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

1 files changed, 231 insertions, 0 deletions

diff --git a/hugolib/paths/paths.go b/hugolib/paths/paths.go
new file mode 100644
index 000000000..cf8792e5a
--- /dev/null
+++ b/hugolib/paths/paths.go
@@ -0,0 +1,231 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package paths
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/langs"
+
+	"github.com/gohugoio/hugo/hugofs"
+)
+
+var FilePathSeparator = string(filepath.Separator)
+
+type Paths struct {
+	Fs  *hugofs.Fs
+	Cfg config.Provider
+
+	BaseURL
+
+	// If the baseURL contains a base path, e.g. https://example.com/docs, then "/docs" will be the BasePath.
+	// This will not be set if canonifyURLs is enabled.
+	BasePath string
+
+	// Directories
+	// TODO(bep) when we have trimmed down mos of the dirs usage outside of this package, make
+	// these into an interface.
+	ContentDir      string
+	ThemesDir       string
+	WorkingDir      string
+	AbsResourcesDir string
+	AbsPublishDir   string
+
+	// pagination path handling
+	PaginatePath string
+
+	PublishDir string
+
+	DisablePathToLower bool
+	RemovePathAccents  bool
+	UglyURLs           bool
+	CanonifyURLs       bool
+
+	Language  *langs.Language
+	Languages langs.Languages
+
+	// The PathSpec looks up its config settings in both the current language
+	// and then in the global Viper config.
+	// Some settings, the settings listed below, does not make sense to be set
+	// on per-language-basis. We have no good way of protecting against this
+	// other than a "white-list". See language.go.
+	defaultContentLanguageInSubdir bool
+	DefaultContentLanguage         string
+	multilingual                   bool
+
+	themes    []string
+	AllThemes []ThemeConfig
+}
+
+func New(fs *hugofs.Fs, cfg config.Provider) (*Paths, error) {
+	baseURLstr := cfg.GetString("baseURL")
+	baseURL, err := newBaseURLFromString(baseURLstr)
+
+	if err != nil {
+		return nil, fmt.Errorf("Failed to create baseURL from %q: %s", baseURLstr, err)
+	}
+
+	// TODO(bep)
+	contentDir := cfg.GetString("contentDir")
+	workingDir := cfg.GetString("workingDir")
+	resourceDir := cfg.GetString("resourceDir")
+	publishDir := cfg.GetString("publishDir")
+
+	defaultContentLanguage := cfg.GetString("defaultContentLanguage")
+
+	var (
+		language  *langs.Language
+		languages langs.Languages
+	)
+
+	if l, ok := cfg.(*langs.Language); ok {
+		language = l
+
+	}
+
+	if l, ok := cfg.Get("languagesSorted").(langs.Languages); ok {
+		languages = l
+	}
+
+	if len(languages) == 0 {
+		// We have some old tests that does not test the entire chain, hence
+		// they have no languages. So create one so we get the proper filesystem.
+		languages = langs.Languages{&langs.Language{Lang: "en", Cfg: cfg, ContentDir: contentDir}}
+	}
+
+	absPublishDir := AbsPathify(workingDir, publishDir)
+	if !strings.HasSuffix(absPublishDir, FilePathSeparator) {
+		absPublishDir += FilePathSeparator
+	}
+	// If root, remove the second '/'
+	if absPublishDir == "//" {
+		absPublishDir = FilePathSeparator
+	}
+	absResourcesDir := AbsPathify(workingDir, resourceDir)
+	if !strings.HasSuffix(absResourcesDir, FilePathSeparator) {
+		absResourcesDir += FilePathSeparator
+	}
+	if absResourcesDir == "//" {
+		absResourcesDir = FilePathSeparator
+	}
+
+	p := &Paths{
+		Fs:      fs,
+		Cfg:     cfg,
+		BaseURL: baseURL,
+
+		DisablePathToLower: cfg.GetBool("disablePathToLower"),
+		RemovePathAccents:  cfg.GetBool("removePathAccents"),
+		UglyURLs:           cfg.GetBool("uglyURLs"),
+		CanonifyURLs:       cfg.GetBool("canonifyURLs"),
+
+		ContentDir: contentDir,
+		ThemesDir:  cfg.GetString("themesDir"),
+		WorkingDir: workingDir,
+
+		AbsResourcesDir: absResourcesDir,
+		AbsPublishDir:   absPublishDir,
+
+		themes: config.GetStringSlicePreserveString(cfg, "theme"),
+
+		multilingual:                   cfg.GetBool("multilingual"),
+		defaultContentLanguageInSubdir: cfg.GetBool("defaultContentLanguageInSubdir"),
+		DefaultContentLanguage:         defaultContentLanguage,
+
+		Language:  language,
+		Languages: languages,
+
+		PaginatePath: cfg.GetString("paginatePath"),
+	}
+
+	if cfg.IsSet("allThemes") {
+		p.AllThemes = cfg.Get("allThemes").([]ThemeConfig)
+	} else {
+		p.AllThemes, err = collectThemeNames(p)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// TODO(bep) remove this, eventually
+	p.PublishDir = absPublishDir
+
+	return p, nil
+}
+
+func (p *Paths) Lang() string {
+	if p == nil || p.Language == nil {
+		return ""
+	}
+	return p.Language.Lang
+}
+
+// ThemeSet checks whether a theme is in use or not.
+func (p *Paths) ThemeSet() bool {
+	return len(p.themes) > 0
+}
+
+func (p *Paths) Themes() []string {
+	return p.themes
+}
+
+func (p *Paths) GetLanguagePrefix() string {
+	if !p.multilingual {
+		return ""
+	}
+
+	defaultLang := p.DefaultContentLanguage
+	defaultInSubDir := p.defaultContentLanguageInSubdir
+
+	currentLang := p.Language.Lang
+	if currentLang == "" || (currentLang == defaultLang && !defaultInSubDir) {
+		return ""
+	}
+	return currentLang
+}
+
+// GetLangSubDir returns the given language's subdir if needed.
+func (p *Paths) GetLangSubDir(lang string) string {
+	if !p.multilingual {
+		return ""
+	}
+
+	if p.Languages.IsMultihost() {
+		return ""
+	}
+
+	if lang == "" || (lang == p.DefaultContentLanguage && !p.defaultContentLanguageInSubdir) {
+		return ""
+	}
+
+	return lang
+}
+
+// AbsPathify creates an absolute path if given a relative path. If already
+// absolute, the path is just cleaned.
+func (p *Paths) AbsPathify(inPath string) string {
+	return AbsPathify(p.WorkingDir, inPath)
+}
+
+// AbsPathify creates an absolute path if given a working dir and arelative path.
+// If already absolute, the path is just cleaned.
+func AbsPathify(workingDir, inPath string) string {
+	if filepath.IsAbs(inPath) {
+		return filepath.Clean(inPath)
+	}
+	return filepath.Join(workingDir, inPath)
+}