cmd/tailcat: keep --version working as an unadvertised alias

RELEASING.md

@@ -44,7 +44,7 @@ both `vX.Y.Z` and `latest`. The image is the static binary in a
 image; see `Dockerfile.goreleaser`.
 
 The binary version is embedded at build time via `-ldflags -X
-main.version=...`; `tailcat --version` prints it. Builds made with
+main.version=...`; `tailcat version` prints it. Builds made with
 `go install github.com/tailscale/tailcat/cmd/tailcat@vX.Y.Z` instead
 report the module version from the Go build info.
 

cmd/tailcat/e2e_test.go

@@ -95,6 +95,22 @@ func buildTailcatBinary(dir string) error {
 	return nil
 }
 
+// TestVersionFlag verifies that "tailcat --version" works even though
+// --version is not a registered flag; main special-cases it on parse
+// failure. The advertised interface is the version subcommand, but
+// nixpkgs' versionCheckHook runs "tailcat --version" and depends on
+// its exit status and output.
+func TestVersionFlag(t *testing.T) {
+	bin := buildTailcat(t)
+	out, err := exec.Command(bin, "--version").CombinedOutput()
+	if err != nil {
+		t.Fatalf("tailcat --version: %v\n%s", err, out)
+	}
+	if v := strings.TrimSpace(string(out)); v == "" || strings.Contains(v, "\n") {
+		t.Errorf("output = %q; want a single non-empty version line", out)
+	}
+}
+
 // testNoopCommand returns a child command that exits successfully,
 // for tests that only care that a wrapper ran it. Windows has no
 // "true" binary.

cmd/tailcat/tailcat.go

@@ -54,7 +54,6 @@ var (
 	flagKey         *string
 	flagAllow       *string
 	flagVerbose     *bool
-	flagVersion     *bool
 	flagFullAddress *bool
 	flagJSON        *bool
 	flagDERPMapURL  *string
@@ -89,7 +88,6 @@ func newRootCommand() *ff.Command {
 	flagServe = rootFS.StringLong("serve", "", "comma-separated list of port numbers, port ranges, or service names to serve; the same list the serve subcommand takes as arguments. Service names are: 'all' (serve all ports), 'exit-node' (run an exit node for all addresses), 'no-auth-ssh' (auth-free SSH server). If empty, it accepts a single connection on any port, writes it to stdout, and exits.")
 	flagKey = rootFS.StringLong("key", "", "'new' for an ephemeral key. If empty, the default saved key is used if it exists ('default' in server mode, 'client-default' in client modes; see genkey), else an ephemeral key. Otherwise the path to a *.private.json or a name like 'foo' to read it from $CONFIG/tailcat/keys/foo.private.json")
 	flagVerbose = rootFS.BoolLong("verbose", "be verbose")
-	flagVersion = rootFS.BoolLong("version", "print the tailcat version and exit")
 	flagJSON = rootFS.BoolLong("json", "in server mode, write {\"listenAddr\": ...} JSON to stdout")
 	flagDERPMapURL = rootFS.StringLong("derpmap-url", tailcat.DefaultDERPMapURL, "URL of the JSON DERP map used to resolve or auto-select a DERP region")
 
@@ -202,7 +200,7 @@ func newRootCommand() *ff.Command {
 			{
 				Name:      "version",
 				Usage:     "tailcat version",
-				ShortHelp: "print the tailcat version (like --version)",
+				ShortHelp: "print the tailcat version",
 				Exec: func(ctx context.Context, args []string) error {
 					fmt.Println(versionString())
 					return nil
@@ -497,13 +495,16 @@ func usagef(format string, args ...any) error {
 func main() {
 	root := newRootCommand()
 	err := root.Parse(os.Args[1:])
+	if err != nil && slices.Contains(os.Args[1:], "--version") {
+		// The advertised way to get the version is the version
+		// subcommand, but nixpkgs' versionCheckHook runs "tailcat
+		// --version", so keep that working as an unadvertised alias.
+		// It's not a registered flag, so it only shows up here as a
+		// parse failure.
+		fmt.Println(versionString())
+		return
+	}
 	if err == nil {
-		// The --version flag short-circuits everything else,
-		// including subcommand dispatch.
-		if *flagVersion {
-			fmt.Println(versionString())
-			return
-		}
 		if *flagVerbose {
 			tailcat.Verbose = true
 		}