed) -> None:
# in eager tasks this waits for the calling task to append this task
# to running_tasks, in regular tasks this wait is a no-op that does
# not yield a future. See gh-124309.
await ok_to_start.wait()
# 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_ok_to_start = locks.Event()
next_task = loop.create_task(run_one_coro(next_ok_to_start, this_failed))
running_tasks.add(next_task)
next_task.add_done_callback(task_done)
# next_task has been appended to running_tasks so next_task is ok to
# start.
next_ok_to_start.set()
# Prepare place to put this coroutine's exceptions if not won
exceptions.append(None)
assert len(exceptions) == this_index + 1
try:
result = await coro_fn()
except (SystemExit, KeyboardInterrupt):
raise
except BaseException as e:
exceptions[this_index] = e
this_failed.set() # Kickstart the next coroutine
else:
# Store winner's results
nonlocal winner_index, winner_result
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
current_task = tasks.current_task(loop)
for t in running_tasks:
if t is not current_task:
t.cancel()
propagate_cancellation_error = None
try:
ok_to_start = locks.Event()
first_task = loop.create_task(run_one_coro(ok_to_start, None))
running_tasks.add(first_task)
first_task.add_done_callback(task_done)
# first_task has been appended to running_tasks so first_task is ok to start.
ok_to_start.set()
propagate_cancellation_error = None
# Make sure no tasks are left running if we leave this function
while running_tasks:
on_completed_fut = loop.create_future()
try:
await on_completed_fut
except exceptions_mod.CancelledError as ex:
propagate_cancellation_error = ex
for task in running_tasks:
task.cancel(*ex.args)
on_completed_fut = None
if __debug__ and unhandled_exceptions:
# If run_one_coro raises an unhandled exception, it's probably a
# programming error, and I want to see it.
raise ExceptionGroup("staggered race failed", unhandled_exceptions)
if propagate_cancellation_error is not None:
raise propagate_cancellation_error
return winner_result, winner_index, exceptions
finally:
del exceptions, propagate_cancellation_error, unhandled_exceptions
