control/controlclient: cut down some NetworkMap stringification & diff allocations

And start of tests.

control/controlclient/netmap.go

@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"reflect"
 	"strings"
 	"time"
 
@@ -83,11 +84,12 @@ func (nm *NetworkMap) Concise() string {
 		for i, e := range p.Endpoints {
 			// Align vertically on the ':' between IP and port
 			colon := strings.IndexByte(e, ':')
-			for colon > 0 && len(e)-colon < 6 {
-				e += " "
+			spaces := 0
+			for colon > 0 && len(e)+spaces-colon < 6 {
+				spaces++
 				colon--
 			}
-			ep[i] = fmt.Sprintf("%21v", e)
+			ep[i] = fmt.Sprintf("%21v", e+strings.Repeat(" ", spaces))
 		}
 
 		derp := p.DERP
@@ -108,6 +110,10 @@ func (nm *NetworkMap) Concise() string {
 }
 
 func (b *NetworkMap) ConciseDiffFrom(a *NetworkMap) string {
+	if reflect.DeepEqual(a, b) {
+		// Fast path that only does one allocation.
+		return ""
+	}
 	out := []string{}
 	ra := strings.Split(a.Concise(), "\n")
 	rb := strings.Split(b.Concise(), "\n")

control/controlclient/netmap_test.go

@@ -0,0 +1,36 @@
+// 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 controlclient
+
+import (
+	"testing"
+
+	"tailscale.com/tailcfg"
+)
+
+func TestNetworkMapConcise(t *testing.T) {
+	nm := &NetworkMap{
+		Peers: []*tailcfg.Node{
+			{
+				Name:      "foo",
+				Endpoints: []string{"192.168.0.100:12", "192.168.0.100:12354"},
+			},
+			{
+				Name:      "bar",
+				Endpoints: []string{"10.2.0.100:12", "10.1.0.100:12345"},
+			},
+		},
+	}
+	var got string
+	n := int(testing.AllocsPerRun(1000, func() {
+		got = nm.Concise()
+	}))
+	t.Logf("Allocs = %d", n)
+	want := "netmap: self: [AAAAA] auth=machine-unknown :0 []\n" +
+		" [AAAAA]                    :    192.168.0.100:12     192.168.0.100:12354\n [AAAAA]                    :       10.2.0.100:12        10.1.0.100:12345\n"
+	if got != want {
+		t.Errorf("Wrong output\n Got: %q\nWant: %q\n## Got (unescaped):\n%s\n## Want (unescaped):\n%s\n", got, want, got, want)
+	}
+}