ipn/ipnlocal: use eventbus.Monitor in LocalBackend (#17225)

This commit does not change the order or meaning of any eventbus activity, it
only updates the way the plumbing is set up.

Updates #15160

Change-Id: I06860ac4e43952a9bb4d85366138c9d9a17fd9cd
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/local.go

@@ -197,18 +197,14 @@ var (
 // state machine generates events back out to zero or more components.
 type LocalBackend struct {
 	// Elements that are thread-safe or constant after construction.
-	ctx                      context.Context         // canceled by [LocalBackend.Shutdown]
-	ctxCancel                context.CancelCauseFunc // cancels ctx
-	logf                     logger.Logf             // general logging
-	keyLogf                  logger.Logf             // for printing list of peers on change
-	statsLogf                logger.Logf             // for printing peers stats on change
-	sys                      *tsd.System
-	eventClient              *eventbus.Client
-	clientVersionSub         *eventbus.Subscriber[tailcfg.ClientVersion]
-	autoUpdateSub            *eventbus.Subscriber[controlclient.AutoUpdate]
-	healthChangeSub          *eventbus.Subscriber[health.Change]
-	changeDeltaSub           *eventbus.Subscriber[netmon.ChangeDelta]
-	subsDoneCh               chan struct{}       // closed when consumeEventbusTopics returns
+	ctx       context.Context         // canceled by [LocalBackend.Shutdown]
+	ctxCancel context.CancelCauseFunc // cancels ctx
+	logf      logger.Logf             // general logging
+	keyLogf   logger.Logf             // for printing list of peers on change
+	statsLogf logger.Logf             // for printing peers stats on change
+	sys       *tsd.System
+	eventSubs eventbus.Monitor
+
 	health                   *health.Tracker     // always non-nil
 	polc                     policyclient.Client // always non-nil
 	metrics                  metrics
@@ -538,13 +534,10 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 		captiveCtx:            captiveCtx,
 		captiveCancel:         nil, // so that we start checkCaptivePortalLoop when Running
 		needsCaptiveDetection: make(chan bool),
-		subsDoneCh:            make(chan struct{}),
 	}
-	b.eventClient = b.Sys().Bus.Get().Client("ipnlocal.LocalBackend")
-	b.clientVersionSub = eventbus.Subscribe[tailcfg.ClientVersion](b.eventClient)
-	b.autoUpdateSub = eventbus.Subscribe[controlclient.AutoUpdate](b.eventClient)
-	b.healthChangeSub = eventbus.Subscribe[health.Change](b.eventClient)
-	b.changeDeltaSub = eventbus.Subscribe[netmon.ChangeDelta](b.eventClient)
+	ec := b.Sys().Bus.Get().Client("ipnlocal.LocalBackend")
+	b.eventSubs = ec.Monitor(b.consumeEventbusTopics(ec))
+
 	nb := newNodeBackend(ctx, b.sys.Bus.Get())
 	b.currentNodeAtomic.Store(nb)
 	nb.ready()
@@ -611,7 +604,6 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 			}
 		}
 	}
-	go b.consumeEventbusTopics()
 	return b, nil
 }
 
@@ -620,21 +612,26 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 // 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 (b *LocalBackend) consumeEventbusTopics() {
-	defer close(b.subsDoneCh)
+func (b *LocalBackend) consumeEventbusTopics(ec *eventbus.Client) func(*eventbus.Client) {
+	clientVersionSub := eventbus.Subscribe[tailcfg.ClientVersion](ec)
+	autoUpdateSub := eventbus.Subscribe[controlclient.AutoUpdate](ec)
+	healthChangeSub := eventbus.Subscribe[health.Change](ec)
+	changeDeltaSub := eventbus.Subscribe[netmon.ChangeDelta](ec)
 
-	for {
-		select {
-		case <-b.eventClient.Done():
-			return
-		case clientVersion := <-b.clientVersionSub.Events():
-			b.onClientVersion(&clientVersion)
-		case au := <-b.autoUpdateSub.Events():
-			b.onTailnetDefaultAutoUpdate(au.Value)
-		case change := <-b.healthChangeSub.Events():
-			b.onHealthChange(change)
-		case changeDelta := <-b.changeDeltaSub.Events():
-			b.linkChange(&changeDelta)
+	return func(ec *eventbus.Client) {
+		for {
+			select {
+			case <-ec.Done():
+				return
+			case clientVersion := <-clientVersionSub.Events():
+				b.onClientVersion(&clientVersion)
+			case au := <-autoUpdateSub.Events():
+				b.onTailnetDefaultAutoUpdate(au.Value)
+			case change := <-healthChangeSub.Events():
+				b.onHealthChange(change)
+			case changeDelta := <-changeDeltaSub.Events():
+				b.linkChange(&changeDelta)
+			}
 		}
 	}
 }
@@ -1103,8 +1100,7 @@ func (b *LocalBackend) Shutdown() {
 	//     they can deadlock with c.Shutdown().
 	//  2. LocalBackend.consumeEventbusTopics event handlers may not guard against
 	//     undesirable post/in-progress LocalBackend.Shutdown() behaviors.
-	b.eventClient.Close()
-	<-b.subsDoneCh
+	b.eventSubs.Close()
 
 	b.em.close()