path: root/taskflow/jobs/backends/impl_zookeeper.py

# -*- coding: utf-8 -*-

#    Copyright (C) 2013 Yahoo! Inc. All Rights Reserved.
#
#    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
import contextlib
import functools
import sys
import threading

import fasteners
import futurist
from kazoo import exceptions as k_exceptions
from kazoo.protocol import paths as k_paths
from kazoo.protocol import states as k_states
from kazoo.recipe import watchers
from oslo_serialization import jsonutils
from oslo_utils import excutils
from oslo_utils import timeutils
from oslo_utils import uuidutils
import six

from taskflow.conductors import base as c_base
from taskflow import exceptions as excp
from taskflow.jobs import base
from taskflow import logging
from taskflow import states
from taskflow.utils import kazoo_utils
from taskflow.utils import misc

LOG = logging.getLogger(__name__)


@functools.total_ordering
class ZookeeperJob(base.Job):
    """A zookeeper job."""

    def __init__(self, board, name, client, path,
                 uuid=None, details=None, book=None, book_data=None,
                 created_on=None, backend=None,
                 priority=base.JobPriority.NORMAL):
        super(ZookeeperJob, self).__init__(board, name,
                                           uuid=uuid, details=details,
                                           backend=backend,
                                           book=book, book_data=book_data)
        self._client = client
        self._path = k_paths.normpath(path)
        self._lock_path = self._path + board.LOCK_POSTFIX
        self._created_on = created_on
        self._node_not_found = False
        basename = k_paths.basename(self._path)
        self._root = self._path[0:-len(basename)]
        self._sequence = int(basename[len(board.JOB_PREFIX):])
        self._priority = priority

    @property
    def lock_path(self):
        """Path the job lock/claim and owner znode is stored."""
        return self._lock_path

    @property
    def priority(self):
        return self._priority

    @property
    def path(self):
        """Path the job data znode is stored."""
        return self._path

    @property
    def sequence(self):
        """Sequence number of the current job."""
        return self._sequence

    @property
    def root(self):
        """The parent path of the job in zookeeper."""
        return self._root

    def _get_node_attr(self, path, attr_name, trans_func=None):
        try:
            _data, node_stat = self._client.get(path)
            attr = getattr(node_stat, attr_name)
            if trans_func is not None:
                return trans_func(attr)
            else:
                return attr
        except k_exceptions.NoNodeError:
            excp.raise_with_cause(
                excp.NotFound,
                "Can not fetch the %r attribute of job %s (%s),"
                " path %s not found" % (attr_name, self.uuid,
                                        self.path, path))
        except self._client.handler.timeout_exception:
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the %r attribute of job %s (%s),"
                " operation timed out" % (attr_name, self.uuid, self.path))
        except k_exceptions.SessionExpiredError:
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the %r attribute of job %s (%s),"
                " session expired" % (attr_name, self.uuid, self.path))
        except (AttributeError, k_exceptions.KazooException):
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the %r attribute of job %s (%s),"
                " internal error" % (attr_name, self.uuid, self.path))

    @property
    def last_modified(self):
        modified_on = None
        try:
            if not self._node_not_found:
                modified_on = self._get_node_attr(
                    self.path, 'mtime',
                    trans_func=misc.millis_to_datetime)
        except excp.NotFound:
            self._node_not_found = True
        return modified_on

    @property
    def created_on(self):
        # This one we can cache (since it won't change after creation).
        if self._node_not_found:
            return None
        if self._created_on is None:
            try:
                self._created_on = self._get_node_attr(
                    self.path, 'ctime',
                    trans_func=misc.millis_to_datetime)
            except excp.NotFound:
                self._node_not_found = True
        return self._created_on

    @property
    def state(self):
        owner = self.board.find_owner(self)
        job_data = {}
        try:
            raw_data, _data_stat = self._client.get(self.path)
            job_data = misc.decode_json(raw_data)
        except k_exceptions.NoNodeError:
            pass
        except k_exceptions.SessionExpiredError:
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the state of %s,"
                " session expired" % (self.uuid))
        except self._client.handler.timeout_exception:
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the state of %s,"
                " operation timed out" % (self.uuid))
        except k_exceptions.KazooException:
            excp.raise_with_cause(
                excp.JobFailure,
                "Can not fetch the state of %s,"
                " internal error" % (self.uuid))
        if not job_data:
            # No data this job has been completed (the owner that we might have
            # fetched will not be able to be fetched again, since the job node
            # is a parent node of the owner/lock node).
            return states.COMPLETE
        if not owner:
            # No owner, but data, still work to be done.
            return states.UNCLAIMED
        return states.CLAIMED

    def __lt__(self, other):
        if not isinstance(other, ZookeeperJob):
            return NotImplemented
        if self.root == other.root:
            if self.priority == other.priority:
                return self.sequence < other.sequence
            else:
                ordered = base.JobPriority.reorder(
                    (self.priority, self), (other.priority, other))
                if ordered[0] is self:
                    return False
                return True
        else:
            # Different jobboards with different roots...
            return self.root < other.root

    def __eq__(self, other):
        if not isinstance(other, ZookeeperJob):
            return NotImplemented
        return ((self.root, self.sequence, self.priority) ==
                (other.root, other.sequence, other.priority))

    def __ne__(self, other):
        return not self.__eq__(other)

    def __hash__(self):
        return hash(self.path)


class ZookeeperJobBoard(base.NotifyingJobBoard):
    """A jobboard backed by `zookeeper`_.

    Powered by the `kazoo <https://kazoo.readthedocs.io/en/latest/>`_ library.

    This jobboard creates *sequenced* persistent znodes in a directory in
    zookeeper and uses zookeeper watches to notify other jobboards of
    jobs which were posted using the :meth:`.post` method (this creates a
    znode with job contents/details encoded in `json`_). The users of these
    jobboard(s) (potentially on disjoint sets of machines) can then iterate
    over the available jobs and decide if they want
    to attempt to claim one of the jobs they have iterated over. If so they
    will then attempt to contact zookeeper and they will attempt to create a
    ephemeral znode using the name of the persistent znode + ".lock" as a
    postfix. If the entity trying to use the jobboard to :meth:`.claim` the
    job is able to create a ephemeral znode with that name then it will be
    allowed (and expected) to perform whatever *work* the contents of that
    job described. Once the claiming entity is finished the ephemeral znode
    and persistent znode will be deleted (if successfully completed) in a
    single transaction. If the claiming entity is not successful (or the
    entity that claimed the znode dies) the ephemeral znode will be
    released (either manually by using :meth:`.abandon` or automatically by
    zookeeper when the ephemeral node and associated session is deemed to
    have been lost).

    Do note that the creation of a kazoo client is achieved
    by :py:func:`~taskflow.utils.kazoo_utils.make_client` and the transfer
    of this jobboard configuration to that function to make a
    client may happen at ``__init__`` time. This implies that certain
    parameters from this jobboard configuration may be provided to
    :py:func:`~taskflow.utils.kazoo_utils.make_client` such
    that if a client was not provided by the caller one will be created
    according to :py:func:`~taskflow.utils.kazoo_utils.make_client`'s
    specification

    .. _zookeeper: http://zookeeper.apache.org/
    .. _json: https://json.org/
    """

    #: Transaction support was added in 3.4.0 so we need at least that version.
    MIN_ZK_VERSION = (3, 4, 0)

    #: Znode **postfix** that lock entries have.
    LOCK_POSTFIX = ".lock"

    #: Znode child path created under root path that contains trashed jobs.
    TRASH_FOLDER = ".trash"

    #: Znode child path created under root path that contains registered
    #: entities.
    ENTITY_FOLDER = ".entities"

    #: Znode **prefix** that job entries have.
    JOB_PREFIX = 'job'

    #: Default znode path used for jobs (data, locks...).
    DEFAULT_PATH = "/taskflow/jobs"

    STATE_HISTORY_LENGTH = 2
    """
    Number of prior state changes to keep a history of, mainly useful
    for history tracking and debugging connectivity issues.
    """

    NO_FETCH_STATES = (k_states.KazooState.LOST, k_states.KazooState.SUSPENDED)
    """
    Client states underwhich we return empty lists from fetching routines,
    during these states the underlying connection either is being recovered
    or may be recovered (aka, it has not full disconnected).
    """

    def __init__(self, name, conf,
                 client=None, persistence=None, emit_notifications=True):
        super(ZookeeperJobBoard, self).__init__(name, conf)
        if client is not None:
            self._client = client
            self._owned = False
        else:
            self._client = kazoo_utils.make_client(self._conf)
            self._owned = True
        path = str(conf.get("path", self.DEFAULT_PATH))
        if not path:
            raise ValueError("Empty zookeeper path is disallowed")
        if not k_paths.isabs(path):
            raise ValueError("Zookeeper path must be absolute")
        self._path = path
        self._trash_path = self._path.replace(k_paths.basename(self._path),
                                              self.TRASH_FOLDER)
        self._entity_path = self._path.replace(
            k_paths.basename(self._path),
            self.ENTITY_FOLDER)
        # The backend to load the full logbooks from, since what is sent over
        # the data connection is only the logbook uuid and name, and not the
        # full logbook.
        self._persistence = persistence
        # Misc. internal details
        self._known_jobs = {}
        self._job_cond = threading.Condition()
        self._open_close_lock = threading.RLock()
        self._client.add_listener(self._state_change_listener)
        self._bad_paths = frozenset([path])
        self._job_watcher = None
        # Since we use sequenced ids this will be the path that the sequences
        # are prefixed with, for example, job0000000001, job0000000002, ...
        self._job_base = k_paths.join(path, self.JOB_PREFIX)
        self._worker = None
        self._emit_notifications = bool(emit_notifications)
        self._connected = False
        self._suspended = False
        self._closing = False
        self._last_states = collections.deque(maxlen=self.STATE_HISTORY_LENGTH)

    def _try_emit(self, state, details):
        # Submit the work to the executor to avoid blocking the kazoo threads
        # and queue(s)...
        worker = self._worker
        if worker is None or not self._emit_notifications:
            # Worker has been destroyed or we aren't supposed to emit anything
            # in the first place...
            return
        try:
            worker.submit(self.notifier.notify, state, details)
        except RuntimeError:
            # Notification thread is/was shutdown just skip submitting a
            # notification...
            pass

    @property
    def path(self):
        """Path where all job znodes will be stored."""
        return self._path

    @property
    def trash_path(self):
        """Path where all trashed job znodes will be stored."""
        return self._trash_path

    @property
    def entity_path(self):
        """Path where all conductor info znodes will be stored."""
        return self._entity_path

    @property
    def job_count(self):
        return len(self._known_jobs)

    def _fetch_jobs(self, ensure_fresh=False):
        try:
            last_state = self._last_states[0]
        except IndexError:
            last_state = None
        if last_state in self.NO_FETCH_STATES:
            # NOTE(harlowja): on lost clear out all known jobs (from the
            # in-memory mapping) as we can not safely assume there are any
            # jobs to continue working on in this state.
            if last_state == k_states.KazooState.LOST and self._known_jobs:
                # This will force the jobboard to drop all (in-memory) jobs
                # that are not in this list (pretty much simulating what
                # would happen if a jobboard data directory was emptied).
                self._on_job_posting([], delayed=False)
            return []
        else:
            if ensure_fresh:
                self._force_refresh()
            with self._job_cond:
                return sorted(six.itervalues(self._known_jobs))

    def _force_refresh(self):
        try:
            maybe_children = self._client.get_children(self.path)
            self._on_job_posting(maybe_children, delayed=False)
        except self._client.handler.timeout_exception:
            excp.raise_with_cause(excp.JobFailure,
                                  "Refreshing failure, operation timed out")
        except k_exceptions.SessionExpiredError:
            excp.raise_with_cause(excp.JobFailure,
                                  "Refreshing failure, session expired")
        except k_exceptions.NoNodeError:
            pass
        except k_exceptions.KazooException:
            excp.raise_with_cause(excp.JobFailure,
                                  "Refreshing failure, internal error")

    def iterjobs(self, only_unclaimed=False, ensure_fresh=False):
        board_removal_func = lambda job: self._remove_job(job.path)
        return base.JobBoardIterator(
            self, LOG, only_unclaimed=only_unclaimed,
            ensure_fresh=ensure_fresh, board_fetch_func=self._fetch_jobs,
            board_removal_func=board_removal_func)

    def _remove_job(self, path):
        if path not in self._known_jobs:
            return False
        with self._job_cond:
            job = self._known_jobs.pop(path, None)
        if job is not None:
            LOG.debug("Removed job that was at path '%s'", path)
            self._try_emit(base.REMOVAL, details={'job': job})
            return True
        else:
            return False

    def _process_child(self, path, request, quiet=True):
        """Receives the result of a child data fetch request."""
        job = None
        try:
            raw_data, node_stat = request.get()
            job_data = misc.decode_json(raw_data)
            job_created_on = misc.millis_to_datetime(node_stat.ctime)
            try:
                job_priority = job_data['priority']
                job_priority = base.JobPriority.convert(job_priority)
            except KeyError:
                job_priority = base.JobPriority.NORMAL
            job_uuid = job_data['uuid']
            job_name = job_data['name']
        except (ValueError, TypeError, KeyError):
            with excutils.save_and_reraise_exception(reraise=not quiet):
                LOG.warning("Incorrectly formatted job data found at path: %s",
                            path, exc_info=True)
        except self._client.handler.timeout_exception:
            with excutils.save_and_reraise_exception(reraise=not quiet):
                LOG.warning("Operation timed out fetching job data from"
                            " from path: %s",
                            path, exc_info=True)
        except k_exceptions.SessionExpiredError:
            with excutils.save_and_reraise_exception(reraise=not quiet):
                LOG.warning("Session expired fetching job data from path: %s",
                            path, exc_info=True)
        except k_exceptions.NoNodeError:
            LOG.debug("No job node found at path: %s, it must have"
                      " disappeared or was removed", path)
        except k_exceptions.KazooException:
            with excutils.save_and_reraise_exception(reraise=not quiet):
                LOG.warning("Internal error fetching job data from path: %s",
                            path, exc_info=True)
        else:
            with self._job_cond:
                # Now we can officially check if someone already placed this
                # jobs information into the known job set (if it's already
                # existing then just leave it alone).
                if path not in self._known_jobs:
                    job = ZookeeperJob(self, job_name,
                                       self._client, path,
                                       backend=self._persistence,
                                       uuid=job_uuid,
                                       book_data=job_data.get("book"),
                                       details=job_data.get("details", {}),
                                       created_on=job_created_on,
                                       priority=job_priority)
                    self._known_jobs[path] = job
                    self._job_cond.notify_all()
        if job is not None:
            self._try_emit(base.POSTED, details={'job': job})

    def _on_job_posting(self, children, delayed=True):
        LOG.debug("Got children %s under path %s", children, self.path)
        child_paths = []
        for c in children:
            if (c.endswith(self.LOCK_POSTFIX) or
                    not c.startswith(self.JOB_PREFIX)):
                # Skip lock paths or non-job-paths (these are not valid jobs)
                continue
            child_paths.append(k_paths.join(self.path, c))
        # Figure out what we really should be investigating and what we
        # shouldn't (remove jobs that exist in our local version, but don't
        # exist in the children anymore) and accumulate all paths that we
        # need to trigger population of (without holding the job lock).
        investigate_paths = []
        pending_removals = []
        with self._job_cond:
            for path in six.iterkeys(self._known_jobs):
                if path not in child_paths:
                    pending_removals.append(path)
        for path in child_paths:
            if path in self._bad_paths:
                continue
            # This pre-check will *not* guarantee that we will not already
            # have the job (if it's being populated elsewhere) but it will
            # reduce the amount of duplicated requests in general; later when
            # the job information has been populated we will ensure that we
            # are not adding duplicates into the currently known jobs...
            if path in self._known_jobs:
                continue
            if path not in investigate_paths:
                investigate_paths.append(path)
        if pending_removals:
            with self._job_cond:
                am_removed = 0
                try:
                    for path in pending_removals:
                        am_removed += int(self._remove_job(path))
                finally:
                    if am_removed:
                        self._job_cond.notify_all()
        for path in investigate_paths:
            # Fire off the request to populate this job.
            #
            # This method is *usually* called from a asynchronous handler so
            # it's better to exit from this quickly to allow other asynchronous
            # handlers to be executed.
            request = self._client.get_async(path)
            if delayed:
                request.rawlink(functools.partial(self._process_child, path))
            else:
                self._process_child(path, request, quiet=False)

    def post(self, name, book=None, details=None,
             priority=base.JobPriority.NORMAL):
        # NOTE(harlowja): Jobs are not ephemeral, they will persist until they
        # are consumed (this may change later, but seems safer to do this until
        # further notice).
        job_priority = base.JobPriority.convert(priority)
        job_uuid = uuidutils.generate_uuid()
        job_posting = base.format_posting(job_uuid, name,
                                          book=book, details=details,
                                          priority=job_priority)
        raw_job_posting = misc.binary_encode(jsonutils.dumps(job_posting))
        with self._wrap(job_uuid, None,
                        fail_msg_tpl="Posting failure: %s",
                        ensure_known=False):
            job_path = self._client.create(self._job_base,
                                           value=raw_job_posting,
                                           sequence=True,
                                           ephemeral=False)
            job = ZookeeperJob(self, name, self._client, job_path,
                               backend=self._persistence,
                               book=book, details=details, uuid=job_uuid,
                               book_data=job_posting.get('book'),
                               priority=job_priority)
            with self._job_cond:
                self._known_jobs[job_path] = job
                self._job_cond.notify_all()
            self._try_emit(base.POSTED, details={'job': job})
            return job

    @base.check_who
    def claim(self, job, who):
        def _unclaimable_try_find_owner(cause):
            try:
                owner = self.find_owner(job)
            except Exception:
                owner = None
            if owner:
                message = "Job %s already claimed by '%s'" % (job.uuid, owner)
            else:
                message = "Job %s already claimed" % (job.uuid)
            excp.raise_with_cause(excp.UnclaimableJob,
                                  message, cause=cause)

        with self._wrap(job.uuid, job.path,
                        fail_msg_tpl="Claiming failure: %s"):
            # NOTE(harlowja): post as json which will allow for future changes
            # more easily than a raw string/text.
            value = jsonutils.dumps({
                'owner': who,
            })
            # Ensure the target job is still existent (at the right version).
            job_data, job_stat = self._client.get(job.path)
            txn = self._client.transaction()
            # This will abort (and not create the lock) if the job has been
            # removed (somehow...) or updated by someone else to a different
            # version...
            txn.check(job.path, version=job_stat.version)
            txn.create(job.lock_path, value=misc.binary_encode(value),
                       ephemeral=True)
            try:
                kazoo_utils.checked_commit(txn)
            except k_exceptions.NodeExistsError as e:
                _unclaimable_try_find_owner(e)
            except kazoo_utils.KazooTransactionException as e:
                if len(e.failures) < 2:
                    raise
                else:
                    if isinstance(e.failures[0], k_exceptions.NoNodeError):
                        excp.raise_with_cause(
                            excp.NotFound,
                            "Job %s not found to be claimed" % job.uuid,
                            cause=e.failures[0])
                    if isinstance(e.failures[1], k_exceptions.NodeExistsError):
                        _unclaimable_try_find_owner(e.failures[1])
                    else:
                        excp.raise_with_cause(
                            excp.UnclaimableJob,
                            "Job %s claim failed due to transaction"
                            " not succeeding" % (job.uuid), cause=e)

    @contextlib.contextmanager
    def _wrap(self, job_uuid, job_path,
              fail_msg_tpl="Failure: %s", ensure_known=True):
        if job_path:
            fail_msg_tpl += " (%s)" % (job_path)
        if ensure_known:
            if not job_path:
                raise ValueError("Unable to check if %r is a known path"
                                 % (job_path))
            if job_path not in self._known_jobs:
                fail_msg_tpl += ", unknown job"
                raise excp.NotFound(fail_msg_tpl % (job_uuid))
        try:
            yield
        except self._client.handler.timeout_exception:
            fail_msg_tpl += ", operation timed out"
            excp.raise_with_cause(excp.JobFailure, fail_msg_tpl % (job_uuid))
        except k_exceptions.SessionExpiredError:
            fail_msg_tpl += ", session expired"
            excp.raise_with_cause(excp.JobFailure, fail_msg_tpl % (job_uuid))
        except k_exceptions.NoNodeError:
            fail_msg_tpl += ", unknown job"
            excp.raise_with_cause(excp.NotFound, fail_msg_tpl % (job_uuid))
        except k_exceptions.KazooException:
            fail_msg_tpl += ", internal error"
            excp.raise_with_cause(excp.JobFailure, fail_msg_tpl % (job_uuid))

    def find_owner(self, job):
        with self._wrap(job.uuid, job.path,
                        fail_msg_tpl="Owner query failure: %s",
                        ensure_known=False):
            try:
                self._client.sync(job.lock_path)
                raw_data, _lock_stat = self._client.get(job.lock_path)
                data = misc.decode_json(raw_data)
                owner = data.get("owner")
            except k_exceptions.NoNodeError:
                owner = None
            return owner

    def _get_owner_and_data(self, job):
        lock_data, lock_stat = self._client.get(job.lock_path)
        job_data, job_stat = self._client.get(job.path)
        return (misc.decode_json(lock_data), lock_stat,
                misc.decode_json(job_data), job_stat)

    def register_entity(self, entity):
        entity_type = entity.kind
        if entity_type == c_base.Conductor.ENTITY_KIND:
            entity_path = k_paths.join(self.entity_path, entity_type)
            try:
                self._client.ensure_path(entity_path)
                self._client.create(k_paths.join(entity_path, entity.name),
                                    value=misc.binary_encode(
                                        jsonutils.dumps(entity.to_dict())),
                                    ephemeral=True)
            except k_exceptions.NodeExistsError:
                pass
            except self._client.handler.timeout_exception:
                excp.raise_with_cause(
                    excp.JobFailure,
                    "Can not register entity %s under %s, operation"
                    " timed out" % (entity.name, entity_path))
            except k_exceptions.SessionExpiredError:
                excp.raise_with_cause(
                    excp.JobFailure,
                    "Can not register entity %s under %s, session"
                    " expired" % (entity.name, entity_path))
            except k_exceptions.KazooException:
                excp.raise_with_cause(
                    excp.JobFailure,
                    "Can not register entity %s under %s, internal"
                    " error" % (entity.name, entity_path))
        else:
            raise excp.NotImplementedError(
                "Not implemented for other entity type '%s'" % entity_type)

    @base.check_who
    def consume(self, job, who):
        with self._wrap(job.uuid, job.path,
                        fail_msg_tpl="Consumption failure: %s"):
            try:
                owner_data = self._get_owner_and_data(job)
                lock_data, lock_stat, data, data_stat = owner_data
            except k_exceptions.NoNodeError:
                excp.raise_with_cause(excp.NotFound,
                                      "Can not consume a job %s"
                                      " which we can not determine"
                                      " the owner of" % (job.uuid))
            if lock_data.get("owner") != who:
                raise excp.JobFailure("Can not consume a job %s"
                                      " which is not owned by %s"
                                      % (job.uuid, who))
            txn = self._client.transaction()
            txn.delete(job.lock_path, version=lock_stat.version)
            txn.delete(job.path, version=data_stat.version)
            kazoo_utils.checked_commit(txn)
            self._remove_job(job.path)

    @base.check_who
    def abandon(self, job, who):
        with self._wrap(job.uuid, job.path,
                        fail_msg_tpl="Abandonment failure: %s"):
            try:
                owner_data = self._get_owner_and_data(job)
                lock_data, lock_stat, data, data_stat = owner_data
            except k_exceptions.NoNodeError:
                excp.raise_with_cause(excp.NotFound,
                                      "Can not abandon a job %s"
                                      " which we can not determine"
                                      " the owner of" % (job.uuid))
            if lock_data.get("owner") != who:
                raise excp.JobFailure("Can not abandon a job %s"
                                      " which is not owned by %s"
                                      % (job.uuid, who))
            txn = self._client.transaction()
            txn.delete(job.lock_path, version=lock_stat.version)
            kazoo_utils.checked_commit(txn)

    @base.check_who
    def trash(self, job, who):
        with self._wrap(job.uuid, job.path,
                        fail_msg_tpl="Trash failure: %s"):
            try:
                owner_data = self._get_owner_and_data(job)
                lock_data, lock_stat, data, data_stat = owner_data
            except k_exceptions.NoNodeError:
                excp.raise_with_cause(excp.NotFound,
                                      "Can not trash a job %s"
                                      " which we can not determine"
                                      " the owner of" % (job.uuid))
            if lock_data.get("owner") != who:
                raise excp.JobFailure("Can not trash a job %s"
                                      " which is not owned by %s"
                                      % (job.uuid, who))
            trash_path = job.path.replace(self.path, self.trash_path)
            value = misc.binary_encode(jsonutils.dumps(data))
            txn = self._client.transaction()
            txn.create(trash_path, value=value)
            txn.delete(job.lock_path, version=lock_stat.version)
            txn.delete(job.path, version=data_stat.version)
            kazoo_utils.checked_commit(txn)

    def _state_change_listener(self, state):
        if self._last_states:
            LOG.debug("Kazoo client has changed to"
                      " state '%s' from prior states '%s'", state,
                      self._last_states)
        else:
            LOG.debug("Kazoo client has changed to state '%s' (from"
                      " its initial/uninitialized state)", state)
        self._last_states.appendleft(state)
        if state == k_states.KazooState.LOST:
            self._connected = False
            # When the client is itself closing itself down this will be
            # triggered, but in that case we expect it, so we don't need
            # to emit a warning message.
            if not self._closing:
                LOG.warning("Connection to zookeeper has been lost")
        elif state == k_states.KazooState.SUSPENDED:
            LOG.warning("Connection to zookeeper has been suspended")
            self._suspended = True
        else:
            # Must be CONNECTED then (as there are only 3 enums)
            if self._suspended:
                self._suspended = False

    def wait(self, timeout=None):
        # Wait until timeout expires (or forever) for jobs to appear.
        watch = timeutils.StopWatch(duration=timeout)
        watch.start()
        with self._job_cond:
            while True:
                if not self._known_jobs:
                    if watch.expired():
                        raise excp.NotFound("Expired waiting for jobs to"
                                            " arrive; waited %s seconds"
                                            % watch.elapsed())
                    # This is done since the given timeout can not be provided
                    # to the condition variable, since we can not ensure that
                    # when we acquire the condition that there will actually
                    # be jobs (especially if we are spuriously awaken), so we
                    # must recalculate the amount of time we really have left.
                    self._job_cond.wait(watch.leftover(return_none=True))
                else:
                    curr_jobs = self._fetch_jobs()
                    fetch_func = lambda ensure_fresh: curr_jobs
                    removal_func = lambda a_job: self._remove_job(a_job.path)
                    return base.JobBoardIterator(
                        self, LOG, board_fetch_func=fetch_func,
                        board_removal_func=removal_func)

    @property
    def connected(self):
        return self._connected and self._client.connected

    @fasteners.locked(lock='_open_close_lock')
    def close(self):
        if self._owned:
            LOG.debug("Stopping client")
            self._closing = True
            kazoo_utils.finalize_client(self._client)
        if self._worker is not None:
            LOG.debug("Shutting down the notifier")
            self._worker.shutdown()
            self._worker = None
        with self._job_cond:
            self._known_jobs.clear()
        LOG.debug("Stopped & cleared local state")
        self._connected = False
        self._last_states.clear()

    @fasteners.locked(lock='_open_close_lock')
    def connect(self, timeout=10.0):

        def try_clean():
            # Attempt to do the needed cleanup if post-connection setup does
            # not succeed (maybe the connection is lost right after it is
            # obtained).
            try:
                self.close()
            except k_exceptions.KazooException:
                LOG.exception("Failed cleaning-up after post-connection"
                              " initialization failed")

        try:
            if timeout is not None:
                timeout = float(timeout)
            self._client.start(timeout=timeout)
            self._closing = False
        except (self._client.handler.timeout_exception,
                k_exceptions.KazooException):
            excp.raise_with_cause(excp.JobFailure,
                                  "Failed to connect to zookeeper")
        try:
            if self._conf.get('check_compatible', True):
                kazoo_utils.check_compatible(self._client, self.MIN_ZK_VERSION)
            if self._worker is None and self._emit_notifications:
                self._worker = futurist.ThreadPoolExecutor(max_workers=1)
            self._client.ensure_path(self.path)
            self._client.ensure_path(self.trash_path)
            if self._job_watcher is None:
                self._job_watcher = watchers.ChildrenWatch(
                    self._client,
                    self.path,
                    func=self._on_job_posting,
                    allow_session_lost=True)
            self._connected = True
        except excp.IncompatibleVersion:
            with excutils.save_and_reraise_exception():
                try_clean()
        except (self._client.handler.timeout_exception,
                k_exceptions.KazooException):
            exc_type, exc, exc_tb = sys.exc_info()
            try:
                try_clean()
                excp.raise_with_cause(excp.JobFailure,
                                      "Failed to do post-connection"
                                      " initialization", cause=exc)
            finally:
                del(exc_type, exc, exc_tb)