Synnefo » snf-ganeti

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#

# Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 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.

#

# 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 Ganeti locking code."""

# pylint: disable=W0212

# W0212 since e.g. LockSet methods use (a lot) the internals of

# SharedLock

import os

import select

import threading

import errno

import weakref

import logging

import heapq

import itertools

from ganeti import errors

from ganeti import utils

from ganeti import compat

from ganeti import query

_EXCLUSIVE_TEXT = "exclusive"

_SHARED_TEXT = "shared"

_DELETED_TEXT = "deleted"

_DEFAULT_PRIORITY = 0

def ssynchronized(mylock, shared=0):

  """Shared Synchronization decorator.

  Calls the function holding the given lock, either in exclusive or shared

  mode. It requires the passed lock to be a SharedLock (or support its

  semantics).

  @type mylock: lockable object or string

  @param mylock: lock to acquire or class member name of the lock to acquire

  """

  def wrap(fn):

    def sync_function(*args, **kwargs):

      if isinstance(mylock, basestring):

        assert args, "cannot ssynchronize on non-class method: self not found"

        # args[0] is "self"

        lock = getattr(args[0], mylock)

      else:

        lock = mylock

      lock.acquire(shared=shared)

      try:

        return fn(*args, **kwargs)

      finally:

        lock.release()

    return sync_function

  return wrap

class _SingleNotifyPipeConditionWaiter(object):

  """Helper class for SingleNotifyPipeCondition

  """

  __slots__ = [

    "_fd",

    "_poller",

    ]

  def __init__(self, poller, fd):

    """Constructor for _SingleNotifyPipeConditionWaiter

    @type poller: select.poll

    @param poller: Poller object

    @type fd: int

    @param fd: File descriptor to wait for

    """

    object.__init__(self)

    self._poller = poller

    self._fd = fd

  def __call__(self, timeout):

    """Wait for something to happen on the pipe.

    @type timeout: float or None

    @param timeout: Timeout for waiting (can be None)

    """

    running_timeout = utils.RunningTimeout(timeout, True)

    while True:

      remaining_time = running_timeout.Remaining()

      if remaining_time is not None:

        if remaining_time < 0.0:

          break

        # Our calculation uses seconds, poll() wants milliseconds

        remaining_time *= 1000

      try:

        result = self._poller.poll(remaining_time)

      except EnvironmentError, err:

        if err.errno != errno.EINTR:

          raise

        result = None

      # Check whether we were notified

      if result and result[0][0] == self._fd:

        break

class _BaseCondition(object):

  """Base class containing common code for conditions.

  Some of this code is taken from python's threading module.

  """

  __slots__ = [

    "_lock",

    "acquire",

    "release",

    "_is_owned",

    "_acquire_restore",

    "_release_save",

    ]

  def __init__(self, lock):

    """Constructor for _BaseCondition.

    @type lock: threading.Lock

    @param lock: condition base lock

    """

    object.__init__(self)

    try:

      self._release_save = lock._release_save

    except AttributeError:

      self._release_save = self._base_release_save

    try:

      self._acquire_restore = lock._acquire_restore

    except AttributeError:

      self._acquire_restore = self._base_acquire_restore

    try:

      self._is_owned = lock.is_owned

    except AttributeError:

      self._is_owned = self._base_is_owned

    self._lock = lock

    # Export the lock's acquire() and release() methods

    self.acquire = lock.acquire

    self.release = lock.release

  def _base_is_owned(self):

    """Check whether lock is owned by current thread.

    """

    if self._lock.acquire(0):

      self._lock.release()

      return False

    return True

  def _base_release_save(self):

    self._lock.release()

  def _base_acquire_restore(self, _):

    self._lock.acquire()

  def _check_owned(self):

    """Raise an exception if the current thread doesn't own the lock.

    """

    if not self._is_owned():

      raise RuntimeError("cannot work with un-aquired lock")

class SingleNotifyPipeCondition(_BaseCondition):

  """Condition which can only be notified once.

  This condition class uses pipes and poll, internally, to be able to wait for

  notification with a timeout, without resorting to polling. It is almost

  compatible with Python's threading.Condition, with the following differences:

    - notifyAll can only be called once, and no wait can happen after that

    - notify is not supported, only notifyAll

  """

  __slots__ = [

    "_poller",

    "_read_fd",

    "_write_fd",

    "_nwaiters",

    "_notified",

    ]

  _waiter_class = _SingleNotifyPipeConditionWaiter

  def __init__(self, lock):

    """Constructor for SingleNotifyPipeCondition

    """

    _BaseCondition.__init__(self, lock)

    self._nwaiters = 0

    self._notified = False

    self._read_fd = None

    self._write_fd = None

    self._poller = None

  def _check_unnotified(self):

    """Throws an exception if already notified.

    """

    if self._notified:

      raise RuntimeError("cannot use already notified condition")

  def _Cleanup(self):

    """Cleanup open file descriptors, if any.

    """

    if self._read_fd is not None:

      os.close(self._read_fd)

      self._read_fd = None

    if self._write_fd is not None:

      os.close(self._write_fd)

      self._write_fd = None

    self._poller = None

  def wait(self, timeout):

    """Wait for a notification.

    @type timeout: float or None

    @param timeout: Waiting timeout (can be None)

    """

    self._check_owned()

    self._check_unnotified()

    self._nwaiters += 1

    try:

      if self._poller is None:

        (self._read_fd, self._write_fd) = os.pipe()

        self._poller = select.poll()

        self._poller.register(self._read_fd, select.POLLHUP)

      wait_fn = self._waiter_class(self._poller, self._read_fd)

      state = self._release_save()

      try:

        # Wait for notification

        wait_fn(timeout)

      finally:

        # Re-acquire lock

        self._acquire_restore(state)

    finally:

      self._nwaiters -= 1

      if self._nwaiters == 0:

        self._Cleanup()

  def notifyAll(self): # pylint: disable=C0103

    """Close the writing side of the pipe to notify all waiters.

    """

    self._check_owned()

    self._check_unnotified()

    self._notified = True

    if self._write_fd is not None:

      os.close(self._write_fd)

      self._write_fd = None

class PipeCondition(_BaseCondition):

  """Group-only non-polling condition with counters.

  This condition class uses pipes and poll, internally, to be able to wait for

  notification with a timeout, without resorting to polling. It is almost

  compatible with Python's threading.Condition, but only supports notifyAll and

  non-recursive locks. As an additional features it's able to report whether

  there are any waiting threads.

  """

  __slots__ = [

    "_waiters",

    "_single_condition",

    ]

  _single_condition_class = SingleNotifyPipeCondition

  def __init__(self, lock):

    """Initializes this class.

    """

    _BaseCondition.__init__(self, lock)

    self._waiters = set()

    self._single_condition = self._single_condition_class(self._lock)

  def wait(self, timeout):

    """Wait for a notification.

    @type timeout: float or None

    @param timeout: Waiting timeout (can be None)

    """

    self._check_owned()

    # Keep local reference to the pipe. It could be replaced by another thread

    # notifying while we're waiting.

    cond = self._single_condition

    self._waiters.add(threading.currentThread())

    try:

      cond.wait(timeout)

    finally:

      self._check_owned()

      self._waiters.remove(threading.currentThread())

  def notifyAll(self): # pylint: disable=C0103

    """Notify all currently waiting threads.

    """

    self._check_owned()

    self._single_condition.notifyAll()

    self._single_condition = self._single_condition_class(self._lock)

  def get_waiting(self):

    """Returns a list of all waiting threads.

    """

    self._check_owned()

    return self._waiters

  def has_waiting(self):

    """Returns whether there are active waiters.

    """

    self._check_owned()

    return bool(self._waiters)

class _PipeConditionWithMode(PipeCondition):

  __slots__ = [

    "shared",

    ]

  def __init__(self, lock, shared):

    """Initializes this class.

    """

    self.shared = shared

    PipeCondition.__init__(self, lock)

class SharedLock(object):

  """Implements a shared lock.

  Multiple threads can acquire the lock in a shared way by calling

  C{acquire(shared=1)}. In order to acquire the lock in an exclusive way

  threads can call C{acquire(shared=0)}.

  Notes on data structures: C{__pending} contains a priority queue (heapq) of

  all pending acquires: C{[(priority1: prioqueue1), (priority2: prioqueue2),

  ...]}. Each per-priority queue contains a normal in-order list of conditions

  to be notified when the lock can be acquired. Shared locks are grouped

  together by priority and the condition for them is stored in

  C{__pending_shared} if it already exists. C{__pending_by_prio} keeps

  references for the per-priority queues indexed by priority for faster access.

  @type name: string

  @ivar name: the name of the lock

  """

  __slots__ = [

    "__weakref__",

    "__deleted",

    "__exc",

    "__lock",

    "__pending",

    "__pending_by_prio",

    "__pending_shared",

    "__shr",

    "name",

    ]

  __condition_class = _PipeConditionWithMode

  def __init__(self, name, monitor=None):

    """Construct a new SharedLock.

    @param name: the name of the lock

    @type monitor: L{LockMonitor}

    @param monitor: Lock monitor with which to register

    """

    object.__init__(self)

    self.name = name

    # Internal lock

    self.__lock = threading.Lock()

    # Queue containing waiting acquires

    self.__pending = []

    self.__pending_by_prio = {}

    self.__pending_shared = {}

    # Current lock holders

    self.__shr = set()

    self.__exc = None

    # is this lock in the deleted state?

    self.__deleted = False

    # Register with lock monitor

    if monitor:

      logging.debug("Adding lock %s to monitor", name)

      monitor.RegisterLock(self)

  def GetLockInfo(self, requested):

    """Retrieves information for querying locks.

    @type requested: set

    @param requested: Requested information, see C{query.LQ_*}

    """

    self.__lock.acquire()

    try:

      # Note: to avoid unintentional race conditions, no references to

      # modifiable objects should be returned unless they were created in this

      # function.

      mode = None

      owner_names = None

      if query.LQ_MODE in requested:

        if self.__deleted:

          mode = _DELETED_TEXT

          assert not (self.__exc or self.__shr)

        elif self.__exc:

          mode = _EXCLUSIVE_TEXT

        elif self.__shr:

          mode = _SHARED_TEXT

      # Current owner(s) are wanted

      if query.LQ_OWNER in requested:

        if self.__exc:

          owner = [self.__exc]

        else:

          owner = self.__shr

        if owner:

          assert not self.__deleted

          owner_names = [i.getName() for i in owner]

      # Pending acquires are wanted

      if query.LQ_PENDING in requested:

        pending = []

        # Sorting instead of copying and using heaq functions for simplicity

        for (_, prioqueue) in sorted(self.__pending):

          for cond in prioqueue:

            if cond.shared:

              pendmode = _SHARED_TEXT

            else:

              pendmode = _EXCLUSIVE_TEXT

            # List of names will be sorted in L{query._GetLockPending}

            pending.append((pendmode, [i.getName()

                                       for i in cond.get_waiting()]))

      else:

        pending = None

      return [(self.name, mode, owner_names, pending)]

    finally:

      self.__lock.release()

  def __check_deleted(self):

    """Raises an exception if the lock has been deleted.

    """

    if self.__deleted:

      raise errors.LockError("Deleted lock %s" % self.name)

  def __is_sharer(self):

    """Is the current thread sharing the lock at this time?

    """

    return threading.currentThread() in self.__shr

  def __is_exclusive(self):

    """Is the current thread holding the lock exclusively at this time?

    """

    return threading.currentThread() == self.__exc

  def __is_owned(self, shared=-1):

    """Is the current thread somehow owning the lock at this time?

    This is a private version of the function, which presumes you're holding

    the internal lock.

    """

    if shared < 0:

      return self.__is_sharer() or self.__is_exclusive()

    elif shared:

      return self.__is_sharer()

    else:

      return self.__is_exclusive()

  def is_owned(self, shared=-1):

    """Is the current thread somehow owning the lock at this time?

    @param shared:

        - < 0: check for any type of ownership (default)

        - 0: check for exclusive ownership

        - > 0: check for shared ownership

    """

    self.__lock.acquire()

    try:

      return self.__is_owned(shared=shared)

    finally:

      self.__lock.release()

  #: Necessary to remain compatible with threading.Condition, which tries to

  #: retrieve a locks' "_is_owned" attribute

  _is_owned = is_owned

  def _count_pending(self):

    """Returns the number of pending acquires.

    @rtype: int

    """

    self.__lock.acquire()

    try:

      return sum(len(prioqueue) for (_, prioqueue) in self.__pending)

    finally:

      self.__lock.release()

  def _check_empty(self):

    """Checks whether there are any pending acquires.

    @rtype: bool

    """

    self.__lock.acquire()

    try:

      # Order is important: __find_first_pending_queue modifies __pending

      (_, prioqueue) = self.__find_first_pending_queue()

      return not (prioqueue or

                  self.__pending or

                  self.__pending_by_prio or

                  self.__pending_shared)

    finally:

      self.__lock.release()

  def __do_acquire(self, shared):

    """Actually acquire the lock.

    """

    if shared:

      self.__shr.add(threading.currentThread())

    else:

      self.__exc = threading.currentThread()

  def __can_acquire(self, shared):

    """Determine whether lock can be acquired.

    """

    if shared:

      return self.__exc is None

    else:

      return len(self.__shr) == 0 and self.__exc is None

  def __find_first_pending_queue(self):

    """Tries to find the topmost queued entry with pending acquires.

    Removes empty entries while going through the list.

    """

    while self.__pending:

      (priority, prioqueue) = self.__pending[0]

      if prioqueue:

        return (priority, prioqueue)

      # Remove empty queue

      heapq.heappop(self.__pending)

      del self.__pending_by_prio[priority]

      assert priority not in self.__pending_shared

    return (None, None)

  def __is_on_top(self, cond):

    """Checks whether the passed condition is on top of the queue.

    The caller must make sure the queue isn't empty.

    """

    (_, prioqueue) = self.__find_first_pending_queue()

    return cond == prioqueue[0]

  def __acquire_unlocked(self, shared, timeout, priority):

    """Acquire a shared lock.

    @param shared: whether to acquire in shared mode; by default an

        exclusive lock will be acquired

    @param timeout: maximum waiting time before giving up

    @type priority: integer

    @param priority: Priority for acquiring lock

    """

    self.__check_deleted()

    # We cannot acquire the lock if we already have it

    assert not self.__is_owned(), ("double acquire() on a non-recursive lock"

                                   " %s" % self.name)

    # Remove empty entries from queue

    self.__find_first_pending_queue()

    # Check whether someone else holds the lock or there are pending acquires.

    if not self.__pending and self.__can_acquire(shared):

      # Apparently not, can acquire lock directly.

      self.__do_acquire(shared)

      return True

    prioqueue = self.__pending_by_prio.get(priority, None)

    if shared:

      # Try to re-use condition for shared acquire

      wait_condition = self.__pending_shared.get(priority, None)

      assert (wait_condition is None or

              (wait_condition.shared and wait_condition in prioqueue))

    else:

      wait_condition = None

    if wait_condition is None:

      if prioqueue is None:

        assert priority not in self.__pending_by_prio

        prioqueue = []

        heapq.heappush(self.__pending, (priority, prioqueue))

        self.__pending_by_prio[priority] = prioqueue

      wait_condition = self.__condition_class(self.__lock, shared)

      prioqueue.append(wait_condition)

      if shared:

        # Keep reference for further shared acquires on same priority. This is

        # better than trying to find it in the list of pending acquires.

        assert priority not in self.__pending_shared

        self.__pending_shared[priority] = wait_condition

    try:

      # Wait until we become the topmost acquire in the queue or the timeout

      # expires.

      # TODO: Decrease timeout with spurious notifications

      while not (self.__is_on_top(wait_condition) and

                 self.__can_acquire(shared)):

        # Wait for notification

        wait_condition.wait(timeout)

        self.__check_deleted()

        # A lot of code assumes blocking acquires always succeed. Loop

        # internally for that case.

        if timeout is not None:

          break

      if self.__is_on_top(wait_condition) and self.__can_acquire(shared):

        self.__do_acquire(shared)

        return True

    finally:

      # Remove condition from queue if there are no more waiters

      if not wait_condition.has_waiting():

        prioqueue.remove(wait_condition)

        if wait_condition.shared:

          # Remove from list of shared acquires if it wasn't while releasing

          # (e.g. on lock deletion)

          self.__pending_shared.pop(priority, None)

    return False

  def acquire(self, shared=0, timeout=None, priority=None,

              test_notify=None):

    """Acquire a shared lock.

    @type shared: integer (0/1) used as a boolean

    @param shared: whether to acquire in shared mode; by default an

        exclusive lock will be acquired

    @type timeout: float

    @param timeout: maximum waiting time before giving up

    @type priority: integer

    @param priority: Priority for acquiring lock

    @type test_notify: callable or None

    @param test_notify: Special callback function for unittesting

    """

    if priority is None:

      priority = _DEFAULT_PRIORITY

    self.__lock.acquire()

    try:

      # We already got the lock, notify now

      if __debug__ and callable(test_notify):

        test_notify()

      return self.__acquire_unlocked(shared, timeout, priority)

    finally:

      self.__lock.release()

  def downgrade(self):

    """Changes the lock mode from exclusive to shared.

    Pending acquires in shared mode on the same priority will go ahead.

    """

    self.__lock.acquire()

    try:

      assert self.__is_owned(), "Lock must be owned"

      if self.__is_exclusive():

        # Do nothing if the lock is already acquired in shared mode

        self.__exc = None

        self.__do_acquire(1)

        # Important: pending shared acquires should only jump ahead if there

        # was a transition from exclusive to shared, otherwise an owner of a

        # shared lock can keep calling this function to push incoming shared

        # acquires

        (priority, prioqueue) = self.__find_first_pending_queue()

        if prioqueue:

          # Is there a pending shared acquire on this priority?

          cond = self.__pending_shared.pop(priority, None)

          if cond:

            assert cond.shared

            assert cond in prioqueue

            # Ensure shared acquire is on top of queue

            if len(prioqueue) > 1:

              prioqueue.remove(cond)

              prioqueue.insert(0, cond)