Assert error in getdns_result(): Condition((rr) != 0) not true.

[delthas]

Stressing Varnish + libvmod-dynamic with many dynamic backends (with probes), I'm sometimes getting the following panic:

Assert error in getdns_result(), dyn_resolver_getdns.c line 259:
  Condition((rr) != 0) not true.
Backtrace:
  0x55bf1d92da92: /usr/sbin/varnishd(+0x5ba92) [0x55bf1d92da92]
  0x55bf1d9aac45: /usr/sbin/varnishd(VAS_Fail+0x45) [0x55bf1d9aac45]
  0x7f845e54e5ec: ./vmod_cache/_vmod_dynamic.29b766e616262ab0ef9252419367ce48fbf642168ba34fc382b1742a77dac5c4(+0xf5ec) [0x7f845e54e5ec]
  0x7f845e544b65: ./vmod_cache/_vmod_dynamic.29b766e616262ab0ef9252419367ce48fbf642168ba34fc382b1742a77dac5c4(+0x5b65) [0x7f845e544b65]
  0x7f84731cd064: /lib/x86_64-linux-gnu/libpthread.so.0(+0x8064) [0x7f84731cd064]
  0x7f8472f0262d: /lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7f8472f0262d]

This refers to the assert here: https://github.com/nigoroll/libvmod-dynamic/blob/master/src/dyn_resolver_getdns.c#L259

I can't reproduce this on my machine to debug locally, I only got this on a remote server. I think the DNS server there might be having issues; but if this is caused by some DNS server issue we should probably not panic (and instead just add no backend).

From my understanding of the function, this can happen if getdns_common_more_answers(state) != 0 from the beginning (or if getdns_common_more_answers(state) == 0 and getdns_list_get_dict(state->answers, state->answer++, &rr) gives a NULL rr, not sure if that's relevant).

[nigoroll]

Nice find, thank you @delthas !