health: remove direct callback and replace with eventbus (#17199)

Pulls out the last callback logic and ensures timers are still running.

The eventbustest package is updated support the absence of events.

Updates #15160

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

control/controlclient/auto.go

@@ -23,6 +23,7 @@ import (
 	"tailscale.com/types/persist"
 	"tailscale.com/types/structs"
 	"tailscale.com/util/clientmetric"
+	"tailscale.com/util/eventbus"
 	"tailscale.com/util/execqueue"
 )
 
@@ -122,7 +123,9 @@ type Auto struct {
 	observerQueue execqueue.ExecQueue
 	shutdownFn    func() // to be called prior to shutdown or nil
 
-	unregisterHealthWatch func()
+	eventClient     *eventbus.Client
+	healthChangeSub *eventbus.Subscriber[health.Change]
+	subsDoneCh      chan struct{} // close-only channel when eventClient has closed
 
 	mu sync.Mutex // mutex guards the following fields
 
@@ -192,21 +195,42 @@ 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)
+
 	c.authCtx, c.authCancel = context.WithCancel(context.Background())
 	c.authCtx = sockstats.WithSockStats(c.authCtx, sockstats.LabelControlClientAuto, opts.Logf)
 
 	c.mapCtx, c.mapCancel = context.WithCancel(context.Background())
 	c.mapCtx = sockstats.WithSockStats(c.mapCtx, sockstats.LabelControlClientAuto, opts.Logf)
 
-	c.unregisterHealthWatch = opts.HealthTracker.RegisterWatcher(func(c health.Change) {
-		if c.WarnableChanged {
-			direct.ReportWarnableChange(c.Warnable, c.UnhealthyState)
-		}
-	})
+	go c.consumeEventbusTopics()
 	return c, nil
 }
 
+// consumeEventbusTopics consumes events from all relevant
+// [eventbus.Subscriber]'s and passes them to their related handler. Events are
+// 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)
+
+	for {
+		select {
+		case <-c.eventClient.Done():
+			return
+		case change := <-c.healthChangeSub.Events():
+			if change.WarnableChanged {
+				c.direct.ReportWarnableChange(change.Warnable, change.UnhealthyState)
+			}
+		}
+	}
+}
+
 // SetPaused controls whether HTTP activity should be paused.
 //
 // The client can be paused and unpaused repeatedly, unlike Start and Shutdown, which can only be used once.
@@ -760,6 +784,9 @@ func (c *Auto) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 }
 
 func (c *Auto) Shutdown() {
+	c.eventClient.Close()
+	<-c.subsDoneCh
+
 	c.mu.Lock()
 	if c.closed {
 		c.mu.Unlock()
@@ -783,7 +810,6 @@ func (c *Auto) Shutdown() {
 		shutdownFn()
 	}
 
-	c.unregisterHealthWatch()
 	<-c.authDone
 	<-c.mapDone
 	<-c.updateDone