In particular, remove the libasyncns import, which was only used by
GUnixResolver, which is only used when threads are not available.
Likewise remove GWin32Resolver, and the hacky broken non-threaded
parts of GIOScheduler.
https://bugzilla.gnome.org/show_bug.cgi?id=616754
In resolve_sync function in gthreadedresolver.c, if g_thread_pool_push
fails due to thread creation failure, we are just simply appending the
data to the queue of work to do. After the failure, we might wait
indefinitely in g_cond_wait. In case of g_thread_pool_push failure,
propagate the error so that this function does not blocks forever in
case of failure.
https://bugzilla.gnome.org/show_bug.cgi?id=651034
When you're using the threaded resolver and using a sync call
without a cancellable the resolve_sync forgot to unlock the
initial req->mutex lock, leading to a deadlock when unrefing
the request.
GResolver provides asynchronous (and synchronous-but-cancellable) APIs
for resolving hostnames, reverse-resolving IP addresses back to
hostnames, and resolving SRV records. Part of #548466.
