148 lines
5.8 KiB
Python
148 lines
5.8 KiB
Python
"""Support for running coroutines in parallel with staggered start times."""
|
|
|
|
__all__ = 'staggered_race',
|
|
|
|
import contextlib
|
|
import typing
|
|
|
|
from . import events
|
|
from . import futures
|
|
from . import locks
|
|
from . import tasks
|
|
|
|
|
|
async def staggered_race(
|
|
coro_fns: typing.Iterable[typing.Callable[[], typing.Awaitable]],
|
|
delay: typing.Optional[float],
|
|
*,
|
|
loop: events.AbstractEventLoop = None,
|
|
) -> typing.Tuple[
|
|
typing.Any,
|
|
typing.Optional[int],
|
|
typing.List[typing.Optional[Exception]]
|
|
]:
|
|
"""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
|
|
exceptions = []
|
|
running_tasks = []
|
|
|
|
async def run_one_coro(
|
|
previous_failed: typing.Optional[locks.Event]) -> None:
|
|
# Wait for the previous task to finish, or for delay seconds
|
|
if previous_failed is not None:
|
|
with contextlib.suppress(futures.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
|
|
|
|
try:
|
|
result = await coro_fn()
|
|
except Exception 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
|
|
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)
|
|
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()
|