Add TOML config file support to mcrctl

Loads defaults from ~/.config/mcrctl.toml (or XDG_CONFIG_HOME).
Resolution order: flag > env (MCR_TOKEN) > config file.
Adds --config flag to specify an explicit path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/mcrctl/config.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	toml "github.com/pelletier/go-toml/v2"
+)
+
+// config holds CLI defaults loaded from a TOML file.
+type config struct {
+	Server string `toml:"server"`
+	GRPC   string `toml:"grpc"`
+	Token  string `toml:"token"`
+	CACert string `toml:"ca_cert"`
+}
+
+// loadConfig reads a TOML config file from the given path. If path is
+// empty it searches the default location (~/.config/mcrctl.toml). A
+// missing file at the default location is not an error — an explicit
+// --config path that doesn't exist is.
+func loadConfig(path string) (config, error) {
+	explicit := path != ""
+	if !explicit {
+		path = defaultConfigPath()
+	}
+
+	data, err := os.ReadFile(path) //nolint:gosec // operator-supplied config path
+	if err != nil {
+		if !explicit && os.IsNotExist(err) {
+			return config{}, nil
+		}
+		return config{}, fmt.Errorf("reading config: %w", err)
+	}
+
+	var cfg config
+	if err := toml.Unmarshal(data, &cfg); err != nil {
+		return config{}, fmt.Errorf("parsing config %s: %w", path, err)
+	}
+	return cfg, nil
+}
+
+// defaultConfigPath returns ~/.config/mcrctl.toml, respecting
+// XDG_CONFIG_HOME if set.
+func defaultConfigPath() string {
+	dir := os.Getenv("XDG_CONFIG_HOME")
+	if dir == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "mcrctl.toml"
+		}
+		dir = filepath.Join(home, ".config")
+	}
+	return filepath.Join(dir, "mcrctl.toml")
+}

cmd/mcrctl/config_test.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadConfigExplicitPath(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "mcrctl.toml")
+	data := []byte(`server = "https://reg.example.com"
+grpc   = "reg.example.com:9443"
+token  = "file-token"
+ca_cert = "/path/to/ca.pem"
+`)
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := loadConfig(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if cfg.Server != "https://reg.example.com" {
+		t.Errorf("Server = %q, want %q", cfg.Server, "https://reg.example.com")
+	}
+	if cfg.GRPC != "reg.example.com:9443" {
+		t.Errorf("GRPC = %q, want %q", cfg.GRPC, "reg.example.com:9443")
+	}
+	if cfg.Token != "file-token" {
+		t.Errorf("Token = %q, want %q", cfg.Token, "file-token")
+	}
+	if cfg.CACert != "/path/to/ca.pem" {
+		t.Errorf("CACert = %q, want %q", cfg.CACert, "/path/to/ca.pem")
+	}
+}
+
+func TestLoadConfigMissingDefaultIsOK(t *testing.T) {
+	// Empty path triggers default search; missing file is not an error.
+	cfg, err := loadConfig("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.Server != "" || cfg.GRPC != "" || cfg.Token != "" {
+		t.Errorf("expected zero config from missing default, got %+v", cfg)
+	}
+}
+
+func TestLoadConfigMissingExplicitIsError(t *testing.T) {
+	_, err := loadConfig("/nonexistent/mcrctl.toml")
+	if err == nil {
+		t.Fatal("expected error for missing explicit config path")
+	}
+}
+
+func TestLoadConfigInvalidTOML(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "bad.toml")
+	if err := os.WriteFile(path, []byte("not valid [[[ toml"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := loadConfig(path)
+	if err == nil {
+		t.Fatal("expected error for invalid TOML")
+	}
+}
+
+func TestLoadConfigPartial(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "mcrctl.toml")
+	data := []byte(`server = "https://reg.example.com"
+`)
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := loadConfig(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if cfg.Server != "https://reg.example.com" {
+		t.Errorf("Server = %q, want %q", cfg.Server, "https://reg.example.com")
+	}
+	if cfg.GRPC != "" {
+		t.Errorf("GRPC should be empty, got %q", cfg.GRPC)
+	}
+}

cmd/mcrctl/main.go

@@ -18,6 +18,7 @@ var version = "dev"
 
 // Global flags, resolved in PersistentPreRunE.
 var (
+	flagConfig string
 	flagServer string
 	flagGRPC   string
 	flagToken  string
@@ -32,15 +33,27 @@ func main() {
 		Use:     "mcrctl",
 		Short:   "Metacircular Container Registry admin CLI",
 		Version: version,
-		PersistentPreRunE: func(_ *cobra.Command, _ []string) error {
-			// Resolve token: flag overrides env.
-			token := flagToken
-			if token == "" {
-				token = os.Getenv("MCR_TOKEN")
+		PersistentPreRunE: func(cmd *cobra.Command, _ []string) error {
+			cfg, err := loadConfig(flagConfig)
+			if err != nil {
+				return err
 			}
 
-			var err error
-			client, err = newClient(flagServer, flagGRPC, token, flagCACert)
+			// Resolution order: flag > env > config file.
+			server := applyDefault(cmd, "server", flagServer, cfg.Server)
+			grpcAddr := applyDefault(cmd, "grpc", flagGRPC, cfg.GRPC)
+			caCert := applyDefault(cmd, "ca-cert", flagCACert, cfg.CACert)
+
+			token := flagToken
+			if !cmd.Flags().Changed("token") {
+				if envToken := os.Getenv("MCR_TOKEN"); envToken != "" {
+					token = envToken
+				} else if cfg.Token != "" {
+					token = cfg.Token
+				}
+			}
+
+			client, err = newClient(server, grpcAddr, token, caCert)
 			if err != nil {
 				return err
 			}
@@ -53,9 +66,10 @@ func main() {
 		},
 	}
 
+	root.PersistentFlags().StringVar(&flagConfig, "config", "", "config file (default ~/.config/mcrctl.toml)")
 	root.PersistentFlags().StringVar(&flagServer, "server", "", "REST API base URL (e.g. https://registry.example.com)")
 	root.PersistentFlags().StringVar(&flagGRPC, "grpc", "", "gRPC server address (e.g. registry.example.com:9443)")
-	root.PersistentFlags().StringVar(&flagToken, "token", "", "bearer token (fallback: MCR_TOKEN env)")
+	root.PersistentFlags().StringVar(&flagToken, "token", "", "bearer token (fallback: MCR_TOKEN env, config file)")
 	root.PersistentFlags().StringVar(&flagCACert, "ca-cert", "", "custom CA certificate PEM file")
 	root.PersistentFlags().BoolVar(&flagJSON, "json", false, "output as JSON instead of table")
 
@@ -740,6 +754,18 @@ func runSnapshot(_ *cobra.Command, _ []string) error {
 
 // ---------- helpers ----------
 
+// applyDefault returns the flag value if the flag was explicitly set on
+// the command line, otherwise falls back to the config file value.
+func applyDefault(cmd *cobra.Command, name, flagVal, cfgVal string) string {
+	if cmd.Flags().Changed(name) {
+		return flagVal
+	}
+	if cfgVal != "" {
+		return cfgVal
+	}
+	return flagVal
+}
+
 // confirmPrompt displays msg and waits for y/n from stdin.
 func confirmPrompt(msg string) bool {
 	fmt.Fprintf(os.Stderr, "%s [y/N] ", msg)