neutron-lib/neutron_lib/_callbacks/manager.py

#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

import collections

from oslo_log import log as logging
from oslo_utils import reflection

from neutron_lib._callbacks import events
from neutron_lib._callbacks import exceptions
from neutron_lib._i18n import _LE
from neutron_lib.db import utils as db_utils

LOG = logging.getLogger(__name__)


class CallbacksManager(object):
    """A callback system that allows objects to cooperate in a loose manner."""

    def __init__(self):
        self.clear()

    def subscribe(self, callback, resource, event):
        """Subscribe callback for a resource event.

        The same callback may register for more than one event.

        :param callback: the callback. It must raise or return a boolean.
        :param resource: the resource. It must be a valid resource.
        :param event: the event. It must be a valid event.
        """
        LOG.debug("Subscribe: %(callback)s %(resource)s %(event)s",
                  {'callback': callback, 'resource': resource, 'event': event})

        callback_id = _get_id(callback)
        try:
            self._callbacks[resource][event][callback_id] = callback
        except KeyError:
            # Initialize the registry for unknown resources and/or events
            # prior to enlisting the callback.
            self._callbacks[resource][event] = {}
            self._callbacks[resource][event][callback_id] = callback
        # We keep a copy of callbacks to speed the unsubscribe operation.
        if callback_id not in self._index:
            self._index[callback_id] = collections.defaultdict(set)
        self._index[callback_id][resource].add(event)

    def unsubscribe(self, callback, resource, event):
        """Unsubscribe callback from the registry.

        :param callback: the callback.
        :param resource: the resource.
        :param event: the event.
        """
        LOG.debug("Unsubscribe: %(callback)s %(resource)s %(event)s",
                  {'callback': callback, 'resource': resource, 'event': event})

        callback_id = self._find(callback)
        if not callback_id:
            LOG.debug("Callback %s not found", callback_id)
            return
        if resource and event:
            del self._callbacks[resource][event][callback_id]
            self._index[callback_id][resource].discard(event)
            if not self._index[callback_id][resource]:
                del self._index[callback_id][resource]
                if not self._index[callback_id]:
                    del self._index[callback_id]
        else:
            value = '%s,%s' % (resource, event)
            raise exceptions.Invalid(element='resource,event', value=value)

    def unsubscribe_by_resource(self, callback, resource):
        """Unsubscribe callback for any event associated to the resource.

        :param callback: the callback.
        :param resource: the resource.
        """
        callback_id = self._find(callback)
        if callback_id:
            if resource in self._index[callback_id]:
                for event in self._index[callback_id][resource]:
                    del self._callbacks[resource][event][callback_id]
                del self._index[callback_id][resource]
                if not self._index[callback_id]:
                    del self._index[callback_id]

    def unsubscribe_all(self, callback):
        """Unsubscribe callback for all events and all resources.


        :param callback: the callback.
        """
        callback_id = self._find(callback)
        if callback_id:
            for resource, resource_events in self._index[callback_id].items():
                for event in resource_events:
                    del self._callbacks[resource][event][callback_id]
            del self._index[callback_id]

    @db_utils.reraise_as_retryrequest
    def notify(self, resource, event, trigger, **kwargs):
        """Notify all subscribed callback(s).

        Dispatch the resource's event to the subscribed callbacks.

        :param resource: the resource.
        :param event: the event.
        :param trigger: the trigger. A reference to the sender of the event.
        """
        errors = self._notify_loop(resource, event, trigger, **kwargs)
        if errors:
            if event.startswith(events.BEFORE):
                abort_event = event.replace(
                    events.BEFORE, events.ABORT)
                self._notify_loop(resource, abort_event, trigger, **kwargs)

                raise exceptions.CallbackFailure(errors=errors)

            if event.startswith(events.PRECOMMIT):
                raise exceptions.CallbackFailure(errors=errors)

    def clear(self):
        """Brings the manager to a clean slate."""
        self._callbacks = collections.defaultdict(dict)
        self._index = collections.defaultdict(dict)

    def _notify_loop(self, resource, event, trigger, **kwargs):
        """The notification loop."""
        LOG.debug("Notify callbacks for %(resource)s, %(event)s",
                  {'resource': resource, 'event': event})

        errors = []
        callbacks = list(self._callbacks[resource].get(event, {}).items())
        # TODO(armax): consider using a GreenPile
        for callback_id, callback in callbacks:
            try:
                LOG.debug("Calling callback %s", callback_id)
                callback(resource, event, trigger, **kwargs)
            except Exception as e:
                LOG.exception(_LE("Error during notification for "
                                  "%(callback)s %(resource)s, %(event)s"),
                              {'callback': callback_id,
                               'resource': resource,
                               'event': event})
                errors.append(exceptions.NotificationError(callback_id, e))
        return errors

    def _find(self, callback):
        """Return the callback_id if found, None otherwise."""
        callback_id = _get_id(callback)
        return callback_id if callback_id in self._index else None


def _get_id(callback):
    """Return a unique identifier for the callback."""
    # TODO(armax): consider using something other than names
    # https://www.python.org/dev/peps/pep-3155/, but this
    # might be okay for now.
    return reflection.get_callable_name(callback)