Add Hugo Modules

This commit implements Hugo Modules.

This is a broad subject, but some keywords include:

* A new `module` configuration section where you can import almost anything. You can configure both your own file mounts nd the file mounts of the modules you import. This is the new recommended way of configuring what you earlier put in `configDir`, `staticDir` etc. And it also allows you to mount folders in non-Hugo-projects, e.g. the `SCSS` folder in the Bootstrap GitHub project.
* A module consists of a set of mounts to the standard 7 component types in Hugo: `static`, `content`, `layouts`, `data`, `assets`, `i18n`, and `archetypes`. Yes, Theme Components can now include content, which should be very useful, especially in bigger multilingual projects.
* Modules not in your local file cache will be downloaded automatically and even "hot replaced" while the server is running.
* Hugo Modules supports and encourages semver versioned modules, and uses the minimal version selection algorithm to resolve versions.
* A new set of CLI commands are provided to manage all of this: `hugo mod init`,  `hugo mod get`,  `hugo mod graph`,  `hugo mod tidy`, and  `hugo mod vendor`.

All of the above is backed by Go Modules.

Fixes #5973
Fixes #5996
Fixes #6010
Fixes #5911
Fixes #5940
Fixes #6074
Fixes #6082
Fixes #6092

20 files changed, 2663 insertions, 791 deletions

diff --git a/hugofs/basepath_real_filename_fs.go b/hugofs/basepath_real_filename_fs.go
deleted file mode 100644
index 1024c4d30..000000000
--- a/hugofs/basepath_real_filename_fs.go
+++ /dev/null
@@ -1,91 +0,0 @@
-// 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 hugofs
-
-import (
-	"os"
-
-	"github.com/spf13/afero"
-)
-
-// RealFilenameInfo is a thin wrapper around os.FileInfo adding the real filename.
-type RealFilenameInfo interface {
-	os.FileInfo
-
-	// This is the real filename to the file in the underlying filesystem.
-	RealFilename() string
-}
-
-type realFilenameInfo struct {
-	os.FileInfo
-	realFilename string
-}
-
-func (f *realFilenameInfo) RealFilename() string {
-	return f.realFilename
-}
-
-// NewBasePathRealFilenameFs returns a new BasePathRealFilenameFs instance
-// using base.
-func NewBasePathRealFilenameFs(base *afero.BasePathFs) *BasePathRealFilenameFs {
-	return &BasePathRealFilenameFs{BasePathFs: base}
-}
-
-// BasePathRealFilenameFs is a thin wrapper around afero.BasePathFs that
-// provides the real filename in Stat and LstatIfPossible.
-type BasePathRealFilenameFs struct {
-	*afero.BasePathFs
-}
-
-// Stat returns the os.FileInfo structure describing a given file.  If there is
-// an error, it will be of type *os.PathError.
-func (b *BasePathRealFilenameFs) Stat(name string) (os.FileInfo, error) {
-	fi, err := b.BasePathFs.Stat(name)
-	if err != nil {
-		return nil, err
-	}
-
-	if _, ok := fi.(RealFilenameInfo); ok {
-		return fi, nil
-	}
-
-	filename, err := b.RealPath(name)
-	if err != nil {
-		return nil, &os.PathError{Op: "stat", Path: name, Err: err}
-	}
-
-	return &realFilenameInfo{FileInfo: fi, realFilename: filename}, nil
-}
-
-// LstatIfPossible returns the os.FileInfo structure describing a given file.
-// It attempts to use Lstat if supported or defers to the os.  In addition to
-// the FileInfo, a boolean is returned telling whether Lstat was called.
-func (b *BasePathRealFilenameFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
-
-	fi, ok, err := b.BasePathFs.LstatIfPossible(name)
-	if err != nil {
-		return nil, false, err
-	}
-
-	if _, ok := fi.(RealFilenameInfo); ok {
-		return fi, ok, nil
-	}
-
-	filename, err := b.RealPath(name)
-	if err != nil {
-		return nil, false, &os.PathError{Op: "lstat", Path: name, Err: err}
-	}
-
-	return &realFilenameInfo{FileInfo: fi, realFilename: filename}, ok, nil
-}
diff --git a/hugofs/decorators.go b/hugofs/decorators.go
new file mode 100644
index 000000000..0a2b39712
--- /dev/null
+++ b/hugofs/decorators.go
@@ -0,0 +1,205 @@
+// Copyright 2019 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 hugofs
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+
+	"github.com/spf13/afero"
+)
+
+func decorateDirs(fs afero.Fs, meta FileMeta) afero.Fs {
+	ffs := &baseFileDecoratorFs{Fs: fs}
+
+	decorator := func(fi os.FileInfo, name string) (os.FileInfo, error) {
+		if !fi.IsDir() {
+			// Leave regular files as they are.
+			return fi, nil
+		}
+
+		return decorateFileInfo(fi, fs, nil, "", "", meta), nil
+	}
+
+	ffs.decorate = decorator
+
+	return ffs
+
+}
+
+func decoratePath(fs afero.Fs, createPath func(name string) string) afero.Fs {
+
+	ffs := &baseFileDecoratorFs{Fs: fs}
+
+	decorator := func(fi os.FileInfo, name string) (os.FileInfo, error) {
+		path := createPath(name)
+
+		return decorateFileInfo(fi, fs, nil, "", path, nil), nil
+	}
+
+	ffs.decorate = decorator
+
+	return ffs
+
+}
+
+// DecorateBasePathFs adds Path info to files and directories in the
+// provided BasePathFs, using the base as base.
+func DecorateBasePathFs(base *afero.BasePathFs) afero.Fs {
+	basePath, _ := base.RealPath("")
+	if !strings.HasSuffix(basePath, filepathSeparator) {
+		basePath += filepathSeparator
+	}
+
+	ffs := &baseFileDecoratorFs{Fs: base}
+
+	decorator := func(fi os.FileInfo, name string) (os.FileInfo, error) {
+		path := strings.TrimPrefix(name, basePath)
+
+		return decorateFileInfo(fi, base, nil, "", path, nil), nil
+	}
+
+	ffs.decorate = decorator
+
+	return ffs
+}
+
+// NewBaseFileDecorator decorates the given Fs to provide the real filename
+// and an Opener func. If
+func NewBaseFileDecorator(fs afero.Fs) afero.Fs {
+
+	ffs := &baseFileDecoratorFs{Fs: fs}
+
+	decorator := func(fi os.FileInfo, filename string) (os.FileInfo, error) {
+		// Store away the original in case it's a symlink.
+		meta := FileMeta{metaKeyName: fi.Name()}
+		isSymlink := isSymlink(fi)
+		if isSymlink {
+			meta[metaKeyOriginalFilename] = filename
+			link, err := filepath.EvalSymlinks(filename)
+			if err != nil {
+				return nil, err
+			}
+
+			fi, err = fs.Stat(link)
+			if err != nil {
+				return nil, err
+			}
+
+			filename = link
+			meta[metaKeyIsSymlink] = true
+
+		}
+
+		opener := func() (afero.File, error) {
+			return ffs.open(filename)
+
+		}
+
+		return decorateFileInfo(fi, ffs, opener, filename, "", meta), nil
+	}
+
+	ffs.decorate = decorator
+	return ffs
+}
+
+type baseFileDecoratorFs struct {
+	afero.Fs
+	decorate func(fi os.FileInfo, filename string) (os.FileInfo, error)
+}
+
+func (fs *baseFileDecoratorFs) Stat(name string) (os.FileInfo, error) {
+	fi, err := fs.Fs.Stat(name)
+	if err != nil {
+		return nil, err
+	}
+
+	return fs.decorate(fi, name)
+
+}
+
+func (fs *baseFileDecoratorFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	var (
+		fi  os.FileInfo
+		err error
+		ok  bool
+	)
+
+	if lstater, isLstater := fs.Fs.(afero.Lstater); isLstater {
+		fi, ok, err = lstater.LstatIfPossible(name)
+	} else {
+		fi, err = fs.Fs.Stat(name)
+	}
+
+	if err != nil {
+		return nil, false, err
+	}
+
+	fi, err = fs.decorate(fi, name)
+
+	return fi, ok, err
+}
+
+func (fs *baseFileDecoratorFs) Open(name string) (afero.File, error) {
+	return fs.open(name)
+}
+
+func (fs *baseFileDecoratorFs) open(name string) (afero.File, error) {
+	f, err := fs.Fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return &baseFileDecoratorFile{File: f, fs: fs}, nil
+}
+
+type baseFileDecoratorFile struct {
+	afero.File
+	fs *baseFileDecoratorFs
+}
+
+func (l *baseFileDecoratorFile) Readdir(c int) (ofi []os.FileInfo, err error) {
+	dirnames, err := l.File.Readdirnames(c)
+	if err != nil {
+		return nil, err
+	}
+
+	fisp := make([]os.FileInfo, 0, len(dirnames))
+
+	for _, dirname := range dirnames {
+		filename := dirname
+
+		if l.Name() != "" && l.Name() != filepathSeparator {
+			filename = filepath.Join(l.Name(), dirname)
+		}
+
+		// We need to resolve any symlink info.
+		fi, _, err := lstatIfPossible(l.fs.Fs, filename)
+		if err != nil {
+			if os.IsNotExist(err) {
+				continue
+			}
+			return nil, err
+		}
+		fi, err = l.fs.decorate(fi, filename)
+		if err != nil {
+			return nil, errors.Wrap(err, "decorate")
+		}
+		fisp = append(fisp, fi)
+	}
+
+	return fisp, err
+}
diff --git a/hugofs/fileinfo.go b/hugofs/fileinfo.go
new file mode 100644
index 000000000..a2f12c429
--- /dev/null
+++ b/hugofs/fileinfo.go
@@ -0,0 +1,297 @@
+// Copyright 2019 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 hugofs provides the file systems used by Hugo.
+package hugofs
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"time"
+
+	"github.com/gohugoio/hugo/hugofs/files"
+	"golang.org/x/text/unicode/norm"
+
+	"github.com/pkg/errors"
+
+	"github.com/spf13/cast"
+
+	"github.com/gohugoio/hugo/common/hreflect"
+
+	"github.com/spf13/afero"
+)
+
+const (
+	metaKeyFilename                   = "filename"
+	metaKeyOriginalFilename           = "originalFilename"
+	metaKeyName                       = "name"
+	metaKeyPath                       = "path"
+	metaKeyPathWalk                   = "pathWalk"
+	metaKeyLang                       = "lang"
+	metaKeyWeight                     = "weight"
+	metaKeyOrdinal                    = "ordinal"
+	metaKeyFs                         = "fs"
+	metaKeyOpener                     = "opener"
+	metaKeyIsOrdered                  = "isOrdered"
+	metaKeyIsSymlink                  = "isSymlink"
+	metaKeySkipDir                    = "skipDir"
+	metaKeyClassifier                 = "classifier"
+	metaKeyTranslationBaseName        = "translationBaseName"
+	metaKeyTranslationBaseNameWithExt = "translationBaseNameWithExt"
+	metaKeyTranslations               = "translations"
+	metaKeyDecoraterPath              = "decoratorPath"
+)
+
+type FileMeta map[string]interface{}
+
+func (f FileMeta) GetInt(key string) int {
+	return cast.ToInt(f[key])
+}
+
+func (f FileMeta) GetString(key string) string {
+	return cast.ToString(f[key])
+}
+
+func (f FileMeta) GetBool(key string) bool {
+	return cast.ToBool(f[key])
+}
+
+func (f FileMeta) Filename() string {
+	return f.stringV(metaKeyFilename)
+}
+
+func (f FileMeta) OriginalFilename() string {
+	return f.stringV(metaKeyOriginalFilename)
+}
+
+func (f FileMeta) SkipDir() bool {
+	return f.GetBool(metaKeySkipDir)
+}
+func (f FileMeta) TranslationBaseName() string {
+	return f.stringV(metaKeyTranslationBaseName)
+}
+
+func (f FileMeta) TranslationBaseNameWithExt() string {
+	return f.stringV(metaKeyTranslationBaseNameWithExt)
+}
+
+func (f FileMeta) Translations() []string {
+	return cast.ToStringSlice(f[metaKeyTranslations])
+}
+
+func (f FileMeta) Name() string {
+	return f.stringV(metaKeyName)
+}
+
+func (f FileMeta) Classifier() string {
+	c := f.stringV(metaKeyClassifier)
+	if c != "" {
+		return c
+	}
+
+	return files.ContentClassFile // For sorting
+}
+
+func (f FileMeta) Lang() string {
+	return f.stringV(metaKeyLang)
+}
+
+func (f FileMeta) Path() string {
+	return f.stringV(metaKeyPath)
+}
+
+func (f FileMeta) Weight() int {
+	return f.GetInt(metaKeyWeight)
+}
+
+func (f FileMeta) Ordinal() int {
+	return f.GetInt(metaKeyOrdinal)
+}
+
+func (f FileMeta) IsOrdered() bool {
+	return f.GetBool(metaKeyIsOrdered)
+}
+
+// IsSymlink returns whether this comes from a symlinked file or directory.
+func (f FileMeta) IsSymlink() bool {
+	return f.GetBool(metaKeyIsSymlink)
+}
+
+func (f FileMeta) Watch() bool {
+	if v, found := f["watch"]; found {
+		return v.(bool)
+	}
+	return false
+}
+
+func (f FileMeta) Fs() afero.Fs {
+	if v, found := f[metaKeyFs]; found {
+		return v.(afero.Fs)
+	}
+	return nil
+}
+
+func (f FileMeta) GetOpener() func() (afero.File, error) {
+	o, found := f[metaKeyOpener]
+	if !found {
+		return nil
+	}
+	return o.(func() (afero.File, error))
+}
+
+func (f FileMeta) Open() (afero.File, error) {
+	v, found := f[metaKeyOpener]
+	if !found {
+		return nil, errors.New("file opener not found")
+	}
+	return v.(func() (afero.File, error))()
+}
+
+func (f FileMeta) stringV(key string) string {
+	if v, found := f[key]; found {
+		return v.(string)
+	}
+	return ""
+}
+
+func (f FileMeta) setIfNotZero(key string, val interface{}) {
+	if !hreflect.IsTruthful(val) {
+		return
+	}
+	f[key] = val
+}
+
+type FileMetaInfo interface {
+	os.FileInfo
+	Meta() FileMeta
+}
+
+type fileInfoMeta struct {
+	os.FileInfo
+	m FileMeta
+}
+
+func (fi *fileInfoMeta) Meta() FileMeta {
+	return fi.m
+}
+
+func NewFileMetaInfo(fi os.FileInfo, m FileMeta) FileMetaInfo {
+
+	if fim, ok := fi.(FileMetaInfo); ok {
+		mergeFileMeta(fim.Meta(), m)
+	}
+	return &fileInfoMeta{FileInfo: fi, m: m}
+}
+
+// Merge metadata, last entry wins.
+func mergeFileMeta(from, to FileMeta) {
+	if from == nil {
+		return
+	}
+	for k, v := range from {
+		if _, found := to[k]; !found {
+			to[k] = v
+		}
+	}
+}
+
+type dirNameOnlyFileInfo struct {
+	name string
+}
+
+func (fi *dirNameOnlyFileInfo) Name() string {
+	return fi.name
+}
+
+func (fi *dirNameOnlyFileInfo) Size() int64 {
+	panic("not implemented")
+}
+
+func (fi *dirNameOnlyFileInfo) Mode() os.FileMode {
+	return os.ModeDir
+}
+
+func (fi *dirNameOnlyFileInfo) ModTime() time.Time {
+	return time.Time{}
+}
+
+func (fi *dirNameOnlyFileInfo) IsDir() bool {
+	return true
+}
+
+func (fi *dirNameOnlyFileInfo) Sys() interface{} {
+	return nil
+}
+
+func newDirNameOnlyFileInfo(name string, isOrdered bool, fileOpener func() (afero.File, error)) FileMetaInfo {
+	name = normalizeFilename(name)
+	_, base := filepath.Split(name)
+	return NewFileMetaInfo(&dirNameOnlyFileInfo{name: base}, FileMeta{
+		metaKeyFilename:  name,
+		metaKeyIsOrdered: isOrdered,
+		metaKeyOpener:    fileOpener})
+}
+
+func decorateFileInfo(
+	fi os.FileInfo,
+	fs afero.Fs, opener func() (afero.File, error),
+	filename, filepath string, inMeta FileMeta) FileMetaInfo {
+
+	var meta FileMeta
+	var fim FileMetaInfo
+
+	filepath = strings.TrimPrefix(filepath, filepathSeparator)
+
+	var ok bool
+	if fim, ok = fi.(FileMetaInfo); ok {
+		meta = fim.Meta()
+	} else {
+		meta = make(FileMeta)
+		fim = NewFileMetaInfo(fi, meta)
+	}
+
+	meta.setIfNotZero(metaKeyOpener, opener)
+	meta.setIfNotZero(metaKeyFs, fs)
+	meta.setIfNotZero(metaKeyPath, normalizeFilename(filepath))
+	meta.setIfNotZero(metaKeyFilename, normalizeFilename(filename))
+
+	mergeFileMeta(inMeta, meta)
+
+	return fim
+
+}
+
+func isSymlink(fi os.FileInfo) bool {
+	return fi != nil && fi.Mode()&os.ModeSymlink == os.ModeSymlink
+}
+
+func fileInfosToFileMetaInfos(fis []os.FileInfo) []FileMetaInfo {
+	fims := make([]FileMetaInfo, len(fis))
+	for i, v := range fis {
+		fims[i] = v.(FileMetaInfo)
+	}
+	return fims
+}
+
+func normalizeFilename(filename string) string {
+	if filename == "" {
+		return ""
+	}
+	if runtime.GOOS == "darwin" {
+		// When a file system is HFS+, its filepath is in NFD form.
+		return norm.NFC.String(filename)
+	}
+	return filename
+}
diff --git a/hugofs/files/classifier.go b/hugofs/files/classifier.go
new file mode 100644
index 000000000..9aa2476b7
--- /dev/null
+++ b/hugofs/files/classifier.go
@@ -0,0 +1,121 @@
+// Copyright 2019 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 files
+
+import (
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+var (
+	// This should be the only list of valid extensions for content files.
+	contentFileExtensions = []string{
+		"html", "htm",
+		"mdown", "markdown", "md",
+		"asciidoc", "adoc", "ad",
+		"rest", "rst",
+		"mmark",
+		"org",
+		"pandoc", "pdc"}
+
+	contentFileExtensionsSet map[string]bool
+)
+
+func init() {
+	contentFileExtensionsSet = make(map[string]bool)
+	for _, ext := range contentFileExtensions {
+		contentFileExtensionsSet[ext] = true
+	}
+}
+
+func IsContentFile(filename string) bool {
+	return contentFileExtensionsSet[strings.TrimPrefix(filepath.Ext(filename), ".")]
+}
+
+func IsContentExt(ext string) bool {
+	return contentFileExtensionsSet[ext]
+}
+
+const (
+	ContentClassLeaf    = "leaf"
+	ContentClassBranch  = "branch"
+	ContentClassFile    = "zfile" // Sort below
+	ContentClassContent = "zcontent"
+)
+
+func ClassifyContentFile(filename string) string {
+	if !IsContentFile(filename) {
+		return ContentClassFile
+	}
+	if strings.HasPrefix(filename, "_index.") {
+		return ContentClassBranch
+	}
+
+	if strings.HasPrefix(filename, "index.") {
+		return ContentClassLeaf
+	}
+
+	return ContentClassContent
+}
+
+const (
+	ComponentFolderArchetypes = "archetypes"
+	ComponentFolderStatic     = "static"
+	ComponentFolderLayouts    = "layouts"
+	ComponentFolderContent    = "content"
+	ComponentFolderData       = "data"
+	ComponentFolderAssets     = "assets"
+	ComponentFolderI18n       = "i18n"
+
+	FolderResources = "resources"
+)
+
+var (
+	ComponentFolders = []string{
+		ComponentFolderArchetypes,
+		ComponentFolderStatic,
+		ComponentFolderLayouts,
+		ComponentFolderContent,
+		ComponentFolderData,
+		ComponentFolderAssets,
+		ComponentFolderI18n,
+	}
+
+	componentFoldersSet = make(map[string]bool)
+)
+
+func init() {
+	sort.Strings(ComponentFolders)
+	for _, f := range ComponentFolders {
+		componentFoldersSet[f] = true
+	}
+}
+
+// ResolveComponentFolder returns "content" from "content/blog/foo.md" etc.
+func ResolveComponentFolder(filename string) string {
+	filename = strings.TrimPrefix(filename, string(os.PathSeparator))
+	for _, cf := range ComponentFolders {
+		if strings.HasPrefix(filename, cf) {
+			return cf
+		}
+	}
+
+	return ""
+}
+
+func IsComponentFolder(name string) bool {
+	return componentFoldersSet[name]
+}
diff --git a/hugofs/files/classifier_test.go b/hugofs/files/classifier_test.go
new file mode 100644
index 000000000..d576b4e58
--- /dev/null
+++ b/hugofs/files/classifier_test.go
@@ -0,0 +1,49 @@
+// Copyright 2019 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 files
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestIsContentFile(t *testing.T) {
+	assert := require.New(t)
+
+	assert.True(IsContentFile(filepath.FromSlash("my/file.md")))
+	assert.True(IsContentFile(filepath.FromSlash("my/file.ad")))
+	assert.False(IsContentFile(filepath.FromSlash("textfile.txt")))
+	assert.True(IsContentExt("md"))
+	assert.False(IsContentExt("json"))
+}
+
+func TestComponentFolders(t *testing.T) {
+	assert := require.New(t)
+
+	// It's important that these are absolutely right and not changed.
+	assert.Equal(len(ComponentFolders), len(componentFoldersSet))
+	assert.True(IsComponentFolder("archetypes"))
+	assert.True(IsComponentFolder("layouts"))
+	assert.True(IsComponentFolder("data"))
+	assert.True(IsComponentFolder("i18n"))
+	assert.True(IsComponentFolder("assets"))
+	assert.False(IsComponentFolder("resources"))
+	assert.True(IsComponentFolder("static"))
+	assert.True(IsComponentFolder("content"))
+	assert.False(IsComponentFolder("foo"))
+	assert.False(IsComponentFolder(""))
+
+}
diff --git a/hugofs/filter_fs.go b/hugofs/filter_fs.go
new file mode 100644
index 000000000..952b276cf
--- /dev/null
+++ b/hugofs/filter_fs.go
@@ -0,0 +1,341 @@
+// Copyright 2019 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 hugofs
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"syscall"
+	"time"
+
+	"github.com/gohugoio/hugo/hugofs/files"
+
+	"github.com/spf13/afero"
+)
+
+var (
+	_ afero.Fs      = (*FilterFs)(nil)
+	_ afero.Lstater = (*FilterFs)(nil)
+	_ afero.File    = (*filterDir)(nil)
+)
+
+func NewLanguageFs(langs map[string]int, fs afero.Fs) (afero.Fs, error) {
+
+	applyMeta := func(fs *FilterFs, name string, fis []os.FileInfo) {
+
+		for i, fi := range fis {
+			if fi.IsDir() {
+				filename := filepath.Join(name, fi.Name())
+				fis[i] = decorateFileInfo(fi, fs, fs.getOpener(filename), "", "", nil)
+				continue
+			}
+
+			meta := fi.(FileMetaInfo).Meta()
+			lang := meta.Lang()
+
+			fileLang, translationBaseName, translationBaseNameWithExt := langInfoFrom(langs, fi.Name())
+			weight := 0
+
+			if fileLang != "" {
+				weight = 1
+				if fileLang == lang {
+					// Give priority to myfile.sv.txt inside the sv filesystem.
+					weight++
+				}
+				lang = fileLang
+			}
+
+			fim := NewFileMetaInfo(fi, FileMeta{
+				metaKeyLang:                       lang,
+				metaKeyWeight: