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

4 files changed, 352 insertions, 35 deletions

diff --git a/hugofs/base_fs.go b/hugofs/base_fs.go
deleted file mode 100644
index 77af66dfe..000000000
--- a/hugofs/base_fs.go
+++ /dev/null
@@ -1,35 +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 (
-	"github.com/spf13/afero"
-)
-
-// BaseFs contains the core base filesystems used by Hugo. The name "base" is used
-// to underline that even if they can be composites, they all have a base path set to a specific
-// resource folder, e.g "/my-project/content". So, no absolute filenames needed.
-type BaseFs struct {
-	// The filesystem used to capture content. This can be a composite and
-	// language aware file system.
-	ContentFs afero.Fs
-
-	// The filesystem used to store resources (processed images etc.).
-	// This usually maps to /my-project/resources.
-	ResourcesFs afero.Fs
-
-	// The filesystem used to publish the rendered site.
-	// This usually maps to /my-project/public.
-	PublishFs afero.Fs
-}
diff --git a/hugofs/noop_fs.go b/hugofs/noop_fs.go
new file mode 100644
index 000000000..2d06622e4
--- /dev/null
+++ b/hugofs/noop_fs.go
@@ -0,0 +1,79 @@
+// 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 (
+	"errors"
+	"os"
+	"time"
+
+	"github.com/spf13/afero"
+)
+
+var (
+	noOpErr          = errors.New("this is a filesystem that does nothing and this operation is not supported")
+	_       afero.Fs = (*noOpFs)(nil)
+	NoOpFs           = &noOpFs{}
+)
+
+type noOpFs struct {
+}
+
+func (fs noOpFs) Create(name string) (afero.File, error) {
+	return nil, noOpErr
+}
+
+func (fs noOpFs) Mkdir(name string, perm os.FileMode) error {
+	return noOpErr
+}
+
+func (fs noOpFs) MkdirAll(path string, perm os.FileMode) error {
+	return noOpErr
+}
+
+func (fs noOpFs) Open(name string) (afero.File, error) {
+	return nil, os.ErrNotExist
+}
+
+func (fs noOpFs) OpenFile(name string, flag int, perm os.FileMode) (afero.File, error) {
+	return nil, os.ErrNotExist
+}
+
+func (fs noOpFs) Remove(name string) error {
+	return noOpErr
+}
+
+func (fs noOpFs) RemoveAll(path string) error {
+	return noOpErr
+}
+
+func (fs noOpFs) Rename(oldname string, newname string) error {
+	return noOpErr
+}
+
+func (fs noOpFs) Stat(name string) (os.FileInfo, error) {
+	return nil, os.ErrNotExist
+}
+
+func (fs noOpFs) Name() string {
+	return "noOpFs"
+}
+
+func (fs noOpFs) Chmod(name string, mode os.FileMode) error {
+	return noOpErr
+}
+
+func (fs noOpFs) Chtimes(name string, atime time.Time, mtime time.Time) error {
+	return noOpErr
+}
diff --git a/hugofs/rootmapping_fs.go b/hugofs/rootmapping_fs.go
new file mode 100644
index 000000000..59f49f3a9
--- /dev/null
+++ b/hugofs/rootmapping_fs.go
@@ -0,0 +1,180 @@
+// 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"
+	"path/filepath"
+	"strings"
+	"time"
+
+	radix "github.com/hashicorp/go-immutable-radix"
+	"github.com/spf13/afero"
+)
+
+var filepathSeparator = string(filepath.Separator)
+
+// A RootMappingFs maps several roots into one. Note that the root of this filesystem
+// is directories only, and they will be returned in Readdir and Readdirnames
+// in the order given.
+type RootMappingFs struct {
+	afero.Fs
+	rootMapToReal *radix.Node
+	virtualRoots  []string
+}
+
+type rootMappingFile struct {
+	afero.File
+	fs   *RootMappingFs
+	name string
+}
+
+type rootMappingFileInfo struct {
+	name string
+}
+
+func (fi *rootMappingFileInfo) Name() string {
+	return fi.name
+}
+
+func (fi *rootMappingFileInfo) Size() int64 {
+	panic("not implemented")
+}
+
+func (fi *rootMappingFileInfo) Mode() os.FileMode {
+	return os.ModeDir
+}
+
+func (fi *rootMappingFileInfo) ModTime() time.Time {
+	panic("not implemented")
+}
+
+func (fi *rootMappingFileInfo) IsDir() bool {
+	return true
+}
+
+func (fi *rootMappingFileInfo) Sys() interface{} {
+	return nil
+}
+
+func newRootMappingDirFileInfo(name string) *rootMappingFileInfo {
+	return &rootMappingFileInfo{name: name}
+}
+
+// NewRootMappingFs creates a new RootMappingFs on top of the provided with
+// a list of from, to string pairs of root mappings.
+// Note that 'from' represents a virtual root that maps to the actual filename in 'to'.
+func NewRootMappingFs(fs afero.Fs, fromTo ...string) (*RootMappingFs, error) {
+	rootMapToReal := radix.New().Txn()
+	var virtualRoots []string
+
+	for i := 0; i < len(fromTo); i += 2 {
+		vr := filepath.Clean(fromTo[i])
+		rr := filepath.Clean(fromTo[i+1])
+
+		// We need to preserve the original order for Readdir
+		virtualRoots = append(virtualRoots, vr)
+
+		rootMapToReal.Insert([]byte(vr), rr)
+	}
+
+	return &RootMappingFs{Fs: fs,
+		virtualRoots:  virtualRoots,
+		rootMapToReal: rootMapToReal.Commit().Root()}, nil
+}
+
+func (fs *RootMappingFs) Stat(name string) (os.FileInfo, error) {
+	if fs.isRoot(name) {
+		return newRootMappingDirFileInfo(name), nil
+	}
+	realName := fs.realName(name)
+	return fs.Fs.Stat(realName)
+}
+
+func (fs *RootMappingFs) isRoot(name string) bool {
+	return name == "" || name == filepathSeparator
+
+}
+
+func (fs *RootMappingFs) Open(name string) (afero.File, error) {
+	if fs.isRoot(name) {
+		return &rootMappingFile{name: name, fs: fs}, nil
+	}
+	realName := fs.realName(name)
+	f, err := fs.Fs.Open(realName)
+	if err != nil {
+		return nil, err
+	}
+	return &rootMappingFile{File: f, name: name, fs: fs}, nil
+}
+
+func (fs *RootMappingFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	if fs.isRoot(name) {
+		return newRootMappingDirFileInfo(name), false, nil
+	}
+	name = fs.realName(name)
+	if ls, ok := fs.Fs.(afero.Lstater); ok {
+		return ls.LstatIfPossible(name)
+	}
+	fi, err := fs.Stat(name)
+	return fi, false, err
+}
+
+func (fs *RootMappingFs) realName(name string) string {
+	key, val, found := fs.rootMapToReal.LongestPrefix([]byte(filepath.Clean(name)))
+	if !found {
+		return name
+	}
+	keystr := string(key)
+
+	return filepath.Join(val.(string), strings.TrimPrefix(name, keystr))
+}
+
+func (f *rootMappingFile) Readdir(count int) ([]os.FileInfo, error) {
+	if f.File == nil {
+		dirsn := make([]os.FileInfo, 0)
+		for i := 0; i < len(f.fs.virtualRoots); i++ {
+			if count != -1 && i >= count {
+				break
+			}
+			dirsn = append(dirsn, newRootMappingDirFileInfo(f.fs.virtualRoots[i]))
+		}
+		return dirsn, nil
+	}
+	return f.File.Readdir(count)
+
+}
+
+func (f *rootMappingFile) Readdirnames(count int) ([]string, error) {
+	dirs, err := f.Readdir(count)
+	if err != nil {
+		return nil, err
+	}
+	dirss := make([]string, len(dirs))
+	for i, d := range dirs {
+		dirss[i] = d.Name()
+	}
+	return dirss, nil
+}
+
+func (f *rootMappingFile) Name() string {
+	return f.name
+}
+
+func (f *rootMappingFile) Close() error {
+	if f.File == nil {
+		return nil
+	}
+	return f.File.Close()
+}
diff --git a/hugofs/rootmapping_fs_test.go b/hugofs/rootmapping_fs_test.go
new file mode 100644
index 000000000..a84f41151
--- /dev/null
+++ b/hugofs/rootmapping_fs_test.go
@@ -0,0 +1,93 @@
+// 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 (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRootMappingFsRealName(t *testing.T) {
+	assert := require.New(t)
+	fs := afero.NewMemMapFs()
+
+	rfs, err := NewRootMappingFs(fs, "f1", "f1t", "f2", "f2t")
+	assert.NoError(err)
+
+	assert.Equal(filepath.FromSlash("f1t/foo/file.txt"), rfs.realName(filepath.Join("f1", "foo", "file.txt")))
+
+}
+
+func TestRootMappingFsDirnames(t *testing.T) {
+	assert := require.New(t)
+	fs := afero.NewMemMapFs()
+
+	testfile := "myfile.txt"
+	assert.NoError(fs.Mkdir("f1t", 0755))
+	assert.NoError(fs.Mkdir("f2t", 0755))
+	assert.NoError(fs.Mkdir("f3t", 0755))
+	assert.NoError(afero.WriteFile(fs, filepath.Join("f2t", testfile), []byte("some content"), 0755))
+
+	rfs, err := NewRootMappingFs(fs, "bf1", "f1t", "cf2", "f2t", "af3", "f3t")
+	assert.NoError(err)
+
+	fif, err := rfs.Stat(filepath.Join("cf2", testfile))
+	assert.NoError(err)
+	assert.Equal("myfile.txt", fif.Name())
+
+	root, err := rfs.Open(filepathSeparator)
+	assert.NoError(err)
+
+	dirnames, err := root.Readdirnames(-1)
+	assert.NoError(err)
+	assert.Equal([]string{"bf1", "cf2", "af3"}, dirnames)
+
+}
+
+func TestRootMappingFsOs(t *testing.T) {
+	assert := require.New(t)
+	fs := afero.NewOsFs()
+
+	d, err := ioutil.TempDir("", "hugo-root-mapping")
+	assert.NoError(err)
+	defer func() {
+		os.RemoveAll(d)
+	}()
+
+	testfile := "myfile.txt"
+	assert.NoError(fs.Mkdir(filepath.Join(d, "f1t"), 0755))
+	assert.NoError(fs.Mkdir(filepath.Join(d, "f2t"), 0755))
+	assert.NoError(fs.Mkdir(filepath.Join(d, "f3t"), 0755))
+	assert.NoError(afero.WriteFile(fs, filepath.Join(d, "f2t", testfile), []byte("some content"), 0755))
+
+	rfs, err := NewRootMappingFs(fs, "bf1", filepath.Join(d, "f1t"), "cf2", filepath.Join(d, "f2t"), "af3", filepath.Join(d, "f3t"))
+	assert.NoError(err)
+
+	fif, err := rfs.Stat(filepath.Join("cf2", testfile))
+	assert.NoError(err)
+	assert.Equal("myfile.txt", fif.Name())
+
+	root, err := rfs.Open(filepathSeparator)
+	assert.NoError(err)
+
+	dirnames, err := root.Readdirnames(-1)
+	assert.NoError(err)
+	assert.Equal([]string{"bf1", "cf2", "af3"}, dirnames)
+
+}