From 3af64765fd5808e72ddd2b29888c3defe9bf5026 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 11 Sep 2020 15:10:29 -0700
Subject: [PATCH] ipn: fix Windows crash from improperly strict assertion

---
 ipn/local.go | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/ipn/local.go b/ipn/local.go
index 6e4b6981c..1d61ffa25 100644
--- a/ipn/local.go
+++ b/ipn/local.go
@@ -1251,11 +1251,21 @@ func (b *LocalBackend) requestEngineStatusAndWait() {
 //  rebooting will fix it.
 func (b *LocalBackend) Logout() {
 	b.mu.Lock()
-	b.assertClientLocked()
 	c := b.c
 	b.netMap = nil
 	b.mu.Unlock()
 
+	if c == nil {
+		// Double Logout can happen via repeated IPN
+		// connections to ipnserver making it repeatedly
+		// transition from 1->0 total connections, which on
+		// Windows by default ("client mode") causes a Logout
+		// on the transition to zero.
+		// Previously this crashed when we asserted that c was non-nil
+		// here.
+		return
+	}
+
 	c.Logout()
 
 	b.mu.Lock()