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.
