Makes leased cron jobs available for leasing again.

def ReturnLeasedCronJobs(self, jobs, cursor=None):
    """Makes leased cron jobs available for leasing again."""
    if not jobs:
      return

    unleased_jobs = []

    conditions = []
    args = []
    for job in jobs:
      if not job.leased_by or not job.leased_until:
        unleased_jobs.append(job)
        continue

      conditions.append("""
      (job_id = %s AND leased_until = FROM_UNIXTIME(%s) AND leased_by = %s)
      """)
      args += [
          job.cron_job_id,
          mysql_utils.RDFDatetimeToTimestamp(job.leased_until), job.leased_by
      ]

    if conditions:
      query = ("UPDATE cron_jobs "
               "SET leased_until=NULL, leased_by=NULL "
               "WHERE ") + " OR ".join(conditions)
      returned = cursor.execute(query, args)

    if unleased_jobs:
      raise ValueError("CronJobs to return are not leased: %s" % unleased_jobs)
    if returned != len(jobs):
      raise ValueError("%d cronjobs in %s could not be returned." %
                       ((len(jobs) - returned), jobs))