wgengine: separately dedupe wireguard configs and router configs.

Otherwise iOS/macOS will reconfigure their routing every time anything
minor changes in the netmap (in particular, endpoints and DERP homes),
which is way too often.

Some users reported "network reconfigured" errors from Chrome when this
happens.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

wgengine/userspace.go

@@ -57,9 +57,10 @@ type userspaceEngine struct {
 	magicConn *magicsock.Conn
 	linkMon   *monitor.Mon
 
-	wgLock       sync.Mutex // serializes all wgdev operations; see lock order comment below
-	lastReconfig string
-	lastCfg      wgcfg.Config
+	wgLock        sync.Mutex // serializes all wgdev operations; see lock order comment below
+	lastEngineSig string
+	lastRouterSig string
+	lastCfg       wgcfg.Config
 
 	mu             sync.Mutex // guards following; see lock order comment below
 	closing        bool       // Close was called (even if we're still closing)
@@ -359,17 +360,18 @@ func (e *userspaceEngine) pinger(peerKey wgcfg.Key, ips []wgcfg.IP) {
 	p.run(ctx, peerKey, ips, srcIP)
 }
 
-func configSignature(cfg *wgcfg.Config, routerCfg *router.Config) (string, error) {
+func configSignatures(cfg *wgcfg.Config, routerCfg *router.Config) (string, string, error) {
 	// TODO(apenwarr): get rid of uapi stuff for in-process comms
 	uapi, err := cfg.ToUAPI()
 	if err != nil {
-		return "", err
+		return "", "", err
 	}
 
-	return fmt.Sprintf("%s %v", uapi, routerCfg), nil
+	return uapi, fmt.Sprintf("%v", routerCfg), nil
 }
 
 func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config) error {
+	e.logf("Reconfig: router.Set: %p %p", cfg, routerCfg)
 	if routerCfg == nil {
 		panic("routerCfg must not be nil")
 	}
@@ -386,35 +388,42 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, routerCfg *router.Config)
 	}
 	e.mu.Unlock()
 
-	rc, err := configSignature(cfg, routerCfg)
+	ec, rc, err := configSignatures(cfg, routerCfg)
 	if err != nil {
 		return err
 	}
-	if rc == e.lastReconfig {
+	engineChanged := ec != e.lastEngineSig
+	routerChanged := rc != e.lastRouterSig
+	if !engineChanged && !routerChanged {
 		return ErrNoChanges
 	}
+	e.lastEngineSig = ec
+	e.lastRouterSig = rc
+	e.lastCfg = cfg.Copy()
 
 	e.logf("wgengine: Reconfig: configuring userspace wireguard engine")
-	e.lastReconfig = rc
-	e.lastCfg = cfg.Copy()
 
-	// Tell magicsock about the new (or initial) private key
-	// (which is needed by DERP) before wgdev gets it, as wgdev
-	// will start trying to handshake, which we want to be able to
-	// go over DERP.
-	if err := e.magicConn.SetPrivateKey(cfg.PrivateKey); err != nil {
-		e.logf("wgengine: Reconfig: SetPrivateKey: %v", err)
-	}
+	if engineChanged {
+		// Tell magicsock about the new (or initial) private key
+		// (which is needed by DERP) before wgdev gets it, as wgdev
+		// will start trying to handshake, which we want to be able to
+		// go over DERP.
+		if err := e.magicConn.SetPrivateKey(cfg.PrivateKey); err != nil {
+			e.logf("wgengine: Reconfig: SetPrivateKey: %v", err)
+		}
 
-	if err := e.wgdev.Reconfig(cfg); err != nil {
-		e.logf("wgdev.Reconfig: %v", err)
-		return err
-	}
+		if err := e.wgdev.Reconfig(cfg); err != nil {
+			e.logf("wgdev.Reconfig: %v", err)
+			return err
+		}
 
-	e.magicConn.UpdatePeers(peerSet)
+		e.magicConn.UpdatePeers(peerSet)
+	}
 
-	if err := e.router.Set(routerCfg); err != nil {
-		return err
+	if routerChanged {
+		if err := e.router.Set(routerCfg); err != nil {
+			return err
+		}
 	}
 
 	e.logf("wgengine: Reconfig done")