forked from goodboy/tractor
Add more detailed docs around nursery logic
The logic in the `ActorNursery` block is critical to cancellation semantics and in particular, understanding how supervisor strategies are invoked. Stick in a bunch of explanatory comments to clear up these details and also prepare to introduce more supervisor strats besides the current one-cancels-all approach.try_trip^2
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@ -177,33 +177,58 @@ class ActorNursery:
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@asynccontextmanager
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async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
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"""Create and yield a new ``ActorNursery``.
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"""
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# TODO: figure out supervisors from erlang
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"""Create and yield a new ``ActorNursery`` to be used for spawning
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structured concurrent subactors.
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When an actor is spawned a new trio task is started which
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invokes one of the process spawning backends to create and start
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a new subprocess. These tasks are started by one of two nurseries
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detailed below. The reason for spawning processes from within
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a new task is because ``trio_run_in_process`` itself creates a new
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internal nursery and the same task that opens a nursery **must**
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close it. It turns out this approach is probably more correct
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anyway since it is more clear from the following nested nurseries
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which cancellation scopes correspond to each spawned subactor set.
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"""
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actor = current_actor()
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if not actor:
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raise RuntimeError("No actor instance has been defined yet?")
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# XXX we use these nurseries because TRIP is doing all its stuff with
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# an `@asynccontextmanager` which has an internal nursery *and* the
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# task that opens a nursery **must also close it**.
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# the collection of errors retreived from spawned sub-actors
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errors: Dict[Tuple[str, str], Exception] = {}
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# This is the outermost level "deamon actor" nursery. It is awaited
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# **after** the below inner "run in actor nursery". This allows for
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# handling errors that are generated by the inner nursery in
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# a supervisor strategy **before** blocking indefinitely to wait for
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# actors spawned in "daemon mode" (aka started using
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# ``ActorNursery.start_actor()``).
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async with trio.open_nursery() as da_nursery:
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try:
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# This is the inner level "run in actor" nursery. It is
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# awaited first since actors spawned in this way (using
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# ``ActorNusery.run_in_actor()``) are expected to only
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# return a single result and then complete (i.e. be canclled
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# gracefully). Errors collected from these actors are
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# immediately raised for handling by a supervisor strategy.
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# As such if the strategy propagates any error(s) upwards
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# the above "daemon actor" nursery will be notified.
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async with trio.open_nursery() as ria_nursery:
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anursery = ActorNursery(
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actor, ria_nursery, da_nursery, errors
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)
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try:
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# spawning of actors happens in this scope after
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# we yield to the caller.
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# spawning of actors happens in the caller's scope
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# after we yield upwards
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yield anursery
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log.debug(
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f"Waiting on subactors {anursery._children}"
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"to complete"
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)
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except (BaseException, Exception) as err:
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# if the caller's scope errored then we activate our
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# one-cancels-all supervisor strategy (don't
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# worry more are coming).
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anursery._join_procs.set()
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try:
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# XXX: hypothetically an error could be raised and then
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@ -219,24 +244,44 @@ async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
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log.exception(
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f"Nursery for {current_actor().uid} "
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f"errored with {err}, ")
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# cancel all subactors
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await anursery.cancel()
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except trio.MultiError as merr:
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# If we receive additional errors while waiting on
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# remaining subactors that were cancelled,
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# aggregate those errors with the original error
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# that triggered this teardown.
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if err not in merr.exceptions:
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raise trio.MultiError(merr.exceptions + [err])
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else:
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raise
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# last bit before first nursery block ends
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# Last bit before first nursery block ends in the case
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# where we didn't error in the caller's scope
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log.debug(f"Waiting on all subactors to complete")
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anursery._join_procs.set()
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# ria_nursery scope
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# ria_nursery scope end
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except (Exception, trio.MultiError) as err:
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# If actor-local error was raised while waiting on
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# ".run_in_actor()" actors then we also want to cancel all
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# remaining sub-actors (due to our lone strategy:
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# one-cancels-all).
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log.warning(f"Nursery cancelling due to {err}")
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if anursery._children:
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with trio.CancelScope(shield=True):
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await anursery.cancel()
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raise
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finally:
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# No errors were raised while awaiting ".run_in_actor()"
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# actors but those actors may have returned remote errors as
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# results (meaning they errored remotely and have relayed
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# those errors back to this parent actor). The errors are
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# collected in ``errors`` so cancel all actors, summarize
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# all errors and re-raise.
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if errors:
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if anursery._children:
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with trio.CancelScope(shield=True):
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@ -245,4 +290,7 @@ async def open_nursery() -> typing.AsyncGenerator[ActorNursery, None]:
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raise trio.MultiError(tuple(errors.values()))
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else:
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raise list(errors.values())[0]
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# ria_nursery scope end
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log.debug(f"Nursery teardown complete")
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