text: #
  # (C) Copyright 2005-2010 Jacek Konieczny <jajcus@jajcus.net>
  #
  # This program is free software; you can redistribute it and/or modify
  # it under the terms of the GNU Lesser General Public License Version
  # 2.1 as published by the Free Software Foundation.
  #
  # 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 Lesser General Public License for more details.
  #
  # You should have received a copy of the GNU Lesser General Public
  # License along with this program; if not, write to the Free Software
  # Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  #
  
  """Caching proxy for Jabber/XMPP objects.
  
  This package provides facilities to retrieve and transparently cache
  cachable objects like Service Discovery responses or e.g. client version
  informations."""
  
  __revision__ = "$Id: cache.py 714 2010-04-05 10:20:10Z jajcus $"
  __docformat__ = "restructuredtext en"
  
  import threading
  from datetime import datetime, timedelta
  
  _state_values = {
          'new': 0,
          'fresh': 1,
          'old': 2,
          'stale': 3,
          'purged': 4
      };
  
  # locking order (anti-deadlock):
  # CacheSuite, Cache, CacheHandler, CacheItem
  
  class CacheItem(object):
      """An item in a cache.
  
      :Ivariables:
          - `value`: item value (cached object).
          - `address`: item address.
          - `state`: current state.
          - `state_value`: numerical value of the current state (lower number means
            fresher item).
          - `timestamp`: time when the object was created.
          - `freshness_time`: time when the object stops being fresh.
          - `expire_time`: time when the object expires.
          - `purge_time`: time when the object should be purged. When 0 then
            item will never be automaticaly purged.
          - `_lock`: lock for thread safety.
      :Types:
          - `value`: `instance`
          - `address`: any hashable
          - `state`: `str`
          - `state_value`: `int`
          - `timestamp`: `datetime`
          - `freshness_time`: `datetime`
          - `expire_time`: `datetime`
          - `purge_time`: `datetime`
          - `_lock`: `threading.RLock`"""
      __slots__ = ['value', 'address', 'state', 'timestamp', 'freshness_time',
              'expire_time', 'purge_time', 'state_value', '_lock']
      def __init__(self, address, value, freshness_period, expiration_period,
              purge_period, state = "new"):
          """Initialize an CacheItem object.
  
          :Parameters:
              - `address`: item address.
              - `value`: item value (cached object).
              - `freshness_period`: time interval after which the object stops being fresh.
              - `expiration_period`: time interval after which the object expires.
              - `purge_period`: time interval after which the object should be purged. When 0 then
                item will never be automaticaly purged.
              - `state`: initial state.
          :Types:
              - `address`: any hashable
              - `value`: `instance`
              - `freshness_period`: `timedelta`
              - `expiration_period`: `timedelta`
              - `purge_period`: `timedelta`
              - `state`: `str`"""
          if freshness_period>expiration_period:
              raise ValueError, "freshness_period greater then expiration_period"
          if expiration_period>purge_period:
              raise ValueError, "expiration_period greater then purge_period"
          self.address = address
          self.value = value
          now = datetime.utcnow()
          self.timestamp = now
          self.freshness_time = now+freshness_period
          self.expire_time = now+expiration_period
          if purge_period:
              self.purge_time = now+purge_period
          else:
              self.purge_time = datetime.max
          self.state = state
          self.state_value = _state_values[state]
          self._lock = threading.RLock()
  
      def update_state(self):
          """Update current status of the item and compute time of the next
          state change.
  
          :return: the new state.
          :returntype: `datetime`"""
          self._lock.acquire()
          try:
              now = datetime.utcnow()
              if self.state == 'new':
                  self.state = 'fresh'
              if self.state == 'fresh':
                  if now > self.freshness_time:
                      self.state = 'old'
              if self.state == 'old':
                  if now > self.expire_time:
                      self.state = 'stale'
              if self.state == 'stale':
                  if now > self.purge_time:
                      self.state = 'purged'
              self.state_value = _state_values[self.state]
              return self.state
          finally:
              self._lock.release()
  
      def __cmp__(self,other):
          try:
              return cmp(
                      (-self.state_value, self.timestamp, id(self)),
                      (-other.state_value, other.timestamp, id(other))
                  )
          except AttributeError:
              return cmp(id(self),id(other))
  
  _hour = timedelta(hours = 1)
  
  class CacheFetcher:
      """Base class for cache object fetchers -- classes responsible for
      retrieving objects from network.
  
      An instance of a fetcher class is created for each object requested and
      not found in the cache, then `fetch` method is called to initialize
      the asynchronous retrieval process. Fetcher object's `got_it` method
      should be called on a successfull retrieval and `error` otherwise.
      `timeout` will be called when the request timeouts.
  
      :Ivariables:
          - `cache`: cache object which created this fetcher.
          - `address`: requested item address.
          - `timeout_time`: timeout time.
          - `active`: `True` as long as the fetcher is active and requestor
            expects one of the handlers to be called.
      :Types:
          - `cache`: `Cache`
          - `address`: any hashable
          - `timeout_time`: `datetime`
          - `active`: `bool`
      """
      def __init__(self, cache, address,
              item_freshness_period, item_expiration_period, item_purge_period,
              object_handler, error_handler, timeout_handler, timeout_period,
              backup_state = None):
          """Initialize an `CacheFetcher` object.
  
          :Parameters:
              - `cache`: cache object which created this fetcher.
              - `address`: requested item address.
              - `item_freshness_period`: freshness period for the requested item.
              - `item_expiration_period`: expiration period for the requested item.
              - `item_purge_period`: purge period for the requested item.
              - `object_handler`: function to be called after the item is fetched.
              - `error_handler`: function to be called on error.
              - `timeout_handler`: function to be called on timeout
              - `timeout_period`: timeout interval.
              - `backup_state`: when not `None` and the fetch fails than an
                object from cache of at least this state will be passed to the
                `object_handler`. If such object is not available, then
                `error_handler` is called.
          :Types:
              - `cache`: `Cache`
              - `address`: any hashable
              - `item_freshness_period`: `timedelta`
              - `item_expiration_period`: `timedelta`
              - `item_purge_period`: `timedelta`
              - `object_handler`: callable(address, value, state)
              - `error_handler`: callable(address, error_data)
              - `timeout_handler`: callable(address)
              - `timeout_period`: `timedelta`
              - `backup_state`: `bool`"""
          self.cache = cache
          self.address = address
          self._item_freshness_period = item_freshness_period
          self._item_expiration_period = item_expiration_period
          self._item_purge_period = item_purge_period
          self._object_handler = object_handler
          self._error_handler = error_handler
          self._timeout_handler = timeout_handler
          if timeout_period:
              self.timeout_time = datetime.utcnow()+timeout_period
          else:
              self.timeout_time = datetime.max
          self._backup_state = backup_state
          self.active = True
  
      def _deactivate(self):
          """Remove the fetcher from cache and mark it not active."""
          self.cache.remove_fetcher(self)
          if self.active:
              self._deactivated()
  
      def _deactivated(self):
          """Mark the fetcher inactive after it is removed from the cache."""
          self.active = False
  
      def fetch(self):
          """Start the retrieval process.
  
          This method must be implemented in any fetcher class."""
          raise RuntimeError, "Pure virtual method called"
  
      def got_it(self, value, state = "new"):
          """Handle a successfull retrieval and call apriopriate handler.
  
          Should be called when retrieval succeeds.
  
          Do nothing when the fetcher is not active any more (after
          one of handlers was already called).
  
          :Parameters:
              - `value`: fetched object.
              - `state`: initial state of the object.
          :Types:
              - `value`: any
              - `state`: `str`"""
          if not self.active:
              return
          item = CacheItem(self.address, value, self._item_freshness_period,
                  self._item_expiration_period, self._item_purge_period, state)
          self._object_handler(item.address, item.value, item.state)
          self.cache.add_item(item)
          self._deactivate()
  
      def error(self, error_data):
          """Handle a retrieval error and call apriopriate handler.
  
          Should be called when retrieval fails.
  
          Do nothing when the fetcher is not active any more (after
          one of handlers was already called).
  
          :Parameters:
              - `error_data`: additional information about the error (e.g. `StanzaError` instance).
          :Types:
              - `error_data`: fetcher dependant
          """
          if not self.active:
              return
          if not self._try_backup_item():
              self._error_handler(self.address, error_data)
          self.cache.invalidate_object(self.address)
          self._deactivate()
  
      def timeout(self):
          """Handle fetcher timeout and call apriopriate handler.
  
          Is called by the cache object and should _not_ be called by fetcher or
          application.
  
          Do nothing when the fetcher is not active any more (after
          one of handlers was already called)."""
          if not self.active:
              return
          if not self._try_backup_item():
              if self._timeout_handler:
                  self._timeout_handler(self.address)
              else:
                  self._error_handler(self.address, None)
          self.cache.invalidate_object(self.address)
          self._deactivate()
  
      def _try_backup_item(self):
          """Check if a backup item is available in cache and call
          the item handler if it is.
  
          :return: `True` if backup item was found.
          :returntype: `bool`"""
          if not self._backup_state:
              return False
          item = self.cache.get_item(self.address, self._backup_state)
          if item:
              self._object_handler(item.address, item.value, item.state)
              return True
          else:
              False
  
  class Cache:
      """Caching proxy for object retrieval and caching.
  
      Object factories ("fetchers") are registered in the `Cache` object and used
      to e.g. retrieve requested objects from network.  They are called only when
      the requested object is not in the cache or is not fresh enough.
  
      A state (freshness level) name may be provided when requesting an object.
      When the cached item state is "less fresh" then requested, then new object
      will be retrieved.
  
      Following states are defined:
  
        - 'new': always a new object should be retrieved.
        - 'fresh': a fresh object (not older than freshness time)
        - 'old': object not fresh, but most probably still valid.
        - 'stale': object known to be expired.
  
      :Ivariables:
          - `default_freshness_period`: default freshness period (in seconds).
          - `default_expiration_period`: default expiration period (in seconds).
          - `default_purge_period`: default purge period (in seconds). When
            0 then items are never purged because of their age.
          - `max_items`: maximum number of items to store.
          - `_items`: dictionary of stored items.
          - `_items_list`: list of stored items with the most suitable for
            purging first.
          - `_fetcher`: fetcher class for this cache.
          - `_active_fetchers`: list of active fetchers sorted by the time of
            its expiration time.
          - `_lock`: lock for thread safety.
      :Types:
          - `default_freshness_period`: timedelta
          - `default_expiration_period`: timedelta
          - `default_purge_period`: timedelta
          - `max_items`: `int`
          - `_items`: `dict` of (`classobj`, addr) -> `CacheItem`
          - `_items_list`: `list` of (`int`, `datetime`, `CacheItem`)
          - `_fetcher`: `CacheFetcher` based class
          - `_active_fetchers`: `list` of (`int`, `CacheFetcher`)
          - `_lock`: `threading.RLock`
      """
      def __init__(self, max_items, default_freshness_period = _hour,
              default_expiration_period = 12*_hour, default_purge_period = 24*_hour):
          """Initialize a `Cache` object.
  
              :Parameters:
                  - `default_freshness_period`: default freshness period (in seconds).
                  - `default_expiration_period`: default expiration period (in seconds).
                  - `default_purge_period`: default purge period (in seconds). When
                    0 then items are never purged because of their age.
                  - `max_items`: maximum number of items to store.
              :Types:
                  - `default_freshness_period`: number
                  - `default_expiration_period`: number
                  - `default_purge_period`: number
                  - `max_items`: number
          """
          self.default_freshness_period = default_freshness_period
          self.default_expiration_period = default_expiration_period
          self.default_purge_period = default_purge_period
          self.max_items = max_items
          self._items = {}
          self._items_list = []
          self._fetcher = None
          self._active_fetchers = []
          self._purged = 0
          self._lock = threading.RLock()
  
      def request_object(self, address, state, object_handler,
              error_handler = None, timeout_handler = None,
              backup_state = None, timeout = timedelta(minutes=60),
              freshness_period = None, expiration_period = None,
              purge_period = None):
          """Request an object with given address and state not worse than
          `state`. The object will be taken from cache if available, and
          created/fetched otherwise. The request is asynchronous -- this metod
          doesn't return the object directly, but the `object_handler` is called
          as soon as the object is available (this may be before `request_object`
          returns and may happen in other thread). On error the `error_handler`
          will be called, and on timeout -- the `timeout_handler`.
  
          :Parameters:
              - `address`: address of the object requested.
              - `state`: the worst acceptable object state. When 'new' then always
                a new object will be created/fetched. 'stale' will select any
                item available in cache.
              - `object_handler`: function to be called when object is available.
                It will be called with the following arguments: address, object
                and its state.
              - `error_handler`: function to be called on object retrieval error.
                It will be called with two arguments: requested address and
                additional error information (fetcher-specific, may be
                StanzaError for XMPP objects).  If not given, then the object
                handler will be called with object set to `None` and state
                "error".
              - `timeout_handler`: function to be called on object retrieval
                timeout.  It will be called with only one argument: the requested
                address. If not given, then the `error_handler` will be called
                instead, with error details set to `None`.
              - `backup_state`: when set and object in state `state` is not
                available in the cache and object retrieval failed then object
                with this state will also be looked-up in the cache and provided
                if available.
              - `timeout`: time interval after which retrieval of the object
                should be given up.
              - `freshness_period`: time interval after which the item created
                should become 'old'.
              - `expiration_period`: time interval after which the item created
                should become 'stale'.
              - `purge_period`: time interval after which the item created
                shuld be removed from the cache.
          :Types:
              - `address`: any hashable
              - `state`: "new", "fresh", "old" or "stale"
              - `object_handler`: callable(address, value, state)
              - `error_handler`: callable(address, error_data)
              - `timeout_handler`: callable(address)
              - `backup_state`: "new", "fresh", "old" or "stale"
              - `timeout`: `timedelta`
              - `freshness_period`: `timedelta`
              - `expiration_period`: `timedelta`
              - `purge_period`: `timedelta`
          """
          self._lock.acquire()
          try:
              if state == 'stale':
                  state = 'purged'
              item = self.get_item(address, state)
              if item:
                  object_handler(item.address, item.value, item.state)
                  return
              if not self._fetcher:
                  raise TypeError, "No cache fetcher defined"
              if not error_handler:
                  def default_error_handler(address, _unused):
                      "Default error handler."
                      return object_handler(address, None, 'error')
                  error_handler = default_error_handler
              if not timeout_handler:
                  def default_timeout_handler(address):
                      "Default timeout handler."
                      return error_handler(address, None)
                  timeout_handler = default_timeout_handler
              if freshness_period is None:
                  freshness_period = self.default_freshness_period
              if expiration_period is None:
                  expiration_period = self.default_expiration_period
              if purge_period is None:
                  purge_period = self.default_purge_period
  
              fetcher = self._fetcher(self, address, freshness_period,
                      expiration_period, purge_period, object_handler, error_handler,
                      timeout_handler, timeout, backup_state)
              fetcher.fetch()
              self._active_fetchers.append((fetcher.timeout_time,fetcher))
              self._active_fetchers.sort()
          finally:
              self._lock.release()
  
      def invalidate_object(self, address, state = 'stale'):
          """Force cache item state change (to 'worse' state only).
  
          :Parameters:
              - `state`: the new state requested.
          :Types:
              - `state`: `str`"""
          self._lock.acquire()
          try:
              item = self.get_item(address)
              if item and item.state_value<_state_values[state]:
                  item.state=state
                  item.update_state()
                  self._items_list.sort()
          finally:
              self._lock.release()
  
      def add_item(self, item):
          """Add an item to the cache.
  
          Item state is updated before adding it (it will not be 'new' any more).
  
          :Parameters:
              - `item`: the item to add.
          :Types:
              - `item`: `CacheItem`
  
          :return: state of the item after addition.
          :returntype: `str`
          """
          self._lock.acquire()
          try:
              state = item.update_state()
              if state != 'purged':
                  if len(self._items_list) >= self.max_items:
                      self.purge_items()
                  self._items[item.address] = item
                  self._items_list.append(item)
                  self._items_list.sort()
              return item.state
          finally:
              self._lock.release()
  
      def get_item(self, address, state = 'fresh'):
          """Get an item from the cache.
  
          :Parameters:
              - `address`: its address.
              - `state`: the worst state that is acceptable.
          :Types:
              - `address`: any hashable
              - `state`: `str`
  
          :return: the item or `None` if it was not found.
          :returntype: `CacheItem`"""
          self._lock.acquire()
          try:
              item = self._items.get(address)
              if not item:
                  return None
              self.update_item(item)
              if _state_values[state] >= item.state_value:
                  return item
              return None
          finally:
              self._lock.release()
  
      def update_item(self, item):
          """Update state of an item in the cache.
  
          Update item's state and remove the item from the cache
          if its new state is 'purged'
  
          :Parameters:
              - `item`: item to update.
          :Types:
              - `item`: `CacheItem`
  
          :return: new state of the item.
          :returntype: `str`"""
  
          self._lock.acquire()
          try:
              state = item.update_state()
              self._items_list.sort()
              if item.state == 'purged':
                  self._purged += 1
                  if self._purged > 0.25*self.max_items:
                      self.purge_items()
              return state
          finally:
              self._lock.release()
  
      def num_items(self):
          """Get the number of items in the cache.
  
          :return: number of items.
          :returntype: `int`"""
          return len(self._items_list)
  
      def purge_items(self):
          """Remove purged and overlimit items from the cache.
  
          TODO: optimize somehow.
  
          Leave no more than 75% of `self.max_items` items in the cache."""
          self._lock.acquire()
          try:
              il=self._items_list
              num_items = len(il)
              need_remove = num_items - int(0.75 * self.max_items)
  
              for _unused in range(need_remove):
                  item=il.pop(0)
                  try:
                      del self._items[item.address]
                  except KeyError:
                      pass
  
              while il and il[0].update_state()=="purged":
                  item=il.pop(0)
                  try:
                      del self._items[item.address]
                  except KeyError:
                      pass
          finally:
              self._lock.release()
  
      def tick(self):
          """Do the regular cache maintenance.
  
          Must be called from time to time for timeouts and cache old items
          purging to work."""
          self._lock.acquire()
          try:
              now = datetime.utcnow()
              for t,f in list(self._active_fetchers):
                  if t > now:
                      break
                  f.timeout()
              self.purge_items()
          finally:
              self._lock.release()
  
      def remove_fetcher(self, fetcher):
          """Remove a running fetcher from the list of active fetchers.
  
          :Parameters:
              - `fetcher`: fetcher instance.
          :Types:
              - `fetcher`: `CacheFetcher`"""
          self._lock.acquire()
          try:
              for t, f in list(self._active_fetchers):
                  if f is fetcher:
                      self._active_fetchers.remove((t, f))
                      f._deactivated()
                      return
          finally:
              self._lock.release()
  
      def set_fetcher(self, fetcher_class):
          """Set the fetcher class.
  
          :Parameters:
              - `fetcher_class`: the fetcher class.
          :Types:
              - `fetcher_class`: `CacheFetcher` based class
          """
          self._lock.acquire()
          try:
              self._fetcher = fetcher_class
          finally:
              self._lock.release()
  
  class CacheSuite:
      """Caching proxy for object retrieval and caching.
  
      Object factories for other classes are registered in the
      `Cache` object and used to e.g. retrieve requested objects from network.
      They are called only when the requested object is not in the cache
      or is not fresh enough.
  
      Objects are addressed using their class and a class dependant address.
      Eg. `pyxmpp.jabber.disco.DiscoInfo` objects are addressed using
      (`pyxmpp.jabber.disco.DiscoInfo`,(jid, node)) tuple.
  
      Additionaly a state (freshness level) name may be provided when requesting
      an object. When the cached item state is "less fresh" then requested, then
      new object will be retrieved.
  
      Following states are defined:
  
        - 'new': always a new object should be retrieved.
        - 'fresh': a fresh object (not older than freshness time)
        - 'old': object not fresh, but most probably still valid.
        - 'stale': object known to be expired.
  
      :Ivariables:
          - `default_freshness_period`: default freshness period (in seconds).
          - `default_expiration_period`: default expiration period (in seconds).
          - `default_purge_period`: default purge period (in seconds). When
            0 then items are never purged because of their age.
          - `max_items`: maximum number of obejects of one class to store.
          - `_caches`: dictionary of per-class caches.
          - `_lock`: lock for thread safety.
      :Types:
          - `default_freshness_period`: timedelta
          - `default_expiration_period`: timedelta
          - `default_purge_period`: timedelta
          - `max_items`: `int`
          - `_caches`: `dict` of (`classobj`, addr) -> `Cache`
          - `_lock`: `threading.RLock`
      """
      def __init__(self, max_items, default_freshness_period = _hour,
              default_expiration_period = 12*_hour, default_purge_period = 24*_hour):
          """Initialize a `Cache` object.
  
              :Parameters:
                  - `default_freshness_period`: default freshness period (in seconds).
                  - `default_expiration_period`: default expiration period (in seconds).
                  - `default_purge_period`: default purge period (in seconds). When
                    0 then items are never purged because of their age.
                  - `max_items`: maximum number of items to store.
              :Types:
                  - `default_freshness_period`: number
                  - `default_expiration_period`: number
                  - `default_purge_period`: number
                  - `max_items`: number
          """
          self.default_freshness_period = default_freshness_period
          self.default_expiration_period = default_expiration_period
          self.default_purge_period = default_purge_period
          self.max_items = max_items
          self._caches = {}
          self._lock = threading.RLock()
  
      def request_object(self, object_class, address, state, object_handler,
              error_handler = None, timeout_handler = None,
              backup_state = None, timeout = None,
              freshness_period = None, expiration_period = None, purge_period = None):
          """Request an object of given class, with given address and state not
          worse than `state`. The object will be taken from cache if available,
          and created/fetched otherwise. The request is asynchronous -- this
          metod doesn't return the object directly, but the `object_handler` is
          called as soon as the object is available (this may be before
          `request_object` returns and may happen in other thread). On error the
          `error_handler` will be called, and on timeout -- the
          `timeout_handler`.
  
          :Parameters:
              - `object_class`: class (type) of the object requested.
              - `address`: address of the object requested.
              - `state`: the worst acceptable object state. When 'new' then always
                a new object will be created/fetched. 'stale' will select any
                item available in cache.
              - `object_handler`: function to be called when object is available.
                It will be called with the following arguments: address, object
                and its state.
              - `error_handler`: function to be called on object retrieval error.
                It will be called with two arguments: requested address and
                additional error information (fetcher-specific, may be
                StanzaError for XMPP objects).  If not given, then the object
                handler will be called with object set to `None` and state
                "error".
              - `timeout_handler`: function to be called on object retrieval
                timeout.  It will be called with only one argument: the requested
                address. If not given, then the `error_handler` will be called
                instead, with error details set to `None`.
              - `backup_state`: when set and object in state `state` is not
                available in the cache and object retrieval failed then object
                with this state will also be looked-up in the cache and provided
                if available.
              - `timeout`: time interval after which retrieval of the object
                should be given up.
              - `freshness_period`: time interval after which the item created
                should become 'old'.
              - `expiration_period`: time interval after which the item created
                should become 'stale'.
              - `purge_period`: time interval after which the item created
                shuld be removed from the cache.
          :Types:
              - `object_class`: `classobj`
              - `address`: any hashable
              - `state`: "new", "fresh", "old" or "stale"
              - `object_handler`: callable(address, value, state)
              - `error_handler`: callable(address, error_data)
              - `timeout_handler`: callable(address)
              - `backup_state`: "new", "fresh", "old" or "stale"
              - `timeout`: `timedelta`
              - `freshness_period`: `timedelta`
              - `expiration_period`: `timedelta`
              - `purge_period`: `timedelta`
          """
  
          self._lock.acquire()
          try:
              if object_class not in self._caches:
                  raise TypeError, "No cache for %r" % (object_class,)
  
              self._caches[object_class].request_object(address, state, object_handler,
                      error_handler, timeout_handler, backup_state, timeout,
                      freshness_period, expiration_period, purge_period)
          finally:
              self._lock.release()
  
      def tick(self):
          """Do the regular cache maintenance.
  
          Must be called from time to time for timeouts and cache old items
          purging to work."""
          self._lock.acquire()
          try:
              for cache in self._caches.values():
                  cache.tick()
          finally:
              self._lock.release()
  
      def register_fetcher(self, object_class, fetcher_class):
          """Register a fetcher class for an object class.
  
          :Parameters:
              - `object_class`: class to be retrieved by the fetcher.
              - `fetcher_class`: the fetcher class.
          :Types:
              - `object_class`: `classobj`
              - `fetcher_class`: `CacheFetcher` based class
          """
          self._lock.acquire()
          try:
              cache = self._caches.get(object_class)
              if not cache:
                  cache = Cache(self.max_items, self.default_freshness_period,
                          self.default_expiration_period, self.default_purge_period)
                  self._caches[object_class] = cache
              cache.set_fetcher(fetcher_class)
          finally:
              self._lock.release()
  
      def unregister_fetcher(self, object_class):
          """Unregister a fetcher class for an object class.
  
          :Parameters:
              - `object_class`: class retrieved by the fetcher.
          :Types:
              - `object_class`: `classobj`
          """
          self._lock.acquire()
          try:
              cache = self._caches.get(object_class)
              if not cache:
                  return
              cache.set_fetcher(None)
          finally:
              self._lock.release()
  
  # vi: sts=4 et sw=4
  