res_query() uses global state in the form of the struct __res_state
which contains the contents of resolv.conf (and other things). On Linux,
this state seems to be thread-local, so there is no problem. On OS X,
however, it is not, and hence multiple res_query() calls from parallel
threads will compete and return bogus results.

The fix for this is to use res_nquery(), introduced in BIND 8.2, which
takes an explicit state argument. This allows us to manually store the
state thread-locally. If res_nquery() isn’t available, we fall back to
res_query(). It should be available on OS X though. As a data point,
it’s available on Fedora 27.

There’s a slight complication in the fact that OS X requires the state
to be freed using res_ndestroy() rather than res_nclose(). Linux uses
res_nclose().

(See, for example, the NetBSD man page:
https://www.unix.com/man-page/netbsd/3/res_ninit/. The Linux one is
incomplete and not so useful:
http://ma...