net/netmon: do not abandon a subscriber when exiting early (#17899)

LinkChangeLogLimiter keeps a subscription to track rate limits for log
messages.  But when its context ended, it would exit the subscription loop,
leaving the subscriber still alive. Ensure the subscriber gets cleaned up
when the context ends, so we don't stall event processing.

Updates tailscale/corp#34311

Change-Id: I82749e482e9a00dfc47f04afbc69dd0237537cb2
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>
(cherry picked from commit ab4b990d51)

net/netmon/loghelper.go

@@ -18,13 +18,13 @@ import (
 // done.
 func LinkChangeLogLimiter(ctx context.Context, logf logger.Logf, nm *Monitor) logger.Logf {
 	var formatSeen sync.Map // map[string]bool
-	nm.b.Monitor(nm.changeDeltaWatcher(nm.b, ctx, func(cd ChangeDelta) {
+	sub := eventbus.SubscribeFunc(nm.b, 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, sub.Close)
 	return func(format string, args ...any) {
 		// We only store 'true' in the map, so if it's present then it
 		// means we've already logged this format string.
@@ -42,19 +42,3 @@ 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)
-			}
-		}
-	}
-}