Публикации с меткой «memcache»

import os, logging
from time import time

logger = logging.getLogger(__name__)

class LockError(Exception):
    def __str__(self):
        return 'Problems with object lock'

class LockedByOther(LockError):
    def __init__(self, user):
        LockError.__init__(self, user)
        self.user = user
    def __str__(self):
        return 'Object is already locked by user: %s (%s)' % \
                                (self.user.name, self.user.login)

class LockIsLost(LockError):
    def __str__(self):
        return 'The object lock is lost'

def create_lock(obj_key, user, force=False):
    '''Marks model object as editted. Returns edit session on success or
    raises exception. obj_key is global identifier of object. When force is
    True the current lock is ignored.'''
    CACHE.clear_cas()
    edit_session = os.urandom(5).encode('hex')
    value = dict(edit_session=edit_session,
                 user_id=user.id,
                 time=time())
    if force:
        if CACHE.set(obj_key, value, time=MODEL_LOCK_TIMEOUT):
            return edit_session
        else:
            raise LockError()
    for i in range(3):
        if CACHE.add(obj_key, value, time=MODEL_LOCK_TIMEOUT):
            return edit_session
        old_value = CACHE.gets(obj_key)
        if old_value is None:
            # Should try add() again
            continue
        if not old_value or time()-old_value['time'] > MODEL_LOCK_TIMEOUT:
            # Somebody's lock is already expired
            if CACHE.cas(obj_key, value, time=MODEL_LOCK_TIMEOUT):
                return edit_session
            else:
                continue
        # Somebody holds active lock, no farther attempts
        break
    else:
        logger.error('Failed to lock model object. Problem with memcached?')
        raise LockError()
    lock_user = get_user(id=old_value['user_id'])
    assert lock_user is not None
    raise LockedByOther(lock_user)

def update_lock(obj_key, user, edit_session):
    '''Updates model object lock as being active. Raises exception on
    error. obj_key is global identifier of object.'''
    CACHE.clear_cas()
    for i in range(3):
        old_value = CACHE.gets(obj_key)
        if not old_value:
            raise LockIsLost()
        elif old_value['edit_session']!=edit_session:
            lock_user = get_user(id=old_value['user_id'])
            assert lock_user is not None
            raise LockedByOther(lock_user)
        new_value = dict(edit_session=edit_session,
                         user_id=user.id,
                         time=time())
        if CACHE.cas(obj_key, new_value, time=MODEL_LOCK_TIMEOUT):
            return
    else:
        # No runtime error here since we want to give user a chance to
        # restore lock.
        raise LockIsLost()

def remove_lock(obj_key, edit_session):
    '''Removes lock for model object. obj_key is global identifier of
    object.'''
    CACHE.clear_cas()
    # We can't garuantee memcache's delete method will remove only our
    # lock, so we update the record with empty value and minimal (1 sec)
    # timeout.
    old_value = CACHE.gets(obj_key)
    # It's too late to do something in case of error, so we just ignore
    # returned value.
    if old_value and old_value['edit_session']==edit_session:
        CACHE.cas(obj_key, '', time=1)