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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package fs
import (
"errors"
"path"
)
// A SubFS is a file system with a Sub method.
type SubFS interface {
FS
// Sub returns an FS corresponding to the subtree rooted at dir.
Sub(dir string) (FS, error)
}
// Sub returns an [FS] corresponding to the subtree rooted at fsys's dir.
//
// If dir is ".", Sub returns fsys unchanged.
// Otherwise, if fs implements [SubFS], Sub returns fsys.Sub(dir).
// Otherwise, Sub returns a new [FS] implementation sub that,
// in effect, implements sub.Open(name) as fsys.Open(path.Join(dir, name)).
// The implementation also translates calls to ReadDir, ReadFile, and Glob appropriately.
//
// Note that Sub(os.DirFS("/"), "prefix") is equivalent to os.DirFS("/prefix")
// and that neither of them guarantees to avoid operating system
// accesses outside "/prefix", because the implementation of [os.DirFS]
// does not check for symbolic links inside "/prefix" that point to
// other directories. That is, [os.DirFS] is not a general substitute for a
// chroot-style security mechanism, and Sub does not change that fact.
func Sub(fsys FS, dir string) (FS, error) {
if !ValidPath(dir) {
return nil, &PathError{Op: "sub", Path: dir, Err: errors.New("invalid name")}
}
if dir == "." {
return fsys, nil
}
if fsys, ok := fsys.(SubFS); ok {
return fsys.Sub(dir)
}
return &subFS{fsys, dir}, nil
}
type subFS struct {
fsys FS
dir string
}
// fullName maps name to the fully-qualified name dir/name.
func (f *subFS) fullName(op string, name string) (string, error) {
if !ValidPath(name) {
return "", &PathError{Op: op, Path: name, Err: errors.New("invalid name")}
}
return path.Join(f.dir, name), nil
}
// shorten maps name, which should start with f.dir, back to the suffix after f.dir.
func (f *subFS) shorten(name string) (rel string, ok bool) {
if name == f.dir {
return ".", true
}
if len(name) >= len(f.dir)+2 && name[len(f.dir)] == '/' && name[:len(f.dir)] == f.dir {
return name[len(f.dir)+1:], true
}
return "", false
}
// fixErr shortens any reported names in PathErrors by stripping f.dir.
func (f *subFS) fixErr(err error) error {
if e, ok := err.(*PathError); ok {
if short, ok := f.shorten(e.Path); ok {
e.Path = short
}
}
return err
}
func (f *subFS) Open(name string) (File, error) {
full, err := f.fullName("open", name)
if err != nil {
return nil, err
}
file, err := f.fsys.Open(full)
return file, f.fixErr(err)
}
func (f *subFS) ReadDir(name string) ([]DirEntry, error) {
full, err := f.fullName("read", name)
if err != nil {
return nil, err
}
dir, err := ReadDir(f.fsys, full)
return dir, f.fixErr(err)
}
func (f *subFS) ReadFile(name string) ([]byte, error) {
full, err := f.fullName("read", name)
if err != nil {
return nil, err
}
data, err := ReadFile(f.fsys, full)
return data, f.fixErr(err)
}
func (f *subFS) Glob(pattern string) ([]string, error) {
// Check pattern is well-formed.
if _, err := path.Match(pattern, ""); err != nil {
return nil, err
}
if pattern == "." {
return []string{"."}, nil
}
full := f.dir + "/" + pattern
list, err := Glob(f.fsys, full)
for i, name := range list {
name, ok := f.shorten(name)
if !ok {
return nil, errors.New("invalid result from inner fsys Glob: " + name + " not in " + f.dir) // can't use fmt in this package
}
list[i] = name
}
return list, f.fixErr(err)
}
func (f *subFS) Sub(dir string) (FS, error) {
if dir == "." {
return f, nil
}
full, err := f.fullName("sub", dir)
if err != nil {
return nil, err
}
return &subFS{f.fsys, full}, nil
}