ipn/localapi,client/local: add services over localapi

Updates tailscale/corp#40052

Signed-off-by: Adriano Sela Aviles <adriano@tailscale.com>

client/local/local.go

@@ -1422,3 +1422,13 @@ func (lc *Client) GetAppConnectorRouteInfo(ctx context.Context) (appctype.RouteI
 	}
 	return decodeJSON[appctype.RouteInfo](body)
 }
+
+// GetServices returns the Services visible to this node,
+// including their names, IP addresses, and ports, keyed by service name.
+func (lc *Client) GetServices(ctx context.Context) (map[tailcfg.ServiceName]tailcfg.ServiceDetails, error) {
+	body, err := lc.get200(ctx, "/localapi/v0/services")
+	if err != nil {
+		return nil, err
+	}
+	return decodeJSON[map[tailcfg.ServiceName]tailcfg.ServiceDetails](body)
+}

ipn/localapi/localapi.go

@@ -82,6 +82,7 @@ var handler = map[string]LocalAPIHandler{
 	"prefs":                (*Handler).servePrefs,
 	"reload-config":        (*Handler).reloadConfig,
 	"reset-auth":           (*Handler).serveResetAuth,
+	"services":             (*Handler).serveServices,
 	"set-expiry-sooner":    (*Handler).serveSetExpirySooner,
 	"shutdown":             (*Handler).serveShutdown,
 	"start":                (*Handler).serveStart,
@@ -1707,6 +1708,20 @@ func (h *Handler) serveShutdown(w http.ResponseWriter, r *http.Request) {
 	eventbus.Publish[Shutdown](ec).Publish(Shutdown{})
 }
 
+func (h *Handler) serveServices(w http.ResponseWriter, r *http.Request) {
+	if r.Method != httpm.GET {
+		http.Error(w, "only GET allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	nm := h.b.NetMap()
+	if nm == nil {
+		http.Error(w, "no netmap", http.StatusServiceUnavailable)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(nm.Services())
+}
+
 func (h *Handler) serveGetAppcRouteInfo(w http.ResponseWriter, r *http.Request) {
 	if !buildfeatures.HasAppConnectors {
 		http.Error(w, feature.ErrUnavailable.Error(), http.StatusNotImplemented)

types/netmap/netmap.go

@@ -149,6 +149,9 @@ func (nm *NetworkMap) GetIPVIPServiceMap() IPServiceMappings {
 // Services returns the Services visible (accessible) to this node,
 // decoded from [tailcfg.NodeAttrPrefixServices]+serviceName entries in the
 // self node's CapMap. Returns nil if nm is nil or SelfNode is invalid.
+//
+// TODO(adrianosela): cache the result of decoding the capmap so
+// we don't have to decode it multiple times after each netmap update.
 func (nm *NetworkMap) Services() map[tailcfg.ServiceName]tailcfg.ServiceDetails {
 	if nm == nil || !nm.SelfNode.Valid() {
 		return nil