Source code for distributed.event

from __future__ import annotations

import asyncio
import logging
import uuid
from collections import defaultdict
from contextlib import suppress

from dask.utils import parse_timedelta

from distributed.client import Client
from distributed.utils import TimeoutError, log_errors
from distributed.worker import get_worker

logger = logging.getLogger(__name__)


class EventExtension:
    """An extension for the scheduler to manage Events

    This adds the following routes to the scheduler

    *  event_wait
    *  event_set
    *  event_clear
    *  event_is_set

    In principle, the implementation logic is quite simple
    as we can reuse the asyncio.Event as much as possible:
    we keep a mapping from name to an asyncio.Event and call
    every function (wait, set, clear, is_set) directly on these
    events.

    However, this would cause a memory leak: created events in the
    dictionary are never removed.
    For this, we also keep a counter for the number of waiters on
    a specific event.
    If an event is set, we need to keep track of this state so
    we can not remove it (the default flag is false).
    If it is unset but there are waiters, we can also not remove
    it, as those waiters would then have dangling futures.
    Therefore the only time we can remove the event from our dict
    is when the number of waiters is 0 and the event flag is cleared.
    """

    def __init__(self, scheduler):
        self.scheduler = scheduler
        # Keep track of all current events, identified by their name
        self._events = defaultdict(asyncio.Event)
        # Keep track on how many waiters are present, so we know when
        # we can remove the event
        self._waiter_count = defaultdict(int)

        self.scheduler.handlers.update(
            {
                "event_wait": self.event_wait,
                "event_set": self.event_set,
                "event_clear": self.event_clear,
                "event_is_set": self.event_is_set,
            }
        )

    @log_errors
    async def event_wait(self, name=None, timeout=None):
        """Wait until the event is set to true.
        Returns false, when this did not happen in the given time
        and true otherwise.
        """
        name = self._normalize_name(name)

        event = self._events[name]
        future = event.wait()
        if timeout is not None:
            future = asyncio.wait_for(future, timeout)

        self._waiter_count[name] += 1
        try:
            await future
        except TimeoutError:
            return False
        finally:
            self._waiter_count[name] -= 1

            if not self._waiter_count[name] and not event.is_set():
                # No one is waiting for this
                # and as the default flag for an event is false
                # we can safely remove it
                self._delete_event(name)

        return True

    @log_errors
    def event_set(self, name=None):
        """Set the event with the given name to true.

        All waiters on this event will be notified.
        """
        name = self._normalize_name(name)
        # No matter if someone is listening or not,
        # we set the event to true
        self._events[name].set()

    @log_errors
    def event_clear(self, name=None):
        """Set the event with the given name to false."""
        name = self._normalize_name(name)
        if not self._waiter_count[name]:
            # No one is waiting for this
            # and as the default flag for an event is false
            # we can safely remove it
            self._delete_event(name)

        else:
            # There are waiters
            # This can happen if an event is "double-cleared"
            # In principle, the event should be unset at this point
            # (because if it is set, all waiters should have been
            # notified). But to prevent race conditions
            # due to unlucky timing, we clear anyways
            assert name in self._events
            event = self._events[name]
            event.clear()

    @log_errors
    def event_is_set(self, name=None):
        name = self._normalize_name(name)
        # the default flag value is false
        # we could also let the defaultdict
        # create a new event for us, but that
        # could produce many unused events
        if name not in self._events:
            return False

        return self._events[name].is_set()

    def _normalize_name(self, name):
        """Helper function to normalize an event name"""
        if isinstance(name, list):
            name = tuple(name)

        return name

    def _delete_event(self, name):
        """Helper function to delete an event"""
        # suppress key errors to make calling this method
        # also possible if we do not even have such an event
        with suppress(KeyError):
            del self._waiter_count[name]
        with suppress(KeyError):
            del self._events[name]


[docs]class Event: """Distributed Centralized Event equivalent to asyncio.Event An event stores a single flag, which is set to false on start. The flag can be set to true (using the set() call) or back to false (with the clear() call). Every call to wait() blocks until the event flag is set to true. Parameters ---------- name: string (optional) Name of the event. Choosing the same name allows two disconnected processes to coordinate an event. If not given, a random name will be generated. client: Client (optional) Client to use for communication with the scheduler. If not given, the default global client will be used. Examples -------- >>> event_1 = Event('a') # doctest: +SKIP >>> event_1.wait(timeout=1) # doctest: +SKIP >>> # in another process >>> event_2 = Event('a') # doctest: +SKIP >>> event_2.set() # doctest: +SKIP >>> # now event_1 will stop waiting """ def __init__(self, name=None, client=None): try: self.client = client or Client.current() except ValueError: # Initialise new client self.client = get_worker().client self.name = name or "event-" + uuid.uuid4().hex def __await__(self): """async constructor Make it possible to write >>> event = await Event("x") # doctest: +SKIP even though no waiting is implied """ async def _(): return self return _().__await__()
[docs] def wait(self, timeout=None): """Wait until the event is set. Parameters ---------- timeout : number or string or timedelta, optional Seconds to wait on the event in the scheduler. This does not include local coroutine time, network transfer time, etc.. Instead of number of seconds, it is also possible to specify a timedelta in string format, e.g. "200ms". Examples -------- >>> event = Event('a') # doctest: +SKIP >>> event.wait(timeout="1s") # doctest: +SKIP Returns ------- True if the event was set of false, if a timeout happened """ timeout = parse_timedelta(timeout) result = self.client.sync( self.client.scheduler.event_wait, name=self.name, timeout=timeout ) return result
[docs] def clear(self): """Clear the event (set its flag to false). All waiters will now block. """ return self.client.sync(self.client.scheduler.event_clear, name=self.name)
[docs] def set(self): """Set the event (set its flag to false). All waiters will now be released. """ result = self.client.sync(self.client.scheduler.event_set, name=self.name) return result
[docs] def is_set(self): """Check if the event is set""" result = self.client.sync(self.client.scheduler.event_is_set, name=self.name) return result
def __reduce__(self): return (Event, (self.name,))