controlclient/auto: switch eventbus to using a monitor

Only changes how the go routine consuming the events starts and stops,
not what it does.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

control/controlclient/auto.go

@@ -123,9 +123,7 @@ type Auto struct {
 	observerQueue execqueue.ExecQueue
 	shutdownFn    func() // to be called prior to shutdown or nil
 
-	eventClient     *eventbus.Client
-	healthChangeSub *eventbus.Subscriber[health.Change]
-	subsDoneCh      chan struct{} // close-only channel when eventClient has closed
+	eventSubs eventbus.Monitor
 
 	mu sync.Mutex // mutex guards the following fields
 
@@ -195,11 +193,11 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 		updateDone: make(chan struct{}),
 		observer:   opts.Observer,
 		shutdownFn: opts.Shutdown,
-		subsDoneCh: make(chan struct{}),
 	}
 
-	c.eventClient = opts.Bus.Client("controlClient.Auto")
-	c.healthChangeSub = eventbus.Subscribe[health.Change](c.eventClient)
+	// Set up eventbus client and subscriber
+	cli := opts.Bus.Client("controlClient.Auto")
+	c.eventSubs = cli.Monitor(c.consumeEventbusTopics(cli))
 
 	c.authCtx, c.authCancel = context.WithCancel(context.Background())
 	c.authCtx = sockstats.WithSockStats(c.authCtx, sockstats.LabelControlClientAuto, opts.Logf)
@@ -207,7 +205,6 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 	c.mapCtx, c.mapCancel = context.WithCancel(context.Background())
 	c.mapCtx = sockstats.WithSockStats(c.mapCtx, sockstats.LabelControlClientAuto, opts.Logf)
 
-	go c.consumeEventbusTopics()
 	return c, nil
 }
 
@@ -216,19 +213,20 @@ func NewNoStart(opts Options) (_ *Auto, err error) {
 // always handled in the order they are received, i.e. the next event is not
 // read until the previous event's handler has returned. It returns when the
 // [eventbus.Client] is closed.
-func (c *Auto) consumeEventbusTopics() {
-	defer close(c.subsDoneCh)
-
+func (c *Auto) consumeEventbusTopics(cli *eventbus.Client) func(*eventbus.Client) {
+	healthChangeSub := eventbus.Subscribe[health.Change](cli)
+	return func(cli *eventbus.Client) {
 		for {
 			select {
-		case <-c.eventClient.Done():
+			case <-cli.Done():
 				return
-		case change := <-c.healthChangeSub.Events():
+			case change := <-healthChangeSub.Events():
 				if change.WarnableChanged {
 					c.direct.ReportWarnableChange(change.Warnable, change.UnhealthyState)
 				}
 			}
 		}
+	}
 }
 
 // SetPaused controls whether HTTP activity should be paused.
@@ -784,8 +782,7 @@ func (c *Auto) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 }
 
 func (c *Auto) Shutdown() {
-	c.eventClient.Close()
-	<-c.subsDoneCh
+	c.eventSubs.Close()
 
 	c.mu.Lock()
 	if c.closed {