cli/serve: shorten help text on error

Our BETA serve help text is long and often hides the actual error
in the user's usage. Instead of printing the full text, prompt
users to use `serve --help` if they want the help info.

Fixes #14274

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

cmd/tailscale/cli/serve.go

@@ -120,6 +120,10 @@ EXAMPLES
 	}
 }
 
+// errHelp is standard error text that prompts users to
+// run `serve --help` for information on how to use serve.
+var errHelp = errors.New("try `tailscale serve --help` for usage info")
+
 func (e *serveEnv) newFlags(name string, setup func(fs *flag.FlagSet)) *flag.FlagSet {
 	onError, out := flag.ExitOnError, Stderr
 	if e.testFlagOut != nil {
@@ -244,7 +248,7 @@ func (e *serveEnv) runServe(ctx context.Context, args []string) error {
 
 	if len(args) < 2 || ((srcType == "https" || srcType == "http") && !turnOff && len(args) < 3) {
 		fmt.Fprintf(os.Stderr, "error: invalid number of arguments\n\n")
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	if srcType == "https" && !turnOff {
@@ -286,7 +290,7 @@ func (e *serveEnv) runServe(ctx context.Context, args []string) error {
 	default:
 		fmt.Fprintf(os.Stderr, "error: invalid serve type %q\n", srcType)
 		fmt.Fprint(os.Stderr, "must be one of: http:<port>, https:<port>, tcp:<port> or tls-terminated-tcp:<port>\n\n", srcType)
-		return flag.ErrHelp
+		return errHelp
 	}
 }
 
@@ -322,13 +326,13 @@ func (e *serveEnv) handleWebServe(ctx context.Context, srvPort uint16, useTLS bo
 		}
 		if !filepath.IsAbs(source) {
 			fmt.Fprintf(os.Stderr, "error: path must be absolute\n\n")
-			return flag.ErrHelp
+			return errHelp
 		}
 		source = filepath.Clean(source)
 		fi, err := os.Stat(source)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error: invalid path: %v\n\n", err)
-			return flag.ErrHelp
+			return errHelp
 		}
 		if fi.IsDir() && !strings.HasSuffix(mount, "/") {
 			// dir mount points must end in /
@@ -354,7 +358,7 @@ func (e *serveEnv) handleWebServe(ctx context.Context, srvPort uint16, useTLS bo
 
 	if sc.IsTCPForwardingOnPort(srvPort) {
 		fmt.Fprintf(os.Stderr, "error: cannot serve web; already serving TCP\n")
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	mak.Set(&sc.TCP, srvPort, &ipn.TCPPortHandler{HTTPS: useTLS, HTTP: !useTLS})
@@ -542,18 +546,18 @@ func (e *serveEnv) handleTCPServe(ctx context.Context, srcType string, srcPort u
 		terminateTLS = true
 	default:
 		fmt.Fprintf(os.Stderr, "error: invalid TCP source %q\n\n", dest)
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	dstURL, err := url.Parse(dest)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "error: invalid TCP source %q: %v\n\n", dest, err)
-		return flag.ErrHelp
+		return errHelp
 	}
 	host, dstPortStr, err := net.SplitHostPort(dstURL.Host)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "error: invalid TCP source %q: %v\n\n", dest, err)
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	switch host {
@@ -562,12 +566,12 @@ func (e *serveEnv) handleTCPServe(ctx context.Context, srcType string, srcPort u
 	default:
 		fmt.Fprintf(os.Stderr, "error: invalid TCP source %q\n", dest)
 		fmt.Fprint(os.Stderr, "must be one of: localhost or 127.0.0.1\n\n", dest)
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	if p, err := strconv.ParseUint(dstPortStr, 10, 16); p == 0 || err != nil {
 		fmt.Fprintf(os.Stderr, "error: invalid port %q\n\n", dstPortStr)
-		return flag.ErrHelp
+		return errHelp
 	}
 
 	cursc, err := e.lc.GetServeConfig(ctx)

cmd/tailscale/cli/serve_test.go

@@ -339,19 +339,19 @@ func TestServeConfigMutations(t *testing.T) {
 	add(step{reset: true})
 	add(step{ // must include scheme for tcp
 		command: cmd("tls-terminated-tcp:443 localhost:5432"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{ // !somehost, must be localhost or 127.0.0.1
 		command: cmd("tls-terminated-tcp:443 tcp://somehost:5432"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{ // bad target port, too low
 		command: cmd("tls-terminated-tcp:443 tcp://somehost:0"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{ // bad target port, too high
 		command: cmd("tls-terminated-tcp:443 tcp://somehost:65536"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{
 		command: cmd("tls-terminated-tcp:443 tcp://localhost:5432"),
@@ -472,7 +472,7 @@ func TestServeConfigMutations(t *testing.T) {
 	})
 	add(step{ // bad path
 		command: cmd("https:443 / bad/path"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{reset: true})
 	add(step{
@@ -666,7 +666,7 @@ func TestServeConfigMutations(t *testing.T) {
 	})
 	add(step{ // try to start a web handler on the same port
 		command: cmd("https:443 / localhost:3000"),
-		wantErr: exactErr(flag.ErrHelp, "flag.ErrHelp"),
+		wantErr: exactErr(errHelp, "errHelp"),
 	})
 	add(step{reset: true})
 	add(step{ // start a web handler on port 443