Synnefo

# Copyright 2012 GRNET S.A. All rights reserved.

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from django.db import connections

from django.db.models import Manager

from django.db.models.query import QuerySet

class ForUpdateManager(Manager):

    """ Model manager implementing SELECT .. FOR UPDATE statement

        This manager implements select_for_update() method in order to use

        row-level locking in the database and guarantee exclusive access, since

        this method is only implemented in Django>=1.4.

        Non-blocking reads are not implemented, and each query including a row

        that is locked by another transaction will block until the lock is

        released. Also care must be taken in order to avoid deadlocks or retry

        transactions that abort due to deadlocks.

        Example:

            networks = Network.objects.select_for_update().filter(public=True)

    """

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

        super(ForUpdateManager, self).__init__(*args, **kwargs)

        self._select_for_update = False

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

        query = self.get_query_set().filter(*args, **kwargs)

        if self._select_for_update:

            self._select_for_update = False

            return for_update(query)

        else:

            return query

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

        if not self._select_for_update:

            return self.get_query_set().get(*args, **kwargs)

        query = self.filter(*args, **kwargs)

        query = list(query)

        num = len(query)

        if num == 1:

            return query[0]

        if not num:

            raise self.model.DoesNotExist(

                    "%s matching query does not exist. "

                    "Lookup parameters were %s" %

                    (self.model._meta.object_name, kwargs))

        raise self.model.MultipleObjectsReturned(

            "get() returned more than one %s -- it returned %s! "

            "Lookup parameters were %s" %

            (self.model._meta.object_name, num, kwargs))

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

        self._select_for_update = True

        return self

def for_update(query):

    """ Rewrite query using SELECT .. FOR UPDATE.

    """

    if 'sqlite' in connections[query.db].settings_dict['ENGINE'].lower():

        # SQLite  does not support FOR UPDATE

        return query

    sql, params = query.query.get_compiler(query.db).as_sql()

    return query.model._default_manager.raw(sql.rstrip() + ' FOR UPDATE',

                                            params)

class ProtectedDeleteManager(ForUpdateManager):

    """ Manager for protecting Backend deletion.

        Call Backend delete() method in order to prevent deletion

        of Backends that host non-deleted VirtualMachines.

    """

    def get_query_set(self):

        return BackendQuerySet(self.model, using=self._db)

class BackendQuerySet(QuerySet):

    def delete(self):

        for backend in self._clone():

            backend.delete()