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>

net/netmon/loghelper.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 
 	"tailscale.com/types/logger"
+	"tailscale.com/util/eventbus"
 )
 
 // LinkChangeLogLimiter returns a new [logger.Logf] that logs each unique
@@ -17,13 +18,12 @@ import (
 // done.
 func LinkChangeLogLimiter(ctx context.Context, logf logger.Logf, nm *Monitor) logger.Logf {
 	var formatSeen sync.Map // map[string]bool
-	unregister := nm.RegisterChangeCallback(func(cd *ChangeDelta) {
+	nm.b.Monitor(nm.changeDeltaWatcher(nm.b, ctx, func(cd ChangeDelta) {
 		// If we're in a major change or a time jump, clear the seen map.
 		if cd.Major || cd.TimeJumped {
 			formatSeen.Clear()
 		}
-	})
-	context.AfterFunc(ctx, unregister)
+	}))
 
 	return func(format string, args ...any) {
 		// We only store 'true' in the map, so if it's present then it
@@ -42,3 +42,19 @@ func LinkChangeLogLimiter(ctx context.Context, logf logger.Logf, nm *Monitor) lo
 		logf(format, args...)
 	}
 }
+
+func (nm *Monitor) changeDeltaWatcher(ec *eventbus.Client, ctx context.Context, fn func(ChangeDelta)) func(*eventbus.Client) {
+	sub := eventbus.Subscribe[ChangeDelta](ec)
+	return func(ec *eventbus.Client) {
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-sub.Done():
+				return
+			case change := <-sub.Events():
+				fn(change)
+			}
+		}
+	}
+}

net/netmon/loghelper_test.go

@@ -11,6 +11,7 @@ import (
 	"testing/synctest"
 
 	"tailscale.com/util/eventbus"
+	"tailscale.com/util/eventbus/eventbustest"
 )
 
 func TestLinkChangeLogLimiter(t *testing.T) { synctest.Test(t, syncTestLinkChangeLogLimiter) }
@@ -61,21 +62,15 @@ func syncTestLinkChangeLogLimiter(t *testing.T) {
 	// string cache and allow the next log to write to our log buffer.
 	//
 	// InjectEvent doesn't work because it's not a major event, so we
-	// instead reach into the netmon and grab the callback, and then call
-	// it ourselves.
-	mon.mu.Lock()
-	var cb func(*ChangeDelta)
-	for _, c := range mon.cbs {
-		cb = c
-		break
-	}
-	mon.mu.Unlock()
-
-	cb(&ChangeDelta{Major: true})
+	// instead inject the event ourselves.
+	injector := eventbustest.NewInjector(t, bus)
+	eventbustest.Inject(injector, ChangeDelta{Major: true})
+	synctest.Wait()
 
 	logf("hello %s", "world")
-	if got := logBuffer.String(); got != "hello world\nother message\nhello world\n" {
-		t.Errorf("unexpected log buffer contents: %q", got)
+	want := "hello world\nother message\nhello world\n"
+	if got := logBuffer.String(); got != want {
+		t.Errorf("unexpected log buffer contents, got: %q, want, %q", got, want)
 	}
 
 	// Canceling the context we passed to LinkChangeLogLimiter should