fs.go

// Copyright 2023 Chainguard, Inc.
//
// 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 tarfs

import (
	"archive/tar"
	"bytes"
	"crypto/sha1" //nolint:gosec // this is what apk tools is using
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"

	"github.com/chainguard-dev/go-apk/pkg/apk"
	apkfs "github.com/chainguard-dev/go-apk/pkg/fs"
	"golang.org/x/sys/unix"
)

const (
	pathSep = "/"
	// maxLinks maximum permitted depths of symlinks, to prevent infinite recursion
	// matches what Linux kernel does from 4.2 onwards, see https://man7.org/linux/man-pages/man7/path_resolution.7.html
	maxLinks = 40

	xattrTarPAXRecordsPrefix = "SCHILY.xattr."
)

var _ apk.WriteHeaderer = (*memFS)(nil)

type tarEntry struct {
	tfs      fs.FS
	header   tar.Header
	checksum []byte
	pkg      *apk.Package
}

type memFS struct {
	tree *node
}

func New() *memFS {
	return &memFS{
		tree: &node{
			dir:      true,
			children: map[string]*node{},
			xattrs:   map[string][]byte{},
			name:     "/",
			mode:     fs.ModeDir | 0o755,
		},
	}
}

func checksumFromHeader(header *tar.Header) ([]byte, error) {
	pax := header.PAXRecords
	if pax == nil {
		return nil, nil
	}

	hexsum, ok := pax["APK-TOOLS.checksum.SHA1"]
	if !ok {
		return nil, nil
	}

	if strings.HasPrefix(hexsum, "Q1") {
		// This is nonstandard but something we did at one point, handle it.
		// In other contexts, this Q1 prefix means "this is sha1 not md5".
		b64 := strings.TrimPrefix(hexsum, "Q1")

		checksum, err := base64.StdEncoding.DecodeString(b64)
		if err != nil {
			return nil, fmt.Errorf("decoding base64 checksum from header for %q: %w", header.Name, err)
		}

		return checksum, nil
	}

	checksum, err := hex.DecodeString(hexsum)
	if err != nil {
		return nil, fmt.Errorf("decoding hex checksum from header for %q: %w", header.Name, err)
	}

	return checksum, nil
}

func (m *memFS) WriteHeader(hdr tar.Header, tfs fs.FS, pkg *apk.Package) (bool, error) {
	switch hdr.Typeflag {
	case tar.TypeDir:
		// special case, if the target already exists, and it is a symlink to a directory, we can accept it as is
		// otherwise, we need to create the directory.
		if fi, err := m.Stat(hdr.Name); err == nil && fi.Mode()&os.ModeSymlink != 0 {
			if target, err := m.Readlink(hdr.Name); err == nil {
				if fi, err = m.Stat(target); err == nil && fi.IsDir() {
					return false, nil
				}
			}
		}
		if err := m.MkdirAll(hdr.Name, hdr.FileInfo().Mode().Perm()); err != nil {
			return false, fmt.Errorf("error creating directory %s: %w", hdr.Name, err)
		}

		for k, v := range hdr.PAXRecords {
			if !strings.HasPrefix(k, xattrTarPAXRecordsPrefix) {
				continue
			}
			attrName := strings.TrimPrefix(k, xattrTarPAXRecordsPrefix)
			if err := m.SetXattr(hdr.Name, attrName, []byte(v)); err != nil {
				return false, fmt.Errorf("error setting xattr %s on %s: %w", attrName, hdr.Name, err)
			}
		}

	case tar.TypeReg:
		// We trust this because we verify it earlier in ExpandAPK.
		checksum, err := checksumFromHeader(&hdr)
		if err != nil {
			return false, err
		}

		if checksum == nil {
			return false, fmt.Errorf("checksum is nil for %s", hdr.Name)
		}

		te := tarEntry{
			tfs:      tfs,
			header:   hdr,
			checksum: checksum,
			pkg:      pkg,
		}

		installed, err := m.writeHeader(hdr.Name, te)
		if err != nil {
			return false, fmt.Errorf("writing header for %q: %w", hdr.Name, err)
		}

		for k, v := range hdr.PAXRecords {
			if !strings.HasPrefix(k, xattrTarPAXRecordsPrefix) {
				continue
			}
			attrName := strings.TrimPrefix(k, xattrTarPAXRecordsPrefix)
			if err := m.SetXattr(hdr.Name, attrName, []byte(v)); err != nil {
				return false, fmt.Errorf("error setting xattr %s on %s: %w", attrName, hdr.Name, err)
			}
		}

		return installed, nil
	case tar.TypeSymlink:
		// some underlying filesystems and some memfs that we use in tests do not support symlinks.
		// attempt it, and if it fails, just copy it.
		// if it already exists, pointing to the same target, we can ignore it
		if target, err := m.Readlink(hdr.Name); err == nil && target == hdr.Linkname {
			return false, nil
		}
		if err := m.Symlink(hdr.Linkname, hdr.Name); err != nil {
			return false, fmt.Errorf("unable to install symlink from %s -> %s: %w", hdr.Name, hdr.Linkname, err)
		}
	case tar.TypeLink:
		if err := m.Link(hdr.Linkname, hdr.Name); err != nil {
			return false, err
		}
	default:
		return false, fmt.Errorf("unsupported file type %s %v", hdr.Name, hdr.Typeflag)
	}

	return true, nil
}

// getNode returns the node for the given path. If the path is not found, it
// returns an error.
func (m *memFS) getNode(path string) (*node, error) {
	return m.getNodeCountLinks(path, 0)
}

func (m *memFS) getNodeCountLinks(path string, linkDepth int) (*node, error) {
	if path == "/" || path == "." {
		return m.tree, nil
	}
	parts := strings.Split(path, pathSep)
	node := m.tree
	traversed := make([]string, 0)
	for _, part := range parts {
		if part == "" {
			continue
		}
		if node.children == nil {
			return nil, fs.ErrNotExist
		}
		var ok bool
		node.mu.Lock()
		childNode, ok := node.children[part]
		// immediately unlock, no need to wait for defer. This is *really* important in the
		// case of symlinks below
		node.mu.Unlock()
		if !ok {
			return nil, fs.ErrNotExist
		}
		// what if it is a symlink?
		if childNode.mode&os.ModeSymlink != 0 {
			newDepth := linkDepth + 1
			if newDepth > maxLinks {
				return nil, fmt.Errorf("maximum symlink depth exceeded")
			}
			// getNode requires working on the absolute path, so we just resolve the path to an absolute path,
			// rather than struggling to clean up the path.
			// But, we have to make sure that we set it relative to where we are currently, rather than the parent of the path.
			// For example, /usr/lib64/foo/bar when /usr/lib64 -> lib, we want to resolve to /usr/lib rather than /usr/lib64/foo/lib
			linkTarget := childNode.linkTarget
			if !filepath.IsAbs(linkTarget) {
				linkTarget = filepath.Join(strings.Join(traversed, pathSep), linkTarget)
			}
			// now we have the absolute path, we can get the node
			// but that absolute path can cause us to try and hit something that is already locked
			// and since we are recursing, it will not get freed until we return
			// leading to a deadlock race condition
			targetNode, err := m.getNodeCountLinks(linkTarget, newDepth)
			if err != nil {
				return nil, err
			}
			childNode = targetNode
		}
		node = childNode
		traversed = append(traversed, part)
	}
	return node, nil
}

func (m *memFS) Mkdir(path string, perms fs.FileMode) error {
	// first see if the parent exists
	parent := filepath.Dir(path)
	anode, err := m.getNode(parent)
	if err != nil {
		return err
	}
	if anode.mode&fs.ModeDir == 0 {
		return fmt.Errorf("parent is not a directory")
	}
	// see if it exists
	anode.mu.Lock()
	defer anode.mu.Unlock()
	if _, ok := anode.children[filepath.Base(path)]; ok {
		return fs.ErrExist
	}
	// now create the directory
	anode.children[filepath.Base(path)] = &node{
		name:       filepath.Base(path),
		mode:       fs.ModeDir | perms,
		dir:        true,
		modTime:    time.Now(),
		createTime: time.Now(),
		children:   map[string]*node{},
		xattrs:     map[string][]byte{},
	}
	return nil
}

func (m *memFS) Stat(path string) (fs.FileInfo, error) {
	node, err := m.getNode(path)
	if err != nil {
		return nil, err
	}
	if node.mode&fs.ModeSymlink != 0 {
		targetNode, err := m.getNode(node.linkTarget)
		if err != nil {
			return nil, err
		}
		node = targetNode
	}
	return node.fileInfo(path), nil
}

func (m *memFS) Lstat(path string) (fs.FileInfo, error) {
	node, err := m.getNode(path)
	if err != nil {
		return nil, err
	}
	return node.fileInfo(path), nil
}

func (m *memFS) MkdirAll(path string, perm fs.FileMode) error {
	parts := strings.Split(path, pathSep)
	traversed := make([]string, 0)
	anode := m.tree
	for _, part := range parts {
		if part == "" {
			continue
		}
		if anode.children == nil {
			anode.children = map[string]*node{}
		}
		var ok bool
		anode.mu.Lock()
		newnode, ok := anode.children[part]
		if !ok {
			newnode = &node{
				name:       part,
				mode:       fs.ModeDir | perm,
				dir:        true,
				modTime:    time.Now(),
				createTime: time.Now(),
				children:   map[string]*node{},
				xattrs:     map[string][]byte{},
			}
			anode.children[part] = newnode
		}
		anode.mu.Unlock()
		// what if it is a symlink?
		if newnode.mode&os.ModeSymlink != 0 {
			linkTarget := newnode.linkTarget
			if !filepath.IsAbs(linkTarget) {
				linkTarget = filepath.Join(strings.Join(traversed, pathSep), linkTarget)
			}

			targetNode, err := m.getNode(linkTarget)
			if err != nil {
				return err
			}
			newnode = targetNode
		}
		if !newnode.dir {
			return fmt.Errorf("path is not a directory")
		}
		anode = newnode
		traversed = append(traversed, part)
	}
	return nil
}

func (m *memFS) Open(name string) (fs.File, error) {
	return m.OpenFile(name, os.O_RDONLY, 0o644)
}

func (m *memFS) OpenFile(name string, flag int, perm fs.FileMode) (apkfs.File, error) {
	return m.openFile(name, flag, perm, 0)
}

func (m *memFS) openFile(name string, flag int, perm fs.FileMode, linkCount int) (*memFile, error) {
	parent := filepath.Dir(name)
	base := filepath.Base(name)
	parentAnode, err := m.getNode(parent)
	if err != nil {
		return nil, err
	}
	if !parentAnode.dir {
		return nil, fmt.Errorf("parent is not a directory")
	}
	if parentAnode.children == nil {
		parentAnode.children = map[string]*node{}
	}
	parentAnode.mu.Lock()
	anode, ok := parentAnode.children[base]
	if !ok && flag&os.O_CREATE == 0 {
		parentAnode.mu.Unlock()

		if parent == name {
			return newMemFile(parentAnode, name, m, flag), nil
		}
		return nil, fs.ErrNotExist
	}
	if anode != nil && anode.dir {
		parentAnode.mu.Unlock()
		return nil, fmt.Errorf("is a directory")
	}
	if flag&os.O_CREATE != 0 {
		if !ok {
			// create the file
			anode = &node{
				name:       base,
				mode:       perm,
				dir:        false,
				modTime:    time.Now(),
				createTime: time.Now(),
				xattrs:     map[string][]byte{},
			}
			parentAnode.children[base] = anode
		}
	}
	parentAnode.mu.Unlock()
	// what if it is a symlink? Follow the symlink
	if anode.mode&os.ModeSymlink != 0 {
		localCount := linkCount + 1
		if localCount > maxLinks {
			return nil, fmt.Errorf("too many links")
		}
		linkTarget := anode.linkTarget
		if !filepath.IsAbs(linkTarget) {
			linkTarget = filepath.Join(parent, linkTarget)
		}
		return m.openFile(linkTarget, flag, perm, localCount)
	}

	// NOTE: This is where we diverge from memfs a bit.
	// If the node has a tarentry, then we may be reading from a tarfs.
	if anode.te != nil && len(anode.data) == 0 && anode.te.header.Size != 0 {
		f, err := anode.te.tfs.Open(anode.te.header.Name)
		if err != nil {
			return nil, err
		}

		// For read-only opens, we just defer to the tarfs.
		write := flag&os.O_APPEND != 0 || flag&os.O_RDWR != 0 || flag&os.O_WRONLY != 0
		if !write {
			mf := newMemFile(anode, name, m, flag)

			// rc gets used in Read() if it's set
			mf.rc = f

			return mf, nil
		}

		// Otherwise, we are opening this file to modify it, so buffer the data.
		anode.data, err = io.ReadAll(f)
		if err != nil {
			return nil, err
		}
	}

	return newMemFile(anode, name, m, flag), nil
}

func (m *memFS) writeHeader(name string, te tarEntry) (bool, error) {
	parent := filepath.Dir(name)
	base := filepath.Base(name)

	parentAnode, err := m.getNode(parent)
	if err != nil {
		return false, err
	}

	if !parentAnode.dir {
		return false, fmt.Errorf("parent is not a directory")
	}
	if parentAnode.children == nil {
		parentAnode.children = map[string]*node{}
	}
	parentAnode.mu.Lock()
	defer parentAnode.mu.Unlock()
	existing, ok := parentAnode.children[base]
	if !ok {
		// create the file
		anode := &node{
			name:       base,
			mode:       te.header.FileInfo().Mode(),
			dir:        false,
			modTime:    time.Now(),
			createTime: time.Now(),
			xattrs:     map[string][]byte{},
			te:         &te,
		}
		parentAnode.children[base] = anode
		return true, nil
	}

	want, got := te, existing.te

	if got == nil {
		if existing.data == nil {
			return false, fmt.Errorf("conflicting file for %q has no tar entry", name)
		}

		// This can happen when go-apk's InitKeyring conflicts with alpine-keys.
		// Since those files will be in memory, quickly compute the checksum and
		// ignore this file if they match.
		h := sha1.New() //nolint:gosec // this is what apk tools is using
		h.Write(existing.data)
		checksum := h.Sum(nil)

		if bytes.Equal(want.checksum, checksum) {
			return false, nil
		}

		return false, fmt.Errorf("conflicting file for %q with checksum %x, existing has checksum %x", name, want.checksum, checksum)
	}

	// Files have the same checksum, that's fine.
	if bytes.Equal(got.checksum, want.checksum) {
		return false, nil
	}

	// If the existing file's package replaces the package we want to install, we don't need to write this file.
	for _, replace := range got.pkg.Replaces {
		if want.pkg.Name == replace {
			return false, nil
		}
	}

	// Otherwise, determine if the package we are installing replaces the existing package.
	replaces := false
	for _, replace := range want.pkg.Replaces {
		if got.pkg.Name == replace {
			replaces = true
			break
		}
	}

	// Or if they're from the same origin.
	sameOrigin := got.pkg.Origin == want.pkg.Origin

	// At this point we know the files conflict, but it's okay if this file replaces that one.
	if !sameOrigin && !replaces {
		return false, fmt.Errorf("conflicting file %q in %q has different origin %q != %q in %q", name, got.pkg.Name, got.pkg.Origin, want.pkg.Origin, want.pkg.Name)
	}

	anode := &node{
		name:       base,
		mode:       te.header.FileInfo().Mode(),
		dir:        false,
		modTime:    time.Now(),
		createTime: time.Now(),
		xattrs:     map[string][]byte{},
		te:         &te,
	}
	parentAnode.children[base] = anode

	// If we got here, they're different, but want replaces got, so it's all cool.
	return true, nil
}

func (m *memFS) ReadFile(name string) ([]byte, error) {
	f, err := m.OpenFile(name, os.O_RDONLY, 0o644)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	b := bytes.NewBuffer(nil)
	if _, err := io.Copy(b, f); err != nil {
		return nil, err
	}
	return b.Bytes(), nil
}

func (m *memFS) WriteFile(name string, b []byte, mode fs.FileMode) error {
	f, err := m.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, mode)
	if err != nil {
		return err
	}
	defer f.Close()
	if _, err := io.Copy(f, bytes.NewBuffer(b)); err != nil {
		return err
	}
	return nil
}

func (m *memFS) ReadDir(name string) ([]fs.DirEntry, error) {
	anode, err := m.getNode(name)
	if err != nil {
		return nil, err
	}
	if !anode.dir {
		return nil, fmt.Errorf("not a directory")
	}
	var de = make([]fs.DirEntry, 0, len(anode.children))
	for name, node := range anode.children {
		de = append(de, fs.FileInfoToDirEntry(node.fileInfo(name)))
	}
	// we need them in a consistent order, so sort them by filename, which is what os.ReadDir() does
	sort.Slice(de, func(i, j int) bool {
		return de[i].Name() < de[j].Name()
	})
	return de, nil
}

func (m *memFS) OpenReaderAt(name string) (apkfs.File, error) {
	return m.OpenFile(name, os.O_RDONLY, 0o644)
}

func (m *memFS) Mknod(path string, mode uint32, dev int) error {
	parent := filepath.Dir(path)
	base := filepath.Base(path)
	anode, err := m.getNode(parent)
	if err != nil {
		return err
	}
	anode.mu.Lock()
	defer anode.mu.Unlock()
	if _, ok := anode.children[base]; ok {
		return fs.ErrExist
	}
	anode.children[base] = &node{
		name:       base,
		mode:       fs.FileMode(mode) | os.ModeCharDevice | os.ModeDevice,
		modTime:    time.Now(),
		createTime: time.Now(),
		major:      unix.Major(uint64(dev)),
		minor:      unix.Minor(uint64(dev)),
		xattrs:     map[string][]byte{},
	}

	return nil
}

func (m *memFS) Readnod(path string) (dev int, err error) {
	parent := filepath.Dir(path)
	base := filepath.Base(path)
	parentNode, err := m.getNode(parent)
	if err != nil {
		return 0, err
	}
	parentNode.mu.Lock()
	defer parentNode.mu.Unlock()
	anode, ok := parentNode.children[base]
	if !ok {
		return 0, fs.ErrNotExist
	}
	if anode.mode&os.ModeDevice != os.ModeDevice || anode.mode&os.ModeCharDevice != os.ModeCharDevice {
		return 0, fmt.Errorf("not a device")
	}
	return int(unix.Mkdev(anode.major, anode.minor)), nil
}

func (m *memFS) Chmod(path string, perm fs.FileMode) error {
	anode, err := m.getNode(path)
	if err != nil {
		return err
	}
	// need to change the mode, but keep the type
	anode.mode = perm | (anode.mode & os.ModeType)
	return nil
}
func (m *memFS) Chown(path string, uid, gid int) error {
	anode, err := m.getNode(path)
	if err != nil {
		return err
	}
	anode.uid = uid
	anode.gid = gid
	return nil
}

func (m *memFS) Create(name string) (apkfs.File, error) {
	return m.OpenFile(name, os.O_CREATE|os.O_TRUNC|os.O_RDWR, 0o666)
}

func (m *memFS) Symlink(oldname, newname string) error {
	parent := filepath.Dir(newname)
	base := filepath.Base(newname)
	anode, err := m.getNode(parent)
	if err != nil {
		return err
	}
	anode.mu.Lock()
	defer anode.mu.Unlock()
	if _, ok := anode.children[base]; ok {
		return fs.ErrExist
	}
	anode.children[base] = &node{
		name:       base,
		mode:       0o777 | os.ModeSymlink,
		modTime:    time.Now(),
		linkTarget: oldname,
		xattrs:     map[string][]byte{},
	}
	return nil
}

func (m *memFS) Link(oldname, newname string) error {
	parent := filepath.Dir(newname)
	base := filepath.Base(newname)
	anode, err := m.getNode(parent)
	if err != nil {
		return err
	}
	target, err := m.getNode(oldname)
	if err != nil {
		return fs.ErrNotExist
	}
	anode.mu.Lock()
	defer anode.mu.Unlock()
	if _, ok := anode.children[base]; ok {
		return fs.ErrExist
	}
	anode.children[base] = target
	target.linkCount++
	return nil
}

func (m *memFS) Readlink(name string) (target string, err error) {
	parent := filepath.Dir(name)
	base := filepath.Base(name)
	parentNode, err := m.getNode(parent)
	if err != nil {
		return "", err
	}
	parentNode.mu.Lock()
	defer parentNode.mu.Unlock()
	anode, ok := parentNode.children[base]
	if !ok {
		return "", fs.ErrNotExist
	}
	if anode.mode&os.ModeSymlink == 0 {
		return "", fmt.Errorf("file is not a link")
	}
	return anode.linkTarget, nil
}

func (m *memFS) Remove(name string) error {
	parent := filepath.Dir(name)
	base := filepath.Base(name)
	anode, err := m.getNode(parent)
	if err != nil {
		return err
	}
	anode.mu.Lock()
	defer anode.mu.Unlock()
	if _, ok := anode.children[base]; !ok {
		return fs.ErrNotExist
	}
	if anode.children[base].linkCount > 0 {
		anode.children[base].linkCount--
	}
	delete(anode.children, base)
	return nil
}

func (m *memFS) SetXattr(path string, attr string, data []byte) error {
	node, err := m.getNode(path)
	if err != nil {
		return os.ErrNotExist
	}
	node.mu.Lock()
	defer node.mu.Unlock()
	node.xattrs[attr] = data
	return nil
}

func (m *memFS) GetXattr(path string, attr string) ([]byte, error) {
	node, err := m.getNode(path)
	if err != nil {
		return nil, os.ErrNotExist
	}
	node.mu.Lock()
	defer node.mu.Unlock()
	data, ok := node.xattrs[attr]
	if !ok {
		return nil, os.ErrNotExist
	}
	return data, nil
}

func (m *memFS) RemoveXattr(path string, attr string) error {
	node, err := m.getNode(path)
	if err != nil {
		return os.ErrNotExist
	}
	node.mu.Lock()
	defer node.mu.Unlock()
	// RemoveXattr is meant to ensure it does not exist; if it does not exist already, that is fine
	if _, ok := node.xattrs[attr]; !ok {
		return nil
	}
	delete(node.xattrs, attr)
	return nil
}

func (m *memFS) ListXattrs(path string) (map[string][]byte, error) {
	node, err := m.getNode(path)
	if err != nil {
		return nil, os.ErrNotExist
	}
	node.mu.Lock()
	defer node.mu.Unlock()
	// do not return the original, as someone might change it by accident.
	// Return a copy
	ret := make(map[string][]byte)
	for k, v := range node.xattrs {
		dst := make([]byte, len(v))
		copy(dst, v)
		ret[k] = dst
	}
	return ret, nil
}

type memFile struct {
	node     *node
	fs       *memFS
	name     string
	offset   int64
	openMode int

	rc io.ReadCloser
}

func newMemFile(node *node, name string, memfs *memFS, openMode int) *memFile {
	m := &memFile{
		node:     node,
		fs:       memfs,
		name:     name,
		openMode: openMode,
	}
	if openMode&os.O_APPEND != 0 {
		m.offset = int64(len(node.data))
	}
	if openMode&os.O_TRUNC != 0 {
		node.data = nil
	}
	return m
}

func (f *memFile) Stat() (fs.FileInfo, error) {
	if f.node == nil || f.fs == nil {
		return nil, fs.ErrClosed
	}
	return f.fs.Stat(f.name)
}

func (f *memFile) Close() error {
	if f.node == nil || f.fs == nil {
		return fs.ErrClosed
	}

	if f.rc != nil {
		if err := f.rc.Close(); err != nil {
			return err
		}
		f.rc = nil
	}

	f.fs = nil
	f.node = nil

	return nil
}

func (f *memFile) Read(b []byte) (int, error) {
	if f.node == nil || f.fs == nil {
		return 0, fs.ErrClosed
	}
	if f.rc != nil {
		// Defer to tarfs-backed file.
		return f.rc.Read(b)
	}
	if f.offset >= int64(len(f.node.data)) {
		return 0, io.EOF
	}
	n := copy(b, f.node.data[f.offset:])
	f.offset += int64(n)
	return n, nil
}

func (f *memFile) ReadAt(p []byte, off int64) (n int, err error) {
	if f.node == nil || f.fs == nil {
		return 0, fs.ErrClosed
	}
	if f.rc != nil {
		// tarfs-backed files don't support ReadAt.
		return 0, fs.ErrInvalid
	}
	if off >= int64(len(f.node.data)) {
		return 0, io.EOF
	}
	n = copy(p, f.node.data[off:])
	return n, nil
}

func (f *memFile) Seek(offset int64, whence int) (int64, error) {
	if f.node == nil || f.fs == nil {
		return 0, fs.ErrClosed
	}
	if f.rc != nil {
		// tarfs-backed files don't support Seek.
		return 0, fs.ErrInvalid
	}
	switch whence {
	case io.SeekStart:
		f.offset = offset
	case io.SeekCurrent:
		f.offset += offset
	case io.SeekEnd:
		f.offset = int64(len(f.node.data)) + offset
	default:
		return 0, errors.New("invalid whence")
	}
	return f.offset, nil
}

func (f *memFile) Write(p []byte) (n int, err error) {
	if f.node == nil || f.fs == nil {
		return 0, fs.ErrClosed
	}
	if f.rc != nil {
		// tarfs-backed files don't support Write.
		return 0, fs.ErrInvalid
	}
	if f.openMode&os.O_APPEND != 0 && f.openMode&os.O_RDWR != 0 && f.openMode&os.O_WRONLY != 0 {
		return 0, errors.New("file not opened in write mode")
	}
	if f.offset+int64(len(p)) > int64(len(f.node.data)) {
		f.node.data = append(f.node.data[:f.offset], p...)
	} else {
		copy(f.node.data[f.offset:], p)
	}
	f.offset += int64(len(p))
	return len(p), nil
}

type node struct {
	mode         fs.FileMode
	uid, gid     int
	dir          bool
	name         string
	data         []byte
	modTime      time.Time
	createTime   time.Time
	linkTarget   string
	linkCount    int // extra links, so 0 means a single pointer. O-based, like most compuuter counting systems.
	major, minor uint32
	children     map[string]*node
	mu           sync.Mutex
	xattrs       map[string][]byte

	// This stores metadata for a tarfs-backed file.
	te *tarEntry
}

func (n *node) fileInfo(name string) fs.FileInfo {
	return &memFileInfo{
		node: n,
		name: name,
	}
}

type memFileInfo struct {
	*node
	name string
}

func (m *memFileInfo) Name() string {
	return m.name
}

func (m *memFileInfo) Size() int64 {
	if m.node.te != nil && len(m.data) == 0 {
		return m.node.te.header.Size
	}
	return int64(len(m.data))
}

func (m *memFileInfo) Mode() fs.FileMode {
	return m.mode
}

func (m *memFileInfo) ModTime() time.Time {
	return m.modTime
}

func (m *memFileInfo) IsDir() bool {
	return m.dir
}

func (m *memFileInfo) Sys() any {
	return &tar.Header{
		Mode: int64(m.mode),
		Uid:  m.uid,
		Gid:  m.gid,
	}
}