[client] Make tunnel notifier Close wait for queued events to drain

Close previously returned immediately after marking the notifier
closed, so deliverLoop could still be invoking listener/DNS callbacks
after Close returned. Close now blocks on a completion channel that
deliverLoop closes once it has drained the queue and exited.

client/internal/tunnelnotifier/notifier.go

@@ -32,6 +32,7 @@ type Notifier struct {
 	cond   *sync.Cond
 	queue  *list.List
 	closed bool
+	done   chan struct{}
 
 	listener   listener.NetworkChangeListener
 	dnsManager dns.IosDnsManager
@@ -40,6 +41,7 @@ type Notifier struct {
 func New(l listener.NetworkChangeListener, dm dns.IosDnsManager) *Notifier {
 	n := &Notifier{
 		queue:      list.New(),
+		done:       make(chan struct{}),
 		listener:   l,
 		dnsManager: dm,
 	}
@@ -64,11 +66,14 @@ func (n *Notifier) ApplyDns(config string) {
 	n.enqueue(event{kind: eventDNS, payload: config})
 }
 
+// Close stops accepting new events and blocks until the delivery loop has
+// drained all queued events and exited.
 func (n *Notifier) Close() {
 	n.mu.Lock()
 	n.closed = true
 	n.cond.Signal()
 	n.mu.Unlock()
+	<-n.done
 }
 
 func (n *Notifier) enqueue(ev event) {
@@ -82,6 +87,7 @@ func (n *Notifier) enqueue(ev event) {
 }
 
 func (n *Notifier) deliverLoop() {
+	defer close(n.done)
 	for {
 		n.mu.Lock()
 		for n.queue.Len() == 0 && !n.closed {

client/internal/tunnelnotifier/notifier_test.go

@@ -183,7 +183,7 @@ func TestCloseDrainsQueue(t *testing.T) {
 	}
 	n.Close()
 
-	require.Eventually(t, func() bool { return rec.count() == events }, 5*time.Second, time.Millisecond)
+	require.Equal(t, events, rec.count(), "Close must not return before all queued events are delivered")
 
 	n.OnNetworkChanged("after-close")
 	n.ApplyDns("after-close")