Source: sub.go in package io/fs


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 fs implements SubFS, Sub calls returns fsys.Sub(dir). Otherwise, if dir is ".", Sub returns fsys unchanged. Otherwise, Sub returns a new FS implementation sub that, in effect, implements sub.Open(dir) 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 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)
}

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)