cmd/tailscale: fix help message for serve funnel

We currently print out "run tailscale serve --help" when the subcmd
might be funnel. This PR ensures the right subcmd is passed.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

cmd/tailscale/cli/serve_v2.go

@@ -100,6 +100,12 @@ func buildShortUsage(subcmd string) string {
 	}, "\n  ")
 }
 
+// errHelpFunc is standard error text that prompts users to
+// run `$subcmd --help` for information on how to use serve.
+var errHelpFunc = func(m serveMode) error {
+	return fmt.Errorf("try `tailscale %s --help` for usage info", infoMap[m].Name)
+}
+
 // newServeV2Command returns a new "serve" subcommand using e as its environment.
 func newServeV2Command(e *serveEnv, subcmd serveMode) *ffcli.Command {
 	if subcmd != serve && subcmd != funnel {
@@ -158,19 +164,19 @@ func validateArgs(subcmd serveMode, args []string) error {
 			fmt.Fprintf(os.Stderr, "\t- %s\n", translation)
 		}
 		fmt.Fprint(os.Stderr, "\nPlease see https://tailscale.com/kb/1242/tailscale-serve for more information.\n")
-		return errHelp
+		return errHelpFunc(subcmd)
 	}
 	if len(args) == 0 {
 		return flag.ErrHelp
 	}
 	if len(args) > 2 {
 		fmt.Fprintf(os.Stderr, "Error: invalid number of arguments (%d)\n", len(args))
-		return errHelp
+		return errHelpFunc(subcmd)
 	}
 	turnOff := args[len(args)-1] == "off"
 	if len(args) == 2 && !turnOff {
 		fmt.Fprintln(os.Stderr, "Error: invalid argument format")
-		return errHelp
+		return errHelpFunc(subcmd)
 	}
 
 	// Given the two checks above, we can assume there
@@ -225,7 +231,7 @@ func (e *serveEnv) runServeCombined(subcmd serveMode) execFunc {
 		srvType, srvPort, err := srvTypeAndPortFromFlags(e)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error: %v\n\n", err)
-			return errHelp
+			return errHelpFunc(subcmd)
 		}
 
 		sc, err := e.lc.GetServeConfig(ctx)
@@ -295,7 +301,7 @@ func (e *serveEnv) runServeCombined(subcmd serveMode) execFunc {
 		}
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "error: %v\n\n", err)
-			return errHelp
+			return errHelpFunc(subcmd)
 		}
 
 		if err := e.lc.SetServeConfig(ctx, parentSC); err != nil {

cmd/tailscale/cli/serve_v2_test.go

@@ -370,15 +370,15 @@ func TestServeDevConfigMutations(t *testing.T) {
 	add(step{reset: true})
 	add(step{ // !somehost, must be localhost or 127.0.0.1
 		command: cmd("serve --tls-terminated-tcp=443 --bg tcp://somehost:5432"),
-		wantErr: exactErr(errHelp, "errHelp"),
+		wantErr: exactErrMsg(errHelp),
 	})
 	add(step{ // bad target port, too low
 		command: cmd("serve --tls-terminated-tcp=443 --bg tcp://somehost:0"),
-		wantErr: exactErr(errHelp, "errHelp"),
+		wantErr: exactErrMsg(errHelp),
 	})
 	add(step{ // bad target port, too high
 		command: cmd("serve --tls-terminated-tcp=443 --bg tcp://somehost:65536"),
-		wantErr: exactErr(errHelp, "errHelp"),
+		wantErr: exactErrMsg(errHelp),
 	})
 	add(step{ // support shorthand
 		command: cmd("serve --tls-terminated-tcp=443 --bg 5432"),
@@ -508,7 +508,7 @@ func TestServeDevConfigMutations(t *testing.T) {
 	})
 	add(step{ // bad path
 		command: cmd("serve --https=443 --bg bad/path"),
-		wantErr: exactErr(errHelp, "errHelp"),
+		wantErr: exactErrMsg(errHelp),
 	})
 	add(step{reset: true})
 	add(step{
@@ -1283,3 +1283,14 @@ func TestIsLegacyInvocation(t *testing.T) {
 		})
 	}
 }
+
+// exactErrMsg returns an error checker that wants exactly the provided want error.
+// If optName is non-empty, it's used in the error message.
+func exactErrMsg(want error) func(error) string {
+	return func(got error) string {
+		if got.Error() == want.Error() {
+			return ""
+		}
+		return fmt.Sprintf("got error %v, want %v", got, want)
+	}
+}