tractor/tractor/_runtime.py

# tractor: structured concurrent "actors".
# Copyright 2018-eternity Tyler Goodlet.

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.

# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

"""
Actor primitives and helpers

"""
from __future__ import annotations
from collections import defaultdict
from functools import partial
from itertools import chain
import importlib
import importlib.util
import inspect
import signal
import sys
from typing import (
    Any,
    Callable,
    Optional,
    Union,
    TYPE_CHECKING,
)
import uuid
from types import ModuleType
import os
from contextlib import ExitStack
import warnings

from async_generator import aclosing
from exceptiongroup import BaseExceptionGroup
import trio  # type: ignore
from trio_typing import TaskStatus

from ._ipc import Channel
from ._streaming import (
    mk_context,
    Context,
)
from .log import get_logger
from ._exceptions import (
    pack_error,
    unpack_error,
    ModuleNotExposed,
    is_multi_cancelled,
    ContextCancelled,
    TransportClosed,
)
from . import _debug
from ._discovery import get_arbiter
from ._portal import Portal
from . import _state
from . import _mp_fixup_main


if TYPE_CHECKING:
    from ._supervise import ActorNursery


log = get_logger('tractor')


async def _invoke(

    actor: 'Actor',
    cid: str,
    chan: Channel,
    func: Callable,
    kwargs: dict[str, Any],

    is_rpc: bool = True,
    task_status: TaskStatus[
        Union[Context, BaseException]
    ] = trio.TASK_STATUS_IGNORED,
):
    '''
    Invoke local func and deliver result(s) over provided channel.

    This is the core "RPC task" starting machinery.

    '''
    __tracebackhide__ = True
    treat_as_gen: bool = False
    failed_resp: bool = False

    # possibly a traceback (not sure what typing is for this..)
    tb = None

    cancel_scope = trio.CancelScope()
    # activated cancel scope ref
    cs: Optional[trio.CancelScope] = None

    ctx = actor.get_context(
        chan,
        cid,
        # We shouldn't ever need to pass this through right?
        # it's up to the soon-to-be called rpc task to
        # open the stream with this option.
        # allow_overruns=True,
    )
    context: bool = False

    if getattr(func, '_tractor_stream_function', False):
        # handle decorated ``@tractor.stream`` async functions
        sig = inspect.signature(func)
        params = sig.parameters

        # compat with old api
        kwargs['ctx'] = ctx

        if 'ctx' in params:
            warnings.warn(
                "`@tractor.stream decorated funcs should now declare "
                "a `stream`  arg, `ctx` is now designated for use with "
                "@tractor.context",
                DeprecationWarning,
                stacklevel=2,
            )

        elif 'stream' in params:
            assert 'stream' in params
            kwargs['stream'] = ctx

        treat_as_gen = True

    elif getattr(func, '_tractor_context_function', False):
        # handle decorated ``@tractor.context`` async function
        kwargs['ctx'] = ctx
        context = True

    # errors raised inside this block are propgated back to caller
    try:
        if not (
            inspect.isasyncgenfunction(func) or
            inspect.iscoroutinefunction(func)
        ):
            raise TypeError(f'{func} must be an async function!')

        try:
            coro = func(**kwargs)
        except TypeError:
            raise

        if inspect.isasyncgen(coro):
            await chan.send({'functype': 'asyncgen', 'cid': cid})
            # XXX: massive gotcha! If the containing scope
            # is cancelled and we execute the below line,
            # any ``ActorNursery.__aexit__()`` WON'T be
            # triggered in the underlying async gen! So we
            # have to properly handle the closing (aclosing)
            # of the async gen in order to be sure the cancel
            # is propagated!
            with cancel_scope as cs:
                ctx._scope = cs
                task_status.started(ctx)
                async with aclosing(coro) as agen:
                    async for item in agen:
                        # TODO: can we send values back in here?
                        # it's gonna require a `while True:` and
                        # some non-blocking way to retrieve new `asend()`
                        # values from the channel:
                        # to_send = await chan.recv_nowait()
                        # if to_send is not None:
                        #     to_yield = await coro.asend(to_send)
                        await chan.send({'yield': item, 'cid': cid})

            log.runtime(f"Finished iterating {coro}")
            # TODO: we should really support a proper
            # `StopAsyncIteration` system here for returning a final
            # value if desired
            await chan.send({'stop': True, 'cid': cid})

        # one way @stream func that gets treated like an async gen
        elif treat_as_gen:
            await chan.send({'functype': 'asyncgen', 'cid': cid})
            # XXX: the async-func may spawn further tasks which push
            # back values like an async-generator would but must
            # manualy construct the response dict-packet-responses as
            # above
            with cancel_scope as cs:
                ctx._scope = cs
                task_status.started(ctx)
                await coro

            if not cs.cancelled_caught:
                # task was not cancelled so we can instruct the
                # far end async gen to tear down
                await chan.send({'stop': True, 'cid': cid})

        elif context:
            # context func with support for bi-dir streaming
            await chan.send({'functype': 'context', 'cid': cid})

            try:
                with cancel_scope as cs:
                    ctx._scope = cs
                    task_status.started(ctx)
                    res = await coro
                    await chan.send({'return': res, 'cid': cid})

            # XXX: do we ever trigger this block any more?
            except (
                BaseExceptionGroup,
                trio.Cancelled,
            ):
                # if a context error was set then likely
                # thei multierror was raised due to that
                if ctx._remote_error is not None:
                    raise ctx._remote_error

                # maybe TODO: pack in ``trio.Cancelled.__traceback__`` here
                # so they can be unwrapped and displayed on the caller
                # side?
                raise

            finally:
                # XXX: only pop the context tracking if
                # a ``@tractor.context`` entrypoint was called
                assert chan.uid

                # don't pop the local context until we know the
                # associated child isn't in debug any more
                await _debug.maybe_wait_for_debugger()
                ctx = actor._contexts.pop((chan.uid, cid))
                if ctx:
                    log.runtime(
                        f'Context entrypoint {func} was terminated:\n{ctx}'
                    )

            assert cs
            if cs.cancelled_caught:

                # first check for and raise any remote error
                # before raising any context cancelled case
                # so that real remote errors don't get masked as
                # ``ContextCancelled``s.
                re = ctx._remote_error
                if re:
                    ctx._maybe_raise_remote_err(re)

                fname = func.__name__
                if cs.cancel_called:
                    canceller = ctx._cancel_called_remote
                    # await _debug.breakpoint()

                    # NOTE / TODO: if we end up having
                    # ``Actor._cancel_task()`` call
                    # ``Context.cancel()`` directly, we're going to
                    # need to change this logic branch since it will
                    # always enter..
                    if ctx._cancel_called:
                        msg = f'`{fname}()`@{actor.uid} cancelled itself'

                    else:
                        msg = (
                            f'`{fname}()`@{actor.uid} '
                            'was remotely cancelled by '
                        )

                    # if the channel which spawned the ctx is the
                    # one that cancelled it then we report that, vs.
                    # it being some other random actor that for ex.
                    # some actor who calls `Portal.cancel_actor()`
                    # and by side-effect cancels this ctx.
                    if canceller == ctx.chan.uid:
                        msg += f'its caller {canceller}'
                    else:
                        msg += f'remote actor {canceller}'

                    # TODO: does this ever get set any more or can
                    # we remove it?
                    if ctx._cancel_msg:
                        msg += f' with msg:\n{ctx._cancel_msg}'

                    # task-contex was either cancelled by request using
                    # ``Portal.cancel_actor()`` or ``Context.cancel()``
                    # on the far end, or it was cancelled by the local
                    # (callee) task, so relay this cancel signal to the
                    # other side.
                    raise ContextCancelled(
                        msg,
                        suberror_type=trio.Cancelled,
                        canceller=canceller,
                    )

        else:
            # regular async function
            try:
                await chan.send({'functype': 'asyncfunc', 'cid': cid})
            except trio.BrokenResourceError:
                failed_resp = True
                if is_rpc:
                    raise
                else:
                    log.warning(
                        f'Failed to respond to non-rpc request: {func}'
                    )

            with cancel_scope as cs:
                ctx._scope = cs
                task_status.started(ctx)
                result = await coro
                fname = func.__name__
                log.runtime(f'{fname}() result: {result}')
                if not failed_resp:
                    # only send result if we know IPC isn't down
                    await chan.send(
                        {'return': result,
                         'cid': cid}
                    )

    except (
        Exception,
        BaseExceptionGroup,
    ) as err:

        if not is_multi_cancelled(err):

            # TODO: maybe we'll want different "levels" of debugging
            # eventualy such as ('app', 'supervisory', 'runtime') ?

            # if not isinstance(err, trio.ClosedResourceError) and (
            # if not is_multi_cancelled(err) and (

            entered_debug: bool = False
            if (
                not isinstance(err, ContextCancelled)
                or (
                    isinstance(err, ContextCancelled)
                    and ctx._cancel_called

                    # if the root blocks the debugger lock request from a child
                    # we will get a remote-cancelled condition.
                    and ctx._enter_debugger_on_cancel
                )
            ):
                # XXX: is there any case where we'll want to debug IPC
                # disconnects as a default?
                #
                # I can't think of a reason that inspecting
                # this type of failure will be useful for respawns or
                # recovery logic - the only case is some kind of strange bug
                # in our transport layer itself? Going to keep this
                # open ended for now.
                entered_debug = await _debug._maybe_enter_pm(err)

                if not entered_debug:
                    log.exception("Actor crashed:")

        # always ship errors back to caller
        err_msg = pack_error(err, tb=tb)
        err_msg['cid'] = cid

        try:
            await chan.send(err_msg)

        # TODO: tests for this scenario:
        # - RPC caller closes connection before getting a response
        # should **not** crash this actor..
        except (
            trio.ClosedResourceError,
            trio.BrokenResourceError,
            BrokenPipeError,
        ):
            # if we can't propagate the error that's a big boo boo
            log.exception(
                f"Failed to ship error to caller @ {chan.uid} !?"
            )

        # error is probably from above coro running code *not from the
        # underlyingn rpc invocation* since a scope was never allocated
        # around actual coroutine await.
        if cs is None:
            # we don't ever raise directly here to allow the
            # msg-loop-scheduler to continue running for this
            # channel.
            task_status.started(err)

    finally:
        # RPC task bookeeping
        try:
            ctx, func, is_complete = actor._rpc_tasks.pop(
                (chan, cid)
            )
            is_complete.set()

        except KeyError:
            if is_rpc:
                # If we're cancelled before the task returns then the
                # cancel scope will not have been inserted yet
                log.warning(
                    f"Task {func} likely errored or cancelled before start")
            else:
                log.cancel(f'{func.__name__}({kwargs}) failed?')

        finally:
            if not actor._rpc_tasks:
                log.runtime("All RPC tasks have completed")
                actor._ongoing_rpc_tasks.set()


def _get_mod_abspath(module):
    return os.path.abspath(module.__file__)


async def try_ship_error_to_parent(
    channel: Channel,
    err: Union[Exception, BaseExceptionGroup],

) -> None:
    with trio.CancelScope(shield=True):
        try:
            # internal error so ship to parent without cid
            await channel.send(pack_error(err))
        except (
            trio.ClosedResourceError,
            trio.BrokenResourceError,
        ):
            # in SC terms this is one of the worst things that can
            # happen and creates the 2-general's dilemma.
            log.critical(
                f"Failed to ship error to parent "
                f"{channel.uid}, channel was closed"
            )


class Actor:
    '''
    The fundamental "runtime" concurrency primitive.

    An *actor* is the combination of a regular Python process executing
    a ``trio`` task tree, communicating with other actors through
    "memory boundary portals" - which provide a native async API around
    IPC transport "channels" which themselves encapsulate various
    (swappable) network protocols.


    Each "actor" is ``trio.run()`` scheduled "runtime" composed of many
    concurrent tasks in a single thread. The "runtime" tasks conduct
    a slew of low(er) level functions to make it possible for message
    passing between actors as well as the ability to create new actors
    (aka new "runtimes" in new processes which are supervised via
    a nursery construct). Each task which sends messages to a task in
    a "peer" (not necessarily a parent-child, depth hierarchy)) is able
    to do so via an "address", which maps IPC connections across memory
    boundaries, and task request id which allows for per-actor
    tasks to send and receive messages to specific peer-actor tasks with
    which there is an ongoing RPC/IPC dialog.

    '''
    # ugh, we need to get rid of this and replace with a "registry" sys
    # https://github.com/goodboy/tractor/issues/216
    is_arbiter: bool = False
    msg_buffer_size: int = 2**6

    # nursery placeholders filled in by `async_main()` after fork
    _root_n: Optional[trio.Nursery] = None
    _service_n: Optional[trio.Nursery] = None
    _server_n: Optional[trio.Nursery] = None

    # Information about `__main__` from parent
    _parent_main_data: dict[str, str]
    _parent_chan_cs: Optional[trio.CancelScope] = None

    # syncs for setup/teardown sequences
    _server_down: Optional[trio.Event] = None

    # if started on ``asycio`` running ``trio`` in guest mode
    _infected_aio: bool = False

    # Process-global stack closed at end on actor runtime teardown.
    # NOTE: this is currently an undocumented public api.
    lifetime_stack: ExitStack = ExitStack()

    def __init__(
        self,
        name: str,
        *,
        enable_modules: list[str] = [],
        uid: str | None = None,
        loglevel: str | None = None,
        arbiter_addr: Optional[tuple[str, int]] = None,
        spawn_method: Optional[str] = None
    ) -> None:
        '''
        This constructor is called in the parent actor **before** the spawning
        phase (aka before a new process is executed).

        '''
        self.name = name
        self.uid = (name, uid or str(uuid.uuid4()))

        self._cancel_complete = trio.Event()
        self._cancel_called_remote: tuple[str, tuple] | None = None
        self._cancel_called: bool = False

        # retreive and store parent `__main__` data which
        # will be passed to children
        self._parent_main_data = _mp_fixup_main._mp_figure_out_main()

        # always include debugging tools module
        enable_modules.append('tractor._debug')

        mods = {}
        for name in enable_modules:
            mod = importlib.import_module(name)
            mods[name] = _get_mod_abspath(mod)

        self.enable_modules = mods
        self._mods: dict[str, ModuleType] = {}
        self.loglevel = loglevel

        self._arb_addr: tuple[str, int] | None = (
            str(arbiter_addr[0]),
            int(arbiter_addr[1])
        ) if arbiter_addr else None

        # marked by the process spawning backend at startup
        # will be None for the parent most process started manually
        # by the user (currently called the "arbiter")
        self._spawn_method = spawn_method

        self._peers: defaultdict = defaultdict(list)
        self._peer_connected: dict = {}
        self._no_more_peers = trio.Event()
        self._no_more_peers.set()
        self._ongoing_rpc_tasks = trio.Event()
        self._ongoing_rpc_tasks.set()

        # (chan, cid) -> (cancel_scope, func)
        self._rpc_tasks: dict[
            tuple[Channel, str],
            tuple[Context, Callable, trio.Event]
        ] = {}

        # map {actor uids -> Context}
        self._contexts: dict[
            tuple[tuple[str, str], str],
            Context
        ] = {}

        self._listeners: list[trio.abc.Listener] = []
        self._parent_chan: Optional[Channel] = None
        self._forkserver_info: Optional[
            tuple[Any, Any, Any, Any, Any]] = None
        self._actoruid2nursery: dict[
            tuple[str, str],
            ActorNursery | None,
        ] = {}  # type: ignore  # noqa

    async def wait_for_peer(
        self, uid: tuple[str, str]
    ) -> tuple[trio.Event, Channel]:
        '''
        Wait for a connection back from a spawned actor with a given
        ``uid``.

        '''
        log.runtime(f"Waiting for peer {uid} to connect")
        event = self._peer_connected.setdefault(uid, trio.Event())
        await event.wait()
        log.runtime(f"{uid} successfully connected back to us")
        return event, self._peers[uid][-1]

    def load_modules(self) -> None:
        '''
        Load allowed RPC modules locally (after fork).

        Since this actor may be spawned on a different machine from
        the original nursery we need to try and load the local module
        code (if it exists).

        '''
        try:
            if self._spawn_method == 'trio':
                parent_data = self._parent_main_data
                if 'init_main_from_name' in parent_data:
                    _mp_fixup_main._fixup_main_from_name(
                        parent_data['init_main_from_name'])
                elif 'init_main_from_path' in parent_data:
                    _mp_fixup_main._fixup_main_from_path(
                        parent_data['init_main_from_path'])

            for modpath, filepath in self.enable_modules.items():
                # XXX append the allowed module to the python path which
                # should allow for relative (at least downward) imports.
                sys.path.append(os.path.dirname(filepath))
                log.runtime(f"Attempting to import {modpath}@{filepath}")
                mod = importlib.import_module(modpath)
                self._mods[modpath] = mod
                if modpath == '__main__':
                    self._mods['__mp_main__'] = mod

        except ModuleNotFoundError:
            # it is expected the corresponding `ModuleNotExposed` error
            # will be raised later
            log.error(f"Failed to import {modpath} in {self.name}")
            raise

    def _get_rpc_func(self, ns, funcname):
        try:
            return getattr(self._mods[ns], funcname)
        except KeyError as err:
            mne = ModuleNotExposed(*err.args)

            if ns == '__main__':
                modpath = '__name__'
            else:
                modpath = f"'{ns}'"

            msg = (
                "\n\nMake sure you exposed the target module, `{ns}`, "
                "using:\n"
                "ActorNursery.start_actor(<name>, enable_modules=[{mod}])"
            ).format(
                ns=ns,
                mod=modpath,
            )

            mne.msg += msg

            raise mne

    async def _stream_handler(

        self,
        stream: trio.SocketStream,

    ) -> None:
        """Entry point for new inbound connections to the channel server.

        """
        self._no_more_peers = trio.Event()  # unset

        chan = Channel.from_stream(stream)
        uid: Optional[tuple[str, str]] = chan.uid
        log.runtime(f"New connection to us {chan}")

        # send/receive initial handshake response
        try:
            uid = await self._do_handshake(chan)

        except (
            # we need this for ``msgspec`` for some reason?
            # for now, it's been put in the stream backend.
            # trio.BrokenResourceError,
            # trio.ClosedResourceError,

            TransportClosed,
        ):
            # XXX: This may propagate up from ``Channel._aiter_recv()``
            # and ``MsgpackStream._inter_packets()`` on a read from the
            # stream particularly when the runtime is first starting up
            # inside ``open_root_actor()`` where there is a check for
            # a bound listener on the "arbiter" addr.  the reset will be
            # because the handshake was never meant took place.
            log.warning(f"Channel {chan} failed to handshake")
            return

        # channel tracking
        event = self._peer_connected.pop(uid, None)
        if event:
            # Instructing connection: this is likely a new channel to
            # a recently spawned actor which we'd like to control via
            # async-rpc calls.
            log.runtime(f"Waking channel waiters {event.statistics()}")
            # Alert any task waiting on this connection to come up
            event.set()

        chans = self._peers[uid]

        # TODO: re-use channels for new connections instead
        # of always new ones; will require changing all the
        # discovery funcs
        if chans:
            log.runtime(
                f"already have channel(s) for {uid}:{chans}?"
            )

        log.runtime(f"Registered {chan} for {uid}")  # type: ignore
        # append new channel
        self._peers[uid].append(chan)

        local_nursery: Optional[ActorNursery] = None  # noqa
        disconnected: bool = False

        # Begin channel management - respond to remote requests and
        # process received reponses.
        try:
            disconnected = await process_messages(self, chan)

        except (
            trio.Cancelled,
        ):
            log.cancel(f"Msg loop was cancelled for {chan}")
            raise

        finally:
            local_nursery = self._actoruid2nursery.get(uid, local_nursery)

            # This is set in ``Portal.cancel_actor()``. So if
            # the peer was cancelled we try to wait for them
            # to tear down their side of the connection before
            # moving on with closing our own side.
            if (
                local_nursery
            ):
                if chan._cancel_called:
                    log.cancel(f"Waiting on cancel request to peer {chan.uid}")
                # XXX: this is a soft wait on the channel (and its
                # underlying transport protocol) to close from the
                # remote peer side since we presume that any channel
                # which is mapped to a sub-actor (i.e. it's managed by
                # one of our local nurseries) has a message is sent to
                # the peer likely by this actor (which is now in
                # a cancelled condition) when the local runtime here is
                # now cancelled while (presumably) in the middle of msg
                # loop processing.
                with trio.move_on_after(0.5) as cs:
                    cs.shield = True
                    # Attempt to wait for the far end to close the channel
                    # and bail after timeout (2-generals on closure).
                    assert chan.msgstream

                    log.runtime(
                        f'Draining lingering msgs from stream {chan.msgstream}'
                    )

                    async for msg in chan.msgstream.drain():
                        # try to deliver any lingering msgs
                        # before we destroy the channel.
                        # This accomplishes deterministic
                        # ``Portal.cancel_actor()`` cancellation by
                        # making sure any RPC response to that call is
                        # delivered the local calling task.
                        # TODO: factor this into a helper?
                        log.runtime(f'drained {msg} for {chan.uid}')
                        cid = msg.get('cid')
                        if cid:
                            # deliver response to local caller/waiter
                            await self._push_result(chan, cid, msg)

                    await local_nursery.exited.wait()

                if disconnected:
                    # if the transport died and this actor is still
                    # registered within a local nursery, we report that the
                    # IPC layer may have failed unexpectedly since it may be
                    # the cause of other downstream errors.
                    entry = local_nursery._children.get(uid)
                    if entry:
                        _, proc, _ = entry

                        poll = getattr(proc, 'poll', None)
                        if poll and poll() is None:
                            log.cancel(
                                f'Actor {uid} IPC broke but proc is alive?'
                            )

            # ``Channel`` teardown and closure sequence

            # Drop ref to channel so it can be gc-ed and disconnected
            log.runtime(f"Releasing channel {chan} from {chan.uid}")
            chans = self._peers.get(chan.uid)
            chans.remove(chan)

            if not chans:
                log.runtime(f"No more channels for {chan.uid}")
                self._peers.pop(uid, None)

            log.runtime(f"Peers is {self._peers}")

            # No more channels to other actors (at all) registered
            # as connected.
            if not self._peers:
                log.runtime("Signalling no more peer channel connections")
                self._no_more_peers.set()

                # NOTE: block this actor from acquiring the
                # debugger-TTY-lock since we have no way to know if we
                # cancelled it and further there is no way to ensure the
                # lock will be released if acquired due to having no
                # more active IPC channels.
                if _state.is_root_process():
                    pdb_lock = _debug.Lock
                    pdb_lock._blocked.add(uid)
                    log.runtime(f"{uid} blocked from pdb locking")

                    # if a now stale local task has the TTY lock still
                    # we cancel it to allow servicing other requests for
                    # the lock.
                    db_cs = pdb_lock._root_local_task_cs_in_debug
                    if (
                        db_cs
                        and not db_cs.cancel_called
                    ):
                        log.warning(
                            f'STALE DEBUG LOCK DETECTED FOR {uid}'
                        )
                        # TODO: figure out why this breaks tests..
                        db_cs.cancel()

            # XXX: is this necessary (GC should do it)?
            if chan.connected():
                # if the channel is still connected it may mean the far
                # end has not closed and we may have gotten here due to
                # an error and so we should at least try to terminate
                # the channel from this end gracefully.

                log.runtime(f"Disconnecting channel {chan}")
                try:
                    # send a msg loop terminate sentinel
                    await chan.send(None)

                    # XXX: do we want this?
                    # causes "[104] connection reset by peer" on other end
                    # await chan.aclose()

                except trio.BrokenResourceError:
                    log.runtime(f"Channel {chan.uid} was already closed")

    async def _push_result(
        self,
        chan: Channel,
        cid: str,
        msg: dict[str, Any],
    ) -> None:
        '''
        Push an RPC result to the local consumer's queue.

        '''
        uid = chan.uid
        assert uid, f"`chan.uid` can't be {uid}"
        try:
            ctx = self._contexts[(uid, cid)]
        except KeyError:
            log.warning(
                f'Ignoring msg from [no-longer/un]known context {uid}:'
                f'\n{msg}')
            return

        return await ctx._deliver_msg(msg)

    def get_context(
        self,
        chan: Channel,
        cid: str,

        msg_buffer_size: int | None = None,
        allow_overruns: bool = False,

    ) -> Context:
        '''
        Look up or create a new inter-actor-task-IPC-linked task
        "context" which encapsulates the local task's scheduling
        enviroment including a ``trio`` cancel scope, a pair of IPC
        messaging "feeder" channels, and an RPC id unique to the
        task-as-function invocation.

        '''
        log.runtime(f"Getting result queue for {chan.uid} cid {cid}")
        actor_uid = chan.uid
        assert actor_uid
        try:
            ctx = self._contexts[(actor_uid, cid)]
            ctx._allow_overruns = allow_overruns

            # adjust buffer size if specified
            state = ctx._send_chan._state  # type: ignore
            if msg_buffer_size and state.max_buffer_size != msg_buffer_size:
                state.max_buffer_size = msg_buffer_size

        except KeyError:
            ctx = mk_context(
                chan,
                cid,
                msg_buffer_size=msg_buffer_size or self.msg_buffer_size,
                _allow_overruns=allow_overruns,
            )
            self._contexts[(actor_uid, cid)] = ctx

        return ctx

    async def start_remote_task(
        self,
        chan: Channel,
        ns: str,
        func: str,
        kwargs: dict,
        msg_buffer_size: int | None = None,
        allow_overruns: bool = False,

    ) -> Context:
        '''
        Send a ``'cmd'`` message to a remote actor, which starts
        a remote task-as-function entrypoint.

        Synchronously validates the endpoint type  and return a caller
        side task ``Context`` that can be used to wait for responses
        delivered by the local runtime's message processing loop.

        '''
        cid = str(uuid.uuid4())
        assert chan.uid
        ctx = self.get_context(
            chan,
            cid,
            msg_buffer_size=msg_buffer_size,
            allow_overruns=allow_overruns,
        )
        log.runtime(f"Sending cmd to {chan.uid}: {ns}.{func}({kwargs})")
        await chan.send(
            {'cmd': (ns, func, kwargs, self.uid, cid)}
        )

        # Wait on first response msg and validate; this should be
        # immediate.
        first_msg = await ctx._recv_chan.receive()
        functype = first_msg.get('functype')

        if 'error' in first_msg:
            raise unpack_error(first_msg, chan)

        elif functype not in (
            'asyncfunc',
            'asyncgen',
            'context',
        ):
            raise ValueError(f"{first_msg} is an invalid response packet?")

        ctx._remote_func_type = functype
        return ctx

    async def _from_parent(
        self,
        parent_addr: Optional[tuple[str, int]],
    ) -> tuple[Channel, Optional[tuple[str, int]]]:
        try:
            # Connect back to the parent actor and conduct initial
            # handshake. From this point on if we error, we
            # attempt to ship the exception back to the parent.
            chan = Channel(
                destaddr=parent_addr,
            )
            await chan.connect()

            # Initial handshake: swap names.
            await self._do_handshake(chan)

            accept_addr: Optional[tuple[str, int]] = None

            if self._spawn_method == "trio":
                # Receive runtime state from our parent
                parent_data: dict[str, Any]
                parent_data = await chan.recv()
                log.runtime(
                    "Received state from parent:\n"
                    f"{parent_data}"
                )
                accept_addr = (
                    parent_data.pop('bind_host'),
                    parent_data.pop('bind_port'),
                )
                rvs = parent_data.pop('_runtime_vars')
                log.runtime(f"Runtime vars are: {rvs}")
                rvs['_is_root'] = False
                _state._runtime_vars.update(rvs)

                for attr, value in parent_data.items():

                    if attr == '_arb_addr':
                        # XXX: ``msgspec`` doesn't support serializing tuples
                        # so just cash manually here since it's what our
                        # internals expect.
                        value = tuple(value) if value else None
                        self._arb_addr = value

                    else:
                        setattr(self, attr, value)

            return chan, accept_addr

        except OSError:  # failed to connect
            log.warning(
                f"Failed to connect to parent @ {parent_addr},"
                " closing server")
            await self.cancel()
            raise

    async def _serve_forever(
        self,
        handler_nursery: trio.Nursery,
        *,
        # (host, port) to bind for channel server
        accept_host: tuple[str, int] | None = None,
        accept_port: int = 0,
        task_status: TaskStatus[trio.Nursery] = trio.TASK_STATUS_IGNORED,
    ) -> None:
        '''
        Start the channel server, begin listening for new connections.

        This will cause an actor to continue living (blocking) until
        ``cancel_server()`` is called.

        '''
        self._server_down = trio.Event()
        try:
            async with trio.open_nursery() as server_n:
                l: list[trio.abc.Listener] = await server_n.start(
                    partial(
                        trio.serve_tcp,
                        self._stream_handler,
                        # new connections will stay alive even if this server
                        # is cancelled
                        handler_nursery=handler_nursery,
                        port=accept_port,
                        host=accept_host,
                    )
                )
                log.runtime(
                    "Started tcp server(s) on"
                    f" {[getattr(l, 'socket', 'unknown socket') for l in l]}")
                self._listeners.extend(l)
                task_status.started(server_n)
        finally:
            # signal the server is down since nursery above terminated
            self._server_down.set()

    def cancel_soon(self) -> None:
        '''
        Cancel this actor asap; can be called from a sync context.

        Schedules `.cancel()` to be run immediately just like when
        cancelled by the parent.

        '''
        assert self._service_n
        self._service_n.start_soon(self.cancel)

    async def cancel(
        self,
        requesting_uid: tuple[str, str],

    ) -> bool:
        '''
        Cancel this actor's runtime.

        The "deterministic" teardown sequence in order is:
            - cancel all ongoing rpc tasks by cancel scope
            - cancel the channel server to prevent new inbound
              connections
            - cancel the "service" nursery reponsible for
              spawning new rpc tasks
            - return control the parent channel message loop

        '''
        log.cancel(f"{self.uid} is trying to cancel")
        self._cancel_called_remote: tuple = requesting_uid
        self._cancel_called = True

        # cancel all ongoing rpc tasks
        with trio.CancelScope(shield=True):

            # kill any debugger request task to avoid deadlock
            # with the root actor in this tree
            dbcs = _debug.Lock._debugger_request_cs
            if dbcs is not None:
                log.cancel("Cancelling active debugger request")
                dbcs.cancel()

            # kill all ongoing tasks
            await self.cancel_rpc_tasks(requesting_uid=requesting_uid)

            # stop channel server
            self.cancel_server()
            if self._server_down is not None:
                await self._server_down.wait()
            else:
                log.warning(
                    f'{self.uid} was likely cancelled before it started')

            # cancel all rpc tasks permanently
            if self._service_n:
                self._service_n.cancel_scope.cancel()

        log.cancel(f"{self.uid} called `Actor.cancel()`")
        self._cancel_complete.set()
        return True

    # XXX: hard kill logic if needed?
    # def _hard_mofo_kill(self):
    #     # If we're the root actor or zombied kill everything
    #     if self._parent_chan is None:  # TODO: more robust check
    #         root = trio.lowlevel.current_root_task()
    #         for n in root.child_nurseries:
    #             n.cancel_scope.cancel()

    async def _cancel_task(
        self,
        cid: str,
        chan: Channel,

        requesting_uid: tuple[str, str] | None = None,
    ) -> bool:
        '''
        Cancel a local task by call-id / channel.

        Note this method will be treated as a streaming function
        by remote actor-callers due to the declaration of ``ctx``
        in the signature (for now).

        '''
        # right now this is only implicitly called by
        # streaming IPC but it should be called
        # to cancel any remotely spawned task
        try:
            # this ctx based lookup ensures the requested task to
            # be cancelled was indeed spawned by a request from this channel
            ctx, func, is_complete = self._rpc_tasks[(chan, cid)]
            scope = ctx._scope
        except KeyError:
            log.cancel(f"{cid} has already completed/terminated?")
            return True

        log.cancel(
            f"Cancelling task:\ncid: {cid}\nfunc: {func}\n"
            f"peer: {chan.uid}\n")

        if (
            ctx._cancel_called_remote is None
            and requesting_uid
        ):
            ctx._cancel_called_remote: tuple = requesting_uid

        # don't allow cancelling this function mid-execution
        # (is this necessary?)
        if func is self._cancel_task:
            return True

        # TODO: shouldn't we eventually be calling ``Context.cancel()``
        # directly here instead (since that method can handle both
        # side's calls into it?
        scope.cancel()

        # wait for _invoke to mark the task complete
        log.runtime(
            'Waiting on task to cancel:\n'
            f'cid: {cid}\nfunc: {func}\n'
            f'peer: {chan.uid}\n'
        )
        await is_complete.wait()

        log.runtime(
            f"Sucessfully cancelled task:\ncid: {cid}\nfunc: {func}\n"
            f"peer: {chan.uid}\n")

        return True

    async def cancel_rpc_tasks(
        self,
        only_chan: Channel | None = None,
        requesting_uid: tuple[str, str] | None = None,

    ) -> None:
        '''
        Cancel all existing RPC responder tasks using the cancel scope
        registered for each.

        '''
        tasks = self._rpc_tasks
        if tasks:
            log.cancel(f"Cancelling all {len(tasks)} rpc tasks:\n{tasks} ")
            for (
                (chan, cid),
                (ctx, func, is_complete),
            ) in tasks.copy().items():
                if only_chan is not None:
                    if only_chan != chan:
                        continue

                # TODO: this should really done in a nursery batch
                if func != self._cancel_task:
                    await self._cancel_task(
                        cid,
                        chan,
                        requesting_uid=requesting_uid,
                    )

            log.cancel(
                f"Waiting for remaining rpc tasks to complete {tasks}")
            await self._ongoing_rpc_tasks.wait()

    def cancel_server(self) -> None:
        '''
        Cancel the internal channel server nursery thereby
        preventing any new inbound connections from being established.

        '''
        if self._server_n:
            log.runtime("Shutting down channel server")
            self._server_n.cancel_scope.cancel()

    @property
    def accept_addr(self) -> Optional[tuple[str, int]]:
        '''
        Primary address to which the channel server is bound.

        '''
        # throws OSError on failure
        return self._listeners[0].socket.getsockname()  # type: ignore

    def get_parent(self) -> Portal:
        '''
        Return a portal to our parent actor.

        '''
        assert self._parent_chan, "No parent channel for this actor?"
        return Portal(self._parent_chan)

    def get_chans(self, uid: tuple[str, str]) -> list[Channel]:
        '''
        Return all channels to the actor with provided uid.

        '''
        return self._peers[uid]

    async def _do_handshake(
        self,
        chan: Channel

    ) -> tuple[str, str]:
        '''
        Exchange (name, UUIDs) identifiers as the first communication step.

        These are essentially the "mailbox addresses" found in actor model
        parlance.

        '''
        await chan.send(self.uid)
        value = await chan.recv()
        uid: tuple[str, str] = (str(value[0]), str(value[1]))

        if not isinstance(uid, tuple):
            raise ValueError(f"{uid} is not a valid uid?!")

        chan.uid = str(uid[0]), str(uid[1])
        log.runtime(f"Handshake with actor {uid}@{chan.raddr} complete")
        return uid

    def is_infected_aio(self) -> bool:
        return self._infected_aio


async def async_main(
    actor: Actor,
    accept_addr: Optional[tuple[str, int]] = None,

    # XXX: currently ``parent_addr`` is only needed for the
    # ``multiprocessing`` backend (which pickles state sent to
    # the child instead of relaying it over the connect-back
    # channel). Once that backend is removed we can likely just
    # change this to a simple ``is_subactor: bool`` which will
    # be False when running as root actor and True when as
    # a subactor.
    parent_addr: Optional[tuple[str, int]] = None,
    task_status: TaskStatus[None] = trio.TASK_STATUS_IGNORED,

) -> None:
    '''
    Actor runtime entrypoint; start the IPC channel server, maybe connect
    back to the parent, and startup all core machinery tasks.

    A "root" (or "top-level") nursery for this actor is opened here and
    when cancelled/terminated effectively closes the actor's "runtime".

    '''
    # attempt to retreive ``trio``'s sigint handler and stash it
    # on our debugger lock state.
    _debug.Lock._trio_handler = signal.getsignal(signal.SIGINT)

    registered_with_arbiter = False
    try:

        # establish primary connection with immediate parent
        actor._parent_chan = None
        if parent_addr is not None:

            actor._parent_chan, accept_addr_rent = await actor._from_parent(
                parent_addr)

            # either it's passed in because we're not a child
            # or because we're running in mp mode
            if accept_addr_rent is not None:
                accept_addr = accept_addr_rent

        # load exposed/allowed RPC modules
        # XXX: do this **after** establishing a channel to the parent
        # but **before** starting the message loop for that channel
        # such that import errors are properly propagated upwards
        actor.load_modules()

        # The "root" nursery ensures the channel with the immediate
        # parent is kept alive as a resilient service until
        # cancellation steps have (mostly) occurred in
        # a deterministic way.
        async with trio.open_nursery() as root_nursery:
            actor._root_n = root_nursery
            assert actor._root_n

            async with trio.open_nursery() as service_nursery:
                # This nursery is used to handle all inbound
                # connections to us such that if the TCP server
                # is killed, connections can continue to process
                # in the background until this nursery is cancelled.
                actor._service_n = service_nursery
                assert actor._service_n

                # Startup up the channel server with,
                # - subactor: the bind address is sent by our parent
                #   over our established channel
                # - root actor: the ``accept_addr`` passed to this method
                assert accept_addr
                host, port = accept_addr

                actor._server_n = await service_nursery.start(
                    partial(
                        actor._serve_forever,
                        service_nursery,
                        accept_host=host,
                        accept_port=port
                    )
                )
                accept_addr = actor.accept_addr
                if _state._runtime_vars['_is_root']:
                    _state._runtime_vars['_root_mailbox'] = accept_addr

                # Register with the arbiter if we're told its addr
                log.runtime(f"Registering {actor} for role `{actor.name}`")
                assert isinstance(actor._arb_addr, tuple)

                async with get_arbiter(*actor._arb_addr) as arb_portal:
                    await arb_portal.run_from_ns(
                        'self',
                        'register_actor',
                        uid=actor.uid,
                        sockaddr=accept_addr,
                    )

                registered_with_arbiter = True

                # init steps complete
                task_status.started()

                # Begin handling our new connection back to our
                # parent. This is done last since we don't want to
                # start processing parent requests until our channel
                # server is 100% up and running.
                if actor._parent_chan:
                    await root_nursery.start(
                        partial(
                            process_messages,
                            actor,
                            actor._parent_chan,
                            shield=True,
                        )
                    )
                log.runtime("Waiting on service nursery to complete")
            log.runtime(
                "Service nursery complete\n"
                "Waiting on root nursery to complete"
            )

        # Blocks here as expected until the root nursery is
        # killed (i.e. this actor is cancelled or signalled by the parent)
    except Exception as err:
        log.runtime("Closing all actor lifetime contexts")
        actor.lifetime_stack.close()

        if not registered_with_arbiter:
            # TODO: I guess we could try to connect back
            # to the parent through a channel and engage a debugger
            # once we have that all working with std streams locking?
            log.exception(
                f"Actor errored and failed to register with arbiter "
                f"@ {actor._arb_addr}?")
            log.error(
                "\n\n\t^^^ THIS IS PROBABLY A TRACTOR BUGGGGG!!! ^^^\n"
                "\tCALMLY CALL THE AUTHORITIES AND HIDE YOUR CHILDREN.\n\n"
                "\tYOUR PARENT CODE IS GOING TO KEEP WORKING FINE!!!\n"
                "\tTHIS IS HOW RELIABlE SYSTEMS ARE SUPPOSED TO WORK!?!?\n"
            )

        if actor._parent_chan:
            await try_ship_error_to_parent(actor._parent_chan, err)

        # always!
        match err:
            case ContextCancelled():
                log.cancel(
                    f'Actor: {actor.uid} was task-context-cancelled with,\n'
                    f'str(err)'
                )
            case _:
                log.exception("Actor errored:")
        raise

    finally:
        log.info("Runtime nursery complete")

        # tear down all lifetime contexts if not in guest mode
        # XXX: should this just be in the entrypoint?
        log.info("Closing all actor lifetime contexts")

        # TODO: we can't actually do this bc the debugger
        # uses the _service_n to spawn the lock task, BUT,
        # in theory if we had the root nursery surround this finally
        # block it might be actually possible to debug THIS
        # machinery in the same way as user task code?
        # if actor.name == 'brokerd.ib':
        #     with trio.CancelScope(shield=True):
        #         await _debug.breakpoint()

        actor.lifetime_stack.close()

        # Unregister actor from the arbiter
        if (
            registered_with_arbiter
            and not actor.is_arbiter
        ):
            failed = False
            assert isinstance(actor._arb_addr, tuple)
            with trio.move_on_after(0.5) as cs:
                cs.shield = True
                try:
                    async with get_arbiter(*actor._arb_addr) as arb_portal:
                        await arb_portal.run_from_ns(
                            'self',
                            'unregister_actor',
                            uid=actor.uid
                        )
                except OSError:
                    failed = True
            if cs.cancelled_caught:
                failed = True
            if failed:
                log.warning(
                    f"Failed to unregister {actor.name} from arbiter")

        # Ensure all peers (actors connected to us as clients) are finished
        if not actor._no_more_peers.is_set():
            if any(
                chan.connected() for chan in chain(*actor._peers.values())
            ):
                log.runtime(
                    f"Waiting for remaining peers {actor._peers} to clear")
                with trio.CancelScope(shield=True):
                    await actor._no_more_peers.wait()
        log.runtime("All peer channels are complete")

    log.runtime("Runtime completed")


async def process_messages(
    actor: Actor,
    chan: Channel,
    shield: bool = False,
    task_status: TaskStatus[trio.CancelScope] = trio.TASK_STATUS_IGNORED,

) -> bool:
    '''
    This is the per-channel, low level RPC task scheduler loop.

    Receive multiplexed RPC request messages from some remote process,
    spawn handler tasks depending on request type and deliver responses
    or boxed errors back to the remote caller (task).

    '''
    # TODO: once https://github.com/python-trio/trio/issues/467 gets
    # worked out we'll likely want to use that!
    msg: dict | None = None
    nursery_cancelled_before_task: bool = False

    log.runtime(f"Entering msg loop for {chan} from {chan.uid}")
    try:
        with trio.CancelScope(shield=shield) as loop_cs:
            # this internal scope allows for keeping this message
            # loop running despite the current task having been
            # cancelled (eg. `open_portal()` may call this method from
            # a locally spawned task) and recieve this scope using
            # ``scope = Nursery.start()``
            task_status.started(loop_cs)
            async for msg in chan:

                if msg is None:  # loop terminate sentinel

                    log.cancel(
                        f"Channel to {chan.uid} terminated?\n"
                        "Cancelling all associated tasks..")

                    for (channel, cid) in actor._rpc_tasks.copy():
                        if channel is chan:
                            await actor._cancel_task(
                                cid,
                                channel,
                            )

                    log.runtime(
                            f"Msg loop signalled to terminate for"
                            f" {chan} from {chan.uid}")

                    break

                log.transport(   # type: ignore
                    f"Received msg {msg} from {chan.uid}")

                cid = msg.get('cid')
                if cid:
                    # deliver response to local caller/waiter
                    # via its per-remote-context memory channel.
                    await actor._push_result(chan, cid, msg)

                    log.runtime(
                        f"Waiting on next msg for {chan} from {chan.uid}")
                    continue

                # TODO: implement with ``match:`` syntax?
                # process command request
                try:
                    ns, funcname, kwargs, actorid, cid = msg['cmd']
                except KeyError:
                    # This is the non-rpc error case, that is, an
                    # error **not** raised inside a call to ``_invoke()``
                    # (i.e. no cid was provided in the msg - see above).
                    # Push this error to all local channel consumers
                    # (normally portals) by marking the channel as errored
                    assert chan.uid
                    exc = unpack_error(msg, chan=chan)
                    chan._exc = exc
                    raise exc

                log.runtime(
                    f"Processing request from {actorid}\n"
                    f"{ns}.{funcname}({kwargs})")

                if ns == 'self':
                    if funcname == 'cancel':
                        func = actor.cancel
                        kwargs['requesting_uid'] = chan.uid

                        # don't start entire actor runtime cancellation
                        # if this actor is currently in debug mode!
                        pdb_complete = _debug.Lock.local_pdb_complete
                        if pdb_complete:
                            await pdb_complete.wait()

                        # we immediately start the runtime machinery
                        # shutdown
                        with trio.CancelScope(shield=True):
                            # actor.cancel() was called so kill this
                            # msg loop and break out into
                            # ``async_main()``
                            log.cancel(
                                "Actor runtime for was remotely cancelled "
                                f"by {chan.uid}"
                            )
                            await _invoke(
                                actor,
                                cid,
                                chan,
                                func,
                                kwargs,
                                is_rpc=False,
                            )

                        log.cancel(
                            f'Cancelling msg loop for {chan.uid}'
                        )
                        loop_cs.cancel()
                        break

                    if funcname == '_cancel_task':
                        func = actor._cancel_task

                        # we immediately start the runtime machinery
                        # shutdown
                        # with trio.CancelScope(shield=True):
                        kwargs['chan'] = chan
                        target_cid = kwargs['cid']
                        kwargs['requesting_uid'] = chan.uid
                        log.cancel(
                            f'Remote request to cancel task\n'
                            f'remote actor: {chan.uid}\n'
                            f'task: {target_cid}'
                        )
                        try:
                            await _invoke(
                                actor,
                                cid,
                                chan,
                                func,
                                kwargs,
                                is_rpc=False,
                            )
                        except BaseException:
                            log.exception("failed to cancel task?")

                        continue
                    else:
                        # normally registry methods, eg.
                        # ``.register_actor()`` etc.
                        func = getattr(actor, funcname)

                else:
                    # complain to client about restricted modules
                    try:
                        func = actor._get_rpc_func(ns, funcname)
                    except (ModuleNotExposed, AttributeError) as err:
                        err_msg = pack_error(err)
                        err_msg['cid'] = cid
                        await chan.send(err_msg)
                        continue

                # spin up a task for the requested function
                log.runtime(f"Spawning task for {func}")
                assert actor._service_n
                try:
                    ctx: Context = await actor._service_n.start(
                        partial(
                            _invoke,
                            actor,
                            cid,
                            chan,
                            func,
                            kwargs,
                        ),
                        name=funcname,
                    )

                except (
                    RuntimeError,
                    BaseExceptionGroup,
                ):
                    # avoid reporting a benign race condition
                    # during actor runtime teardown.
                    nursery_cancelled_before_task: bool = True
                    break

                # in the lone case where a ``Context`` is not
                # delivered, it's likely going to be a locally
                # scoped exception from ``_invoke()`` itself.
                if isinstance(ctx, Exception):
                    log.warning(
                        f"Task for RPC func {func} failed with"
                        f"{ctx}"
                    )
                    continue

                else:
                    # mark that we have ongoing rpc tasks
                    actor._ongoing_rpc_tasks = trio.Event()
                    log.runtime(f"RPC func is {func}")

                    # store cancel scope such that the rpc task can be
                    # cancelled gracefully if requested
                    actor._rpc_tasks[(chan, cid)] = (
                        ctx,
                        func,
                        trio.Event(),
                    )

                log.runtime(
                    f"Waiting on next msg for {chan} from {chan.uid}")

            # end of async for, channel disconnect vis
            # ``trio.EndOfChannel``
            log.runtime(
                f"{chan} for {chan.uid} disconnected, cancelling tasks"
            )
            await actor.cancel_rpc_tasks(chan)

    except (
        TransportClosed,
    ):
        # channels "breaking" (for TCP streams by EOF or 104
        # connection-reset) is ok since we don't have a teardown
        # handshake for them (yet) and instead we simply bail out of
        # the message loop and expect the teardown sequence to clean
        # up.
        log.runtime(
            f'channel from {chan.uid} closed abruptly:\n'
            f'-> {chan.raddr}\n'
        )

        # transport **was** disconnected
        return True

    except (
        Exception,
        BaseExceptionGroup,
    ) as err:
        if nursery_cancelled_before_task:
            sn = actor._service_n
            assert sn and sn.cancel_scope.cancel_called
            log.cancel(
                f'Service nursery cancelled before it handled {funcname}'
            )
        else:
            # ship any "internal" exception (i.e. one from internal
            # machinery not from an rpc task) to parent
            match err:
                case ContextCancelled():
                    log.cancel(
                        f'Actor: {actor.uid} was context-cancelled with,\n'
                        f'str(err)'
                    )
                case _:
                    log.exception("Actor errored:")

            if actor._parent_chan:
                await try_ship_error_to_parent(actor._parent_chan, err)

        # if this is the `MainProcess` we expect the error broadcasting
        # above to trigger an error at consuming portal "checkpoints"
        raise

    finally:
        # msg debugging for when he machinery is brokey
        log.runtime(
            f"Exiting msg loop for {chan} from {chan.uid} "
            f"with last msg:\n{msg}"
        )

    # transport **was not** disconnected
    return False


class Arbiter(Actor):
    '''
    A special actor who knows all the other actors and always has
    access to a top level nursery.

    The arbiter is by default the first actor spawned on each host
    and is responsible for keeping track of all other actors for
    coordination purposes. If a new main process is launched and an
    arbiter is already running that arbiter will be used.

    '''
    is_arbiter = True

    def __init__(self, *args, **kwargs) -> None:

        self._registry: dict[
            tuple[str, str],
            tuple[str, int],
        ] = {}
        self._waiters: dict[
            str,
            # either an event to sync to receiving an actor uid (which
            # is filled in once the actor has sucessfully registered),
            # or that uid after registry is complete.
            list[trio.Event | tuple[str, str]]
        ] = {}

        super().__init__(*args, **kwargs)

    async def find_actor(
        self,
        name: str,

    ) -> tuple[str, int] | None:

        for uid, sockaddr in self._registry.items():
            if name in uid:
                return sockaddr

        return None

    async def get_registry(
        self

    ) -> dict[str, tuple[str, int]]:
        '''
        Return current name registry.

        This method is async to allow for cross-actor invocation.

        '''
        # NOTE: requires ``strict_map_key=False`` to the msgpack
        # unpacker since we have tuples as keys (not this makes the
        # arbiter suscetible to hashdos):
        # https://github.com/msgpack/msgpack-python#major-breaking-changes-in-msgpack-10
        return {'.'.join(key): val for key, val in self._registry.items()}

    async def wait_for_actor(
        self,
        name: str,

    ) -> list[tuple[str, int]]:
        '''
        Wait for a particular actor to register.

        This is a blocking call if no actor by the provided name is currently
        registered.

        '''
        sockaddrs: list[tuple[str, int]] = []
        sockaddr: tuple[str, int]

        for (aname, _), sockaddr in self._registry.items():
            if name == aname:
                sockaddrs.append(sockaddr)

        if not sockaddrs:
            waiter = trio.Event()
            self._waiters.setdefault(name, []).append(waiter)
            await waiter.wait()

            for uid in self._waiters[name]:
                if not isinstance(uid, trio.Event):
                    sockaddrs.append(self._registry[uid])

        return sockaddrs

    async def register_actor(
        self,
        uid: tuple[str, str],
        sockaddr: tuple[str, int]

    ) -> None:
        uid = name, _ = (str(uid[0]), str(uid[1]))
        self._registry[uid] = (str(sockaddr[0]), int(sockaddr[1]))

        # pop and signal all waiter events
        events = self._waiters.pop(name, [])
        self._waiters.setdefault(name, []).append(uid)
        for event in events:
            if isinstance(event, trio.Event):
                event.set()

    async def unregister_actor(
        self,
        uid: tuple[str, str]

    ) -> None:
        uid = (str(uid[0]), str(uid[1]))
        self._registry.pop(uid)