Implement configuration in a directory for modules

Fixes #8654

modules/client.go

@@ -653,6 +653,9 @@ type ClientConfig struct {
 	// Absolute path to the project's themes dir.
 	ThemesDir string
 
+	// Eg. "production"
+	Environment string
+
 	CacheDir     string // Module cache
 	ModuleConfig Config
 }

modules/collect.go

@@ -396,17 +396,16 @@ func (c *collector) applyMounts(moduleImport Import, mod *moduleAdapter) error {
 func (c *collector) applyThemeConfig(tc *moduleAdapter) error {
 	var (
 		configFilename string
-		cfg            config.Provider
 		themeCfg       map[string]interface{}
-		hasConfig      bool
+		hasConfigFile  bool
 		err            error
 	)
 
 	// Viper supports more, but this is the sub-set supported by Hugo.
 	for _, configFormats := range config.ValidConfigFileExtensions {
 		configFilename = filepath.Join(tc.Dir(), "config."+configFormats)
-		hasConfig, _ = afero.Exists(c.fs, configFilename)
-		if hasConfig {
+		hasConfigFile, _ = afero.Exists(c.fs, configFilename)
+		if hasConfigFile {
 			break
 		}
 	}
@@ -428,20 +427,38 @@ func (c *collector) applyThemeConfig(tc *moduleAdapter) error {
 		}
 	}
 
-	if hasConfig {
+	if hasConfigFile {
 		if configFilename != "" {
 			var err error
-			cfg, err = config.FromFile(c.fs, configFilename)
+			tc.cfg, err = config.FromFile(c.fs, configFilename)
 			if err != nil {
 				return errors.Wrapf(err, "failed to read module config for %q in %q", tc.Path(), configFilename)
 			}
 		}
 
-		tc.configFilename = configFilename
-		tc.cfg = cfg
+		tc.configFilenames = append(tc.configFilenames, configFilename)
+
 	}
 
-	config, err := decodeConfig(cfg, c.moduleConfig.replacementsMap)
+	// Also check for a config dir, which we overlay on top of the file configuration.
+	configDir := filepath.Join(tc.Dir(), "config")
+	dcfg, dirnames, err := config.LoadConfigFromDir(c.fs, configDir, c.ccfg.Environment)
+	if err != nil {
+		return err
+	}
+
+	if len(dirnames) > 0 {
+		tc.configFilenames = append(tc.configFilenames, dirnames...)
+
+		if hasConfigFile {
+			// Set will overwrite existing keys.
+			tc.cfg.Set("", dcfg.Get(""))
+		} else {
+			tc.cfg = dcfg
+		}
+	}
+
+	config, err := decodeConfig(tc.cfg, c.moduleConfig.replacementsMap)
 	if err != nil {
 		return err
 	}

modules/module.go

@@ -30,10 +30,10 @@ type Module interface {
 	// The decoded module config and mounts.
 	Config() Config
 
-	// Optional configuration filename (e.g. "/themes/mytheme/config.json").
+	// Optional configuration filenames (e.g. "/themes/mytheme/config.json").
 	// This will be added to the special configuration watch list when in
 	// server mode.
-	ConfigFilename() string
+	ConfigFilenames() []string
 
 	// Directory holding files for this module.
 	Dir() string
@@ -82,9 +82,9 @@ type moduleAdapter struct {
 
 	mounts []Mount
 
-	configFilename string
-	cfg            config.Provider
-	config         Config
+	configFilenames []string
+	cfg             config.Provider
+	config          Config
 
 	// Set if a Go module.
 	gomod *goModule
@@ -98,8 +98,8 @@ func (m *moduleAdapter) Config() Config {
 	return m.config
 }
 
-func (m *moduleAdapter) ConfigFilename() string {
-	return m.configFilename
+func (m *moduleAdapter) ConfigFilenames() []string {
+	return m.configFilenames
 }
 
 func (m *moduleAdapter) Dir() string {