cmd/tailscale/cli: allow empty text ("") serve

Current behavior is broken. `tailscale serve text / ""` returns no error
and shows up in `tailscale serve status` but requests return a 500
"empty handler".

The commit changes the handler field `Text` to type of `*string`.

Closes #6405

Signed-off-by: Shayne Sweeney <shayne@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -53,6 +53,12 @@ func outln(a ...any) {
 	fmt.Fprintln(Stdout, a...)
 }
 
+// ptrTo returns a pointer to the provided value.
+// It's a convenience function to avoid having to write &value,
+// or where the language doesn't allow it.
+// Used primarily in tests.
+func ptrTo[T any](v T) *T { return &v }
+
 // ActLikeCLI reports whether a GUI application should act like the
 // CLI based on os.Args, GOOS, the context the process is running in
 // (pty, parent PID), etc.

cmd/tailscale/cli/serve.go

@@ -285,7 +285,7 @@ func (e *serveEnv) runServe(ctx context.Context, args []string) error {
 		}
 		h.Proxy = t
 	case "text":
-		h.Text = args[2]
+		h.Text = ptrTo(args[2])
 	default:
 		fmt.Fprintf(os.Stderr, "error: unknown serve type %q\n\n", args[1])
 		return flag.ErrHelp
@@ -547,8 +547,8 @@ func printWebStatusTree(sc *ipn.ServeConfig, hp ipn.HostPort) {
 			return "path", h.Path
 		case h.Proxy != "":
 			return "proxy", h.Proxy
-		case h.Text != "":
-			return "text", "\"" + elipticallyTruncate(h.Text, 20) + "\""
+		case h.Text != nil:
+			return "text", "\"" + elipticallyTruncate(*h.Text, 20) + "\""
 		}
 		return "", ""
 	}

cmd/tailscale/cli/serve_test.go

@@ -151,7 +151,7 @@ func TestServeConfigMutations(t *testing.T) {
 					"/abc": {Proxy: "http://127.0.0.1:3001"},
 				}},
 				"foo.test.ts.net:10000": {Handlers: map[string]*ipn.HTTPHandler{
-					"/": {Text: "hi"},
+					"/": {Text: ptrTo("hi")},
 				}},
 			},
 		},
@@ -314,7 +314,7 @@ func TestServeConfigMutations(t *testing.T) {
 			TCP: map[uint16]*ipn.TCPPortHandler{443: {HTTPS: true}},
 			Web: map[ipn.HostPort]*ipn.WebServerConfig{
 				"foo.test.ts.net:443": {Handlers: map[string]*ipn.HTTPHandler{
-					"/": {Text: "hello"},
+					"/": {Text: ptrTo("hello")},
 				}},
 			},
 		},
@@ -547,7 +547,7 @@ func TestServeConfigMutations(t *testing.T) {
 			TCP: map[uint16]*ipn.TCPPortHandler{443: {HTTPS: true}},
 			Web: map[ipn.HostPort]*ipn.WebServerConfig{
 				"foo.test.ts.net:443": {Handlers: map[string]*ipn.HTTPHandler{
-					"/tcp": {Text: "foo"},
+					"/tcp": {Text: ptrTo("foo")},
 				}},
 			},
 		},

cmd/tailscale/cli/set_test.go

@@ -13,8 +13,6 @@ import (
 	"tailscale.com/net/tsaddr"
 )
 
-func ptrTo[T any](v T) *T { return &v }
-
 func TestCalcAdvertiseRoutesForSet(t *testing.T) {
 	pfx := netip.MustParsePrefix
 	tests := []struct {

ipn/ipn_clone.go

@@ -118,6 +118,10 @@ func (src *HTTPHandler) Clone() *HTTPHandler {
 	}
 	dst := new(HTTPHandler)
 	*dst = *src
+	if dst.Text != nil {
+		dst.Text = new(string)
+		*dst.Text = *src.Text
+	}
 	return dst
 }
 
@@ -125,7 +129,7 @@ func (src *HTTPHandler) Clone() *HTTPHandler {
 var _HTTPHandlerCloneNeedsRegeneration = HTTPHandler(struct {
 	Path  string
 	Proxy string
-	Text  string
+	Text  *string
 }{})
 
 // Clone makes a deep copy of WebServerConfig.

ipn/ipn_view.go

@@ -290,13 +290,19 @@ func (v *HTTPHandlerView) UnmarshalJSON(b []byte) error {
 
 func (v HTTPHandlerView) Path() string  { return v.ж.Path }
 func (v HTTPHandlerView) Proxy() string { return v.ж.Proxy }
-func (v HTTPHandlerView) Text() string  { return v.ж.Text }
+func (v HTTPHandlerView) Text() *string {
+	if v.ж.Text == nil {
+		return nil
+	}
+	x := *v.ж.Text
+	return &x
+}
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _HTTPHandlerViewNeedsRegeneration = HTTPHandler(struct {
 	Path  string
 	Proxy string
-	Text  string
+	Text  *string
 }{})
 
 // View returns a readonly view of WebServerConfig.

ipn/ipnlocal/serve.go

@@ -395,9 +395,9 @@ func (b *LocalBackend) serveWebHandler(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if s := h.Text(); s != "" {
+	if s := h.Text(); s != nil {
 		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
-		io.WriteString(w, s)
+		io.WriteString(w, *s)
 		return
 	}
 	if v := h.Path(); v != "" {

ipn/serve.go

@@ -65,7 +65,7 @@ type HTTPHandler struct {
 	Path  string `json:",omitempty"` // absolute path to directory or file to serve
 	Proxy string `json:",omitempty"` // http://localhost:3000/, localhost:3030, 3030
 
-	Text string `json:",omitempty"` // plaintext to serve (primarily for testing)
+	Text *string `json:",omitempty"` // plaintext to serve (primarily for testing)
 
 	// TODO(bradfitz): bool to not enumerate directories? TTL on mapping for
 	// temporary ones? Error codes? Redirects?