net/netmon: remove usage of direct callbacks from netmon (#17292)

The callback itself is not removed as it is used in other repos, making
it simpler for those to slowly transition to the eventbus.

Updates #15160

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

logtail/logtail.go

@@ -32,6 +32,7 @@ import (
 	"tailscale.com/tstime"
 	tslogger "tailscale.com/types/logger"
 	"tailscale.com/types/logid"
+	"tailscale.com/util/eventbus"
 	"tailscale.com/util/set"
 	"tailscale.com/util/truncate"
 	"tailscale.com/util/zstdframe"
@@ -120,6 +121,10 @@ func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
 		shutdownStart: make(chan struct{}),
 		shutdownDone:  make(chan struct{}),
 	}
+
+	if cfg.Bus != nil {
+		l.eventClient = cfg.Bus.Client("logtail.Logger")
+	}
 	l.SetSockstatsLabel(sockstats.LabelLogtailLogger)
 	l.compressLogs = cfg.CompressLogs
 
@@ -156,6 +161,7 @@ type Logger struct {
 	privateID      logid.PrivateID
 	httpDoCalls    atomic.Int32
 	sockstatsLabel atomicSocktatsLabel
+	eventClient    *eventbus.Client
 
 	procID              uint32
 	includeProcSequence bool
@@ -221,6 +227,9 @@ func (l *Logger) Shutdown(ctx context.Context) error {
 		l.httpc.CloseIdleConnections()
 	}()
 
+	if l.eventClient != nil {
+		l.eventClient.Close()
+	}
 	l.shutdownStartMu.Lock()
 	select {
 	case <-l.shutdownStart:
@@ -417,6 +426,10 @@ func (l *Logger) internetUp() bool {
 }
 
 func (l *Logger) awaitInternetUp(ctx context.Context) {
+	if l.eventClient != nil {
+		l.awaitInternetUpBus(ctx)
+		return
+	}
 	upc := make(chan bool, 1)
 	defer l.netMonitor.RegisterChangeCallback(func(delta *netmon.ChangeDelta) {
 		if delta.New.AnyInterfaceUp() {
@@ -436,6 +449,24 @@ 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.