host-resolver: Do not cancel threads waiting for new queries during cleanup

While it is currently unclear why it happens, canceling threads waiting in the
new_query condvar does not work as expected. The behavior is not fully
reproducible: Either cancel(), join() or destroying the condvar hangs.

The issue has been seen in the http-fetcher unit tests, where the stream service
triggers the use of the resolver for "localhost" hosts. It is reproducible with
any cleanup following a host_create_from_dns() use on a Ubuntu 14.04 x64 system.
Further, the issue is related to the use of libunwind, as only builds with
--enable-unwind-backtraces are affected.

As we broadcast() the new_query condvar before destruction, a hard cancel() of
these threads is actually not required. Instead we let these threads clean up
themselves after receiving the condvar signal.

src/libstrongswan/networking/host_resolver.c

@@ -169,17 +169,19 @@ static void *resolve_hosts(private_host_resolver_t *this)
 	while (TRUE)
 	{
 		this->mutex->lock(this->mutex);
-		thread_cleanup_push((thread_cleanup_t)this->mutex->unlock, this->mutex);
 		while (this->queue->remove_first(this->queue,
 										(void**)&query) != SUCCESS)
 		{
-			old = thread_cancelability(TRUE);
-			timed_out = this->new_query->timed_wait(this->new_query,
-									this->mutex, NEW_QUERY_WAIT_TIMEOUT * 1000);
-			thread_cancelability(old);
 			if (this->disabled)
 			{
-				thread_cleanup_pop(TRUE);
+				this->mutex->unlock(this->mutex);
+				return NULL;
+			}
+			timed_out = this->new_query->timed_wait(this->new_query,
+									this->mutex, NEW_QUERY_WAIT_TIMEOUT * 1000);
+			if (this->disabled)
+			{
+				this->mutex->unlock(this->mutex);
 				return NULL;
 			}
 			else if (timed_out && (this->threads > this->min_threads))
@@ -188,13 +190,13 @@ static void *resolve_hosts(private_host_resolver_t *this)
 
 				this->threads--;
 				this->pool->remove(this->pool, thread, NULL);
-				thread_cleanup_pop(TRUE);
+				this->mutex->unlock(this->mutex);
 				thread->detach(thread);
 				return NULL;
 			}
 		}
 		this->busy_threads++;
-		thread_cleanup_pop(TRUE);
+		this->mutex->unlock(this->mutex);
 
 		memset(&hints, 0, sizeof(hints));
 		hints.ai_family = query->family;