Philip Withnall 40be86bb0e gio: Port GThreadedResolver to use res_nquery() to fix thread-safety
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://man7.org/linux/man-pages/man3/resolver.3.html.)

The new code will call res_ninit() once per res_nquery() task. This is
not optimal, but no worse than before — since res_query() was being
called in a worker thread, on Linux, it would implicitly initialise the
thread-local struct __res_state when it was called. We’ve essentially
just made that explicit. In practical terms, this means a
stat("/etc/resolv.conf") call per res_nquery() task.

In future, we could improve this by using an explicit thread pool with
some manually-created worker threads, each of which initialises a struct
__res_state on spawning, and only updates it on receiving
the #GResolver::reload signal.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=792050
2018-02-02 18:05:27 +01:00
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