scheduler/monitor_db.py

#!/usr/bin/python -u

"""
Autotest scheduler
"""


import common
import datetime, errno, optparse, os, pwd, Queue, re, shutil, signal
import smtplib, socket, stat, subprocess, sys, tempfile, time, traceback, urllib
import itertools, logging, weakref, gc

import MySQLdb

from autotest_lib.scheduler import scheduler_logging_config
from autotest_lib.frontend import setup_django_environment

import django.db

from autotest_lib.client.common_lib import global_config, logging_manager
from autotest_lib.client.common_lib import host_protections, utils
from autotest_lib.database import database_connection
from autotest_lib.frontend.afe import models, rpc_utils, readonly_connection
from autotest_lib.frontend.afe import model_attributes
from autotest_lib.scheduler import drone_manager, drones, email_manager
from autotest_lib.scheduler import monitor_db_cleanup
from autotest_lib.scheduler import status_server, scheduler_config
from autotest_lib.scheduler import gc_stats, metahost_scheduler
from autotest_lib.scheduler import scheduler_models
BABYSITTER_PID_FILE_PREFIX = 'monitor_db_babysitter'
PID_FILE_PREFIX = 'monitor_db'

RESULTS_DIR = '.'
AUTOSERV_NICE_LEVEL = 10
DB_CONFIG_SECTION = 'AUTOTEST_WEB'
AUTOTEST_PATH = os.path.join(os.path.dirname(__file__), '..')

if os.environ.has_key('AUTOTEST_DIR'):
    AUTOTEST_PATH = os.environ['AUTOTEST_DIR']
AUTOTEST_SERVER_DIR = os.path.join(AUTOTEST_PATH, 'server')
AUTOTEST_TKO_DIR = os.path.join(AUTOTEST_PATH, 'tko')

if AUTOTEST_SERVER_DIR not in sys.path:
    sys.path.insert(0, AUTOTEST_SERVER_DIR)

# error message to leave in results dir when an autoserv process disappears
# mysteriously
_LOST_PROCESS_ERROR = """\
Autoserv failed abnormally during execution for this job, probably due to a
system error on the Autotest server.  Full results may not be available.  Sorry.
"""

_db = None
_shutdown = False
_autoserv_path = os.path.join(drones.AUTOTEST_INSTALL_DIR, 'server', 'autoserv')
_testing_mode = False
_drone_manager = None

def _parser_path_default(install_dir):
    return os.path.join(install_dir, 'tko', 'parse')
_parser_path_func = utils.import_site_function(
        __file__, 'autotest_lib.scheduler.site_monitor_db',
        'parser_path', _parser_path_default)
_parser_path = _parser_path_func(drones.AUTOTEST_INSTALL_DIR)


def _get_pidfile_timeout_secs():
    """@returns How long to wait for autoserv to write pidfile."""
    pidfile_timeout_mins = global_config.global_config.get_config_value(
            scheduler_config.CONFIG_SECTION, 'pidfile_timeout_mins', type=int)
    return pidfile_timeout_mins * 60


def _site_init_monitor_db_dummy():
    return {}


get_site_metahost_schedulers = utils.import_site_function(
        __file__, 'autotest_lib.scheduler.site_metahost_scheduler',
        'get_metahost_schedulers', lambda : ())


def _verify_default_drone_set_exists():
    if (models.DroneSet.drone_sets_enabled() and
            not models.DroneSet.default_drone_set_name()):
        raise SchedulerError('Drone sets are enabled, but no default is set')


def _sanity_check():
    """Make sure the configs are consistent before starting the scheduler"""
    _verify_default_drone_set_exists()


def main():
    try:
        try:
            main_without_exception_handling()
        except SystemExit:
            raise
        except:
            logging.exception('Exception escaping in monitor_db')
            raise
    finally:
        utils.delete_pid_file_if_exists(PID_FILE_PREFIX)


def main_without_exception_handling():
    setup_logging()

    usage = 'usage: %prog [options] results_dir'
    parser = optparse.OptionParser(usage)
    parser.add_option('--recover-hosts', help='Try to recover dead hosts',
                      action='store_true')
    parser.add_option('--test', help='Indicate that scheduler is under ' +
                      'test and should use dummy autoserv and no parsing',
                      action='store_true')
    (options, args) = parser.parse_args()
    if len(args) != 1:
        parser.print_usage()
        return

    scheduler_enabled = global_config.global_config.get_config_value(
        scheduler_config.CONFIG_SECTION, 'enable_scheduler', type=bool)

    if not scheduler_enabled:
        msg = ("Scheduler not enabled, set enable_scheduler to true in the "
               "global_config's SCHEDULER section to enabled it. Exiting.")
        logging.error(msg)
        sys.exit(1)

    global RESULTS_DIR
    RESULTS_DIR = args[0]

    site_init = utils.import_site_function(__file__,
        "autotest_lib.scheduler.site_monitor_db", "site_init_monitor_db",
        _site_init_monitor_db_dummy)
    site_init()

    # Change the cwd while running to avoid issues incase we were launched from
    # somewhere odd (such as a random NFS home directory of the person running
    # sudo to launch us as the appropriate user).
    os.chdir(RESULTS_DIR)

    # This is helpful for debugging why stuff a scheduler launches is
    # misbehaving.
    logging.info('os.environ: %s', os.environ)

    if options.test:
        global _autoserv_path
        _autoserv_path = 'autoserv_dummy'
        global _testing_mode
        _testing_mode = True

    server = status_server.StatusServer()
    server.start()

    try:
        initialize()
        dispatcher = Dispatcher()
        dispatcher.initialize(recover_hosts=options.recover_hosts)

        while not _shutdown:
            dispatcher.tick()
            time.sleep(scheduler_config.config.tick_pause_sec)
    except:
        email_manager.manager.log_stacktrace(
            "Uncaught exception; terminating monitor_db")

    email_manager.manager.send_queued_emails()
    server.shutdown()
    _drone_manager.shutdown()
    _db.disconnect()


def setup_logging():
    log_dir = os.environ.get('AUTOTEST_SCHEDULER_LOG_DIR', None)
    log_name = os.environ.get('AUTOTEST_SCHEDULER_LOG_NAME', None)
    logging_manager.configure_logging(
            scheduler_logging_config.SchedulerLoggingConfig(), log_dir=log_dir,
            logfile_name=log_name)


def handle_sigint(signum, frame):
    global _shutdown
    _shutdown = True
    logging.info("Shutdown request received.")


def initialize():
    logging.info("%s> dispatcher starting", time.strftime("%X %x"))
    logging.info("My PID is %d", os.getpid())

    if utils.program_is_alive(PID_FILE_PREFIX):
        logging.critical("monitor_db already running, aborting!")
        sys.exit(1)
    utils.write_pid(PID_FILE_PREFIX)

    if _testing_mode:
        global_config.global_config.override_config_value(
            DB_CONFIG_SECTION, 'database', 'stresstest_autotest_web')

    os.environ['PATH'] = AUTOTEST_SERVER_DIR + ':' + os.environ['PATH']
    global _db
    _db = database_connection.DatabaseConnection(DB_CONFIG_SECTION)
    _db.connect(db_type='django')

    # ensure Django connection is in autocommit
    setup_django_environment.enable_autocommit()
    # bypass the readonly connection
    readonly_connection.ReadOnlyConnection.set_globally_disabled(True)

    logging.info("Setting signal handler")
    signal.signal(signal.SIGINT, handle_sigint)

    initialize_globals()
    scheduler_models.initialize()

    drones = global_config.global_config.get_config_value(
        scheduler_config.CONFIG_SECTION, 'drones', default='localhost')
    drone_list = [hostname.strip() for hostname in drones.split(',')]
    results_host = global_config.global_config.get_config_value(
        scheduler_config.CONFIG_SECTION, 'results_host', default='localhost')
    _drone_manager.initialize(RESULTS_DIR, drone_list, results_host)

    logging.info("Connected! Running...")


def initialize_globals():
    global _drone_manager
    _drone_manager = drone_manager.instance()


def _autoserv_command_line(machines, extra_args, job=None, queue_entry=None,
                           verbose=True):
    """
    @returns The autoserv command line as a list of executable + parameters.

    @param machines - string - A machine or comma separated list of machines
            for the (-m) flag.
    @param extra_args - list - Additional arguments to pass to autoserv.
    @param job - Job object - If supplied, -u owner and -l name parameters
            will be added.
    @param queue_entry - A HostQueueEntry object - If supplied and no Job
            object was supplied, this will be used to lookup the Job object.
    """
    autoserv_argv = [_autoserv_path, '-p',
                     '-r', drone_manager.WORKING_DIRECTORY]
    if machines:
        autoserv_argv += ['-m', machines]
    if job or queue_entry:
        if not job:
            job = queue_entry.job
        autoserv_argv += ['-u', job.owner, '-l', job.name]
        if job.is_image_update_job():
            autoserv_argv += ['--image', job.update_image_path]
    if verbose:
        autoserv_argv.append('--verbose')
    return autoserv_argv + extra_args


class SchedulerError(Exception):
    """Raised by HostScheduler when an inconsistent state occurs."""


class HostScheduler(metahost_scheduler.HostSchedulingUtility):
    """Handles the logic for choosing when to run jobs and on which hosts.

    This class makes several queries to the database on each tick, building up
    some auxiliary data structures and using them to determine which hosts are
    eligible to run which jobs, taking into account all the various factors that
    affect that.

    In the past this was done with one or two very large, complex database
    queries.  It has proven much simpler and faster to build these auxiliary
    data structures and perform the logic in Python.
    """
    def __init__(self):
        self._metahost_schedulers = metahost_scheduler.get_metahost_schedulers()

        # load site-specific scheduler selected in global_config
        site_schedulers_str = global_config.global_config.get_config_value(
                scheduler_config.CONFIG_SECTION, 'site_metahost_schedulers',
                default='')
        site_schedulers = set(site_schedulers_str.split(','))
        for scheduler in get_site_metahost_schedulers():
            if type(scheduler).__name__ in site_schedulers:
                # always prepend, so site schedulers take precedence
                self._metahost_schedulers = (
                        [scheduler] + self._metahost_schedulers)
        logging.info('Metahost schedulers: %s',
                     ', '.join(type(scheduler).__name__ for scheduler
                               in self._metahost_schedulers))


    def _get_ready_hosts(self):
        # avoid any host with a currently active queue entry against it
        hosts = scheduler_models.Host.fetch(
            joins='LEFT JOIN afe_host_queue_entries AS active_hqe '
                  'ON (afe_hosts.id = active_hqe.host_id AND '
                      'active_hqe.active)',
            where="active_hqe.host_id IS NULL "
                  "AND NOT afe_hosts.locked "
                  "AND (afe_hosts.status IS NULL "
                          "OR afe_hosts.status = 'Ready')")
        return dict((host.id, host) for host in hosts)


    @staticmethod
    def _get_sql_id_list(id_list):
        return ','.join(str(item_id) for item_id in id_list)


    @classmethod
    def _get_many2many_dict(cls, query, id_list, flip=False):
        if not id_list:
            return {}
        query %= cls._get_sql_id_list(id_list)
        rows = _db.execute(query)
        return cls._process_many2many_dict(rows, flip)


    @staticmethod
    def _process_many2many_dict(rows, flip=False):
        result = {}
        for row in rows:
            left_id, right_id = int(row[0]), int(row[1])
            if flip:
                left_id, right_id = right_id, left_id
            result.setdefault(left_id, set()).add(right_id)
        return result


    @classmethod
    def _get_job_acl_groups(cls, job_ids):
        query = """
        SELECT afe_jobs.id, afe_acl_groups_users.aclgroup_id
        FROM afe_jobs
        INNER JOIN afe_users ON afe_users.login = afe_jobs.owner
        INNER JOIN afe_acl_groups_users ON
                afe_acl_groups_users.user_id = afe_users.id
        WHERE afe_jobs.id IN (%s)
        """
        return cls._get_many2many_dict(query, job_ids)


    @classmethod
    def _get_job_ineligible_hosts(cls, job_ids):
        query = """
        SELECT job_id, host_id
        FROM afe_ineligible_host_queues
        WHERE job_id IN (%s)
        """
        return cls._get_many2many_dict(query, job_ids)


    @classmethod
    def _get_job_dependencies(cls, job_ids):
        query = """
        SELECT job_id, label_id
        FROM afe_jobs_dependency_labels
        WHERE job_id IN (%s)
        """
        return cls._get_many2many_dict(query, job_ids)


    @classmethod
    def _get_host_acls(cls, host_ids):
        query = """
        SELECT host_id, aclgroup_id
        FROM afe_acl_groups_hosts
        WHERE host_id IN (%s)
        """
        return cls._get_many2many_dict(query, host_ids)


    @classmethod
    def _get_label_hosts(cls, host_ids):
        if not host_ids:
            return {}, {}
        query = """
        SELECT label_id, host_id
        FROM afe_hosts_labels
        WHERE host_id IN (%s)
        """ % cls._get_sql_id_list(host_ids)
        rows = _db.execute(query)
        labels_to_hosts = cls._process_many2many_dict(rows)
        hosts_to_labels = cls._process_many2many_dict(rows, flip=True)
        return labels_to_hosts, hosts_to_labels


    @classmethod
    def _get_labels(cls):
        return dict((label.id, label) for label
                    in scheduler_models.Label.fetch())


    def recovery_on_startup(self):
        for metahost_scheduler in self._metahost_schedulers:
            metahost_scheduler.recovery_on_startup()


    def refresh(self, pending_queue_entries):
        self._hosts_available = self._get_ready_hosts()

        relevant_jobs = [queue_entry.job_id
                         for queue_entry in pending_queue_entries]
        self._job_acls = self._get_job_acl_groups(relevant_jobs)
        self._ineligible_hosts = self._get_job_ineligible_hosts(relevant_jobs)
        self._job_dependencies = self._get_job_dependencies(relevant_jobs)

        host_ids = self._hosts_available.keys()
        self._host_acls = self._get_host_acls(host_ids)
        self._label_hosts, self._host_labels = self._get_label_hosts(host_ids)

        self._labels = self._get_labels()


    def tick(self):
        for metahost_scheduler in self._metahost_schedulers:
            metahost_scheduler.tick()


    def hosts_in_label(self, label_id):
        return set(self._label_hosts.get(label_id, ()))


    def remove_host_from_label(self, host_id, label_id):
        self._label_hosts[label_id].remove(host_id)


    def pop_host(self, host_id):
        return self._hosts_available.pop(host_id)


    def ineligible_hosts_for_entry(self, queue_entry):
        return set(self._ineligible_hosts.get(queue_entry.job_id, ()))


    def _is_acl_accessible(self, host_id, queue_entry):
        job_acls = self._job_acls.get(queue_entry.job_id, set())
        host_acls = self._host_acls.get(host_id, set())
        return len(host_acls.intersection(job_acls)) > 0


    def _check_job_dependencies(self, job_dependencies, host_labels):
        missing = job_dependencies - host_labels
        return len(missing) == 0


    def _check_only_if_needed_labels(self, job_dependencies, host_labels,
                                     queue_entry):
        if not queue_entry.meta_host:
            # bypass only_if_needed labels when a specific host is selected
            return True

        for label_id in host_labels:
            label = self._labels[label_id]
            if not label.only_if_needed:
                # we don't care about non-only_if_needed labels
                continue
            if queue_entry.meta_host == label_id:
                # if the label was requested in a metahost it's OK
                continue
            if label_id not in job_dependencies:
                return False
        return True


    def _check_atomic_group_labels(self, host_labels, queue_entry):
        """
        Determine if the given HostQueueEntry's atomic group settings are okay
        to schedule on a host with the given labels.

        @param host_labels: A list of label ids that the host has.
        @param queue_entry: The HostQueueEntry being considered for the host.

        @returns True if atomic group settings are okay, False otherwise.
        """
        return (self._get_host_atomic_group_id(host_labels, queue_entry) ==
                queue_entry.atomic_group_id)


    def _get_host_atomic_group_id(self, host_labels, queue_entry=None):
        """
        Return the atomic group label id for a host with the given set of
        labels if any, or None otherwise.  Raises an exception if more than
        one atomic group are found in the set of labels.

        @param host_labels: A list of label ids that the host has.
        @param queue_entry: The HostQueueEntry we're testing.  Only used for
                extra info in a potential logged error message.

        @returns The id of the atomic group found on a label in host_labels
                or None if no atomic group label is found.
        """
        atomic_labels = [self._labels[label_id] for label_id in host_labels
                         if self._labels[label_id].atomic_group_id is not None]
        atomic_ids = set(label.atomic_group_id for label in atomic_labels)
        if not atomic_ids:
            return None
        if len(atomic_ids) > 1:
            logging.error('More than one Atomic Group on HQE "%s" via: %r',
                          queue_entry, atomic_labels)
        return atomic_ids.pop()


    def _get_atomic_group_labels(self, atomic_group_id):
        """
        Lookup the label ids that an atomic_group is associated with.

        @param atomic_group_id - The id of the AtomicGroup to look up.

        @returns A generator yeilding Label ids for this atomic group.
        """
        return (id for id, label in self._labels.iteritems()
                if label.atomic_group_id == atomic_group_id
                and not label.invalid)


    def _get_eligible_host_ids_in_group(self, group_hosts, queue_entry):
        """
        @param group_hosts - A sequence of Host ids to test for usability
                and eligibility against the Job associated with queue_entry.
        @param queue_entry - The HostQueueEntry that these hosts are being
                tested for eligibility against.

        @returns A subset of group_hosts Host ids that are eligible for the
                supplied queue_entry.
        """
        return set(host_id for host_id in group_hosts
                   if self.is_host_usable(host_id)
                   and self.is_host_eligible_for_job(host_id, queue_entry))


    def is_host_eligible_for_job(self, host_id, queue_entry):
        if self._is_host_invalid(host_id):
            # if an invalid host is scheduled for a job, it's a one-time host
            # and it therefore bypasses eligibility checks. note this can only
            # happen for non-metahosts, because invalid hosts have their label
            # relationships cleared.
            return True

        job_dependencies = self._job_dependencies.get(queue_entry.job_id, set())
        host_labels = self._host_labels.get(host_id, set())

        return (self._is_acl_accessible(host_id, queue_entry) and
                self._check_job_dependencies(job_dependencies, host_labels) and
                self._check_only_if_needed_labels(
                    job_dependencies, host_labels, queue_entry) and
                self._check_atomic_group_labels(host_labels, queue_entry))


    def _is_host_invalid(self, host_id):
        host_object = self._hosts_available.get(host_id, None)
        return host_object and host_object.invalid


    def _schedule_non_metahost(self, queue_entry):
        if not self.is_host_eligible_for_job(queue_entry.host_id, queue_entry):
            return None
        return self._hosts_available.pop(queue_entry.host_id, None)


    def is_host_usable(self, host_id):
        if host_id not in self._hosts_available:
            # host was already used during this scheduling cycle
            return False
        if self._hosts_available[host_id].invalid:
            # Invalid hosts cannot be used for metahosts.  They're included in
            # the original query because they can be used by non-metahosts.
            return False
        return True


    def schedule_entry(self, queue_entry):
        if queue_entry.host_id is not None:
            return self._schedule_non_metahost(queue_entry)

        for scheduler in self._metahost_schedulers:
            if scheduler.can_schedule_metahost(queue_entry):
                scheduler.schedule_metahost(queue_entry, self)
                return None

        raise SchedulerError('No metahost scheduler to handle %s' % queue_entry)


    def find_eligible_atomic_group(self, queue_entry):
        """
        Given an atomic group host queue entry, locate an appropriate group
        of hosts for the associated job to run on.

        The caller is responsible for creating new HQEs for the additional
        hosts returned in order to run the actual job on them.

        @returns A list of Host instances in a ready state to satisfy this
                atomic group scheduling.  Hosts will all belong to the same
                atomic group label as specified by the queue_entry.
                An empty list will be returned if no suitable atomic
                group could be found.

        TODO(gps): what is responsible for kicking off any attempted repairs on
        a group of hosts?  not this function, but something needs to.  We do
        not communicate that reason for returning [] outside of here...
        For now, we'll just be unschedulable if enough hosts within one group
        enter Repair Failed state.
        """
        assert queue_entry.atomic_group_id is not None
        job = queue_entry.job
        assert job.synch_count and job.synch_count > 0
        atomic_group = queue_entry.atomic_group
        if job.synch_count > atomic_group.max_number_of_machines:
            # Such a Job and HostQueueEntry should never be possible to
            # create using the frontend.  Regardless, we can't process it.
            # Abort it immediately and log an error on the scheduler.
            queue_entry.set_status(models.HostQueueEntry.Status.ABORTED)
            logging.error(
                'Error: job %d synch_count=%d > requested atomic_group %d '
                'max_number_of_machines=%d.  Aborted host_queue_entry %d.',
                job.id, job.synch_count, atomic_group.id,
                 atomic_group.max_number_of_machines, queue_entry.id)
            return []
        hosts_in_label = self.hosts_in_label(queue_entry.meta_host)
        ineligible_host_ids = self.ineligible_hosts_for_entry(queue_entry)

        # Look in each label associated with atomic_group until we find one with
        # enough hosts to satisfy the job.
        for atomic_label_id in self._get_atomic_group_labels(atomic_group.id):
            group_hosts = set(self.hosts_in_label(atomic_label_id))
            if queue_entry.meta_host is not None:
                # If we have a metahost label, only allow its hosts.
                group_hosts.intersection_update(hosts_in_label)
            group_hosts -= ineligible_host_ids
            eligible_host_ids_in_group = self._get_eligible_host_ids_in_group(
                    group_hosts, queue_entry)

            # Job.synch_count is treated as "minimum synch count" when
            # scheduling for an atomic group of hosts.  The atomic group
            # number of machines is the maximum to pick out of a single
            # atomic group label for scheduling at one time.
            min_hosts = job.synch_count
            max_hosts = atomic_group.max_number_of_machines

            if len(eligible_host_ids_in_group) < min_hosts:
                # Not enough eligible hosts in this atomic group label.
                continue

            eligible_hosts_in_group = [self._hosts_available[id]
                                       for id in eligible_host_ids_in_group]
            # So that they show up in a sane order when viewing the job.
            eligible_hosts_in_group.sort(cmp=scheduler_models.Host.cmp_for_sort)

            # Limit ourselves to scheduling the atomic group size.
            if len(eligible_hosts_in_group) > max_hosts:
                eligible_hosts_in_group = eligible_hosts_in_group[:max_hosts]

            # Remove the selected hosts from our cached internal state
            # of available hosts in order to return the Host objects.
            host_list = []
            for host in eligible_hosts_in_group:
                hosts_in_label.discard(host.id)
                self._hosts_available.pop(host.id)
                host_list.append(host)
            return host_list

        return []


class Dispatcher(object):
    def __init__(self):
        self._agents = []
        self._last_clean_time = time.time()
        self._host_scheduler = HostScheduler()
        user_cleanup_time = scheduler_config.config.clean_interval
        self._periodic_cleanup = monitor_db_cleanup.UserCleanup(
            _db, user_cleanup_time)
        self._24hr_upkeep = monitor_db_cleanup.TwentyFourHourUpkeep(_db)
        self._host_agents = {}
        self._queue_entry_agents = {}
        self._tick_count = 0
        self._last_garbage_stats_time = time.time()
        self._seconds_between_garbage_stats = 60 * (
                global_config.global_config.get_config_value(
                        scheduler_config.CONFIG_SECTION,
                        'gc_stats_interval_mins', type=int, default=6*60))


    def initialize(self, recover_hosts=True):
        self._periodic_cleanup.initialize()
        self._24hr_upkeep.initialize()

        # always recover processes
        self._recover_processes()

        if recover_hosts:
            self._recover_hosts()

        self._host_scheduler.recovery_on_startup()


    def tick(self):
        self._garbage_collection()
        _drone_manager.refresh()
        self._run_cleanup()
        self._find_aborting()
        self._process_recurring_runs()
        self._schedule_delay_tasks()
        self._schedule_running_host_queue_entries()
        self._schedule_special_tasks()
        self._schedule_new_jobs()
        self._handle_agents()
        self._host_scheduler.tick()
        _drone_manager.execute_actions()
        email_manager.manager.send_queued_emails()
        django.db.reset_queries()
        self._tick_count += 1


    def _run_cleanup(self):
        self._periodic_cleanup.run_cleanup_maybe()
        self._24hr_upkeep.run_cleanup_maybe()


    def _garbage_collection(self):
        threshold_time = time.time() - self._seconds_between_garbage_stats
        if threshold_time < self._last_garbage_stats_time:
            # Don't generate these reports very often.
            return

        self._last_garbage_stats_time = time.time()
        # Force a full level 0 collection (because we can, it doesn't hurt
        # at this interval).
        gc.collect()
        logging.info('Logging garbage collector stats on tick %d.',
                     self._tick_count)
        gc_stats._log_garbage_collector_stats()


    def _register_agent_for_ids(self, agent_dict, object_ids, agent):
        for object_id in object_ids:
            agent_dict.setdefault(object_id, set()).add(agent)


    def _unregister_agent_for_ids(self, agent_dict, object_ids, agent):
        for object_id in object_ids:
            assert object_id in agent_dict
            agent_dict[object_id].remove(agent)


    def add_agent_task(self, agent_task):
        agent = Agent(agent_task)
        self._agents.append(agent)
        agent.dispatcher = self
        self._register_agent_for_ids(self._host_agents, agent.host_ids, agent)
        self._register_agent_for_ids(self._queue_entry_agents,
                                     agent.queue_entry_ids, agent)


    def get_agents_for_entry(self, queue_entry):
        """
        Find agents corresponding to the specified queue_entry.
        """
        return list(self._queue_entry_agents.get(queue_entry.id, set()))


    def host_has_agent(self, host):
        """
        Determine if there is currently an Agent present using this host.
        """
        return bool(self._host_agents.get(host.id, None))


    def remove_agent(self, agent):
        self._agents.remove(agent)
        self._unregister_agent_for_ids(self._host_agents, agent.host_ids,
                                       agent)
        self._unregister_agent_for_ids(self._queue_entry_agents,
                                       agent.queue_entry_ids, agent)


    def _host_has_scheduled_special_task(self, host):
        return bool(models.SpecialTask.objects.filter(host__id=host.id,
                                                      is_active=False,
                                                      is_complete=False))


    def _recover_processes(self):
        agent_tasks = self._create_recovery_agent_tasks()
        self._register_pidfiles(agent_tasks)
        _drone_manager.refresh()
        self._recover_tasks(agent_tasks)
        self._recover_pending_entries()
        self._check_for_unrecovered_verifying_entries()
        self._reverify_remaining_hosts()
        # reinitialize drones after killing orphaned processes, since they can
        # leave around files when they die
        _drone_manager.execute_actions()
        _drone_manager.reinitialize_drones()


    def _create_recovery_agent_tasks(self):
        return (self._get_queue_entry_agent_tasks()
                + self._get_special_task_agent_tasks(is_active=True))


    def _get_queue_entry_agent_tasks(self):
        # host queue entry statuses handled directly by AgentTasks (Verifying is
        # handled through SpecialTasks, so is not listed here)
        statuses = (models.HostQueueEntry.Status.STARTING,
                    models.HostQueueEntry.Status.RUNNING,
                    models.HostQueueEntry.Status.GATHERING,
                    models.HostQueueEntry.Status.PARSING,
                    models.HostQueueEntry.Status.ARCHIVING)
        status_list = ','.join("'%s'" % status for status in statuses)
        queue_entries = scheduler_models.HostQueueEntry.fetch(
                where='status IN (%s)' % status_list)

        agent_tasks = []
        used_queue_entries = set()
        for entry in queue_entries:
            if self.get_agents_for_entry(entry):
                # already being handled
                continue
            if entry in used_queue_entries:
                # already picked up by a synchronous job
                continue
            agent_task = self._get_agent_task_for_queue_entry(entry)
            agent_tasks.append(agent_task)
            used_queue_entries.update(agent_task.queue_entries)
        return agent_tasks


    def _get_special_task_agent_tasks(self, is_active=False):
        special_tasks = models.SpecialTask.objects.filter(
                is_active=is_active, is_complete=False)
        return [self._get_agent_task_for_special_task(task)
                for task in special_tasks]


    def _get_agent_task_for_queue_entry(self, queue_entry):
        """
        Construct an AgentTask instance for the given active HostQueueEntry,
        if one can currently run it.
        @param queue_entry: a HostQueueEntry
        @returns an AgentTask to run the queue entry
        """
        task_entries = queue_entry.job.get_group_entries(queue_entry)
        self._check_for_duplicate_host_entries(task_entries)

        if queue_entry.status in (models.HostQueueEntry.Status.STARTING,
                                  models.HostQueueEntry.Status.RUNNING):
            if queue_entry.is_hostless():
                return HostlessQueueTask(queue_entry=queue_entry)
            return QueueTask(queue_entries=task_entries)
        if queue_entry.status == models.HostQueueEntry.Status.GATHERING:
            return GatherLogsTask(queue_entries=task_entries)
        if queue_entry.status == models.HostQueueEntry.Status.PARSING:
            return FinalReparseTask(queue_entries=task_entries)
        if queue_entry.status == models.HostQueueEntry.Status.ARCHIVING:
            return ArchiveResultsTask(queue_entries=task_entries)

        raise SchedulerError('_get_agent_task_for_queue_entry got entry with '
                             'invalid status %s: %s'
                             % (queue_entry.status, queue_entry))


    def _check_for_duplicate_host_entries(self, task_entries):
        non_host_statuses = (models.HostQueueEntry.Status.PARSING,
                             models.HostQueueEntry.Status.ARCHIVING)
        for task_entry in task_entries:
            using_host = (task_entry.host is not None
                          and task_entry.status not in non_host_statuses)
            if using_host:
                self._assert_host_has_no_agent(task_entry)


    def _assert_host_has_no_agent(self, entry):
        """
        @param entry: a HostQueueEntry or a SpecialTask
        """
        if self.host_has_agent(entry.host):
            agent = tuple(self._host_agents.get(entry.host.id))[0]
            raise SchedulerError(
                    'While scheduling %s, host %s already has a host agent %s'
                    % (entry, entry.host, agent.task))


    def _get_agent_task_for_special_task(self, special_task):
        """
        Construct an AgentTask class to run the given SpecialTask and add it
        to this dispatcher.
        @param special_task: a models.SpecialTask instance
        @returns an AgentTask to run this SpecialTask
        """
        self._assert_host_has_no_agent(special_task)

        special_agent_task_classes = (CleanupTask, VerifyTask, RepairTask)
        for agent_task_class in special_agent_task_classes:
            if agent_task_class.TASK_TYPE == special_task.task:
                return agent_task_class(task=special_task)

        raise SchedulerError('No AgentTask class for task', str(special_task))


    def _register_pidfiles(self, agent_tasks):
        for agent_task in agent_tasks:
            agent_task.register_necessary_pidfiles()


    def _recover_tasks(self, agent_tasks):
        orphans = _drone_manager.get_orphaned_autoserv_processes()

        for agent_task in agent_tasks:
            agent_task.recover()
            if agent_task.monitor and agent_task.monitor.has_process():
                orphans.discard(agent_task.monitor.get_process())
            self.add_agent_task(agent_task)

        self._check_for_remaining_orphan_processes(orphans)


    def _get_unassigned_entries(self, status):
        for entry in scheduler_models.HostQueueEntry.fetch(where="status = '%s'"
                                                           % status):
            if entry.status == status and not self.get_agents_for_entry(entry):
                # The status can change during iteration, e.g., if job.run()
                # sets a group of queue entries to Starting
                yield entry


    def _check_for_remaining_orphan_processes(self, orphans):
        if not orphans:
            return
        subject = 'Unrecovered orphan autoserv processes remain'
        message = '\n'.join(str(process) for process in orphans)
        email_manager.manager.enqueue_notify_email(subject, message)

        die_on_orphans = global_config.global_config.get_config_value(
            scheduler_config.CONFIG_SECTION, 'die_on_orphans', type=bool)

        if die_on_orphans:
            raise RuntimeError(subject + '\n' + message)


    def _recover_pending_entries(self):
        for entry in self._get_unassigned_entries(
                models.HostQueueEntry.Status.PENDING):
            logging.info('Recovering Pending entry %s', entry)
            entry.on_pending()


    def _check_for_unrecovered_verifying_entries(self):
        queue_entries = scheduler_models.HostQueueEntry.fetch(
                where='status = "%s"' % models.HostQueueEntry.Status.VERIFYING)
        unrecovered_hqes = []
        for queue_entry in queue_entries:
            special_tasks = models.SpecialTask.objects.filter(
                    task__in=(models.SpecialTask.Task.CLEANUP,
                              models.SpecialTask.Task.VERIFY),
                    queue_entry__id=queue_entry.id,
                    is_complete=False)
            if special_tasks.count() == 0:
                unrecovered_hqes.append(queue_entry)

        if unrecovered_hqes:
            message = '\n'.join(str(hqe) for hqe in unrecovered_hqes)
            raise SchedulerError(
                    '%d unrecovered verifying host queue entries:\n%s' %
                    (len(unrecovered_hqes), message))


    def _get_prioritized_special_tasks(self):
        """
        Returns all queued SpecialTasks prioritized for repair first, then
        cleanup, then verify.
        """
        queued_tasks = models.SpecialTask.objects.filter(is_active=False,
                                                         is_complete=False,
                                                         host__locked=False)
        # exclude hosts with active queue entries unless the SpecialTask is for
        # that queue entry
        queued_tasks = models.SpecialTask.objects.add_join(
                queued_tasks, 'afe_host_queue_entries', 'host_id',
                join_condition='afe_host_queue_entries.active',
                join_from_key='host_id', force_left_join=True)
        queued_tasks = queued_tasks.extra(
                where=['(afe_host_queue_entries.id IS NULL OR '
                       'afe_host_queue_entries.id = '
                               'afe_special_tasks.queue_entry_id)'])

        # reorder tasks by priority
        task_priority_order = [models.SpecialTask.Task.REPAIR,
                               models.SpecialTask.Task.CLEANUP,
                               models.SpecialTask.Task.VERIFY]
        def task_priority_key(task):
            return task_priority_order.index(task.task)
        return sorted(queued_tasks, key=task_priority_key)


    def _schedule_special_tasks(self):
        """
        Execute queued SpecialTasks that are ready to run on idle hosts.
        """
        for task in self._get_prioritized_special_tasks():
            if self.host_has_agent(task.host):
                continue
            self.add_agent_task(self._get_agent_task_for_special_task(task))


    def _reverify_remaining_hosts(self):
        # recover active hosts that have not yet been recovered, although this
        # should never happen
        message = ('Recovering active host %s - this probably indicates a '
                   'scheduler bug')
        self._reverify_hosts_where(
                "status IN ('Repairing', 'Verifying', 'Cleaning')",
                print_message=message)


    def _reverify_hosts_where(self, where,
                              print_message='Reverifying host %s'):
        full_where='locked = 0 AND invalid = 0 AND ' + where
        for host in scheduler_models.Host.fetch(where=full_where):
            if self.host_has_agent(host):
                # host has already been recovered in some way
                continue
            if self._host_has_scheduled_special_task(host):
                # host will have a special task scheduled on the next cycle
                continue
            if print_message:
                logging.info(print_message, host.hostname)
            models.SpecialTask.objects.create(
                    task=models.SpecialTask.Task.CLEANUP,
                    host=models.Host.objects.get(id=host.id))


    def _recover_hosts(self):
        # recover "Repair Failed" hosts
        message = 'Reverifying dead host %s'
        self._reverify_hosts_where("status = 'Repair Failed'",
                                   print_message=message)



    def _get_pending_queue_entries(self):
        # prioritize by job priority, then non-metahost over metahost, then FIFO
        return list(scheduler_models.HostQueueEntry.fetch(
            joins='INNER JOIN afe_jobs ON (job_id=afe_jobs.id)',
            where='NOT complete AND NOT active AND status="Queued"',
            order_by='afe_jobs.priority DESC, meta_host, job_id'))


    def _refresh_pending_queue_entries(self):
        """
        Lookup the pending HostQueueEntries and call our HostScheduler
        refresh() method given that list.  Return the list.

        @returns A list of pending HostQueueEntries sorted in priority order.
        """
        queue_entries = self._get_pending_queue_entries()
        if not queue_entries:
            return []

        self._host_scheduler.refresh(queue_entries)

        return queue_entries


    def _schedule_atomic_group(self, queue_entry):
        """
        Schedule the given queue_entry on an atomic group of hosts.

        Returns immediately if there are insufficient available hosts.

        Creates new HostQueueEntries based off of queue_entry for the
        scheduled hosts and starts them all running.
        """
        # This is a virtual host queue entry representing an entire
        # atomic group, find a group and schedule their hosts.
        group_hosts = self._host_scheduler.find_eligible_atomic_group(
                queue_entry)
        if not group_hosts:
            return

        logging.info('Expanding atomic group entry %s with hosts %s',
                     queue_entry,
                     ', '.join(host.hostname for host in group_hosts))

        for assigned_host in group_hosts[1:]:
            # Create a new HQE for every additional assigned_host.
            new_hqe = scheduler_models.HostQueueEntry.clone(queue_entry)
            new_hqe.save()
            new_hqe.set_host(assigned_host)
            self._run_queue_entry(new_hqe)

        # The first assigned host uses the original HostQueueEntry
        queue_entry.set_host(group_hosts[0])
        self._run_queue_entry(queue_entry)


    def _schedule_hostless_job(self, queue_entry):
        self.add_agent_task(HostlessQueueTask(queue_entry))
        queue_entry.set_status(models.HostQueueEntry.Status.STARTING)


    def _schedule_new_jobs(self):
        queue_entries = self._refresh_pending_queue_entries()
        if not queue_entries:
            return

        for queue_entry in queue_entries:
            is_unassigned_atomic_group = (
                    queue_entry.atomic_group_id is not None
                    and queue_entry.host_id is None)

            if queue_entry.is_hostless():
                self._schedule_hostless_job(queue_entry)
            elif is_unassigned_atomic_group:
                self._schedule_atomic_group(queue_entry)
            else:
                assigned_host = self._host_scheduler.schedule_entry(queue_entry)
                if assigned_host and not self.host_has_agent(assigned_host):
                    assert assigned_host.id == queue_entry.host_id
                    self._run_queue_entry(queue_entry)


    def _schedule_running_host_queue_entries(self):
        for agent_task in self._get_queue_entry_agent_tasks():
            self.add_agent_task(agent_task)


    def _schedule_delay_tasks(self):
        for entry in scheduler_models.HostQueueEntry.fetch(
                where='status = "%s"' % models.HostQueueEntry.Status.WAITING):
            task = entry.job.schedule_delayed_callback_task(entry)
            if task:
                self.add_agent_task(task)


    def _run_queue_entry(self, queue_entry):
        queue_entry.schedule_pre_job_tasks()


    def _find_aborting(self):
        jobs_to_stop = set()
        for entry in scheduler_models.HostQueueEntry.fetch(
                where='aborted and not complete'):
            logging.info('Aborting %s', entry)
            for agent in self.get_agents_for_entry(entry):
                agent.abort()
            entry.abort(self)
            jobs_to_stop.add(entry.job)
        for job in jobs_to_stop:
            job.stop_if_necessary()


    def _can_start_agent(self, agent, num_started_this_cycle,
                         have_reached_limit):
        # always allow zero-process agents to run
        if agent.task.num_processes == 0:
            return True
        # don't allow any nonzero-process agents to run after we've reached a
        # limit (this avoids starvation of many-process agents)
        if have_reached_limit:
            return False
        # total process throttling
        max_runnable_processes = _drone_manager.max_runnable_processes(
                agent.task.owner_username,
                agent.task.get_drone_hostnames_allowed())
        if agent.task.num_processes > max_runnable_processes:
            return False
        # if a single agent exceeds the per-cycle throttling, still allow it to
        # run when it's the first agent in the cycle
        if num_started_this_cycle == 0:
            return True
        # per-cycle throttling
        if (num_started_this_cycle + agent.task.num_processes >
                scheduler_config.config.max_processes_started_per_cycle):
            return False
        return True


    def _handle_agents(self):
        num_started_this_cycle = 0
        have_reached_limit = False
        # iterate over copy, so we can remove agents during iteration
        for agent in list(self._agents):
            if not agent.started:
                if not self._can_start_agent(agent, num_started_this_cycle,
                                             have_reached_limit):
                    have_reached_limit = True
                    continue
                num_started_this_cycle += agent.task.num_processes
            agent.tick()
            if agent.is_done():
                logging.info("agent finished")
                self.remove_agent(agent)
        logging.info('%d running processes',
                     _drone_manager.total_running_processes())


    def _process_recurring_runs(self):
        recurring_runs = models.RecurringRun.objects.filter(
            start_date__lte=datetime.datetime.now())
        for rrun in recurring_runs:
            # Create job from template
            job = rrun.job
            info = rpc_utils.get_job_info(job)
            options = job.get_object_dict()

            host_objects = info['hosts']
            one_time_hosts = info['one_time_hosts']
            metahost_objects = info['meta_hosts']
            dependencies = info['dependencies']
            atomic_group = info['atomic_group']

            for host in one_time_hosts or []:
                this_host = models.Host.create_one_time_host(host.hostname)
                host_objects.append(this_host)

            try:
                rpc_utils.create_new_job(owner=rrun.owner.login,
                                         options=options,
                                         host_objects=host_objects,
                                         metahost_objects=metahost_objects,
                                         atomic_group=atomic_group)

            except Exception, ex:
                logging.exception(ex)
                #TODO send email

            if rrun.loop_count == 1:
                rrun.delete()
            else:
                if rrun.loop_count != 0: # if not infinite loop
                    # calculate new start_date
                    difference = datetime.timedelta(seconds=rrun.loop_period)
                    rrun.start_date = rrun.start_date + difference
                    rrun.loop_count -= 1
                    rrun.save()


class PidfileRunMonitor(object):
    """
    Client must call either run() to start a new process or
    attach_to_existing_process().
    """

    class _PidfileException(Exception):
        """
        Raised when there's some unexpected behavior with the pid file, but only
        used internally (never allowed to escape this class).
        """


    def __init__(self):
        self.lost_process = False
        self._start_time = None
        self.pidfile_id = None
        self._state = drone_manager.PidfileContents()


    def _add_nice_command(self, command, nice_level):
        if not nice_level:
            return command
        return ['nice', '-n', str(nice_level)] + command


    def _set_start_time(self):
        self._start_time = time.time()


    def run(self, command, working_directory, num_processes, nice_level=None,
            log_file=None, pidfile_name=None, paired_with_pidfile=None,
            username=None, drone_hostnames_allowed=None):
        assert command is not None
        if nice_level is not None:
            command = ['nice', '-n', str(nice_level)] + command
        self._set_start_time()
        self.pidfile_id = _drone_manager.execute_command(
            command, working_directory, pidfile_name=pidfile_name,
            num_processes=num_processes, log_file=log_file,
            paired_with_pidfile=paired_with_pidfile, username=username,
            drone_hostnames_allowed=drone_hostnames_allowed)


    def attach_to_existing_process(self, execution_path,
                                   pidfile_name=drone_manager.AUTOSERV_PID_FILE,
                                   num_processes=None):
        self._set_start_time()
        self.pidfile_id = _drone_manager.get_pidfile_id_from(
            execution_path, pidfile_name=pidfile_name)
        if num_processes is not None:
            _drone_manager.declare_process_count(self.pidfile_id, num_processes)


    def kill(self):
        if self.has_process():
            _drone_manager.kill_process(self.get_process())


    def has_process(self):
        self._get_pidfile_info()
        return self._state.process is not None


    def get_process(self):
        self._get_pidfile_info()
        assert self._state.process is not None
        return self._state.process


    def _read_pidfile(self, use_second_read=False):
        assert self.pidfile_id is not None, (
            'You must call run() or attach_to_existing_process()')
        contents = _drone_manager.get_pidfile_contents(
            self.pidfile_id, use_second_read=use_second_read)
        if contents.is_invalid():
            self._state = drone_manager.PidfileContents()
            raise self._PidfileException(contents)
        self._state = contents


    def _handle_pidfile_error(self, error, message=''):
        message = error + '\nProcess: %s\nPidfile: %s\n%s' % (
            self._state.process, self.pidfile_id, message)
        email_manager.manager.enqueue_notify_email(error, message)
        self.on_lost_process(self._state.process)


    def _get_pidfile_info_helper(self):
        if self.lost_process:
            return

        self._read_pidfile()

        if self._state.process is None:
            self._handle_no_process()
            return

        if self._state.exit_status is None:
            # double check whether or not autoserv is running
            if _drone_manager.is_process_running(self._state.process):
                return

            # pid but no running process - maybe process *just* exited
            self._read_pidfile(use_second_read=True)
            if self._state.exit_status is None:
                # autoserv exited without writing an exit code
                # to the pidfile
                self._handle_pidfile_error(
                    'autoserv died without writing exit code')


    def _get_pidfile_info(self):
        """\
        After completion, self._state will contain:
         pid=None, exit_status=None if autoserv has not yet run
         pid!=None, exit_status=None if autoserv is running
         pid!=None, exit_status!=None if autoserv has completed
        """
        try:
            self._get_pidfile_info_helper()
        except self._PidfileException, exc:
            self._handle_pidfile_error('Pidfile error', traceback.format_exc())


    def _handle_no_process(self):
        """\
        Called when no pidfile is found or no pid is in the pidfile.
        """
        message = 'No pid found at %s' % self.pidfile_id
        if time.time() - self._start_time > _get_pidfile_timeout_secs():
            email_manager.manager.enqueue_notify_email(
                'Process has failed to write pidfile', message)
            self.on_lost_process()


    def on_lost_process(self, process=None):
        """\
        Called when autoserv has exited without writing an exit status,
        or we've timed out waiting for autoserv to write a pid to the
        pidfile.  In either case, we just return failure and the caller
        should signal some kind of warning.

        process is unimportant here, as it shouldn't be used by anyone.
        """
        self.lost_process = True
        self._state.process = process
        self._state.exit_status = 1
        self._state.num_tests_failed = 0


    def exit_code(self):
        self._get_pidfile_info()
        return self._state.exit_status


    def num_tests_failed(self):
        """@returns The number of tests that failed or -1 if unknown."""
        self._get_pidfile_info()
        if self._state.num_tests_failed is None:
            return -1
        return self._state.num_tests_failed


    def try_copy_results_on_drone(self, **kwargs):
        if self.has_process():
            # copy results logs into the normal place for job results
            _drone_manager.copy_results_on_drone(self.get_process(), **kwargs)


    def try_copy_to_results_repository(self, source, **kwargs):
        if self.has_process():
            _drone_manager.copy_to_results_repository(self.get_process(),
                                                      source, **kwargs)


class Agent(object):
    """
    An agent for use by the Dispatcher class to perform a task.

    The following methods are required on all task objects:
        poll() - Called periodically to let the task check its status and
                update its internal state.  If the task succeeded.
        is_done() - Returns True if the task is finished.
        abort() - Called when an abort has been requested.  The task must
                set its aborted attribute to True if it actually aborted.

    The following attributes are required on all task objects:
        aborted - bool, True if this task was aborted.
        success - bool, True if this task succeeded.
        queue_entry_ids - A sequence of HostQueueEntry ids this task handles.
        host_ids - A sequence of Host ids this task represents.
    """


    def __init__(self, task):
        """
        @param task: A task as described in the class docstring.
        """
        self.task = task

        # This is filled in by Dispatcher.add_agent()
        self.dispatcher = None

        self.queue_entry_ids = task.queue_entry_ids
        self.host_ids = task.host_ids

        self.started = False
        self.finished = False


    def tick(self):
        self.started = True
        if not self.finished:
            self.task.poll()
            if self.task.is_done():
                self.finished = True


    def is_done(self):
        return self.finished


    def abort(self):
        if self.task:
            self.task.abort()
            if self.task.aborted:
                # tasks can choose to ignore aborts
                self.finished = True


class AgentTask(object):
    class _NullMonitor(object):
        pidfile_id = None

        def has_process(self):
            return True


    def __init__(self, log_file_name=None):
        """
        @param log_file_name: (optional) name of file to log command output to
        """
        self.done = False
        self.started = False
        self.success = None
        self.aborted = False
        self.monitor = None
        self.queue_entry_ids = []
        self.host_ids = []
        self._log_file_name = log_file_name


    def _set_ids(self, host=None, queue_entries=None):
        if queue_entries and queue_entries != [None]:
            self.host_ids = [entry.host.id for entry in queue_entries]
            self.queue_entry_ids = [entry.id for entry in queue_entries]
        else:
            assert host
            self.host_ids = [host.id]


    def poll(self):
        if not self.started:
            self.start()
        if not self.done:
            self.tick()


    def tick(self):
        assert self.monitor
        exit_code = self.monitor.exit_code()
        if exit_code is None:
            return

        success = (exit_code == 0)
        self.finished(success)


    def is_done(self):
        return self.done


    def finished(self, success):
        if self.done:
            assert self.started
            return
        self.started = True
        self.done = True
        self.success = success
        self.epilog()


    def prolog(self):
        """
        To be overridden.
        """
        assert not self.monitor
        self.register_necessary_pidfiles()


    def _log_file(self):
        if not self._log_file_name:
            return None
        return os.path.join(self._working_directory(), self._log_file_name)


    def cleanup(self):
        log_file = self._log_file()
        if self.monitor and log_file:
            self.monitor.try_copy_to_results_repository(log_file)


    def epilog(self):
        """
        To be overridden.
        """
        self.cleanup()
        logging.info("%s finished with success=%s", type(self).__name__,
                     self.success)



    def start(self):
        if not self.started:
            self.prolog()
            self.run()

        self.started = True


    def abort(self):
        if self.monitor:
            self.monitor.kill()
        self.done = True
        self.aborted = True
        self.cleanup()


    def _get_consistent_execution_path(self, execution_entries):
        first_execution_path = execution_entries[0].execution_path()
        for execution_entry in execution_entries[1:]:
            assert execution_entry.execution_path() == first_execution_path, (
                '%s (%s) != %s (%s)' % (execution_entry.execution_path(),
                                        execution_entry,
                                        first_execution_path,
                                        execution_entries[0]))
        return first_execution_path


    def _copy_results(self, execution_entries, use_monitor=None):
        """
        @param execution_entries: list of objects with execution_path() method
        """
        if use_monitor is not None and not use_monitor.has_process():
            return

        assert len(execution_entries) > 0
        if use_monitor is None:
            assert self.monitor
            use_monitor = self.monitor
        assert use_monitor.has_process()
        execution_path = self._get_consistent_execution_path(execution_entries)
        results_path = execution_path + '/'
        use_monitor.try_copy_to_results_repository(results_path)


    def _parse_results(self, queue_entries):
        for queue_entry in queue_entries:
            queue_entry.set_status(models.HostQueueEntry.Status.PARSING)


    def _archive_results(self, queue_entries):
        for queue_entry in queue_entries:
            queue_entry.set_status(models.HostQueueEntry.Status.ARCHIVING)


    def _command_line(self):
        """
        Return the command line to run.  Must be overridden.
        """
        raise NotImplementedError


    @property
    def num_processes(self):
        """
        Return the number of processes forked by this AgentTask's process.  It
        may only be approximate.  To be overridden if necessary.
        """
        return 1


    def _paired_with_monitor(self):
        """
        If this AgentTask's process must run on the same machine as some
        previous process, this method should be overridden to return a
        PidfileRunMonitor for that process.
        """
        return self._NullMonitor()


    @property
    def owner_username(self):
        """
        Return login of user responsible for this task.  May be None.  Must be
        overridden.
        """
        raise NotImplementedError


    def _working_directory(self):
        """
        Return the directory where this AgentTask's process executes.  Must be
        overridden.
        """
        raise NotImplementedError


    def _pidfile_name(self):
        """
        Return the name of the pidfile this AgentTask's process uses.  To be
        overridden if necessary.
        """
        return drone_manager.AUTOSERV_PID_FILE


    def _check_paired_results_exist(self):
        if not self._paired_with_monitor().has_process():
            email_manager.manager.enqueue_notify_email(
                    'No paired results in task',
                    'No paired results in task %s at %s'
                    % (self, self._paired_with_monitor().pidfile_id))
            self.finished(False)
            return False
        return True


    def _create_monitor(self):
        assert not self.monitor
        self.monitor = PidfileRunMonitor()


    def run(self):
        if not self._check_paired_results_exist():
            return

        self._create_monitor()
        self.monitor.run(
                self._command_line(), self._working_directory(),
                num_processes=self.num_processes,
                nice_level=AUTOSERV_NICE_LEVEL, log_file=self._log_file(),
                pidfile_name=self._pidfile_name(),
                paired_with_pidfile=self._paired_with_monitor().pidfile_id,
                username=self.owner_username,
                drone_hostnames_allowed=self.get_drone_hostnames_allowed())


    def get_drone_hostnames_allowed(self):
        if not models.DroneSet.drone_sets_enabled():
            return None

        hqes = models.HostQueueEntry.objects.filter(id__in=self.queue_entry_ids)
        if not hqes:
            # Only special tasks could be missing host queue entries
            assert isinstance(self, SpecialAgentTask)
            return self._user_or_global_default_drone_set(
                    self.task, self.task.requested_by)

        job_ids = hqes.values_list('job', flat=True).distinct()
        assert job_ids.count() == 1, ("AgentTask's queue entries "
                                      "span multiple jobs")

        job = models.Job.objects.get(id=job_ids[0])
        drone_set = job.drone_set
        if not drone_set:
            return self._user_or_global_default_drone_set(job, job.user())

        return drone_set.get_drone_hostnames()


    def _user_or_global_default_drone_set(self, obj_with_owner, user):
        """
        Returns the user's default drone set, if present.

        Otherwise, returns the global default drone set.
        """
        default_hostnames = models.DroneSet.get_default().get_drone_hostnames()
        if not user:
            logging.warn('%s had no owner; using default drone set',
                         obj_with_owner)
            return default_hostnames
        if not user.drone_set:
            logging.warn('User %s has no default drone set, using global '
                         'default', user.login)
            return default_hostnames
        return user.drone_set.get_drone_hostnames()


    def register_necessary_pidfiles(self):
        pidfile_id = _drone_manager.get_pidfile_id_from(
                self._working_directory(), self._pidfile_name())
        _drone_manager.register_pidfile(pidfile_id)

        paired_pidfile_id = self._paired_with_monitor().pidfile_id
        if paired_pidfile_id:
            _drone_manager.register_pidfile(paired_pidfile_id)


    def recover(self):
        if not self._check_paired_results_exist():
            return

        self._create_monitor()
        self.monitor.attach_to_existing_process(
                self._working_directory(), pidfile_name=self._pidfile_name(),
                num_processes=self.num_processes)
        if not self.monitor.has_process():
            # no process to recover; wait to be started normally
            self.monitor = None
            return

        self.started = True
        logging.info('Recovering process %s for %s at %s'
                     % (self.monitor.get_process(), type(self).__name__,
                        self._working_directory()))


    def _check_queue_entry_statuses(self, queue_entries, allowed_hqe_statuses,
                                    allowed_host_statuses=None):
        class_name = self.__class__.__name__
        for entry in queue_entries:
            if entry.status not in allowed_hqe_statuses:
                raise SchedulerError('%s attempting to start '
                                     'entry with invalid status %s: %s'
                                     % (class_name, entry.status, entry))
            invalid_host_status = (
                    allowed_host_statuses is not None
                    and entry.host.status not in allowed_host_statuses)
            if invalid_host_status:
                raise SchedulerError('%s attempting to start on queue '
                                     'entry with invalid host status %s: %s'
                                     % (class_name, entry.host.status, entry))


class TaskWithJobKeyvals(object):
    """AgentTask mixin providing functionality to help with job keyval files."""
    _KEYVAL_FILE = 'keyval'
    def _format_keyval(self, key, value):
        return '%s=%s' % (key, value)


    def _keyval_path(self):
        """Subclasses must override this"""
        raise NotImplementedError


    def _write_keyval_after_job(self, field, value):
        assert self.monitor
        if not self.monitor.has_process():
            return
        _drone_manager.write_lines_to_file(
            self._keyval_path(), [self._format_keyval(field, value)],
            paired_with_process=self.monitor.get_process())


    def _job_queued_keyval(self, job):
        return 'job_queued', int(time.mktime(job.created_on.timetuple()))


    def _write_job_finished(self):
        self._write_keyval_after_job("job_finished", int(time.time()))


    def _write_keyvals_before_job_helper(self, keyval_dict, keyval_path):
        keyval_contents = '\n'.join(self._format_keyval(key, value)
                                    for key, value in keyval_dict.iteritems())
        # always end with a newline to allow additional keyvals to be written
        keyval_contents += '\n'
        _drone_manager.attach_file_to_execution(self._working_directory(),
                                                keyval_contents,
                                                file_path=keyval_path)


    def _write_keyvals_before_job(self, keyval_dict):
        self._write_keyvals_before_job_helper(keyval_dict, self._keyval_path())


    def _write_host_keyvals(self, host):
        keyval_path = os.path.join(self._working_directory(), 'host_keyvals',
                                   host.hostname)
        platform, all_labels = host.platform_and_labels()
        all_labels = [ urllib.quote(label) for label in all_labels ]
        keyval_dict = dict(platform=platform, labels=','.join(all_labels))
        self._write_keyvals_before_job_helper(keyval_dict, keyval_path)


class SpecialAgentTask(AgentTask, TaskWithJobKeyvals):
    """
    Subclass for AgentTasks that correspond to a SpecialTask entry in the DB.
    """

    TASK_TYPE = None
    host = None
    queue_entry = None

    def __init__(self, task, extra_command_args):
        super(SpecialAgentTask, self).__init__()

        assert self.TASK_TYPE is not None, 'self.TASK_TYPE must be overridden'

        self.host = scheduler_models.Host(id=task.host.id)
        self.queue_entry = None
        if task.queue_entry:
            self.queue_entry = scheduler_models.HostQueueEntry(
                    id=task.queue_entry.id)

        self.task = task
        self._extra_command_args = extra_command_args


    def _keyval_path(self):
        return os.path.join(self._working_directory(), self._KEYVAL_FILE)


    def _command_line(self):
        return _autoserv_command_line(self.host.hostname,
                                      self._extra_command_args,
                                      queue_entry=self.queue_entry)


    def _working_directory(self):
        return self.task.execution_path()


    @property
    def owner_username(self):
        if self.task.requested_by:
            return self.task.requested_by.login
        return None


    def prolog(self):
        super(SpecialAgentTask, self).prolog()
        self.task.activate()
        self._write_host_keyvals(self.host)


    def _fail_queue_entry(self):
        assert self.queue_entry

        if self.queue_entry.meta_host:
            return # don't fail metahost entries, they'll be reassigned

        self.queue_entry.update_from_database()
        if self.queue_entry.status != models.HostQueueEntry.Status.QUEUED:
            return # entry has been aborted

        self.queue_entry.set_execution_subdir()
        queued_key, queued_time = self._job_queued_keyval(
            self.queue_entry.job)
        self._write_keyval_after_job(queued_key, queued_time)
        self._write_job_finished()

        # copy results logs into the normal place for job results
        self.monitor.try_copy_results_on_drone(
                source_path=self._working_directory() + '/',
                destination_path=self.queue_entry.execution_path() + '/')

        pidfile_id = _drone_manager.get_pidfile_id_from(
                self.queue_entry.execution_path(),
                pidfile_name=drone_manager.AUTOSERV_PID_FILE)
        _drone_manager.register_pidfile(pidfile_id)

        if self.queue_entry.job.parse_failed_repair:
            self._parse_results([self.queue_entry])
        else:
            self._archive_results([self.queue_entry])


    def cleanup(self):
        super(SpecialAgentTask, self).cleanup()

        # We will consider an aborted task to be "Failed"
        self.task.finish(bool(self.success))

        if self.monitor:
            if self.monitor.has_process():
                self._copy_results([self.task])
            if self.monitor.pidfile_id is not None:
                _drone_manager.unregister_pidfile(self.monitor.pidfile_id)


class RepairTask(SpecialAgentTask):
    TASK_TYPE = models.SpecialTask.Task.REPAIR


    def __init__(self, task):
        """\
        queue_entry: queue entry to mark failed if this repair fails.
        """
        protection = host_protections.Protection.get_string(
                task.host.protection)
        # normalize the protection name
        protection = host_protections.Protection.get_attr_name(protection)

        super(RepairTask, self).__init__(
                task, ['-R', '--host-protection', protection])

        # *don't* include the queue entry in IDs -- if the queue entry is
        # aborted, we want to leave the repair task running
        self._set_ids(host=self.host)


    def prolog(self):
        super(RepairTask, self).prolog()
        logging.info("repair_task starting")
        self.host.set_status(models.Host.Status.REPAIRING)


    def epilog(self):
        super(RepairTask, self).epilog()

        if self.success:
            self.host.set_status(models.Host.Status.READY)
        else:
            self.host.set_status(models.Host.Status.REPAIR_FAILED)
            if self.queue_entry:
                self._fail_queue_entry()


class PreJobTask(SpecialAgentTask):
    def _copy_to_results_repository(self):
        if not self.queue_entry or self.queue_entry.meta_host:
            return

        self.queue_entry.set_execution_subdir()
        log_name = os.path.basename(self.task.execution_path())
        source = os.path.join(self.task.execution_path(), 'debug',
                              'autoserv.DEBUG')
        destination = os.path.join(
                self.queue_entry.execution_path(), log_name)

        self.monitor.try_copy_to_results_repository(
                source, destination_path=destination)


    def epilog(self):
        super(PreJobTask, self).epilog()

        if self.success:
            return

        self._copy_to_results_repository()

        if self.host.protection == host_protections.Protection.DO_NOT_VERIFY:
            # effectively ignore failure for these hosts
            self.success = True
            return

        if self.queue_entry:
            self.queue_entry.requeue()

            if models.SpecialTask.objects.filter(
                    task=models.SpecialTask.Task.REPAIR,
                    queue_entry__id=self.queue_entry.id):
                self.host.set_status(models.Host.Status.REPAIR_FAILED)
                self._fail_queue_entry()
                return

            queue_entry = models.HostQueueEntry.objects.get(
                    id=self.queue_entry.id)
        else:
            queue_entry = None

        models.SpecialTask.objects.create(
                host=models.Host.objects.get(id=self.host.id),
                task=models.SpecialTask.Task.REPAIR,
                queue_entry=queue_entry,
                requested_by=self.task.requested_by)


class VerifyTask(PreJobTask):
    TASK_TYPE = models.SpecialTask.Task.VERIFY


    def __init__(self, task):
        super(VerifyTask, self).__init__(task, ['-v'])
        self._set_ids(host=self.host, queue_entries=[self.queue_entry])


    def prolog(self):
        super(VerifyTask, self).prolog()

        logging.info("starting verify on %s", self.host.hostname)
        if self.queue_entry:
            self.queue_entry.set_status(models.HostQueueEntry.Status.VERIFYING)
        self.host.set_status(models.Host.Status.VERIFYING)

        # Delete any queued manual reverifies for this host.  One verify will do
        # and there's no need to keep records of other requests.
        queued_verifies = models.SpecialTask.objects.filter(
            host__id=self.host.id,
            task=models.SpecialTask.Task.VERIFY,
            is_active=False, is_complete=False, queue_entry=None)
        queued_verifies = queued_verifies.exclude(id=self.task.id)
        queued_verifies.delete()


    def epilog(self):
        super(VerifyTask, self).epilog()
        if self.success:
            if self.queue_entry:
                self.queue_entry.on_pending()
            else:
                self.host.set_status(models.Host.Status.READY)


class CleanupTask(PreJobTask):
    # note this can also run post-job, but when it does, it's running standalone
    # against the host (not related to the job), so it's not considered a
    # PostJobTask

    TASK_TYPE = models.SpecialTask.Task.CLEANUP


    def __init__(self, task, recover_run_monitor=None):
        super(CleanupTask, self).__init__(task, ['--cleanup'])
        self._set_ids(host=self.host, queue_entries=[self.queue_entry])


    def prolog(self):
        super(CleanupTask, self).prolog()
        logging.info("starting cleanup task for host: %s", self.host.hostname)
        self.host.set_status(models.Host.Status.CLEANING)
        if self.queue_entry:
            self.queue_entry.set_status(models.HostQueueEntry.Status.VERIFYING)


    def _finish_epilog(self):
        if not self.queue_entry or not self.success:
            return

        do_not_verify_protection = host_protections.Protection.DO_NOT_VERIFY
        should_run_verify = (
                self.queue_entry.job.run_verify
                and self.host.protection != do_not_verify_protection)
        if should_run_verify:
            entry = models.HostQueueEntry.objects.get(id=self.queue_entry.id)
            models.SpecialTask.objects.create(
                    host=models.Host.objects.get(id=self.host.id),
                    queue_entry=entry,
                    task=models.SpecialTask.Task.VERIFY)
        else:
            self.queue_entry.on_pending()


    def epilog(self):
        super(CleanupTask, self).epilog()

        if self.success:
            self.host.update_field('dirty', 0)
            self.host.set_status(models.Host.Status.READY)

        self._finish_epilog()


class AbstractQueueTask(AgentTask, TaskWithJobKeyvals):
    """
    Common functionality for QueueTask and HostlessQueueTask
    """
    def __init__(self, queue_entries):
        super(AbstractQueueTask, self).__init__()
        self.job = queue_entries[0].job
        self.queue_entries = queue_entries


    def _keyval_path(self):
        return os.path.join(self._working_directory(), self._KEYVAL_FILE)


    def _write_control_file(self, execution_path):
        control_path = _drone_manager.attach_file_to_execution(
                execution_path, self.job.control_file)
        return control_path


    def _command_line(self):
        execution_path = self.queue_entries[0].execution_path()
        control_path = self._write_control_file(execution_path)
        hostnames = ','.join(entry.host.hostname
                             for entry in self.queue_entries
                             if not entry.is_hostless())

        execution_tag = self.queue_entries[0].execution_tag()
        params = _autoserv_command_line(
            hostnames,
            ['-P', execution_tag, '-n',
             _drone_manager.absolute_path(control_path)],
            job=self.job, verbose=False)

        if not self.job.is_server_job():
            params.append('-c')

        return params


    @property
    def num_processes(self):
        return len(self.queue_entries)


    @property
    def owner_username(self):
        return self.job.owner


    def _working_directory(self):
        return self._get_consistent_execution_path(self.queue_entries)


    def prolog(self):
        queued_key, queued_time = self._job_queued_keyval(self.job)
        keyval_dict = self.job.keyval_dict()
        keyval_dict[queued_key] = queued_time
        group_name = self.queue_entries[0].get_group_name()
        if group_name:
            keyval_dict['host_group_name'] = group_name
        self._write_keyvals_before_job(keyval_dict)
        for queue_entry in self.queue_entries:
            queue_entry.set_status(models.HostQueueEntry.Status.RUNNING)
            queue_entry.set_started_on_now()


    def _write_lost_process_error_file(self):
        error_file_path = os.path.join(self._working_directory(), 'job_failure')
        _drone_manager.write_lines_to_file(error_file_path,
                                           [_LOST_PROCESS_ERROR])


    def _finish_task(self):
        if not self.monitor:
            return

        self._write_job_finished()

        if self.monitor.lost_process:
            self._write_lost_process_error_file()


    def _write_status_comment(self, comment):
        _drone_manager.write_lines_to_file(
            os.path.join(self._working_directory(), 'status.log'),
            ['INFO\t----\t----\t' + comment],
            paired_with_process=self.monitor.get_process())


    def _log_abort(self):
        if not self.monitor or not self.monitor.has_process():
            return

        # build up sets of all the aborted_by and aborted_on values
        aborted_by, aborted_on = set(), set()
        for queue_entry in self.queue_entries:
            if queue_entry.aborted_by:
                aborted_by.add(queue_entry.aborted_by)
                t = int(time.mktime(queue_entry.aborted_on.timetuple()))
                aborted_on.add(t)

        # extract some actual, unique aborted by value and write it out
        # TODO(showard): this conditional is now obsolete, we just need to leave
        # it in temporarily for backwards compatibility over upgrades.  delete
        # soon.
        assert len(aborted_by) <= 1
        if len(aborted_by) == 1:
            aborted_by_value = aborted_by.pop()
            aborted_on_value = max(aborted_on)
        else:
            aborted_by_value = 'autotest_system'
            aborted_on_value = int(time.time())

        self._write_keyval_after_job("aborted_by", aborted_by_value)
        self._write_keyval_after_job("aborted_on", aborted_on_value)

        aborted_on_string = str(datetime.datetime.fromtimestamp(
            aborted_on_value))
        self._write_status_comment('Job aborted by %s on %s' %
                                   (aborted_by_value, aborted_on_string))


    def abort(self):
        super(AbstractQueueTask, self).abort()
        self._log_abort()
        self._finish_task()


    def epilog(self):
        super(AbstractQueueTask, self).epilog()
        self._finish_task()


class QueueTask(AbstractQueueTask):
    def __init__(self, queue_entries):
        super(QueueTask, self).__init__(queue_entries)
        self._set_ids(queue_entries=queue_entries)


    def prolog(self):
        self._check_queue_entry_statuses(
                self.queue_entries,
                allowed_hqe_statuses=(models.HostQueueEntry.Status.STARTING,
                                      models.HostQueueEntry.Status.RUNNING),
                allowed_host_statuses=(models.Host.Status.PENDING,
                                       models.Host.Status.RUNNING))

        super(QueueTask, self).prolog()

        for queue_entry in self.queue_entries:
            self._write_host_keyvals(queue_entry.host)
            queue_entry.host.set_status(models.Host.Status.RUNNING)
            queue_entry.host.update_field('dirty', 1)
        if self.job.synch_count == 1 and len(self.queue_entries) == 1:
            # TODO(gps): Remove this if nothing needs it anymore.
            # A potential user is: tko/parser
            self.job.write_to_machines_file(self.queue_entries[0])


    def _finish_task(self):
        super(QueueTask, self)._finish_task()

        for queue_entry in self.queue_entries:
            queue_entry.set_status(models.HostQueueEntry.Status.GATHERING)
            queue_entry.host.set_status(models.Host.Status.RUNNING)


class HostlessQueueTask(AbstractQueueTask):
    def __init__(self, queue_entry):
        super(HostlessQueueTask, self).__init__([queue_entry])
        self.queue_entry_ids = [queue_entry.id]


    def prolog(self):
        self.queue_entries[0].update_field('execution_subdir', 'hostless')
        super(HostlessQueueTask, self).prolog()


    def _finish_task(self):
        super(HostlessQueueTask, self)._finish_task()
        self.queue_entries[0].set_status(models.HostQueueEntry.Status.PARSING)


class PostJobTask(AgentTask):
    def __init__(self, queue_entries, log_file_name):
        super(PostJobTask, self).__init__(log_file_name=log_file_name)

        self.queue_entries = queue_entries

        self._autoserv_monitor = PidfileRunMonitor()
        self._autoserv_monitor.attach_to_existing_process(
                self._working_directory())


    def _command_line(self):
        if _testing_mode:
            return 'true'
        return self._generate_command(
                _drone_manager.absolute_path(self._working_directory()))


    def _generate_command(self, results_dir):
        raise NotImplementedError('Subclasses must override this')


    @property
    def owner_username(self):
        return self.queue_entries[0].job.owner


    def _working_directory(self):
        return self._get_consistent_execution_path(self.queue_entries)


    def _paired_with_monitor(self):
        return self._autoserv_monitor


    def _job_was_aborted(self):
        was_aborted = None
        for queue_entry in self.queue_entries:
            queue_entry.update_from_database()
            if was_aborted is None: # first queue entry
                was_aborted = bool(queue_entry.aborted)
            elif was_aborted != bool(queue_entry.aborted): # subsequent entries
                entries = ['%s (aborted: %s)' % (entry, entry.aborted)
                           for entry in self.queue_entries]
                email_manager.manager.enqueue_notify_email(
                        'Inconsistent abort state',
                        'Queue entries have inconsistent abort state:\n' +
                        '\n'.join(entries))
                # don't crash here, just assume true
                return True
        return was_aborted


    def _final_status(self):
        if self._job_was_aborted():
            return models.HostQueueEntry.Status.ABORTED

        # we'll use a PidfileRunMonitor to read the autoserv exit status
        if self._autoserv_monitor.exit_code() == 0:
            return models.HostQueueEntry.Status.COMPLETED
        return models.HostQueueEntry.Status.FAILED


    def _set_all_statuses(self, status):
        for queue_entry in self.queue_entries:
            queue_entry.set_status(status)


    def abort(self):
        # override AgentTask.abort() to avoid killing the process and ending
        # the task.  post-job tasks continue when the job is aborted.
        pass


    def _pidfile_label(self):
        # '.autoserv_execute' -> 'autoserv'
        return self._pidfile_name()[1:-len('_execute')]


class GatherLogsTask(PostJobTask):
    """
    Task responsible for
    * gathering uncollected logs (if Autoserv crashed hard or was killed)
    * copying logs to the results repository
    * spawning CleanupTasks for hosts, if necessary
    * spawning a FinalReparseTask for the job
    """
    def __init__(self, queue_entries, recover_run_monitor=None):
        self._job = queue_entries[0].job
        super(GatherLogsTask, self).__init__(
            queue_entries, log_file_name='.collect_crashinfo.log')
        self._set_ids(queue_entries=queue_entries)


    def _generate_command(self, results_dir):
        host_list = ','.join(queue_entry.host.hostname
                             for queue_entry in self.queue_entries)
        return [_autoserv_path , '-p',
                '--pidfile-label=%s' % self._pidfile_label(),
                '--use-existing-results', '--collect-crashinfo',
                '-m', host_list, '-r', results_dir]


    @property
    def num_processes(self):
        return len(self.queue_entries)


    def _pidfile_name(self):
        return drone_manager.CRASHINFO_PID_FILE


    def prolog(self):
        self._check_queue_entry_statuses(
                self.queue_entries,
                allowed_hqe_statuses=(models.HostQueueEntry.Status.GATHERING,),
                allowed_host_statuses=(models.Host.Status.RUNNING,))

        super(GatherLogsTask, self).prolog()


    def epilog(self):
        super(GatherLogsTask, self).epilog()
        self._parse_results(self.queue_entries)
        self._reboot_hosts()


    def _reboot_hosts(self):
        if self._autoserv_monitor.has_process():
            final_success = (self._final_status() ==
                             models.HostQueueEntry.Status.COMPLETED)
            num_tests_failed = self._autoserv_monitor.num_tests_failed()
        else:
            final_success = False
            num_tests_failed = 0

        reboot_after = self._job.reboot_after
        do_reboot = (
                # always reboot after aborted jobs
                self._final_status() == models.HostQueueEntry.Status.ABORTED
                or reboot_after == model_attributes.RebootAfter.ALWAYS
                or (reboot_after == model_attributes.RebootAfter.IF_ALL_TESTS_PASSED
                    and final_success and num_tests_failed == 0))

        for queue_entry in self.queue_entries:
            if do_reboot:
                # don't pass the queue entry to the CleanupTask. if the cleanup
                # fails, the job doesn't care -- it's over.
                models.SpecialTask.objects.create(
                        host=models.Host.objects.get(id=queue_entry.host.id),
                        task=models.SpecialTask.Task.CLEANUP,
                        requested_by=self._job.owner_model())
            else:
                queue_entry.host.set_status(models.Host.Status.READY)


    def run(self):
        autoserv_exit_code = self._autoserv_monitor.exit_code()
        # only run if Autoserv exited due to some signal. if we have no exit
        # code, assume something bad (and signal-like) happened.
        if autoserv_exit_code is None or os.WIFSIGNALED(autoserv_exit_code):
            super(GatherLogsTask, self).run()
        else:
            self.finished(True)


class SelfThrottledPostJobTask(PostJobTask):
    """
    Special AgentTask subclass that maintains its own global process limit.
    """
    _num_running_processes = 0


    @classmethod
    def _increment_running_processes(cls):
        cls._num_running_processes += 1


    @classmethod
    def _decrement_running_processes(cls):
        cls._num_running_processes -= 1


    @classmethod
    def _max_processes(cls):
        raise NotImplementedError


    @classmethod
    def _can_run_new_process(cls):
        return cls._num_running_processes < cls._max_processes()


    def _process_started(self):
        return bool(self.monitor)


    def tick(self):
        # override tick to keep trying to start until the process count goes
        # down and we can, at which point we revert to default behavior
        if self._process_started():
            super(SelfThrottledPostJobTask, self).tick()
        else:
            self._try_starting_process()


    def run(self):
        # override run() to not actually run unless we can
        self._try_starting_process()


    def _try_starting_process(self):
        if not self._can_run_new_process():
            return

        # actually run the command
        super(SelfThrottledPostJobTask, self).run()
        if self._process_started():
            self._increment_running_processes()


    def finished(self, success):
        super(SelfThrottledPostJobTask, self).finished(success)
        if self._process_started():
            self._decrement_running_processes()


class FinalReparseTask(SelfThrottledPostJobTask):
    def __init__(self, queue_entries):
        super(FinalReparseTask, self).__init__(queue_entries,
                                               log_file_name='.parse.log')
        # don't use _set_ids, since we don't want to set the host_ids
        self.queue_entry_ids = [entry.id for entry in queue_entries]


    def _generate_command(self, results_dir):
        return [_parser_path, '--write-pidfile', '-l', '2', '-r', '-o',
                results_dir]


    @property
    def num_processes(self):
        return 0 # don't include parser processes in accounting


    def _pidfile_name(self):
        return drone_manager.PARSER_PID_FILE


    @classmethod
    def _max_processes(cls):
        return scheduler_config.config.max_parse_processes


    def prolog(self):
        self._check_queue_entry_statuses(
                self.queue_entries,
                allowed_hqe_statuses=(models.HostQueueEntry.Status.PARSING,))

        super(FinalReparseTask, self).prolog()


    def epilog(self):
        super(FinalReparseTask, self).epilog()
        self._archive_results(self.queue_entries)


class ArchiveResultsTask(SelfThrottledPostJobTask):
    _ARCHIVING_FAILED_FILE = '.archiver_failed'

    def __init__(self, queue_entries):
        super(ArchiveResultsTask, self).__init__(queue_entries,
                                                 log_file_name='.archiving.log')
        # don't use _set_ids, since we don't want to set the host_ids
        self.queue_entry_ids = [entry.id for entry in queue_entries]


    def _pidfile_name(self):
        return drone_manager.ARCHIVER_PID_FILE


    def _generate_command(self, results_dir):
        return [_autoserv_path , '-p',
                '--pidfile-label=%s' % self._pidfile_label(), '-r', results_dir,
                '--use-existing-results', '--control-filename=control.archive',
                os.path.join(drones.AUTOTEST_INSTALL_DIR, 'scheduler',
                             'archive_results.control.srv')]


    @classmethod
    def _max_processes(cls):
        return scheduler_config.config.max_transfer_processes


    def prolog(self):
        self._check_queue_entry_statuses(
                self.queue_entries,
                allowed_hqe_statuses=(models.HostQueueEntry.Status.ARCHIVING,))

        super(ArchiveResultsTask, self).prolog()


    def epilog(self):
        super(ArchiveResultsTask, self).epilog()
        if not self.success and self._paired_with_monitor().has_process():
            failed_file = os.path.join(self._working_directory(),
                                       self._ARCHIVING_FAILED_FILE)
            paired_process = self._paired_with_monitor().get_process()
            _drone_manager.write_lines_to_file(
                    failed_file, ['Archiving failed with exit code %s'
                                  % self.monitor.exit_code()],
                    paired_with_process=paired_process)
        self._set_all_statuses(self._final_status())


if __name__ == '__main__':
    main()