util: Add afero relpath implementation

This new filesystem implements a relative filesystem modifier which can
be useful for converting between absolute filesystems rooted at / and
relative ones. This is particularly useful when interfacing with the
golang embed package.

The upstream requires a CLA, so we'll just store this here instead.
spf13/afero#417

util/afero_relpath.go

@@ -0,0 +1,301 @@
+// Mgmt
+// Copyright (C) 2013-2024+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// This filesystem implementation is based on the afero.BasePathFs code.
+
+package util
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/spf13/afero"
+)
+
+const (
+	// RelPathFsScheme returns a unique name for this type of filesystem.
+	RelPathFsScheme = "RelPathFs"
+)
+
+var (
+	_ afero.Lstater  = (*RelPathFs)(nil)
+	_ fs.ReadDirFile = (*RelPathFile)(nil)
+)
+
+// RelPathFs removes a prefix from all operations to a given path within an Fs.
+// The given file name to the operations on this Fs will have a prefix removed
+// before calling the base Fs.
+//
+// When initializing it with "/", a call to `/foo` turns into `foo`.
+//
+// Note that it does not clean the error messages on return, so you may reveal
+// the real path on errors.
+type RelPathFs struct {
+	source afero.Fs
+
+	prefix string
+}
+
+// RelPathFile represents a file node.
+type RelPathFile struct {
+	afero.File
+
+	prefix string
+}
+
+// Name returns the path of the file.
+func (obj *RelPathFile) Name() string {
+	sourcename := obj.File.Name()
+	//return strings.TrimPrefix(sourcename, filepath.Clean(obj.prefix))
+	return filepath.Clean(obj.prefix) + sourcename // add prefix back on
+}
+
+// ReadDir lists the contents of the directory and returns a list of file info
+// objects for each entry.
+func (obj *RelPathFile) ReadDir(n int) ([]fs.DirEntry, error) {
+	if rdf, ok := obj.File.(fs.ReadDirFile); ok {
+		return rdf.ReadDir(n)
+	}
+	return readDirFile{obj.File}.ReadDir(n)
+}
+
+// NewRelPathFs creates a new RelPathFs.
+func NewRelPathFs(source afero.Fs, prefix string) afero.Fs {
+	return &RelPathFs{source: source, prefix: prefix}
+}
+
+// RealPath returns the correct path with the prefix removed.
+func (obj *RelPathFs) RealPath(name string) (string, error) {
+	if name == "/" {
+		return ".", nil // special trim
+	}
+	if name == "" {
+		return filepath.Clean(name), nil // returns a single period
+	}
+	path := filepath.Clean(name)         // actual path
+	prefix := filepath.Clean(obj.prefix) // is often a / and we trim it off
+
+	//if strings.HasPrefix(path, prefix) { // redundant
+	path = strings.TrimPrefix(path, prefix)
+	//}
+
+	return path, nil
+}
+
+// Chtimes changes the access and modification times of the named file, similar
+// to the Unix utime() or utimes() functions. The underlying filesystem may
+// truncate or round the values to a less precise time unit. If there is an
+// error, it will be of type *PathError.
+func (obj *RelPathFs) Chtimes(name string, atime, mtime time.Time) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "chtimes", Path: name, Err: err}
+	}
+	return obj.source.Chtimes(name, atime, mtime)
+}
+
+// Chmod changes the mode of a file.
+func (obj *RelPathFs) Chmod(name string, mode os.FileMode) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "chmod", Path: name, Err: err}
+	}
+	return obj.source.Chmod(name, mode)
+}
+
+// Chown changes the ownership of a file. It is the equivalent of os.Chown.
+func (obj *RelPathFs) Chown(name string, uid, gid int) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "chown", Path: name, Err: err}
+	}
+	return obj.source.Chown(name, uid, gid)
+}
+
+// Name returns the name of this filesystem.
+func (obj *RelPathFs) Name() string {
+	return RelPathFsScheme
+}
+
+// Stat returns some information about the particular path.
+func (obj *RelPathFs) Stat(name string) (fi os.FileInfo, err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return nil, &os.PathError{Op: "stat", Path: name, Err: err}
+	}
+	return obj.source.Stat(name)
+}
+
+// Rename moves or renames a file or directory.
+func (obj *RelPathFs) Rename(oldname, newname string) (err error) {
+	if oldname, err = obj.RealPath(oldname); err != nil {
+		return &os.PathError{Op: "rename", Path: oldname, Err: err}
+	}
+	if newname, err = obj.RealPath(newname); err != nil {
+		return &os.PathError{Op: "rename", Path: newname, Err: err}
+	}
+	return obj.source.Rename(oldname, newname)
+}
+
+// RemoveAll removes path and any children it contains. It removes everything it
+// can but returns the first error it encounters. If the path does not exist,
+// RemoveAll returns nil (no error).
+func (obj *RelPathFs) RemoveAll(name string) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "remove_all", Path: name, Err: err}
+	}
+	return obj.source.RemoveAll(name)
+}
+
+// Remove removes a path.
+func (obj *RelPathFs) Remove(name string) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "remove", Path: name, Err: err}
+	}
+	return obj.source.Remove(name)
+}
+
+// OpenFile opens a path with a particular flag and permission.
+func (obj *RelPathFs) OpenFile(name string, flag int, mode os.FileMode) (f afero.File, err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return nil, &os.PathError{Op: "openfile", Path: name, Err: err}
+	}
+	sourcef, err := obj.source.OpenFile(name, flag, mode)
+	if err != nil {
+		return nil, err
+	}
+	return &RelPathFile{File: sourcef, prefix: obj.prefix}, nil
+}
+
+// Open opens a path. It will be opened read-only.
+func (obj *RelPathFs) Open(name string) (f afero.File, err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return nil, &os.PathError{Op: "open", Path: name, Err: err}
+	}
+	sourcef, err := obj.source.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return &RelPathFile{File: sourcef, prefix: obj.prefix}, nil
+}
+
+// Mkdir makes a new directory.
+func (obj *RelPathFs) Mkdir(name string, mode os.FileMode) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "mkdir", Path: name, Err: err}
+	}
+	return obj.source.Mkdir(name, mode)
+}
+
+// MkdirAll creates a directory named path, along with any necessary parents,
+// and returns nil, or else returns an error. The permission bits perm are used
+// for all directories that MkdirAll creates. If path is already a directory,
+// MkdirAll does nothing and returns nil.
+func (obj *RelPathFs) MkdirAll(name string, mode os.FileMode) (err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return &os.PathError{Op: "mkdir", Path: name, Err: err}
+	}
+	return obj.source.MkdirAll(name, mode)
+}
+
+// Create creates a new file.
+func (obj *RelPathFs) Create(name string) (f afero.File, err error) {
+	if name, err = obj.RealPath(name); err != nil {
+		return nil, &os.PathError{Op: "create", Path: name, Err: err}
+	}
+	sourcef, err := obj.source.Create(name)
+	if err != nil {
+		return nil, err
+	}
+	return &RelPathFile{File: sourcef, prefix: obj.prefix}, nil
+}
+
+// LstatIfPossible is for the lstater interface.
+func (obj *RelPathFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
+	name, err := obj.RealPath(name)
+	if err != nil {
+		return nil, false, &os.PathError{Op: "lstat", Path: name, Err: err}
+	}
+	if lstater, ok := obj.source.(afero.Lstater); ok {
+		return lstater.LstatIfPossible(name)
+	}
+	fi, err := obj.source.Stat(name)
+	return fi, false, err
+}
+
+// SymlinkIfPossible is for the weird Afero symlink API.
+func (obj *RelPathFs) SymlinkIfPossible(oldname, newname string) error {
+	oldname, err := obj.RealPath(oldname)
+	if err != nil {
+		return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: err}
+	}
+	newname, err = obj.RealPath(newname)
+	if err != nil {
+		return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: err}
+	}
+	if linker, ok := obj.source.(afero.Linker); ok {
+		return linker.SymlinkIfPossible(oldname, newname)
+	}
+	return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: afero.ErrNoSymlink}
+}
+
+// ReadlinkIfPossible is for the weird Afero readlink API.
+func (obj *RelPathFs) ReadlinkIfPossible(name string) (string, error) {
+	name, err := obj.RealPath(name)
+	if err != nil {
+		return "", &os.PathError{Op: "readlink", Path: name, Err: err}
+	}
+	if reader, ok := obj.source.(afero.LinkReader); ok {
+		return reader.ReadlinkIfPossible(name)
+	}
+	return "", &os.PathError{Op: "readlink", Path: name, Err: afero.ErrNoReadlink}
+}
+
+// readDirFile provides adapter from afero.File to fs.ReadDirFile needed for
+// correct Open
+type readDirFile struct {
+	afero.File
+}
+
+var _ fs.ReadDirFile = readDirFile{}
+
+func (r readDirFile) ReadDir(n int) ([]fs.DirEntry, error) {
+	items, err := r.File.Readdir(n)
+	if err != nil {
+		return nil, err
+	}
+
+	ret := make([]fs.DirEntry, len(items))
+	for i := range items {
+		//ret[i] = common.FileInfoDirEntry{FileInfo: items[i]}
+		ret[i] = FileInfoDirEntry{FileInfo: items[i]}
+	}
+
+	return ret, nil
+}
+
+var _ fs.DirEntry = FileInfoDirEntry{}
+
+// FileInfoDirEntry provides an adapter from os.FileInfo to fs.DirEntry
+type FileInfoDirEntry struct {
+	fs.FileInfo
+}
+
+// Type returns the FileMode for this DirEntry.
+func (obj FileInfoDirEntry) Type() fs.FileMode { return obj.FileInfo.Mode().Type() }
+
+// Info returns the FileInfo for this DirEntry.
+func (obj FileInfoDirEntry) Info() (fs.FileInfo, error) { return obj.FileInfo, nil }