ipn/ipnlocal: use eventbus.SubscribeFunc in expiryManager

Updates #15160
Updates #17487

Change-Id: I8721e3ac1af505630edca7c5cb50695b0aad832a
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/expiry.go

@@ -43,7 +43,7 @@ type expiryManager struct {
 	logf  logger.Logf
 	clock tstime.Clock
 
-	eventSubs eventbus.Monitor
+	eventClient *eventbus.Client
 }
 
 func newExpiryManager(logf logger.Logf, bus *eventbus.Bus) *expiryManager {
@@ -53,30 +53,13 @@ func newExpiryManager(logf logger.Logf, bus *eventbus.Bus) *expiryManager {
 		clock:             tstime.StdClock{},
 	}
 
-	cli := bus.Client("ipnlocal.expiryManager")
+	em.eventClient = bus.Client("ipnlocal.expiryManager")
-	em.eventSubs = cli.Monitor(em.consumeEventbusTopics(cli))
+	eventbus.SubscribeFunc(em.eventClient, func(ct controlclient.ControlTime) {
+		em.onControlTime(ct.Value)
+	})
 	return em
 }
 
-// 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 (em *expiryManager) consumeEventbusTopics(cli *eventbus.Client) func(*eventbus.Client) {
-	controlTimeSub := eventbus.Subscribe[controlclient.ControlTime](cli)
-	return func(cli *eventbus.Client) {
-		for {
-			select {
-			case <-cli.Done():
-				return
-			case time := <-controlTimeSub.Events():
-				em.onControlTime(time.Value)
-			}
-		}
-	}
-}
-
 // onControlTime is called whenever we receive a new timestamp from the control
 // server to store the delta.
 func (em *expiryManager) onControlTime(t time.Time) {
@@ -245,7 +228,7 @@ func (em *expiryManager) nextPeerExpiry(nm *netmap.NetworkMap, localNow time.Tim
 	return nextExpiry
 }
 
-func (em *expiryManager) close() { em.eventSubs.Close() }
+func (em *expiryManager) close() { em.eventClient.Close() }
 
 // ControlNow estimates the current time on the control server, calculated as
 // localNow + the delta between local and control server clocks as recorded