Add support for theme composition and inheritance

This commit adds support for theme composition and inheritance in Hugo.

With this, it helps thinking about a theme as a set of ordered components:

```toml
theme = ["my-shortcodes", "base-theme", "hyde"]
```

The theme definition example above in `config.toml` creates a theme with the 3 components with presedence from left to right.

So, Hugo will, for any given file, data entry etc., look first in the project, and then in `my-shortcode`, `base-theme` and lastly `hyde`.

Hugo uses two different algorithms to merge the filesystems, depending on the file type:

* For `i18n` and `data` files, Hugo merges deeply using the translation id and data key inside the files.
* For `static`, `layouts` (templates) and `archetypes` files, these are merged on file level. So the left-most file will be chosen.

The name used in the `theme` definition above must match a folder in `/your-site/themes`, e.g. `/your-site/themes/my-shortcodes`. There are  plans to improve on this and get a URL scheme so this can be resolved automatically.

Also note that a component that is part of a theme can have its own configuration file, e.g. `config.toml`. There are currently some restrictions to what a theme component can configure:

* `params` (global and per language)
* `menu` (global and per language)
* `outputformats` and `mediatypes`

The same rules apply here: The left-most param/menu etc. with the same ID will win. There are some hidden and experimental namespace support in the above, which we will work to improve in the future, but theme authors are encouraged to create their own namespaces to avoid naming conflicts.

A final note: Themes/components can also have a `theme` definition in their `config.toml` and similar, which is the "inheritance" part of this commit's title. This is currently not supported by the Hugo theme site. We will have to wait for some "auto dependency" feature to be implemented for that to happen, but this can be a powerful feature if you want to create your own theme-variant based on others.

Fixes #4460
Fixes #4450

hugolib/config.go

@@ -16,11 +16,14 @@ package hugolib
 import (
 	"errors"
 	"fmt"
-	"path/filepath"
+
+	"github.com/gohugoio/hugo/hugolib/paths"
 
 	"io"
 	"strings"
 
+	"github.com/gohugoio/hugo/langs"
+
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/config/privacy"
 	"github.com/gohugoio/hugo/config/services"
@@ -81,6 +84,8 @@ func LoadConfigDefault(fs afero.Fs) (*viper.Viper, error) {
 	return v, err
 }
 
+var ErrNoConfigFile = errors.New("Unable to locate Config file. Perhaps you need to create a new site.\n       Run `hugo help new` for details.\n")
+
 // LoadConfig loads Hugo configuration into a new Viper and then adds
 // a set of defaults.
 func LoadConfig(d ConfigSourceDescriptor, doWithConfig ...func(cfg config.Provider) error) (*viper.Viper, []string, error) {
@@ -100,41 +105,50 @@ func LoadConfig(d ConfigSourceDescriptor, doWithConfig ...func(cfg config.Provid
 	v.SetConfigFile(configFilenames[0])
 	v.AddConfigPath(d.Path)
 
+	var configFileErr error
+
 	err := v.ReadInConfig()
 	if err != nil {
 		if _, ok := err.(viper.ConfigParseError); ok {
 			return nil, configFiles, err
 		}
-		return nil, configFiles, fmt.Errorf("Unable to locate Config file. Perhaps you need to create a new site.\n       Run `hugo help new` for details. (%s)\n", err)
+		configFileErr = ErrNoConfigFile
 	}
 
-	if cf := v.ConfigFileUsed(); cf != "" {
-		configFiles = append(configFiles, cf)
-	}
+	if configFileErr == nil {
 
-	for _, configFile := range configFilenames[1:] {
-		var r io.Reader
-		var err error
-		if r, err = fs.Open(configFile); err != nil {
-			return nil, configFiles, fmt.Errorf("Unable to open Config file.\n (%s)\n", err)
+		if cf := v.ConfigFileUsed(); cf != "" {
+			configFiles = append(configFiles, cf)
 		}
-		if err = v.MergeConfig(r); err != nil {
-			return nil, configFiles, fmt.Errorf("Unable to parse/merge Config file (%s).\n (%s)\n", configFile, err)
+
+		for _, configFile := range configFilenames[1:] {
+			var r io.Reader
+			var err error
+			if r, err = fs.Open(configFile); err != nil {
+				return nil, configFiles, fmt.Errorf("Unable to open Config file.\n (%s)\n", err)
+			}
+			if err = v.MergeConfig(r); err != nil {
+				return nil, configFiles, fmt.Errorf("Unable to parse/merge Config file (%s).\n (%s)\n", configFile, err)
+			}
+			configFiles = append(configFiles, configFile)
 		}
-		configFiles = append(configFiles, configFile)
+
 	}
 
 	if err := loadDefaultSettingsFor(v); err != nil {
 		return v, configFiles, err
 	}
 
-	themeConfigFile, err := loadThemeConfig(d, v)
-	if err != nil {
-		return v, configFiles, err
-	}
+	if configFileErr == nil {
 
-	if themeConfigFile != "" {
-		configFiles = append(configFiles, themeConfigFile)
+		themeConfigFiles, err := loadThemeConfig(d, v)
+		if err != nil {
+			return v, configFiles, err
+		}
+
+		if len(themeConfigFiles) > 0 {
+			configFiles = append(configFiles, themeConfigFiles...)
+		}
 	}
 
 	// We create languages based on the settings, so we need to make sure that
@@ -149,11 +163,11 @@ func LoadConfig(d ConfigSourceDescriptor, doWithConfig ...func(cfg config.Provid
 		return v, configFiles, err
 	}
 
-	return v, configFiles, nil
+	return v, configFiles, configFileErr
 
 }
 
-func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error {
+func loadLanguageSettings(cfg config.Provider, oldLangs langs.Languages) error {
 
 	defaultLang := cfg.GetString("defaultContentLanguage")
 
@@ -182,14 +196,14 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 	}
 
 	var (
-		langs helpers.Languages
-		err   error
+		languages2 langs.Languages
+		err        error
 	)
 
 	if len(languages) == 0 {
-		langs = append(langs, helpers.NewDefaultLanguage(cfg))
+		languages2 = append(languages2, langs.NewDefaultLanguage(cfg))
 	} else {
-		langs, err = toSortedLanguages(cfg, languages)
+		languages2, err = toSortedLanguages(cfg, languages)
 		if err != nil {
 			return fmt.Errorf("Failed to parse multilingual config: %s", err)
 		}
@@ -201,10 +215,10 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 		// The validation below isn't complete, but should cover the most
 		// important cases.
 		var invalid bool
-		if langs.IsMultihost() != oldLangs.IsMultihost() {
+		if languages2.IsMultihost() != oldLangs.IsMultihost() {
 			invalid = true
 		} else {
-			if langs.IsMultihost() && len(langs) != len(oldLangs) {
+			if languages2.IsMultihost() && len(languages2) != len(oldLangs) {
 				invalid = true
 			}
 		}
@@ -213,10 +227,10 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 			return errors.New("language change needing a server restart detected")
 		}
 
-		if langs.IsMultihost() {
+		if languages2.IsMultihost() {
 			// We need to transfer any server baseURL to the new language
 			for i, ol := range oldLangs {
-				nl := langs[i]
+				nl := languages2[i]
 				nl.Set("baseURL", ol.GetString("baseURL"))
 			}
 		}
@@ -225,7 +239,7 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 	// The defaultContentLanguage is something the user has to decide, but it needs
 	// to match a language in the language definition list.
 	langExists := false
-	for _, lang := range langs {
+	for _, lang := range languages2 {
 		if lang.Lang == defaultLang {
 			langExists = true
 			break
@@ -236,10 +250,10 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 		return fmt.Errorf("site config value %q for defaultContentLanguage does not match any language definition", defaultLang)
 	}
 
-	cfg.Set("languagesSorted", langs)
-	cfg.Set("multilingual", len(langs) > 1)
+	cfg.Set("languagesSorted", languages2)
+	cfg.Set("multilingual", len(languages2) > 1)
 
-	multihost := langs.IsMultihost()
+	multihost := languages2.IsMultihost()
 
 	if multihost {
 		cfg.Set("defaultContentLanguageInSubdir", true)
@@ -250,7 +264,7 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 		// The baseURL may be provided at the language level. If that is true,
 		// then every language must have a baseURL. In this case we always render
 		// to a language sub folder, which is then stripped from all the Permalink URLs etc.
-		for _, l := range langs {
+		for _, l := range languages2 {
 			burl := l.GetLocal("baseURL")
 			if burl == nil {
 				return errors.New("baseURL must be set on all or none of the languages")
@@ -262,49 +276,32 @@ func loadLanguageSettings(cfg config.Provider, oldLangs helpers.Languages) error
 	return nil
 }
 
-func loadThemeConfig(d ConfigSourceDescriptor, v1 *viper.Viper) (string, error) {
+func loadThemeConfig(d ConfigSourceDescriptor, v1 *viper.Viper) ([]string, error) {
+	themesDir := paths.AbsPathify(d.WorkingDir, v1.GetString("themesDir"))
+	themes := config.GetStringSlicePreserveString(v1, "theme")
 
-	theme := v1.GetString("theme")
-	if theme == "" {
-		return "", nil
-	}
-
-	themesDir := helpers.AbsPathify(d.WorkingDir, v1.GetString("themesDir"))
-	configDir := filepath.Join(themesDir, theme)
-
-	var (
-		configPath string
-		exists     bool
-		err        error
-	)
-
-	// Viper supports more, but this is the sub-set supported by Hugo.
-	for _, configFormats := range []string{"toml", "yaml", "yml", "json"} {
-		configPath = filepath.Join(configDir, "config."+configFormats)
-		exists, err = helpers.Exists(configPath, d.Fs)
-		if err != nil {
-			return "", err
-		}
-		if exists {
-			break
-		}
-	}
-
-	if !exists {
-		// No theme config set.
-		return "", nil
-	}
-
-	v2 := viper.New()
-	v2.SetFs(d.Fs)
-	v2.AutomaticEnv()
-	v2.SetEnvPrefix("hugo")
-	v2.SetConfigFile(configPath)
-
-	err = v2.ReadInConfig()
+	//  CollectThemes(fs afero.Fs, themesDir string, themes []strin
+	themeConfigs, err := paths.CollectThemes(d.Fs, themesDir, themes)
 	if err != nil {
-		return "", err
+		return nil, err
 	}
+	v1.Set("allThemes", themeConfigs)
+
+	var configFilenames []string
+	for _, tc := range themeConfigs {
+		if tc.ConfigFilename != "" {
+			configFilenames = append(configFilenames, tc.ConfigFilename)
+			if err := applyThemeConfig(v1, tc); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return configFilenames, nil
+
+}
+
+func applyThemeConfig(v1 *viper.Viper, theme paths.ThemeConfig) error {
 
 	const (
 		paramsKey    = "params"
@@ -312,11 +309,13 @@ func loadThemeConfig(d ConfigSourceDescriptor, v1 *viper.Viper) (string, error)
 		menuKey      = "menu"
 	)
 
+	v2 := theme.Cfg
+
 	for _, key := range []string{paramsKey, "outputformats", "mediatypes"} {
 		mergeStringMapKeepLeft("", key, v1, v2)
 	}
 
-	themeLower := strings.ToLower(theme)
+	themeLower := strings.ToLower(theme.Name)
 	themeParamsNamespace := paramsKey + "." + themeLower
 
 	// Set namespaced params
@@ -371,11 +370,11 @@ func loadThemeConfig(d ConfigSourceDescriptor, v1 *viper.Viper) (string, error)
 		}
 	}
 
-	return v2.ConfigFileUsed(), nil
+	return nil
 
 }
 
-func mergeStringMapKeepLeft(rootKey, key string, v1, v2 *viper.Viper) {
+func mergeStringMapKeepLeft(rootKey, key string, v1, v2 config.Provider) {
 	if !v2.IsSet(key) {
 		return
 	}