Forked from  Issue 845172 

Relevant code:

        job_ids = set([])
        <snip>

        if cls.FETCH_READONLY_JOBS:
            #TODO(jkop): Get rid of this kludge when we update Django to >=1.7
            #correct usage would be .raw(..., using='readonly')
            old_db = Job.objects._db
            try:
                Job.objects._db = 'readonly'
                job_ids |= set([j.id for j in Job.objects.raw(raw_sql)])
            finally:
                Job.objects._db = old_db
        if not job_ids:
            #If the replica is down or we're in a test, fetch from master.
            job_ids |= set([j.id for j in Job.objects.raw(raw_sql)])

The master is consulted if the first query returns no results. However, it is not consulted if the first queries times out or fails and throws an exception, which is what I believe was happening here.

I don't think this is the intended before for two reasons:
 1) It doesn't failover on the common case of the slave being unavailable or very slow.
 2) It might failover incorrectly in the case where the jobs_ids set is truly empty.