deadlock hangs named

Ubuntu bug: https://bugs.launchpad.net/maas/+bug/1710278

quick summary:

deadlock in bind9 code; thread A runs ns_client_endrequest->dns_view_detach->view_flushanddetach, which includes:

  LOCK(&view->lock);
  if (!RESSHUTDOWN(view))
    dns_resolver_shutdown(view->resolver);

inside dns_resolver_shutdown, for each resolver bucket, it does:

  for (i = 0; i < res->nbuckets; i++) {
    LOCK(&res->buckets[i].lock);

at this point, one of the bucket locks is held, and thread A is blocked holding view->lock, but waiting for the view->resolver->bucket[i].lock.

meanwhile, thread B runs dispatch->validated, and does:

  bucketnum = fctx->bucketnum;
  LOCK(&res->buckets[bucketnum].lock);

then while still holding that lock calls dns_validator_destroy->destroy->dns_view_weakdetach

which does:

  LOCK(&view->lock);

leaving thread A and thread B in a deadlock, with thread A waiting for the buckets[bucketnum].lock that thread B holds, and thread B waiting for the view->lock that thread A holds.