From 747c7d7ce27265922eaffb78c6dd9fa43ed637e9 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 14 Feb 2020 13:09:19 -0800
Subject: [PATCH] types/empty: add Message, stop using mysterious *struct{}

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 control/controlclient/auto.go               |  7 ++++---
 control/controlclient/controlclient_test.go |  4 +++-
 ipn/backend.go                              | 19 ++++++++++---------
 ipn/local.go                                |  4 ++--
 types/empty/message.go                      | 14 ++++++++++++++
 5 files changed, 33 insertions(+), 15 deletions(-)
 create mode 100644 types/empty/message.go

diff --git a/control/controlclient/auto.go b/control/controlclient/auto.go
index ba99884bc..54ff7d77c 100644
--- a/control/controlclient/auto.go
+++ b/control/controlclient/auto.go
@@ -21,6 +21,7 @@ import (
 	"tailscale.com/logger"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/empty"
 )
 
 // TODO(apenwarr): eliminate the 'state' variable, as it's now obsolete.
@@ -60,7 +61,7 @@ func (s state) String() string {
 }
 
 type Status struct {
-	LoginFinished *struct{}
+	LoginFinished *empty.Message
 	Err           string
 	URL           string
 	Persist       *Persist         // locally persisted configuration
@@ -507,9 +508,9 @@ func (c *Client) sendStatus(who string, err error, url string, nm *NetworkMap) {
 	c.logf("sendStatus: %s: %v\n", who, state)
 
 	var p *Persist
-	var fin *struct{}
+	var fin *empty.Message
 	if state == stateAuthenticated {
-		fin = &struct{}{}
+		fin = new(empty.Message)
 	}
 	if nm != nil && loggedIn && synced {
 		pp := c.direct.GetPersist()
diff --git a/control/controlclient/controlclient_test.go b/control/controlclient/controlclient_test.go
index a764a8c71..4790eec13 100644
--- a/control/controlclient/controlclient_test.go
+++ b/control/controlclient/controlclient_test.go
@@ -7,6 +7,8 @@ package controlclient
 import (
 	"reflect"
 	"testing"
+
+	"tailscale.com/types/empty"
 )
 
 func fieldsOf(t reflect.Type) (fields []string) {
@@ -55,7 +57,7 @@ func TestStatusEqual(t *testing.T) {
 		},
 		{
 			&Status{LoginFinished: nil},
-			&Status{LoginFinished: new(struct{})},
+			&Status{LoginFinished: new(empty.Message)},
 			false,
 		},
 	}
diff --git a/ipn/backend.go b/ipn/backend.go
index 593ab915c..6742e3302 100644
--- a/ipn/backend.go
+++ b/ipn/backend.go
@@ -9,6 +9,7 @@ import (
 
 	"tailscale.com/control/controlclient"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/empty"
 	"tailscale.com/wgengine"
 )
 
@@ -39,15 +40,15 @@ type NetworkMap = controlclient.NetworkMap
 // In any given notification, any or all of these may be nil, meaning
 // that they have not changed.
 type Notify struct {
-	Version       string        // version number of IPN backend
-	ErrMessage    *string       // critical error message, if any
-	LoginFinished *struct{}     // event: login process succeeded
-	State         *State        // current IPN state has changed
-	Prefs         *Prefs        // preferences were changed
-	NetMap        *NetworkMap   // new netmap received
-	Engine        *EngineStatus // wireguard engine stats
-	BrowseToURL   *string       // UI should open a browser right now
-	BackendLogID  *string       // public logtail id used by backend
+	Version       string         // version number of IPN backend
+	ErrMessage    *string        // critical error message, if any
+	LoginFinished *empty.Message // event: non-nil when login process succeeded
+	State         *State         // current IPN state has changed
+	Prefs         *Prefs         // preferences were changed
+	NetMap        *NetworkMap    // new netmap received
+	Engine        *EngineStatus  // wireguard engine stats
+	BrowseToURL   *string        // UI should open a browser right now
+	BackendLogID  *string        // public logtail id used by backend
 }
 
 // StateKey is an opaque identifier for a set of LocalBackend state
diff --git a/ipn/local.go b/ipn/local.go
index d4ee3e609..81ad20c43 100644
--- a/ipn/local.go
+++ b/ipn/local.go
@@ -17,6 +17,7 @@ import (
 	"tailscale.com/logger"
 	"tailscale.com/portlist"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/empty"
 	"tailscale.com/version"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
@@ -194,8 +195,7 @@ func (b *LocalBackend) Start(opts Options) error {
 			// Auth completed, unblock the engine
 			b.blockEngineUpdates(false)
 			b.authReconfig()
-			noargs := struct{}{}
-			b.send(Notify{LoginFinished: &noargs})
+			b.send(Notify{LoginFinished: &empty.Message{}})
 		}
 		if new.Persist != nil {
 			persist := *new.Persist // copy
diff --git a/types/empty/message.go b/types/empty/message.go
new file mode 100644
index 000000000..4e208ca49
--- /dev/null
+++ b/types/empty/message.go
@@ -0,0 +1,14 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package empty defines an empty struct type.
+package empty
+
+// Message is an empty message. Its purpose is to be used as pointer
+// type where nil and non-nil distinguish whether it's set. This is
+// used instead of a bool when we want to marshal it as a JSON empty
+// object (or null) for the future ability to add other fields, at
+// which point callers would define a new struct and not use
+// empty.Message.
+type Message struct{}