From ab4b990d51c41aff8e1ae7a08435dedfe621ce0d Mon Sep 17 00:00:00 2001
From: "M. J. Fromberger" <fromberger@tailscale.com>
Date: Fri, 14 Nov 2025 11:57:41 -0800
Subject: [PATCH] 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>
---
 net/netmon/loghelper.go | 22 +++-------------------
 1 file changed, 3 insertions(+), 19 deletions(-)

diff --git a/net/netmon/loghelper.go b/net/netmon/loghelper.go
index 2e28e8cda..675762cd1 100644
--- a/net/netmon/loghelper.go
+++ b/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)
-			}
-		}
-	}
-}