expand the path to support working in systemd service

internal/config/config.go

@@ -37,8 +37,8 @@ func Load(path string) (*Config, error) {
 		return nil, fmt.Errorf("parsing config file: %w", err)
 	}
 
-	cfg.NetrcPath = os.ExpandEnv(cfg.NetrcPath)
-	cfg.ClientSecretFile = os.ExpandEnv(cfg.ClientSecretFile)
+	cfg.NetrcPath = expandPath(cfg.NetrcPath)
+	cfg.ClientSecretFile = expandPath(cfg.ClientSecretFile)
 
 	if cfg.ClientSecretFile != "" {
 		secret, err := os.ReadFile(cfg.ClientSecretFile)
@@ -71,6 +71,29 @@ func (c *Config) validate() error {
 	return nil
 }
 
+// expandPath expands environment variables in a path. XDG base directory
+// variables are resolved using the xdg library, which applies the XDG spec
+// fallbacks (e.g. $HOME/.config when $XDG_CONFIG_HOME is unset). This ensures
+// correct behaviour in systemd user services, which do not set XDG variables.
+func expandPath(s string) string {
+	return os.Expand(s, func(key string) string {
+		switch key {
+		case "XDG_CONFIG_HOME":
+			return xdg.ConfigHome
+		case "XDG_DATA_HOME":
+			return xdg.DataHome
+		case "XDG_CACHE_HOME":
+			return xdg.CacheHome
+		case "XDG_STATE_HOME":
+			return xdg.StateHome
+		case "XDG_RUNTIME_DIR":
+			return xdg.RuntimeDir
+		default:
+			return os.Getenv(key)
+		}
+	})
+}
+
 // RefreshTokenPath returns the path to the stored refresh token.
 func RefreshTokenPath() string {
 	return filepath.Join(xdg.ConfigHome, "nix-cache-login", "refresh-token")