From 256db54c9d25e06297017c3145028ab3ed41b259 Mon Sep 17 00:00:00 2001
From: David Crawshaw <crawshaw@tailscale.com>
Date: Wed, 19 May 2021 14:22:26 -0400
Subject: [PATCH] ipn: allow b to be nil in NewBackendServer

A couple of code paths in ipnserver use a NewBackendServer with a nil
backend just to call the callback with an encapsulated error message.
This covers a panic case seen in logs.

For #1920

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
---
 ipn/message.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/ipn/message.go b/ipn/message.go
index 7f8d2be21..7080e4e2d 100644
--- a/ipn/message.go
+++ b/ipn/message.go
@@ -104,7 +104,9 @@ func NewBackendServer(logf logger.Logf, b Backend, sendNotifyMsg func(b []byte))
 		b:             b,
 		sendNotifyMsg: sendNotifyMsg,
 	}
-	if sendNotifyMsg != nil {
+	// b may be nil if the BackendServer is being created just to
+	// encapsulate and send an error message.
+	if sendNotifyMsg != nil && b != nil {
 		b.SetNotifyCallback(bs.send)
 	}
 	return bs