logtail: avoid racing eventbus subscriptions with Shutdown (tailscale#17639)

When the eventbus is enabled, set up the subscription for change deltas at the
beginning when the client is created, rather than waiting for the first
awaitInternetUp check.

Otherwise, it is possible for a check to race with the client close in
Shutdown, which triggers a panic.

Updates tailscale#17638

Change-Id: I461c07939eca46699072b14b1814ecf28eec750c
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>
(cherry picked from commit 4346615)

Author: creachadair

logtail/logtail.go

@@ -124,6 +124,7 @@ func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
 
 	if cfg.Bus != nil {
 		l.eventClient = cfg.Bus.Client("logtail.Logger")
+		l.changeDeltaSub = eventbus.Subscribe[netmon.ChangeDelta](l.eventClient)
 	}
 	l.SetSockstatsLabel(sockstats.LabelLogtailLogger)
 	l.compressLogs = cfg.CompressLogs
@@ -162,6 +163,7 @@ type Logger struct {
 	httpDoCalls    atomic.Int32
 	sockstatsLabel atomicSocktatsLabel
 	eventClient    *eventbus.Client
+	changeDeltaSub *eventbus.Subscriber[netmon.ChangeDelta]
 
 	procID              uint32
 	includeProcSequence bool
@@ -427,8 +429,23 @@ func (l *Logger) internetUp() bool {
 
 func (l *Logger) awaitInternetUp(ctx context.Context) {
 	if l.eventClient != nil {
-		l.awaitInternetUpBus(ctx)
-		return
+		for {
+			if l.internetUp() {
+				return
+			}
+			select {
+			case <-ctx.Done():
+				return // give up
+			case <-l.changeDeltaSub.Done():
+				return // give up (closing down)
+			case delta := <-l.changeDeltaSub.Events():
+				if delta.New.AnyInterfaceUp() || l.internetUp() {
+					fmt.Fprintf(l.stderr, "logtail: internet back up\n")
+					return
+				}
+				fmt.Fprintf(l.stderr, "logtail: network changed, but is not up")
+			}
+		}
 	}
 	upc := make(chan bool, 1)
 	defer l.netMonitor.RegisterChangeCallback(func(delta *netmon.ChangeDelta) {
@@ -449,24 +466,6 @@ func (l *Logger) awaitInternetUp(ctx context.Context) {
 	}
 }
 
-func (l *Logger) awaitInternetUpBus(ctx context.Context) {
-	if l.internetUp() {
-		return
-	}
-	sub := eventbus.Subscribe[netmon.ChangeDelta](l.eventClient)
-	defer sub.Close()
-	select {
-	case delta := <-sub.Events():
-		if delta.New.AnyInterfaceUp() {
-			fmt.Fprintf(l.stderr, "logtail: internet back up\n")
-			return
-		}
-		fmt.Fprintf(l.stderr, "logtail: network changed, but is not up")
-	case <-ctx.Done():
-		return
-	}
-}
-
 // upload uploads body to the log server.
 // origlen indicates the pre-compression body length.
 // origlen of -1 indicates that the body is not compressed.