ipn: allow b to be nil in NewBackendServercrawshaw/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>

1 files changed, 14 insertions, 0 deletions

diff --git a/ipn/message_test.go b/ipn/message_test.go
index 79c4a76b6..194ac3b89 100644
--- a/ipn/message_test.go
+++ b/ipn/message_test.go
@@ -187,3 +187,17 @@ func TestClientServer(t *testing.T) {
 	})
 	flushUntil(Running)
 }
+
+func TestNilBackend(t *testing.T) {
+	var called *Notify
+	bs := NewBackendServer(t.Logf, nil, func(n Notify) {
+		called = &n
+	})
+	bs.SendErrorMessage("Danger, Will Robinson!")
+	if called == nil {
+		t.Errorf("expect callback to be called, wasn't")
+	}
+	if called.ErrMessage == nil || *called.ErrMessage != "Danger, Will Robinson!" {
+		t.Errorf("callback got wrong error: %v", called.ErrMessage)
+	}
+}