Quota reservation not rolled back if security group create fails

In SecurityGroupAPI.create() we do the following (simplified):

        reservations = QUOTAS.reserve(context, security_groups=1)

        if self.db.security_group_exists(context, context.project_id, name):
            msg = _('Security group %s already exists') % name
            self.raise_group_already_exists(msg)

 try:
            self.db.security_group_create(context, group)
            QUOTAS.commit(context, reservations)
 except Exception:
            with excutils.save_and_reraise_exception():
                QUOTAS.rollback(context, reservations)

the problem is obvious, if you do:

  $> nova secgroup-create foo bar
  $> nova secgroup-create foo bar

the second one fails with:

  ERROR: Security group foo already exists (HTTP 400)

if you keep repeating it you get:

  ERROR: SecurityGroupLimitExceeded: Quota exceeded, too many security groups.

and, by default, you have to wait a day for the leaked reservations to expire.