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#
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#
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# Copyright (C) 2006, 2007 Google Inc.
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful, but
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# WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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# General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program; if not, write to the Free Software
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# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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# 02110-1301, USA.
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"""Module implementing the Ganeti locking code."""
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# pylint: disable-msg=W0613,W0201
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import threading
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# Wouldn't it be better to define LockingError in the locking module?
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# Well, for now that's how the rest of the code does it...
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from ganeti import errors
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class SharedLock:
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  """Implements a shared lock.
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  Multiple threads can acquire the lock in a shared way, calling
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  acquire_shared().  In order to acquire the lock in an exclusive way threads
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  can call acquire_exclusive().
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  The lock prevents starvation but does not guarantee that threads will acquire
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  the shared lock in the order they queued for it, just that they will
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  eventually do so.
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  """
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  def __init__(self):
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    """Construct a new SharedLock"""
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    # we have two conditions, c_shr and c_exc, sharing the same lock.
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    self.__lock = threading.Lock()
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    self.__turn_shr = threading.Condition(self.__lock)
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    self.__turn_exc = threading.Condition(self.__lock)
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    # current lock holders
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    self.__shr = set()
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    self.__exc = None
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    # lock waiters
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    self.__nwait_exc = 0
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    self.__nwait_shr = 0
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    # is this lock in the deleted state?
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    self.__deleted = False
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  def __is_sharer(self):
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    """Is the current thread sharing the lock at this time?"""
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    return threading.currentThread() in self.__shr
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  def __is_exclusive(self):
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    """Is the current thread holding the lock exclusively at this time?"""
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    return threading.currentThread() == self.__exc
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  def __is_owned(self, shared=-1):
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    """Is the current thread somehow owning the lock at this time?
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    This is a private version of the function, which presumes you're holding
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    the internal lock.
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    """
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    if shared < 0:
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      return self.__is_sharer() or self.__is_exclusive()
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    elif shared:
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      return self.__is_sharer()
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    else:
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      return self.__is_exclusive()
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  def _is_owned(self, shared=-1):
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    """Is the current thread somehow owning the lock at this time?
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    Args:
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      shared:
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        < 0: check for any type of ownership (default)
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        0: check for exclusive ownership
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        > 0: check for shared ownership
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    """
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    self.__lock.acquire()
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    try:
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      result = self.__is_owned(shared)
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    finally:
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      self.__lock.release()
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    return result
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  def __wait(self,c):
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    """Wait on the given condition, and raise an exception if the current lock
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    is declared deleted in the meantime.
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    Args:
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      c: condition to wait on
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    """
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    c.wait()
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    if self.__deleted:
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      raise errors.LockError('deleted lock')
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  def __exclusive_acquire(self):
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    """Acquire the lock exclusively.
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    This is a private function that presumes you are already holding the
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    internal lock. It's defined separately to avoid code duplication between
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    acquire() and delete()
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    """
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    self.__nwait_exc += 1
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    try:
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      # This is to save ourselves from a nasty race condition that could
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      # theoretically make the sharers starve.
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      if self.__nwait_shr > 0 or self.__nwait_exc > 1:
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        self.__wait(self.__turn_exc)
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      while len(self.__shr) > 0 or self.__exc is not None:
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        self.__wait(self.__turn_exc)
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      self.__exc = threading.currentThread()
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    finally:
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      self.__nwait_exc -= 1
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  def acquire(self, blocking=1, shared=0):
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    """Acquire a shared lock.
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    Args:
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      shared: whether to acquire in shared mode. By default an exclusive lock
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              will be acquired.
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      blocking: whether to block while trying to acquire or to operate in try-lock mode.
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                this locking mode is not supported yet.
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    """
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    if not blocking:
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      # We don't have non-blocking mode for now
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      raise NotImplementedError
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    self.__lock.acquire()
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    try:
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      if self.__deleted:
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        raise errors.LockError('deleted lock')
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      # We cannot acquire the lock if we already have it
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      assert not self.__is_owned(), "double acquire() on a non-recursive lock"
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      if shared:
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        self.__nwait_shr += 1
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        try:
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          # If there is an exclusive holder waiting we have to wait.  We'll
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          # only do this once, though, when we start waiting for the lock. Then
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          # we'll just wait while there are no exclusive holders.
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          if self.__nwait_exc > 0:
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            # TODO: if !blocking...
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            self.__wait(self.__turn_shr)
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          while self.__exc is not None:
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            # TODO: if !blocking...
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            self.__wait(self.__turn_shr)
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          self.__shr.add(threading.currentThread())
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        finally:
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          self.__nwait_shr -= 1
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      else:
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        # TODO: if !blocking...
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        # (or modify __exclusive_acquire for non-blocking mode)
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        self.__exclusive_acquire()
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    finally:
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      self.__lock.release()
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    return True
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  def release(self):
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    """Release a Shared Lock.
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    You must have acquired the lock, either in shared or in exclusive mode,
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    before calling this function.
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    """
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    self.__lock.acquire()
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    try:
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      # Autodetect release type
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      if self.__is_exclusive():
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        self.__exc = None
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        # An exclusive holder has just had the lock, time to put it in shared
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        # mode if there are shared holders waiting. Otherwise wake up the next
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        # exclusive holder.
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        if self.__nwait_shr > 0:
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          self.__turn_shr.notifyAll()
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        elif self.__nwait_exc > 0:
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         self.__turn_exc.notify()
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      elif self.__is_sharer():
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        self.__shr.remove(threading.currentThread())
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        # If there are shared holders waiting there *must* be an exclusive holder
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        # waiting as well; otherwise what were they waiting for?
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        assert (self.__nwait_shr == 0 or self.__nwait_exc > 0,
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                "Lock sharers waiting while no exclusive is queueing")
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        # If there are no more shared holders and some exclusive holders are
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        # waiting let's wake one up.
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        if len(self.__shr) == 0 and self.__nwait_exc > 0:
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          self.__turn_exc.notify()
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      else:
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        assert False, "Cannot release non-owned lock"
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    finally:
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      self.__lock.release()
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  def delete(self, blocking=1):
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    """Delete a Shared Lock.
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    This operation will declare the lock for removal. First the lock will be
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    acquired in exclusive mode if you don't already own it, then the lock
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    will be put in a state where any future and pending acquire() fail.
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    Args:
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      blocking: whether to block while trying to acquire or to operate in
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                try-lock mode.  this locking mode is not supported yet unless
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                you are already holding exclusively the lock.
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    """
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    self.__lock.acquire()
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    try:
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      assert not self.__is_sharer(), "cannot delete() a lock while sharing it"
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      if self.__deleted:
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        raise errors.LockError('deleted lock')
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      if not self.__is_exclusive():
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        if not blocking:
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          # We don't have non-blocking mode for now
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          raise NotImplementedError
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        self.__exclusive_acquire()
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      self.__deleted = True
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      self.__exc = None
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      # Wake up everybody, they will fail acquiring the lock and
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      # raise an exception instead.
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      self.__turn_exc.notifyAll()
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      self.__turn_shr.notifyAll()
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    finally:
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      self.__lock.release()
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class LockSet:
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  """Implements a set of locks.
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  This abstraction implements a set of shared locks for the same resource type,
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  distinguished by name. The user can lock a subset of the resources and the
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  LockSet will take care of acquiring the locks always in the same order, thus
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  preventing deadlock.
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  All the locks needed in the same set must be acquired together, though.
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  """
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  def __init__(self, members=None):
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    """Constructs a new LockSet.
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    Args:
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      members: initial members of the set
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    """
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    # Used internally to guarantee coherency.
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    self.__lock = SharedLock()
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    # The lockdict indexes the relationship name -> lock
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    # The order-of-locking is implied by the alphabetical order of names
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    self.__lockdict = {}
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    if members is not None:
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      for name in members:
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        self.__lockdict[name] = SharedLock()
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    # The owner dict contains the set of locks each thread owns. For
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    # performance each thread can access its own key without a global lock on
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    # this structure. It is paramount though that *no* other type of access is
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    # done to this structure (eg. no looping over its keys). *_owner helper
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    # function are defined to guarantee access is correct, but in general never
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    # do anything different than __owners[threading.currentThread()], or there
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    # will be trouble.
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    self.__owners = {}
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  def _is_owned(self):
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    """Is the current thread a current level owner?"""
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    return threading.currentThread() in self.__owners
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  def _add_owned(self, name):
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    """Note the current thread owns the given lock"""
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    if self._is_owned():
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      self.__owners[threading.currentThread()].add(name)
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    else:
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       self.__owners[threading.currentThread()] = set([name])
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  def _del_owned(self, name):
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    """Note the current thread owns the given lock"""
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    self.__owners[threading.currentThread()].remove(name)
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    if not self.__owners[threading.currentThread()]:
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      del self.__owners[threading.currentThread()]
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  def _list_owned(self):
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    """Get the set of resource names owned by the current thread"""
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    if self._is_owned():
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      return self.__owners[threading.currentThread()].copy()
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    else:
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      return set()
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  def __names(self):
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    """Return the current set of names.
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    Only call this function while holding __lock and don't iterate on the
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    result after releasing the lock.
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    """
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    return set(self.__lockdict.keys())
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  def _names(self):
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    """Return a copy of the current set of elements.
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338
    Used only for debugging purposes.
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    """
340
    self.__lock.acquire(shared=1)
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    try:
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      result = self.__names()
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    finally:
344
      self.__lock.release()
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    return result
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  def acquire(self, names, blocking=1, shared=0):
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    """Acquire a set of resource locks.
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350
    Args:
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      names: the names of the locks which shall be acquired.
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             (special lock names, or instance/node names)
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      shared: whether to acquire in shared mode. By default an exclusive lock
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              will be acquired.
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      blocking: whether to block while trying to acquire or to operate in try-lock mode.
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                this locking mode is not supported yet.
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358
    Returns:
359
      True: when all the locks are successfully acquired
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361
    Raises:
362
      errors.LockError: when any lock we try to acquire has been deleted
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      before we succeed. In this case none of the locks requested will be
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      acquired.
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366
    """
367
    if not blocking:
368
      # We don't have non-blocking mode for now
369
      raise NotImplementedError
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371
    # Check we don't already own locks at this level
372
    assert not self._is_owned(), "Cannot acquire locks in the same set twice"
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374
    # Support passing in a single resource to acquire rather than many
375
    if isinstance(names, basestring):
376
      names = [names]
377
    else:
378
      names.sort()
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380
    acquire_list = []
381
    # First we look the locks up on __lockdict. We have no way of being sure
382
    # they will still be there after, but this makes it a lot faster should
383
    # just one of them be the already wrong
384
    try:
385
      for lname in names:
386
        lock = self.__lockdict[lname] # raises KeyError if the lock is not there
387
        acquire_list.append((lname, lock))
388
    except (KeyError):
389
      raise errors.LockError('non-existing lock in set (%s)' % lname)
390

    
391
    # Now acquire_list contains a sorted list of resources and locks we want.
392
    # In order to get them we loop on this (private) list and acquire() them.
393
    # We gave no real guarantee they will still exist till this is done but
394
    # .acquire() itself is safe and will alert us if the lock gets deleted.
395
    try:
396
      for (lname, lock) in acquire_list:
397
        lock.acquire(shared=shared) # raises LockError if the lock is deleted
398
        try:
399
          # now the lock cannot be deleted, we have it!
400
          self._add_owned(lname)
401
        except:
402
          # We shouldn't have problems adding the lock to the owners list, but
403
          # if we did we'll try to release this lock and re-raise exception.
404
          # Of course something is going to be really wrong, after this.
405
          lock.release()
406
          raise
407

    
408
    except (errors.LockError):
409
      name_fail = lname
410
      for lname in self._list_owned():
411
        self.__lockdict[lname].release()
412
        self._del_owned(lname)
413
      raise errors.LockError('non-existing lock in set (%s)' % name_fail)
414

    
415
    return True
416

    
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  def release(self, names=None):
418
    """Release a set of resource locks, at the same level.
419

420
    You must have acquired the locks, either in shared or in exclusive mode,
421
    before releasing them.
422

423
    Args:
424
      names: the names of the locks which shall be released.
425
             (defaults to all the locks acquired at that level).
426

427
    """
428

    
429
    assert self._is_owned(), "release() on lock set while not owner"
430

    
431
    # Support passing in a single resource to release rather than many
432
    if isinstance(names, basestring):
433
      names = [names]
434

    
435
    if names is None:
436
      names = self._list_owned()
437
    else:
438
      names = set(names)
439
      assert self._list_owned().issuperset(names), (
440
               "release() on unheld resources %s" %
441
               names.difference(self._list_owned()))
442

    
443
    for lockname in names:
444
      # If we are sure the lock doesn't leave __lockdict without being
445
      # exclusively held we can do this...
446
      self.__lockdict[lockname].release()
447
      self._del_owned(lockname)
448

    
449
  def add(self, names, acquired=0, shared=0):
450
    """Add a new set of elements to the set
451

452
    Args:
453
      names: names of the new elements to add
454
      acquired: pre-acquire the new resource?
455
      shared: is the pre-acquisition shared?
456

457
    """
458
    # Support passing in a single resource to add rather than many
459
    if isinstance(names, basestring):
460
      names = [names]
461

    
462
    # Acquire the internal lock in an exclusive way, so there cannot be a
463
    # conflicting add()
464
    self.__lock.acquire()
465
    try:
466
      invalid_names = self.__names().intersection(names)
467
      if invalid_names:
468
        # This must be an explicit raise, not an assert, because assert is
469
        # turned off when using optimization, and this can happen because of
470
        # concurrency even if the user doesn't want it.
471
        raise errors.LockError("duplicate add() (%s)" % invalid_names)
472

    
473
      for lockname in names:
474
        lock = SharedLock()
475

    
476
        if acquired:
477
          lock.acquire(shared=shared)
478
          # now the lock cannot be deleted, we have it!
479
          try:
480
            self._add_owned(lockname)
481
          except:
482
            # We shouldn't have problems adding the lock to the owners list,
483
            # but if we did we'll try to release this lock and re-raise
484
            # exception.  Of course something is going to be really wrong,
485
            # after this.  On the other hand the lock hasn't been added to the
486
            # __lockdict yet so no other threads should be pending on it. This
487
            # release is just a safety measure.
488
            lock.release()
489
            raise
490

    
491
        self.__lockdict[lockname] = lock
492

    
493
    finally:
494
      self.__lock.release()
495

    
496
    return True
497

    
498
  def remove(self, names, blocking=1):
499
    """Remove elements from the lock set.
500

501
    You can either not hold anything in the lockset or already hold a superset
502
    of the elements you want to delete, exclusively.
503

504
    Args:
505
      names: names of the resource to remove.
506
      blocking: whether to block while trying to acquire or to operate in
507
                try-lock mode.  this locking mode is not supported yet unless
508
                you are already holding exclusively the locks.
509

510
    Returns:
511
      A list of lock which we failed to delete. The list is always empty if we
512
      were holding all the locks exclusively.
513

514
    """
515
    if not blocking and not self._is_owned():
516
      # We don't have non-blocking mode for now
517
      raise NotImplementedError
518

    
519
    # Support passing in a single resource to remove rather than many
520
    if isinstance(names, basestring):
521
      names = [names]
522

    
523
    # If we own any subset of this lock it must be a superset of what we want
524
    # to delete. The ownership must also be exclusive, but that will be checked
525
    # by the lock itself.
526
    assert not self._is_owned() or self._list_owned().issuperset(names), (
527
      "remove() on acquired lockset while not owning all elements")
528

    
529
    delete_failed=[]
530

    
531
    for lname in names:
532
      # Calling delete() acquires the lock exclusively if we don't already own
533
      # it, and causes all pending and subsequent lock acquires to fail. It's
534
      # fine to call it out of order because delete() also implies release(),
535
      # and the assertion above guarantees that if we either already hold
536
      # everything we want to delete, or we hold none.
537
      try:
538
        self.__lockdict[lname].delete()
539
      except (KeyError, errors.LockError):
540
        delete_failed.append(lname)
541
        # This cannot happen if we were already holding it, verify:
542
        assert not self._is_owned(), "remove failed while holding lockset"
543
      else:
544
        # If no LockError was raised we are the ones who deleted the lock.
545
        # This means we can safely remove it from lockdict, as any further or
546
        # pending delete() or acquire() will fail (and nobody can have the lock
547
        # since before our call to delete()).
548
        #
549
        # This is done in an else clause because if the exception was thrown
550
        # it's the job of the one who actually deleted it.
551
        del self.__lockdict[lname]
552
        # And let's remove it from our private list if we owned it.
553
        if self._is_owned():
554
          self._del_owned(lname)
555

    
556
    return delete_failed
557