From 2ad2d4d409e6b5eac5dbecb59ce307eb3297587c Mon Sep 17 00:00:00 2001
From: Jordan Whited <jordan@tailscale.com>
Date: Mon, 10 Nov 2025 21:08:13 -0800
Subject: [PATCH] wgengine/magicsock: fix UDPRelayAllocReq/Resp deadlock
 (#17831)

Updates #17830

Signed-off-by: Jordan Whited <jordan@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 3d7b16f30..f1721e1d9 100644
--- a/wgengine/magicsock/magicsock.go
+++ b/wgengine/magicsock/magicsock.go
@@ -2444,7 +2444,10 @@ func (c *Conn) handleDiscoMessage(msg []byte, src epAddr, shouldBeRelayHandshake
 		if !nodeHasCap(c.filt, c.peers.At(peerI), c.self, tailcfg.PeerCapabilityRelay) {
 			return
 		}
-		c.allocRelayEndpointPub.Publish(UDPRelayAllocReq{
+		// [Conn.mu] must not be held while publishing, or [Conn.onUDPRelayAllocResp]
+		// can deadlock as the req sub and resp pub are the same goroutine.
+		// See #17830.
+		go c.allocRelayEndpointPub.Publish(UDPRelayAllocReq{
 			RxFromDiscoKey: sender,
 			RxFromNodeKey:  nodeKey,
 			Message:        req,