Vastly improve error triggered cancellation
At the expense of a bit more complexity in `ActorNursery.wait()` (which I commented the heck out of fwiw) this adds far superior and correct cancellation semantics for when a nursery is cancelled due to (remote) errors in subactors. This includes: - `wait()` will now raise a `trio.MultiError` if multiple subactors error with the same semantics as in `trio`. - in `wait()` portals which are paired with `run_in_actor()` spawned subactors (versus `start_actor()`) are waited on separately and if the nursery **hasn't** been cancelled but there are errors those are raised immediately before waiting on `start_actor()` subactors which will block indefinitely if they haven't been explicitly cancelled. - if `wait()` does raise when the nursery hasn't yet been cancelled it's expected that it will be called again depending on the actor supervision strategy (i.e. right now we operate with a one-cancels-all strategy, the same as `trio`, so `ActorNursery.__aexit__() calls `cancel()` if any error is raised by `wait()`). Oh and I added `is_main_process()` helper; can't remember why..improved_errors
Tyler Goodlet
parent
e75b25dc21
commit
835d1fa07a
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@ -32,7 +32,8 @@ class ActorNursery:
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Tuple[str, str],
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Tuple[Actor, mp.Process, Optional[Portal]]
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] = {}
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# portals spawned with ``run_in_actor()``
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# portals spawned with ``run_in_actor()`` are
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# cancelled when their "main" result arrives
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self._cancel_after_result_on_exit: set = set()
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self.cancelled: bool = False
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self._forkserver: forkserver.ForkServer = None
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@ -132,6 +133,8 @@ class ActorNursery:
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bind_addr=bind_addr,
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statespace=statespace,
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)
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# this marks the actor to be cancelled after its portal result
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# is retreived, see ``wait()`` below.
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self._cancel_after_result_on_exit.add(portal)
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await portal._submit_for_result(
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mod_path,
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@ -142,29 +145,65 @@ class ActorNursery:
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async def wait(self) -> None:
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"""Wait for all subactors to complete.
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This is probably the most complicated (and confusing, sorry)
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function that does all the clever crap to deal with cancellation,
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error propagation, and graceful subprocess tear down.
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"""
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async def maybe_consume_result(portal, actor):
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if (
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portal in self._cancel_after_result_on_exit and
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(portal._result is None and portal._exc is None)
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):
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log.debug(f"Waiting on final result from {subactor.uid}")
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res = await portal.result()
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# if it's an async-gen then we should alert the user
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# that we're cancelling it
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async def exhaust_portal(portal, actor):
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"""Pull final result from portal (assuming it was one).
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If the main task is an async generator do our best to consume
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what's left of it.
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"""
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try:
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log.debug(f"Waiting on final result from {actor.uid}")
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final = res = await portal.result()
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# if it's an async-gen then alert that we're cancelling it
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if inspect.isasyncgen(res):
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final = []
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log.warning(
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f"Blindly consuming asyncgen for {actor.uid}")
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with trio.fail_after(1):
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async with aclosing(res) as agen:
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async for item in agen:
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log.debug(f"Consuming item {item}")
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final.append(item)
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except Exception as err:
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# we reraise in the parent task via a ``trio.MultiError``
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return err
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else:
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return final
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async def cancel_on_completion(
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portal: Portal,
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actor: Actor,
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task_status=trio.TASK_STATUS_IGNORED,
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) -> None:
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"""Cancel actor gracefully once it's "main" portal's
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result arrives.
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Should only be called for actors spawned with `run_in_actor()`.
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"""
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with trio.open_cancel_scope() as cs:
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task_status.started(cs)
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# this may error in which case we expect the far end
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# actor to have already terminated itself
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result = await exhaust_portal(portal, actor)
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if isinstance(result, Exception):
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errors.append(result)
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log.info(f"Cancelling {portal.channel.uid} gracefully")
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await portal.cancel_actor()
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if cs.cancelled_caught:
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log.warning(
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"Result waiter was cancelled, process may have died")
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async def wait_for_proc(
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proc: mp.Process,
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actor: Actor,
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portal: Portal,
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cancel_scope: trio._core._run.CancelScope,
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cancel_scope: Optional[trio._core._run.CancelScope] = None,
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) -> None:
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# TODO: timeout block here?
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if proc.is_alive():
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@ -172,42 +211,57 @@ class ActorNursery:
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# please god don't hang
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proc.join()
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log.debug(f"Joined {proc}")
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await maybe_consume_result(portal, actor)
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self._children.pop(actor.uid)
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# proc terminated, cancel result waiter
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# proc terminated, cancel result waiter that may have
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# been spawned in tandem
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if cancel_scope:
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log.warning(
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f"Cancelling existing result waiter task for {actor.uid}")
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cancel_scope.cancel()
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async def wait_for_actor(
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portal: Portal,
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actor: Actor,
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task_status=trio.TASK_STATUS_IGNORED,
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) -> None:
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# cancel the actor gracefully
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with trio.open_cancel_scope() as cs:
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task_status.started(cs)
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await maybe_consume_result(portal, actor)
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log.info(f"Cancelling {portal.channel.uid} gracefully")
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await portal.cancel_actor()
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if cs.cancelled_caught:
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log.warning("Result waiter was cancelled")
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# unblocks when all waiter tasks have completed
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log.debug(f"Waiting on all subactors to complete")
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children = self._children.copy()
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errors = []
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# wait on run_in_actor() tasks, unblocks when all complete
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async with trio.open_nursery() as nursery:
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for subactor, proc, portal in children.values():
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cs = None
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# portal from ``run_in_actor()``
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if portal in self._cancel_after_result_on_exit:
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cs = await nursery.start(wait_for_actor, portal, subactor)
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cs = await nursery.start(
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cancel_on_completion, portal, subactor)
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# TODO: how do we handle remote host spawned actors?
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nursery.start_soon(
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wait_for_proc, proc, subactor, portal, cs)
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if errors:
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if not self.cancelled:
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# halt here and expect to be called again once the nursery
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# has been cancelled externally (ex. from within __aexit__()
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# if an error is captured from ``wait()`` then ``cancel()``
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# is called immediately after which in turn calls ``wait()``
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# again.)
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raise trio.MultiError(errors)
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# wait on all `start_actor()` subactors to complete
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# if errors were captured above and we have not been cancelled
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# then these ``start_actor()`` spawned actors will block until
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# cancelled externally
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children = self._children.copy()
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async with trio.open_nursery() as nursery:
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for subactor, proc, portal in children.values():
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# TODO: how do we handle remote host spawned actors?
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nursery.start_soon(wait_for_proc, proc, subactor, portal, cs)
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log.debug(f"All subactors for {self} have terminated")
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if errors:
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# always raise any error if we're also cancelled
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raise trio.MultiError(errors)
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async def cancel(self, hard_kill: bool = False) -> None:
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"""Cancel this nursery by instructing each subactor to cancel
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iteslf and wait for all subprocesses to terminate.
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itself and wait for all subactors to terminate.
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If ``hard_killl`` is set to ``True`` then kill the processes
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directly without any far end graceful ``trio`` cancellation.
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@ -234,56 +288,57 @@ class ActorNursery:
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# channel/portal should now be up
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_, _, portal = self._children[subactor.uid]
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if portal is None:
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# cancelled while waiting on the event?
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# cancelled while waiting on the event
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# to arrive
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chan = self._actor._peers[subactor.uid][-1]
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if chan:
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portal = Portal(chan)
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else: # there's no other choice left
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do_hard_kill(proc)
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# spawn cancel tasks async
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# spawn cancel tasks
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assert portal
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n.start_soon(portal.cancel_actor)
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log.debug(f"Waiting on all subactors to complete")
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await self.wait()
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# mark ourselves as having (tried to have) cancelled all subactors
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self.cancelled = True
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log.debug(f"All subactors for {self} have terminated")
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await self.wait()
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async def __aexit__(self, etype, value, tb):
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"""Wait on all subactor's main routines to complete.
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"""
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try:
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if etype is not None:
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if etype is not None:
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try:
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# XXX: hypothetically an error could be raised and then
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# a cancel signal shows up slightly after in which case the
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# else block here might not complete? Should both be shielded?
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# a cancel signal shows up slightly after in which case
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# the `else:` block here might not complete?
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# For now, shield both.
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with trio.open_cancel_scope(shield=True):
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if etype is trio.Cancelled:
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log.warning(
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f"{current_actor().uid} was cancelled with {etype}"
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", cancelling actor nursery")
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await self.cancel()
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f"Nursery for {current_actor().uid} was "
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f"cancelled with {etype}")
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else:
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log.exception(
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f"{current_actor().uid} errored with {etype}, "
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"cancelling actor nursery")
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await self.cancel()
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else:
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# XXX: this is effectively the lone cancellation/supervisor
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# strategy which exactly mimicks trio's behaviour
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log.debug(f"Waiting on subactors {self._children} to complete")
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try:
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await self.wait()
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except Exception as err:
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log.warning(f"Nursery caught {err}, cancelling")
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f"Nursery for {current_actor().uid} "
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f"errored with {etype}, ")
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await self.cancel()
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raise
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log.debug(f"Nursery teardown complete")
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except Exception:
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log.exception("Error on nursery exit:")
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await self.wait()
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raise
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except trio.MultiError as merr:
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if value not in merr.exceptions:
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raise trio.MultiError(merr.exceptions + [value])
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raise
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else:
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# XXX: this is effectively the (for now) lone
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# cancellation/supervisor strategy which exactly
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# mimicks trio's behaviour
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log.debug(f"Waiting on subactors {self._children} to complete")
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try:
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await self.wait()
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except Exception as err:
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log.warning(f"Nursery caught {err}, cancelling")
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await self.cancel()
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raise
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log.debug(f"Nursery teardown complete")
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@asynccontextmanager
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@ -297,3 +352,8 @@ async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
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# TODO: figure out supervisors from erlang
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async with ActorNursery(current_actor()) as nursery:
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yield nursery
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def is_main_process():
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"Bool determining if this actor is running in the top-most process."
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return mp.current_process().name == 'MainProcess'
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