Babel: Possible segfault in bird unfeasible update handling code

[Daniel Gröber]

Hi Babelers,

I've been working on the babel proto in bird and found some code where I
can't convince myself it won't segfault. The problematic bit is, I think,
's' in babel_handle_update can be NULL because nothing ensures the
babel_source for a particular neighbour actually exists here:

      /* Regular update */
      [...]
      s = babel_find_source(e, msg->router_id); /* for feasibility */
      [...]

      /* RFC section 3.8.2.2 - Dealing with unfeasible updates */
      if (!feasible && (metric != BABEL_INFINITY) &&
          (!best || (r == best) || (metric < best->metric)))
        babel_add_seqno_request(p, e, s->router_id, s->seqno + 1, 0, nbr);
        //^ BUG: Can 's' be NULL here?

The only place that allocates sources is babel_send_update_ which just
happens at it's own pace and has nothing to do with incoming update
handling AFAICT.

Am I missing something or is this a bug? Perhaps find should just be
replaced by babel_get_source here?

--Daniel