ipn: drop unchanged network map updates

This should never happen. But it is, so defend against it while
I fix the bug elsewhere.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

ipn/local.go

@@ -5,6 +5,8 @@
 package ipn
 
 import (
+	"crypto/sha256"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"log"
@@ -199,7 +201,24 @@ func (b *LocalBackend) Start(opts Options) error {
 		cli.UpdateEndpoints(0, endpoints)
 	}
 
-	cli.SetStatusFunc(func(newSt controlclient.Status) {
+	cli.SetStatusFunc(b.controlStatusUpdate)
+	b.e.SetStatusCallback(b.engineStatusUpdate)
+
+	b.mu.Lock()
+	prefs := b.prefs.Clone()
+	b.mu.Unlock()
+
+	blid := b.backendLogID
+	b.logf("Backend: logs: be:%v fe:%v\n", blid, opts.FrontendLogID)
+	b.send(Notify{BackendLogID: &blid})
+	b.send(Notify{Prefs: prefs})
+
+	cli.Login(nil, controlclient.LoginDefault)
+	return nil
+}
+
+// controlStatusUpdate is passed as a callback to controlclient SetStatusFunc.
+func (b *LocalBackend) controlStatusUpdate(newSt controlclient.Status) {
 	if newSt.LoginFinished != nil {
 		// Auth completed, unblock the engine
 		b.blockEngineUpdates(false)
@@ -223,8 +242,19 @@ func (b *LocalBackend) Start(opts Options) error {
 		b.send(Notify{Prefs: prefs})
 	}
 	if newSt.NetMap != nil {
+		changed := true
+
 		b.mu.Lock()
-			if b.netMapCache != nil && b.cmpDiff != nil {
+		if b.netMapCache != nil {
+			hash1, err1 := jsonHash(newSt.NetMap)
+			hash2, err2 := jsonHash(b.netMapCache)
+			if err1 == nil && err2 == nil {
+				changed = hash1 != hash2
+			} else {
+				b.logf("[unexpected] netmap hash encode failed: %v, %v", err1, err2)
+			}
+
+			if changed && b.cmpDiff != nil {
 				s1 := strings.Split(b.netMapCache.Concise(), "\n")
 				s2 := strings.Split(newSt.NetMap.Concise(), "\n")
 				diff := b.cmpDiff(s1, s2)
@@ -232,11 +262,21 @@ func (b *LocalBackend) Start(opts Options) error {
 					b.logf("netmap diff:\n%v\n", diff)
 				}
 			}
+		}
 		b.netMapCache = newSt.NetMap
 		b.mu.Unlock()
 
+		if changed {
 			b.send(Notify{NetMap: newSt.NetMap})
 			b.updateFilter(newSt.NetMap)
+		} else {
+			// This should never happen. A properly
+			// functioning CONTROL should skip sending
+			// equal netmaps. We detect and log here both
+			// as defense-in-depth and to give us warning
+			// (in the logs) that something is wrong.
+			b.logf("[unexpected] unchanged netmap, skipping")
+		}
 	}
 	if newSt.URL != "" {
 		b.logf("Received auth URL: %.20v...\n", newSt.URL)
@@ -266,9 +306,10 @@ func (b *LocalBackend) Start(opts Options) error {
 		b.SetPrefs(prefs)
 	}
 	b.stateMachine()
-	})
+}
 
-	b.e.SetStatusCallback(func(s *wgengine.Status, err error) {
+// engineStatusUpdate is passed as a callback to wgengine SetStatusCallback.
+func (b *LocalBackend) engineStatusUpdate(s *wgengine.Status, err error) {
 	if err != nil {
 		b.logf("wgengine status error: %#v", err)
 		return
@@ -295,19 +336,13 @@ func (b *LocalBackend) Start(opts Options) error {
 	b.statusLock.Unlock()
 
 	b.send(Notify{Engine: &es})
-	})
+}
 
-	b.mu.Lock()
-	prefs := b.prefs.Clone()
-	b.mu.Unlock()
-
-	blid := b.backendLogID
-	b.logf("Backend: logs: be:%v fe:%v\n", blid, opts.FrontendLogID)
-	b.send(Notify{BackendLogID: &blid})
-	b.send(Notify{Prefs: prefs})
-
-	cli.Login(nil, controlclient.LoginDefault)
-	return nil
+func jsonHash(v interface{}) (hash [sha256.Size]byte, err error) {
+	h := sha256.New()
+	err = json.NewEncoder(h).Encode(v)
+	h.Sum(hash[:0])
+	return hash, err
 }
 
 func (b *LocalBackend) updateFilter(netMap *controlclient.NetworkMap) {