Add new relpath filesystem

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.

relpath.go

@@ -0,0 +1,216 @@
+// Copyright (C) 2024+ James Shubin
+// Written by James Shubin <james@shubin.ca>
+// This filesystem implementation is based on the afero.BasePathFs code.
+package afero
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+var (
+	_ Lstater        = (*RelPathFs)(nil)
+	_ fs.ReadDirFile = (*RelPathFile)(nil)
+	_ fs.ReadDirFile = readDirFile{}
+)
+
+// 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 Fs
+
+	prefix string
+}
+
+type RelPathFile struct {
+	File
+
+	prefix string
+}
+
+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
+}
+
+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 Fs, prefix string) Fs {
+	return &RelPathFs{source: source, prefix: prefix}
+}
+
+// on a file outside the base path it returns the given file name and an error,
+// else the given file with the base path 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
+}
+
+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)
+}
+
+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)
+}
+
+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)
+}
+
+func (obj *RelPathFs) Name() string {
+	return "RelPathFs"
+}
+
+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)
+}
+
+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)
+}
+
+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)
+}
+
+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)
+}
+
+func (obj *RelPathFs) OpenFile(name string, flag int, mode os.FileMode) (f 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
+}
+
+func (obj *RelPathFs) Open(name string) (f 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
+}
+
+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)
+}
+
+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)
+}
+
+func (obj *RelPathFs) Create(name string) (f 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
+}
+
+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.(Lstater); ok {
+		return lstater.LstatIfPossible(name)
+	}
+	fi, err := obj.source.Stat(name)
+	return fi, false, err
+}
+
+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.(Linker); ok {
+		return linker.SymlinkIfPossible(oldname, newname)
+	}
+	return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: ErrNoSymlink}
+}
+
+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.(LinkReader); ok {
+		return reader.ReadlinkIfPossible(name)
+	}
+	return "", &os.PathError{Op: "readlink", Path: name, Err: ErrNoReadlink}
+}