From 42ce5c88bed24817def3049d75e5a6810f172c7a Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Mon, 10 Nov 2025 16:51:46 -0800
Subject: [PATCH] wgengine/magicsock: unblock Conn.Synchronize on Conn.Close

I noticed a deadlock in a test in a in-development PR where during a
shutdown storm of things (from a tsnet.Server.Close), LocalBackend was
trying to call magicsock.Conn.Synchronize but the magicsock and/or
eventbus was already shut down and no longer processing events.

Updates #16369

Change-Id: I58b1f86c8959303c3fb46e2e3b7f38f6385036f1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 wgengine/magicsock/magicsock.go | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/wgengine/magicsock/magicsock.go b/wgengine/magicsock/magicsock.go
index f1721e1d9..d44cf1c11 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -665,7 +665,10 @@ func (c *Conn) Synchronize() {
 	}
 	sp := syncPoint(make(chan struct{}))
 	c.syncPub.Publish(sp)
-	sp.Wait()
+	select {
+	case <-sp:
+	case <-c.donec:
+	}
 }
 
 // NewConn creates a magic Conn listening on opts.Port.