ipn/localapi: add debug (root-required) access to inject debug Notify

For testing of Windows GUI client.

Updates #6480

Change-Id: I42f7526d95723e14bed7085fb759e371b43aa9da
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1820,6 +1820,13 @@ func (b *LocalBackend) pollRequestEngineStatus(ctx context.Context) {
 	}
 }
 
+// DebugNotify injects a fake notify message to clients.
+//
+// It should only be used via the LocalAPI's debug handler.
+func (b *LocalBackend) DebugNotify(n ipn.Notify) {
+	b.send(n)
+}
+
 // send delivers n to the connected frontend and any API watchers from
 // LocalBackend.WatchNotifications (via the LocalAPI).
 //
@@ -1828,7 +1835,9 @@ func (b *LocalBackend) pollRequestEngineStatus(ctx context.Context) {
 //
 // b.mu must not be held.
 func (b *LocalBackend) send(n ipn.Notify) {
+	if n.Version == "" {
 		n.Version = version.Long
+	}
 
 	b.mu.Lock()
 	notifyFunc := b.notify

ipn/localapi/localapi.go

@@ -434,7 +434,16 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "POST required", http.StatusMethodNotAllowed)
 		return
 	}
-	action := r.FormValue("action")
+	// The action is normally in a POST form parameter, but
+	// some actions (like "notify") want a full JSON body, so
+	// permit some to have their action in a header.
+	var action string
+	switch v := r.Header.Get("Debug-Action"); v {
+	case "notify":
+		action = v
+	default:
+		action = r.FormValue("action")
+	}
 	var err error
 	switch action {
 	case "rebind":
@@ -455,6 +464,14 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 		// client already did. A future change will remove this, so don't depend
 		// on it.
 		h.b.RequestEngineStatus()
+	case "notify":
+		var n ipn.Notify
+		err = json.NewDecoder(r.Body).Decode(&n)
+		if err != nil {
+			break
+		}
+		h.b.DebugNotify(n)
+
 	case "":
 		err = fmt.Errorf("missing parameter 'action'")
 	default: