Synnefo » snf-ganeti

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#

# Copyright (C) 2006, 2007, 2008 Google Inc.

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# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

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# This program is distributed in the hope that it will be useful, but

# WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

# General Public License for more details.

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# You should have received a copy of the GNU General Public License

# along with this program; if not, write to the Free Software

# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

# 02110-1301, USA.

"""Module implementing the job queue handling.

Locking: there's a single, large lock in the L{JobQueue} class. It's

used by all other classes in this module.

@var JOBQUEUE_THREADS: the number of worker threads we start for

    processing jobs

"""

import os

import logging

import threading

import errno

import re

import time

import weakref

from ganeti import constants

from ganeti import serializer

from ganeti import workerpool

from ganeti import opcodes

from ganeti import errors

from ganeti import mcpu

from ganeti import utils

from ganeti import jstore

from ganeti import rpc

JOBQUEUE_THREADS = 25

def TimeStampNow():

  """Returns the current timestamp.

  @rtype: tuple

  @return: the current time in the (seconds, microseconds) format

  """

  return utils.SplitTime(time.time())

class _QueuedOpCode(object):

  """Encasulates an opcode object.

  @ivar log: holds the execution log and consists of tuples

  of the form C{(log_serial, timestamp, level, message)}

  @ivar input: the OpCode we encapsulate

  @ivar status: the current status

  @ivar result: the result of the LU execution

  @ivar start_timestamp: timestamp for the start of the execution

  @ivar stop_timestamp: timestamp for the end of the execution

  """

  def __init__(self, op):

    """Constructor for the _QuededOpCode.

    @type op: L{opcodes.OpCode}

    @param op: the opcode we encapsulate

    """

    self.input = op

    self.status = constants.OP_STATUS_QUEUED

    self.result = None

    self.log = []

    self.start_timestamp = None

    self.end_timestamp = None

  @classmethod

  def Restore(cls, state):

    """Restore the _QueuedOpCode from the serialized form.

    @type state: dict

    @param state: the serialized state

    @rtype: _QueuedOpCode

    @return: a new _QueuedOpCode instance

    """

    obj = _QueuedOpCode.__new__(cls)

    obj.input = opcodes.OpCode.LoadOpCode(state["input"])

    obj.status = state["status"]

    obj.result = state["result"]

    obj.log = state["log"]

    obj.start_timestamp = state.get("start_timestamp", None)

    obj.end_timestamp = state.get("end_timestamp", None)

    return obj

  def Serialize(self):

    """Serializes this _QueuedOpCode.

    @rtype: dict

    @return: the dictionary holding the serialized state

    """

    return {

      "input": self.input.__getstate__(),

      "status": self.status,

      "result": self.result,

      "log": self.log,

      "start_timestamp": self.start_timestamp,

      "end_timestamp": self.end_timestamp,

      }

class _QueuedJob(object):

  """In-memory job representation.

  This is what we use to track the user-submitted jobs. Locking must

  be taken care of by users of this class.

  @type queue: L{JobQueue}

  @ivar queue: the parent queue

  @ivar id: the job ID

  @type ops: list

  @ivar ops: the list of _QueuedOpCode that constitute the job

  @type run_op_index: int

  @ivar run_op_index: the currently executing opcode, or -1 if

      we didn't yet start executing

  @type log_serial: int

  @ivar log_serial: holds the index for the next log entry

  @ivar received_timestamp: the timestamp for when the job was received

  @ivar start_timestmap: the timestamp for start of execution

  @ivar end_timestamp: the timestamp for end of execution

  @ivar change: a Condition variable we use for waiting for job changes

  """

  def __init__(self, queue, job_id, ops):

    """Constructor for the _QueuedJob.

    @type queue: L{JobQueue}

    @param queue: our parent queue

    @type job_id: job_id

    @param job_id: our job id

    @type ops: list

    @param ops: the list of opcodes we hold, which will be encapsulated

        in _QueuedOpCodes

    """

    if not ops:

      # TODO: use a better exception

      raise Exception("No opcodes")

    self.queue = queue

    self.id = job_id

    self.ops = [_QueuedOpCode(op) for op in ops]

    self.run_op_index = -1

    self.log_serial = 0

    self.received_timestamp = TimeStampNow()

    self.start_timestamp = None

    self.end_timestamp = None

    # Condition to wait for changes

    self.change = threading.Condition(self.queue._lock)

  @classmethod

  def Restore(cls, queue, state):

    """Restore a _QueuedJob from serialized state:

    @type queue: L{JobQueue}

    @param queue: to which queue the restored job belongs

    @type state: dict

    @param state: the serialized state

    @rtype: _JobQueue

    @return: the restored _JobQueue instance

    """

    obj = _QueuedJob.__new__(cls)

    obj.queue = queue

    obj.id = state["id"]

    obj.run_op_index = state["run_op_index"]

    obj.received_timestamp = state.get("received_timestamp", None)

    obj.start_timestamp = state.get("start_timestamp", None)

    obj.end_timestamp = state.get("end_timestamp", None)

    obj.ops = []

    obj.log_serial = 0

    for op_state in state["ops"]:

      op = _QueuedOpCode.Restore(op_state)

      for log_entry in op.log:

        obj.log_serial = max(obj.log_serial, log_entry[0])

      obj.ops.append(op)

    # Condition to wait for changes

    obj.change = threading.Condition(obj.queue._lock)

    return obj

  def Serialize(self):

    """Serialize the _JobQueue instance.

    @rtype: dict

    @return: the serialized state

    """

    return {

      "id": self.id,

      "ops": [op.Serialize() for op in self.ops],

      "run_op_index": self.run_op_index,

      "start_timestamp": self.start_timestamp,

      "end_timestamp": self.end_timestamp,

      "received_timestamp": self.received_timestamp,

      }

  def CalcStatus(self):

    """Compute the status of this job.

    This function iterates over all the _QueuedOpCodes in the job and

    based on their status, computes the job status.

    The algorithm is:

      - if we find a cancelled, or finished with error, the job

        status will be the same

      - otherwise, the last opcode with the status one of:

          - waitlock

          - running

        will determine the job status

      - otherwise, it means either all opcodes are queued, or success,

        and the job status will be the same

    @return: the job status

    """

    status = constants.JOB_STATUS_QUEUED

    all_success = True

    for op in self.ops:

      if op.status == constants.OP_STATUS_SUCCESS:

        continue

      all_success = False

      if op.status == constants.OP_STATUS_QUEUED:

        pass

      elif op.status == constants.OP_STATUS_WAITLOCK:

        status = constants.JOB_STATUS_WAITLOCK

      elif op.status == constants.OP_STATUS_RUNNING:

        status = constants.JOB_STATUS_RUNNING

      elif op.status == constants.OP_STATUS_ERROR:

        status = constants.JOB_STATUS_ERROR

        # The whole job fails if one opcode failed

        break

      elif op.status == constants.OP_STATUS_CANCELED:

        status = constants.OP_STATUS_CANCELED

        break

    if all_success:

      status = constants.JOB_STATUS_SUCCESS

    return status

  def GetLogEntries(self, newer_than):

    """Selectively returns the log entries.

    @type newer_than: None or int

    @param newer_than: if this is None, return all log enties,

        otherwise return only the log entries with serial higher

        than this value

    @rtype: list

    @return: the list of the log entries selected

    """

    if newer_than is None:

      serial = -1

    else:

      serial = newer_than

    entries = []

    for op in self.ops:

      entries.extend(filter(lambda entry: entry[0] > serial, op.log))

    return entries

class _JobQueueWorker(workerpool.BaseWorker):

  """The actual job workers.

  """

  def _NotifyStart(self):

    """Mark the opcode as running, not lock-waiting.

    This is called from the mcpu code as a notifier function, when the

    LU is finally about to start the Exec() method. Of course, to have

    end-user visible results, the opcode must be initially (before

    calling into Processor.ExecOpCode) set to OP_STATUS_WAITLOCK.

    """

    assert self.queue, "Queue attribute is missing"

    assert self.opcode, "Opcode attribute is missing"

    self.queue.acquire()

    try:

      self.opcode.status = constants.OP_STATUS_RUNNING

    finally:

      self.queue.release()

  def RunTask(self, job):

    """Job executor.

    This functions processes a job. It is closely tied to the _QueuedJob and

    _QueuedOpCode classes.

    @type job: L{_QueuedJob}

    @param job: the job to be processed

    """

    logging.debug("Worker %s processing job %s",

                  self.worker_id, job.id)

    proc = mcpu.Processor(self.pool.queue.context)

    self.queue = queue = job.queue

    try:

      try:

        count = len(job.ops)

        for idx, op in enumerate(job.ops):

          try:

            logging.debug("Op %s/%s: Starting %s", idx + 1, count, op)

            queue.acquire()

            try:

              job.run_op_index = idx

              op.status = constants.OP_STATUS_WAITLOCK

              op.result = None

              op.start_timestamp = TimeStampNow()

              if idx == 0: # first opcode

                job.start_timestamp = op.start_timestamp

              queue.UpdateJobUnlocked(job)

              input_opcode = op.input

            finally:

              queue.release()

            def _Log(*args):

              """Append a log entry.

              """

              assert len(args) < 3

              if len(args) == 1:

                log_type = constants.ELOG_MESSAGE

                log_msg = args[0]

              else:

                log_type, log_msg = args

              # The time is split to make serialization easier and not lose

              # precision.

              timestamp = utils.SplitTime(time.time())

              queue.acquire()

              try:

                job.log_serial += 1

                op.log.append((job.log_serial, timestamp, log_type, log_msg))

                job.change.notifyAll()

              finally:

                queue.release()

            # Make sure not to hold lock while _Log is called

            self.opcode = op

            result = proc.ExecOpCode(input_opcode, _Log, self._NotifyStart)

            queue.acquire()

            try:

              op.status = constants.OP_STATUS_SUCCESS

              op.result = result

              op.end_timestamp = TimeStampNow()

              queue.UpdateJobUnlocked(job)

            finally:

              queue.release()

            logging.debug("Op %s/%s: Successfully finished %s",

                          idx + 1, count, op)

          except Exception, err:

            queue.acquire()

            try:

              try:

                op.status = constants.OP_STATUS_ERROR

                op.result = str(err)

                op.end_timestamp = TimeStampNow()

                logging.debug("Op %s/%s: Error in %s", idx + 1, count, op)

              finally:

                queue.UpdateJobUnlocked(job)

            finally:

              queue.release()

            raise

      except errors.GenericError, err:

        logging.exception("Ganeti exception")

      except:

        logging.exception("Unhandled exception")

    finally:

      queue.acquire()

      try:

        try:

          job.run_op_idx = -1

          job.end_timestamp = TimeStampNow()

          queue.UpdateJobUnlocked(job)

        finally:

          job_id = job.id

          status = job.CalcStatus()

      finally:

        queue.release()

      logging.debug("Worker %s finished job %s, status = %s",

                    self.worker_id, job_id, status)

class _JobQueueWorkerPool(workerpool.WorkerPool):

  """Simple class implementing a job-processing workerpool.

  """

  def __init__(self, queue):

    super(_JobQueueWorkerPool, self).__init__(JOBQUEUE_THREADS,

                                              _JobQueueWorker)

    self.queue = queue

class JobQueue(object):

  """Quue used to manaage the jobs.

  @cvar _RE_JOB_FILE: regex matching the valid job file names

  """

  _RE_JOB_FILE = re.compile(r"^job-(%s)$" % constants.JOB_ID_TEMPLATE)

  def _RequireOpenQueue(fn):

    """Decorator for "public" functions.

    This function should be used for all 'public' functions. That is,

    functions usually called from other classes.

    @warning: Use this decorator only after utils.LockedMethod!

    Example::

      @utils.LockedMethod

      @_RequireOpenQueue

      def Example(self):

        pass

    """

    def wrapper(self, *args, **kwargs):

      assert self._queue_lock is not None, "Queue should be open"

      return fn(self, *args, **kwargs)

    return wrapper

  def __init__(self, context):

    """Constructor for JobQueue.

    The constructor will initialize the job queue object and then

    start loading the current jobs from disk, either for starting them

    (if they were queue) or for aborting them (if they were already

    running).

    @type context: GanetiContext

    @param context: the context object for access to the configuration

        data and other ganeti objects

    """

    self.context = context

    self._memcache = weakref.WeakValueDictionary()

    self._my_hostname = utils.HostInfo().name

    # Locking

    self._lock = threading.Lock()

    self.acquire = self._lock.acquire

    self.release = self._lock.release

    # Initialize

    self._queue_lock = jstore.InitAndVerifyQueue(must_lock=True)

    # Read serial file

    self._last_serial = jstore.ReadSerial()

    assert self._last_serial is not None, ("Serial file was modified between"

                                           " check in jstore and here")

    # Get initial list of nodes

    self._nodes = dict((n.name, n.primary_ip)

                       for n in self.context.cfg.GetAllNodesInfo().values())

    # Remove master node

    try:

      del self._nodes[self._my_hostname]

    except ValueError:

      pass

    # TODO: Check consistency across nodes

    # Setup worker pool

    self._wpool = _JobQueueWorkerPool(self)

    # We need to lock here because WorkerPool.AddTask() may start a job while

    # we're still doing our work.

    self.acquire()

    try:

      for job in self._GetJobsUnlocked(None):

        # a failure in loading the job can cause 'None' to be returned

        if job is None:

          continue

        status = job.CalcStatus()

        if status in (constants.JOB_STATUS_QUEUED, ):

          self._wpool.AddTask(job)

        elif status in (constants.JOB_STATUS_RUNNING,

                        constants.JOB_STATUS_WAITLOCK):

          logging.warning("Unfinished job %s found: %s", job.id, job)

          try:

            for op in job.ops:

              op.status = constants.OP_STATUS_ERROR

              op.result = "Unclean master daemon shutdown"

          finally:

            self.UpdateJobUnlocked(job)

    finally:

      self.release()

  @utils.LockedMethod

  @_RequireOpenQueue

  def AddNode(self, node):

    """Register a new node with the queue.

    @type node: L{objects.Node}

    @param node: the node object to be added

    """

    node_name = node.name

    assert node_name != self._my_hostname

    # Clean queue directory on added node

    rpc.RpcRunner.call_jobqueue_purge(node_name)

    # Upload the whole queue excluding archived jobs

    files = [self._GetJobPath(job_id) for job_id in self._GetJobIDsUnlocked()]

    # Upload current serial file

    files.append(constants.JOB_QUEUE_SERIAL_FILE)

    for file_name in files:

      # Read file content

      fd = open(file_name, "r")

      try:

        content = fd.read()

      finally:

        fd.close()

      result = rpc.RpcRunner.call_jobqueue_update([node_name],

                                                  [node.primary_ip],

                                                  file_name, content)

      if not result[node_name]:

        logging.error("Failed to upload %s to %s", file_name, node_name)

    self._nodes[node_name] = node.primary_ip

  @utils.LockedMethod

  @_RequireOpenQueue

  def RemoveNode(self, node_name):

    """Callback called when removing nodes from the cluster.

    @type node_name: str

    @param node_name: the name of the node to remove

    """

    try:

      # The queue is removed by the "leave node" RPC call.

      del self._nodes[node_name]

    except KeyError:

      pass

  def _CheckRpcResult(self, result, nodes, failmsg):

    """Verifies the status of an RPC call.

    Since we aim to keep consistency should this node (the current

    master) fail, we will log errors if our rpc fail, and especially

    log the case when more than half of the nodes failes.

    @param result: the data as returned from the rpc call

    @type nodes: list

    @param nodes: the list of nodes we made the call to

    @type failmsg: str

    @param failmsg: the identifier to be used for logging

    """

    failed = []

    success = []

    for node in nodes:

      if result[node]:

        success.append(node)

      else:

        failed.append(node)

    if failed:

      logging.error("%s failed on %s", failmsg, ", ".join(failed))

    # +1 for the master node

    if (len(success) + 1) < len(failed):

      # TODO: Handle failing nodes

      logging.error("More than half of the nodes failed")

  def _GetNodeIp(self):

    """Helper for returning the node name/ip list.

    @rtype: (list, list)

    @return: a tuple of two lists, the first one with the node

        names and the second one with the node addresses

    """

    name_list = self._nodes.keys()

    addr_list = [self._nodes[name] for name in name_list]

    return name_list, addr_list

  def _WriteAndReplicateFileUnlocked(self, file_name, data):

    """Writes a file locally and then replicates it to all nodes.

    This function will replace the contents of a file on the local

    node and then replicate it to all the other nodes we have.

    @type file_name: str

    @param file_name: the path of the file to be replicated

    @type data: str

    @param data: the new contents of the file

    """

    utils.WriteFile(file_name, data=data)

    names, addrs = self._GetNodeIp()

    result = rpc.RpcRunner.call_jobqueue_update(names, addrs, file_name, data)

    self._CheckRpcResult(result, self._nodes,

                         "Updating %s" % file_name)

  def _RenameFileUnlocked(self, old, new):

    """Renames a file locally and then replicate the change.

    This function will rename a file in the local queue directory

    and then replicate this rename to all the other nodes we have.

    @type old: str

    @param old: the current name of the file

    @type new: str

    @param new: the new name of the file

    """

    os.rename(old, new)

    names, addrs = self._GetNodeIp()

    result = rpc.RpcRunner.call_jobqueue_rename(names, addrs, old, new)

    self._CheckRpcResult(result, self._nodes,

                         "Moving %s to %s" % (old, new))

  def _FormatJobID(self, job_id):

    """Convert a job ID to string format.

    Currently this just does C{str(job_id)} after performing some

    checks, but if we want to change the job id format this will

    abstract this change.

    @type job_id: int or long

    @param job_id: the numeric job id

    @rtype: str

    @return: the formatted job id

    """

    if not isinstance(job_id, (int, long)):

      raise errors.ProgrammerError("Job ID '%s' not numeric" % job_id)

    if job_id < 0:

      raise errors.ProgrammerError("Job ID %s is negative" % job_id)

    return str(job_id)

  def _NewSerialUnlocked(self):

    """Generates a new job identifier.

    Job identifiers are unique during the lifetime of a cluster.

    @rtype: str

    @return: a string representing the job identifier.

    """

    # New number

    serial = self._last_serial + 1

    # Write to file

    self._WriteAndReplicateFileUnlocked(constants.JOB_QUEUE_SERIAL_FILE,

                                        "%s\n" % serial)

    # Keep it only if we were able to write the file

    self._last_serial = serial

    return self._FormatJobID(serial)

  @staticmethod

  def _GetJobPath(job_id):

    """Returns the job file for a given job id.

    @type job_id: str

    @param job_id: the job identifier

    @rtype: str

    @return: the path to the job file

    """

    return os.path.join(constants.QUEUE_DIR, "job-%s" % job_id)

  @staticmethod

  def _GetArchivedJobPath(job_id):

    """Returns the archived job file for a give job id.

    @type job_id: str

    @param job_id: the job identifier

    @rtype: str

    @return: the path to the archived job file

    """

    return os.path.join(constants.JOB_QUEUE_ARCHIVE_DIR, "job-%s" % job_id)

  @classmethod

  def _ExtractJobID(cls, name):

    """Extract the job id from a filename.

    @type name: str

    @param name: the job filename

    @rtype: job id or None

    @return: the job id corresponding to the given filename,

        or None if the filename does not represent a valid

        job file

    """

    m = cls._RE_JOB_FILE.match(name)

    if m:

      return m.group(1)

    else:

      return None

  def _GetJobIDsUnlocked(self, archived=False):

    """Return all known job IDs.

    If the parameter archived is True, archived jobs IDs will be

    included. Currently this argument is unused.

    The method only looks at disk because it's a requirement that all

    jobs are present on disk (so in the _memcache we don't have any

    extra IDs).

    @rtype: list

    @return: the list of job IDs

    """

    jlist = [self._ExtractJobID(name) for name in self._ListJobFiles()]

    jlist = utils.NiceSort(jlist)

    return jlist

  def _ListJobFiles(self):

    """Returns the list of current job files.

    @rtype: list

    @return: the list of job file names

    """

    return [name for name in utils.ListVisibleFiles(constants.QUEUE_DIR)

            if self._RE_JOB_FILE.match(name)]

  def _LoadJobUnlocked(self, job_id):

    """Loads a job from the disk or memory.

    Given a job id, this will return the cached job object if

    existing, or try to load the job from the disk. If loading from

    disk, it will also add the job to the cache.

    @param job_id: the job id

    @rtype: L{_QueuedJob} or None

    @return: either None or the job object

    """

    job = self._memcache.get(job_id, None)

    if job:

      logging.debug("Found job %s in memcache", job_id)

      return job

    filepath = self._GetJobPath(job_id)

    logging.debug("Loading job from %s", filepath)

    try:

      fd = open(filepath, "r")

    except IOError, err:

      if err.errno in (errno.ENOENT, ):

        return None

      raise

    try:

      data = serializer.LoadJson(fd.read())

    finally:

      fd.close()

    try:

      job = _QueuedJob.Restore(self, data)

    except Exception, err:

      new_path = self._GetArchivedJobPath(job_id)

      if filepath == new_path:

        # job already archived (future case)

        logging.exception("Can't parse job %s", job_id)

      else:

        # non-archived case

        logging.exception("Can't parse job %s, will archive.", job_id)

        self._RenameFileUnlocked(filepath, new_path)

      return None

    self._memcache[job_id] = job

    logging.debug("Added job %s to the cache", job_id)

    return job

  def _GetJobsUnlocked(self, job_ids):

    """Return a list of jobs based on their IDs.

    @type job_ids: list

    @param job_ids: either an empty list (meaning all jobs),

        or a list of job IDs

    @rtype: list

    @return: the list of job objects

    """

    if not job_ids:

      job_ids = self._GetJobIDsUnlocked()

    return [self._LoadJobUnlocked(job_id) for job_id in job_ids]

  @staticmethod

  def _IsQueueMarkedDrain():

    """Check if the queue is marked from drain.

    This currently uses the queue drain file, which makes it a

    per-node flag. In the future this can be moved to the config file.

    @rtype: boolean

    @return: True of the job queue is marked for draining

    """

    return os.path.exists(constants.JOB_QUEUE_DRAIN_FILE)

  @staticmethod

  def SetDrainFlag(drain_flag):

    """Sets the drain flag for the queue.

    This is similar to the function L{backend.JobQueueSetDrainFlag},

    and in the future we might merge them.

    @type drain_flag: boolean

    @param drain_flag: wheter to set or unset the drain flag

    """

    if drain_flag:

      utils.WriteFile(constants.JOB_QUEUE_DRAIN_FILE, data="", close=True)

    else:

      utils.RemoveFile(constants.JOB_QUEUE_DRAIN_FILE)

    return True

  @utils.LockedMethod

  @_RequireOpenQueue

  def SubmitJob(self, ops):

    """Create and store a new job.

    This enters the job into our job queue and also puts it on the new

    queue, in order for it to be picked up by the queue processors.

    @type ops: list

    @param ops: The list of OpCodes that will become the new job.

    @rtype: job ID

    @return: the job ID of the newly created job

    @raise errors.JobQueueDrainError: if the job is marked for draining

    """

    if self._IsQueueMarkedDrain():

      raise errors.JobQueueDrainError()

    # Get job identifier

    job_id = self._NewSerialUnlocked()

    job = _QueuedJob(self, job_id, ops)

    # Write to disk

    self.UpdateJobUnlocked(job)

    logging.debug("Adding new job %s to the cache", job_id)

    self._memcache[job_id] = job

    # Add to worker pool

    self._wpool.AddTask(job)

    return job.id

  @_RequireOpenQueue

  def UpdateJobUnlocked(self, job):

    """Update a job's on disk storage.

    After a job has been modified, this function needs to be called in

    order to write the changes to disk and replicate them to the other

    nodes.

    @type job: L{_QueuedJob}

    @param job: the changed job

    """

    filename = self._GetJobPath(job.id)

    data = serializer.DumpJson(job.Serialize(), indent=False)

    logging.debug("Writing job %s to %s", job.id, filename)

    self._WriteAndReplicateFileUnlocked(filename, data)

    # Notify waiters about potential changes

    job.change.notifyAll()

  @utils.LockedMethod

  @_RequireOpenQueue

  def WaitForJobChanges(self, job_id, fields, prev_job_info, prev_log_serial,

                        timeout):

    """Waits for changes in a job.

    @type job_id: string

    @param job_id: Job identifier

    @type fields: list of strings

    @param fields: Which fields to check for changes

    @type prev_job_info: list or None

    @param prev_job_info: Last job information returned

    @type prev_log_serial: int

    @param prev_log_serial: Last job message serial number

    @type timeout: float

    @param timeout: maximum time to wait

    @rtype: tuple (job info, log entries)

    @return: a tuple of the job information as required via

        the fields parameter, and the log entries as a list

        if the job has not changed and the timeout has expired,

        we instead return a special value,

        L{constants.JOB_NOTCHANGED}, which should be interpreted

        as such by the clients

    """

    logging.debug("Waiting for changes in job %s", job_id)

    end_time = time.time() + timeout

    while True:

      delta_time = end_time - time.time()

      if delta_time < 0:

        return constants.JOB_NOTCHANGED

      job = self._LoadJobUnlocked(job_id)

      if not job:

        logging.debug("Job %s not found", job_id)

        break

      status = job.CalcStatus()

      job_info = self._GetJobInfoUnlocked(job, fields)

      log_entries = job.GetLogEntries(prev_log_serial)

      # Serializing and deserializing data can cause type changes (e.g. from

      # tuple to list) or precision loss. We're doing it here so that we get

      # the same modifications as the data received from the client. Without

      # this, the comparison afterwards might fail without the data being

      # significantly different.

      job_info = serializer.LoadJson(serializer.DumpJson(job_info))

      log_entries = serializer.LoadJson(serializer.DumpJson(log_entries))

      if status not in (constants.JOB_STATUS_QUEUED,

                        constants.JOB_STATUS_RUNNING,

                        constants.JOB_STATUS_WAITLOCK):

        # Don't even try to wait if the job is no longer running, there will be

        # no changes.

        break

      if (prev_job_info != job_info or

          (log_entries and prev_log_serial != log_entries[0][0])):

        break

      logging.debug("Waiting again")

      # Release the queue lock while waiting

      job.change.wait(delta_time)

    logging.debug("Job %s changed", job_id)

    return (job_info, log_entries)

  @utils.LockedMethod

  @_RequireOpenQueue

  def CancelJob(self, job_id):

    """Cancels a job.

    This will only succeed if the job has not started yet.

    @type job_id: string

    @param job_id: job ID of job to be cancelled.

    """

    logging.debug("Cancelling job %s", job_id)

    job = self._LoadJobUnlocked(job_id)

    if not job:

      logging.debug("Job %s not found", job_id)

      return

    if job.CalcStatus() not in (constants.JOB_STATUS_QUEUED,):

      logging.debug("Job %s is no longer in the queue", job.id)

      return

    try:

      for op in job.ops:

        op.status = constants.OP_STATUS_ERROR

        op.result = "Job cancelled by request"

    finally:

      self.UpdateJobUnlocked(job)

  @_RequireOpenQueue

  def _ArchiveJobUnlocked(self, job_id):

    """Archives a job.

    @type job_id: string

    @param job_id: Job ID of job to be archived.

    """

    logging.info("Archiving job %s", job_id)

    job = self._LoadJobUnlocked(job_id)

    if not job:

      logging.debug("Job %s not found", job_id)

      return

    if job.CalcStatus() not in (constants.JOB_STATUS_CANCELED,

                                constants.JOB_STATUS_SUCCESS,

                                constants.JOB_STATUS_ERROR):

      logging.debug("Job %s is not yet done", job.id)

      return

    old = self._GetJobPath(job.id)

    new = self._GetArchivedJobPath(job.id)

    self._RenameFileUnlocked(old, new)

    logging.debug("Successfully archived job %s", job.id)

  @utils.LockedMethod

  @_RequireOpenQueue

  def ArchiveJob(self, job_id):

    """Archives a job.

    This is just a wrapper over L{_ArchiveJobUnlocked}.