types/empty: add Message, stop using mysterious *struct{}

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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()

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,
 		},
 	}

ipn/backend.go

@@ -9,6 +9,7 @@ import (
 
 	"tailscale.com/control/controlclient"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/empty"
 	"tailscale.com/wgengine"
 )
 
@@ -41,7 +42,7 @@ type NetworkMap = controlclient.NetworkMap
 type Notify struct {
 	Version       string         // version number of IPN backend
 	ErrMessage    *string        // critical error message, if any
-	LoginFinished *struct{}     // event: login process succeeded
+	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

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

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{}