Adding upstream version 1.22.1.upstream/1.22.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

31 files changed, 5088 insertions, 0 deletions

diff --git a/src/io/example_test.go b/src/io/example_test.go
new file mode 100644
index 0000000..818020e
--- /dev/null
+++ b/src/io/example_test.go
@@ -0,0 +1,284 @@
+// Copyright 2015 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 io_test
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"strings"
+)
+
+func ExampleCopy() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+
+	if _, err := io.Copy(os.Stdout, r); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// some io.Reader stream to be read
+}
+
+func ExampleCopyBuffer() {
+	r1 := strings.NewReader("first reader\n")
+	r2 := strings.NewReader("second reader\n")
+	buf := make([]byte, 8)
+
+	// buf is used here...
+	if _, err := io.CopyBuffer(os.Stdout, r1, buf); err != nil {
+		log.Fatal(err)
+	}
+
+	// ... reused here also. No need to allocate an extra buffer.
+	if _, err := io.CopyBuffer(os.Stdout, r2, buf); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// first reader
+	// second reader
+}
+
+func ExampleCopyN() {
+	r := strings.NewReader("some io.Reader stream to be read")
+
+	if _, err := io.CopyN(os.Stdout, r, 4); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// some
+}
+
+func ExampleReadAtLeast() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+
+	buf := make([]byte, 14)
+	if _, err := io.ReadAtLeast(r, buf, 4); err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s\n", buf)
+
+	// buffer smaller than minimal read size.
+	shortBuf := make([]byte, 3)
+	if _, err := io.ReadAtLeast(r, shortBuf, 4); err != nil {
+		fmt.Println("error:", err)
+	}
+
+	// minimal read size bigger than io.Reader stream
+	longBuf := make([]byte, 64)
+	if _, err := io.ReadAtLeast(r, longBuf, 64); err != nil {
+		fmt.Println("error:", err)
+	}
+
+	// Output:
+	// some io.Reader
+	// error: short buffer
+	// error: unexpected EOF
+}
+
+func ExampleReadFull() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+
+	buf := make([]byte, 4)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%s\n", buf)
+
+	// minimal read size bigger than io.Reader stream
+	longBuf := make([]byte, 64)
+	if _, err := io.ReadFull(r, longBuf); err != nil {
+		fmt.Println("error:", err)
+	}
+
+	// Output:
+	// some
+	// error: unexpected EOF
+}
+
+func ExampleWriteString() {
+	if _, err := io.WriteString(os.Stdout, "Hello World"); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output: Hello World
+}
+
+func ExampleLimitReader() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	lr := io.LimitReader(r, 4)
+
+	if _, err := io.Copy(os.Stdout, lr); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// some
+}
+
+func ExampleMultiReader() {
+	r1 := strings.NewReader("first reader ")
+	r2 := strings.NewReader("second reader ")
+	r3 := strings.NewReader("third reader\n")
+	r := io.MultiReader(r1, r2, r3)
+
+	if _, err := io.Copy(os.Stdout, r); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// first reader second reader third reader
+}
+
+func ExampleTeeReader() {
+	var r io.Reader = strings.NewReader("some io.Reader stream to be read\n")
+
+	r = io.TeeReader(r, os.Stdout)
+
+	// Everything read from r will be copied to stdout.
+	if _, err := io.ReadAll(r); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// some io.Reader stream to be read
+}
+
+func ExampleSectionReader() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	if _, err := io.Copy(os.Stdout, s); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// io.Reader stream
+}
+
+func ExampleSectionReader_Read() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	buf := make([]byte, 9)
+	if _, err := s.Read(buf); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s\n", buf)
+
+	// Output:
+	// io.Reader
+}
+
+func ExampleSectionReader_ReadAt() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	buf := make([]byte, 6)
+	if _, err := s.ReadAt(buf, 10); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s\n", buf)
+
+	// Output:
+	// stream
+}
+
+func ExampleSectionReader_Seek() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	if _, err := s.Seek(10, io.SeekStart); err != nil {
+		log.Fatal(err)
+	}
+
+	if _, err := io.Copy(os.Stdout, s); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// stream
+}
+
+func ExampleSectionReader_Size() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	fmt.Println(s.Size())
+
+	// Output:
+	// 17
+}
+
+func ExampleSeeker_Seek() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+
+	r.Seek(5, io.SeekStart) // move to the 5th char from the start
+	if _, err := io.Copy(os.Stdout, r); err != nil {
+		log.Fatal(err)
+	}
+
+	r.Seek(-5, io.SeekEnd)
+	if _, err := io.Copy(os.Stdout, r); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// io.Reader stream to be read
+	// read
+}
+
+func ExampleMultiWriter() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+
+	var buf1, buf2 strings.Builder
+	w := io.MultiWriter(&buf1, &buf2)
+
+	if _, err := io.Copy(w, r); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Print(buf1.String())
+	fmt.Print(buf2.String())
+
+	// Output:
+	// some io.Reader stream to be read
+	// some io.Reader stream to be read
+}
+
+func ExamplePipe() {
+	r, w := io.Pipe()
+
+	go func() {
+		fmt.Fprint(w, "some io.Reader stream to be read\n")
+		w.Close()
+	}()
+
+	if _, err := io.Copy(os.Stdout, r); err != nil {
+		log.Fatal(err)
+	}
+
+	// Output:
+	// some io.Reader stream to be read
+}
+
+func ExampleReadAll() {
+	r := strings.NewReader("Go is a general-purpose language designed with systems programming in mind.")
+
+	b, err := io.ReadAll(r)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s", b)
+
+	// Output:
+	// Go is a general-purpose language designed with systems programming in mind.
+}
diff --git a/src/io/export_test.go b/src/io/export_test.go
new file mode 100644
index 0000000..06853f9
--- /dev/null
+++ b/src/io/export_test.go
@@ -0,0 +1,10 @@
+// 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 io
+
+// exported for test
+var ErrInvalidWrite = errInvalidWrite
+var ErrWhence = errWhence
+var ErrOffset = errOffset
diff --git a/src/io/fs/example_test.go b/src/io/fs/example_test.go
new file mode 100644
index 0000000..c902703
--- /dev/null
+++ b/src/io/fs/example_test.go
@@ -0,0 +1,25 @@
+// Copyright 2021 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_test
+
+import (
+	"fmt"
+	"io/fs"
+	"log"
+	"os"
+)
+
+func ExampleWalkDir() {
+	root := "/usr/local/go/bin"
+	fileSystem := os.DirFS(root)
+
+	fs.WalkDir(fileSystem, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Println(path)
+		return nil
+	})
+}
diff --git a/src/io/fs/format.go b/src/io/fs/format.go
new file mode 100644
index 0000000..60b40df
--- /dev/null
+++ b/src/io/fs/format.go
@@ -0,0 +1,76 @@
+// Copyright 2023 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 (
+	"time"
+)
+
+// FormatFileInfo returns a formatted version of info for human readability.
+// Implementations of [FileInfo] can call this from a String method.
+// The output for a file named "hello.go", 100 bytes, mode 0o644, created
+// January 1, 1970 at noon is
+//
+//	-rw-r--r-- 100 1970-01-01 12:00:00 hello.go
+func FormatFileInfo(info FileInfo) string {
+	name := info.Name()
+	b := make([]byte, 0, 40+len(name))
+	b = append(b, info.Mode().String()...)
+	b = append(b, ' ')
+
+	size := info.Size()
+	var usize uint64
+	if size >= 0 {
+		usize = uint64(size)
+	} else {
+		b = append(b, '-')
+		usize = uint64(-size)
+	}
+	var buf [20]byte
+	i := len(buf) - 1
+	for usize >= 10 {
+		q := usize / 10
+		buf[i] = byte('0' + usize - q*10)
+		i--
+		usize = q
+	}
+	buf[i] = byte('0' + usize)
+	b = append(b, buf[i:]...)
+	b = append(b, ' ')
+
+	b = append(b, info.ModTime().Format(time.DateTime)...)
+	b = append(b, ' ')
+
+	b = append(b, name...)
+	if info.IsDir() {
+		b = append(b, '/')
+	}
+
+	return string(b)
+}
+
+// FormatDirEntry returns a formatted version of dir for human readability.
+// Implementations of [DirEntry] can call this from a String method.
+// The outputs for a directory named subdir and a file named hello.go are:
+//
+//	d subdir/
+//	- hello.go
+func FormatDirEntry(dir DirEntry) string {
+	name := dir.Name()
+	b := make([]byte, 0, 5+len(name))
+
+	// The Type method does not return any permission bits,
+	// so strip them from the string.
+	mode := dir.Type().String()
+	mode = mode[:len(mode)-9]
+
+	b = append(b, mode...)
+	b = append(b, ' ')
+	b = append(b, name...)
+	if dir.IsDir() {
+		b = append(b, '/')
+	}
+	return string(b)
+}
diff --git a/src/io/fs/format_test.go b/src/io/fs/format_test.go
new file mode 100644
index 0000000..a5f5066
--- /dev/null
+++ b/src/io/fs/format_test.go
@@ -0,0 +1,123 @@
+// Copyright 2023 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_test
+
+import (
+	. "io/fs"
+	"testing"
+	"time"
+)
+
+// formatTest implements FileInfo to test FormatFileInfo,
+// and implements DirEntry to test FormatDirEntry.
+type formatTest struct {
+	name    string
+	size    int64
+	mode    FileMode
+	modTime time.Time
+	isDir   bool
+}
+
+func (fs *formatTest) Name() string {
+	return fs.name
+}
+
+func (fs *formatTest) Size() int64 {
+	return fs.size
+}
+
+func (fs *formatTest) Mode() FileMode {
+	return fs.mode
+}
+
+func (fs *formatTest) ModTime() time.Time {
+	return fs.modTime
+}
+
+func (fs *formatTest) IsDir() bool {
+	return fs.isDir
+}
+
+func (fs *formatTest) Sys() any {
+	return nil
+}
+
+func (fs *formatTest) Type() FileMode {
+	return fs.mode.Type()
+}
+
+func (fs *formatTest) Info() (FileInfo, error) {
+	return fs, nil
+}
+
+var formatTests = []struct {
+	input        formatTest
+	wantFileInfo string
+	wantDirEntry string
+}{
+	{
+		formatTest{
+			name:    "hello.go",
+			size:    100,
+			mode:    0o644,
+			modTime: time.Date(1970, time.January, 1, 12, 0, 0, 0, time.UTC),
+			isDir:   false,
+		},
+		"-rw-r--r-- 100 1970-01-01 12:00:00 hello.go",
+		"- hello.go",
+	},
+	{
+		formatTest{
+			name:    "home/gopher",
+			size:    0,
+			mode:    ModeDir | 0o755,
+			modTime: time.Date(1970, time.January, 1, 12, 0, 0, 0, time.UTC),
+			isDir:   true,
+		},
+		"drwxr-xr-x 0 1970-01-01 12:00:00 home/gopher/",
+		"d home/gopher/",
+	},
+	{
+		formatTest{
+			name:    "big",
+			size:    0x7fffffffffffffff,
+			mode:    ModeIrregular | 0o644,
+			modTime: time.Date(1970, time.January, 1, 12, 0, 0, 0, time.UTC),
+			isDir:   false,
+		},
+		"?rw-r--r-- 9223372036854775807 1970-01-01 12:00:00 big",
+		"? big",
+	},
+	{
+		formatTest{
+			name:    "small",
+			size:    -0x8000000000000000,
+			mode:    ModeSocket | ModeSetuid | 0o644,
+			modTime: time.Date(1970, time.January, 1, 12, 0, 0, 0, time.UTC),
+			isDir:   false,
+		},
+		"Surw-r--r-- -9223372036854775808 1970-01-01 12:00:00 small",
+		"S small",
+	},
+}
+
+func TestFormatFileInfo(t *testing.T) {
+	for i, test := range formatTests {
+		got := FormatFileInfo(&test.input)
+		if got != test.wantFileInfo {
+			t.Errorf("%d: FormatFileInfo(%#v) = %q, want %q", i, test.input, got, test.wantFileInfo)
+		}
+	}
+}
+
+func TestFormatDirEntry(t *testing.T) {
+	for i, test := range formatTests {
+		got := FormatDirEntry(&test.input)
+		if got != test.wantDirEntry {
+			t.Errorf("%d: FormatDirEntry(%#v) = %q, want %q", i, test.input, got, test.wantDirEntry)
+		}
+	}
+
+}
diff --git a/src/io/fs/fs.go b/src/io/fs/fs.go
new file mode 100644
index 0000000..6891d75
--- /dev/null
+++ b/src/io/fs/fs.go
@@ -0,0 +1,264 @@
+// 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 defines basic interfaces to a file system.
+// A file system can be provided by the host operating system
+// but also by other packages.
+//
+// See the [testing/fstest] package for support with testing
+// implementations of file systems.
+package fs
+
+import (
+	"internal/oserror"
+	"time"
+	"unicode/utf8"
+)
+
+// An FS provides access to a hierarchical file system.
+//
+// The FS interface is the minimum implementation required of the file system.
+// A file system may implement additional interfaces,
+// such as [ReadFileFS], to provide additional or optimized functionality.
+//
+// [testing/fstest.TestFS] may be used to test implementations of an FS for
+// correctness.
+type FS interface {
+	// Open opens the named file.
+	//
+	// When Open returns an error, it should be of type *PathError
+	// with the Op field set to "open", the Path field set to name,
+	// and the Err field describing the problem.
+	//
+	// Open should reject attempts to open names that do not satisfy
+	// ValidPath(name), returning a *PathError with Err set to
+	// ErrInvalid or ErrNotExist.
+	Open(name string) (File, error)
+}
+
+// ValidPath reports whether the given path name
+// is valid for use in a call to Open.
+//
+// Path names passed to open are UTF-8-encoded,
+// unrooted, slash-separated sequences of path elements, like “x/y/z”.
+// Path names must not contain an element that is “.” or “..” or the empty string,
+// except for the special case that the root directory is named “.”.
+// Paths must not start or end with a slash: “/x” and “x/” are invalid.
+//
+// Note that paths are slash-separated on all systems, even Windows.
+// Paths containing other characters such as backslash and colon
+// are accepted as valid, but those characters must never be
+// interpreted by an [FS] implementation as path element separators.
+func ValidPath(name string) bool {
+	if !utf8.ValidString(name) {
+		return false
+	}
+
+	if name == "." {
+		// special case
+		return true
+	}
+
+	// Iterate over elements in name, checking each.
+	for {
+		i := 0
+		for i < len(name) && name[i] != '/' {
+			i++
+		}
+		elem := name[:i]
+		if elem == "" || elem == "." || elem == ".." {
+			return false
+		}
+		if i == len(name) {
+			return true // reached clean ending
+		}
+		name = name[i+1:]
+	}
+}
+
+// A File provides access to a single file.
+// The File interface is the minimum implementation required of the file.
+// Directory files should also implement [ReadDirFile].
+// A file may implement [io.ReaderAt] or [io.Seeker] as optimizations.
+type File interface {
+	Stat() (FileInfo, error)
+	Read([]byte) (int, error)
+	Close() error
+}
+
+// A DirEntry is an entry read from a directory
+// (using the [ReadDir] function or a [ReadDirFile]'s ReadDir method).
+type DirEntry interface {
+	// Name returns the name of the file (or subdirectory) described by the entry.
+	// This name is only the final element of the path (the base name), not the entire path.
+	// For example, Name would return "hello.go" not "home/gopher/hello.go".
+	Name() string
+
+	// IsDir reports whether the entry describes a directory.
+	IsDir() bool
+
+	// Type returns the type bits for the entry.
+	// The type bits are a subset of the usual FileMode bits, those returned by the FileMode.Type method.
+	Type() FileMode
+
+	// Info returns the FileInfo for the file or subdirectory described by the entry.
+	// The returned FileInfo may be from the time of the original directory read
+	// or from the time of the call to Info. If the file has been removed or renamed
+	// since the directory read, Info may return an error satisfying errors.Is(err, ErrNotExist).
+	// If the entry denotes a symbolic link, Info reports the information about the link itself,
+	// not the link's target.
+	Info() (FileInfo, error)
+}
+
+// A ReadDirFile is a directory file whose entries can be read with the ReadDir method.
+// Every directory file should implement this interface.
+// (It is permissible for any file to implement this interface,
+// but if so ReadDir should return an error for non-directories.)
+type ReadDirFile interface {
+	File
+
+	// ReadDir reads the contents of the directory and returns
+	// a slice of up to n DirEntry values in directory order.
+	// Subsequent calls on the same file will yield further DirEntry values.
+	//
+	// If n > 0, ReadDir returns at most n DirEntry structures.
+	// In this case, if ReadDir returns an empty slice, it will return
+	// a non-nil error explaining why.
+	// At the end of a directory, the error is io.EOF.
+	// (ReadDir must return io.EOF itself, not an error wrapping io.EOF.)
+	//
+	// If n <= 0, ReadDir returns all the DirEntry values from the directory
+	// in a single slice. In this case, if ReadDir succeeds (reads all the way
+	// to the end of the directory), it returns the slice and a nil error.
+	// If it encounters an error before the end of the directory,
+	// ReadDir returns the DirEntry list read until that point and a non-nil error.
+	ReadDir(n int) ([]DirEntry, error)
+}
+
+// Generic file system errors.
+// Errors returned by file systems can be tested against these errors
+// using [errors.Is].
+var (
+	ErrInvalid    = errInvalid()    // "invalid argument"
+	ErrPermission = errPermission() // "permission denied"
+	ErrExist      = errExist()      // "file already exists"
+	ErrNotExist   = errNotExist()   // "file does not exist"
+	ErrClosed     = errClosed()     // "file already closed"
+)
+
+func errInvalid() error    { return oserror.ErrInvalid }
+func errPermission() error { return oserror.ErrPermission }
+func errExist() error      { return oserror.ErrExist }
+func errNotExist() error   { return oserror.ErrNotExist }
+func errClosed() error     { return oserror.ErrClosed }
+
+// A FileInfo describes a file and is returned by [Stat].
+type FileInfo interface {
+	Name() string       // base name of the file
+	Size() int64        // length in bytes for regular files; system-dependent for others
+	Mode() FileMode     // file mode bits
+	ModTime() time.Time // modification time
+	IsDir() bool        // abbreviation for Mode().IsDir()
+	Sys() any           // underlying data source (can return nil)
+}
+
+// A FileMode represents a file's mode and permission bits.
+// The bits have the same definition on all systems, so that
+// information about files can be moved from one system
+// to another portably. Not all bits apply to all systems.
+// The only required bit is [ModeDir] for directories.
+type FileMode uint32
+
+// The defined file mode bits are the most significant bits of the [FileMode].
+// The nine least-significant bits are the standard Unix rwxrwxrwx permissions.
+// The values of these bits should be considered part of the public API and
+// may be used in wire protocols or disk representations: they must not be
+// changed, although new bits might be added.
+const (
+	// The single letters are the abbreviations
+	// used by the String method's formatting.
+	ModeDir        FileMode = 1 << (32 - 1 - iota) // d: is a directory
+	ModeAppend                                     // a: append-only
+	ModeExclusive                                  // l: exclusive use
+	ModeTemporary                                  // T: temporary file; Plan 9 only
+	ModeSymlink                                    // L: symbolic link
+	ModeDevice                                     // D: device file
+	ModeNamedPipe                                  // p: named pipe (FIFO)
+	ModeSocket                                     // S: Unix domain socket
+	ModeSetuid                                     // u: setuid
+	ModeSetgid                                     // g: setgid
+	ModeCharDevice                                 // c: Unix character device, when ModeDevice is set
+	ModeSticky                                     // t: sticky
+	ModeIrregular                                  // ?: non-regular file; nothing else is known about this file
+
+	// Mask for the type bits. For regular files, none will be set.
+	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice | ModeCharDevice | ModeIrregular
+
+	ModePerm FileMode = 0777 // Unix permission bits
+)
+
+func (m FileMode) String() string {
+	const str = "dalTLDpSugct?"
+	var buf [32]byte // Mode is uint32.
+	w := 0
+	for i, c := range str {
+		if m&(1<<uint(32-1-i)) != 0 {
+			buf[w] = byte(c)
+			w++
+		}
+	}
+	if w == 0 {
+		buf[w] = '-'
+		w++
+	}
+	const rwx = "rwxrwxrwx"
+	for i, c := range rwx {
+		if m&(1<<uint(9-1-i)) != 0 {
+			buf[w] = byte(c)
+		} else {
+			buf[w] = '-'
+		}
+		w++
+	}
+	return string(buf[:w])
+}
+
+// IsDir reports whether m describes a directory.
+// That is, it tests for the [ModeDir] bit being set in m.
+func (m FileMode) IsDir() bool {
+	return m&ModeDir != 0
+}
+
+// IsRegular reports whether m describes a regular file.
+// That is, it tests that no mode type bits are set.
+func (m FileMode) IsRegular() bool {
+	return m&ModeType == 0
+}
+
+// Perm returns the Unix permission bits in m (m & [ModePerm]).
+func (m FileMode) Perm() FileMode {
+	return m & ModePerm
+}
+
+// Type returns type bits in m (m & [ModeType]).
+func (m FileMode) Type() FileMode {
+	return m & ModeType
+}
+
+// PathError records an error and the operation and file path that caused it.
+type PathError struct {
+	Op   string
+	Path string
+	Err  error
+}
+
+func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }
+
+func (e *PathError) Unwrap() error { return e.Err }
+
+// Timeout reports whether this error represents a timeout.
+func (e *PathError) Timeout() bool {
+	t, ok := e.Err.(interface{ Timeout() bool })
+	return ok && t.Timeout()
+}
diff --git a/src/io/fs/fs_test.go b/src/io/fs/fs_test.go
new file mode 100644
index 0000000..aae1a76
--- /dev/null
+++ b/src/io/fs/fs_test.go
@@ -0,0 +1,49 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"testing"
+)
+
+var isValidPathTests = []struct {
+	name string
+	ok   bool
+}{
+	{".", true},
+	{"x", true},
+	{"x/y", true},
+
+	{"", false},
+	{"..", false},
+	{"/", false},
+	{"x/", false},
+	{"/x", false},
+	{"x/y/", false},
+	{"/x/y", false},
+	{"./", false},
+	{"./x", false},
+	{"x/.", false},
+	{"x/./y", false},
+	{"../", false},
+	{"../x", false},
+	{"x/..", false},
+	{"x/../y", false},
+	{"x//y", false},
+	{`x\`, true},
+	{`x\y`, true},
+	{`x:y`, true},
+	{`\x`, true},
+}
+
+func TestValidPath(t *testing.T) {
+	for _, tt := range isValidPathTests {
+		ok := ValidPath(tt.name)
+		if ok != tt.ok {
+			t.Errorf("ValidPath(%q) = %v, want %v", tt.name, ok, tt.ok)
+		}
+	}
+}
diff --git a/src/io/fs/glob.go b/src/io/fs/glob.go
new file mode 100644
index 0000000..db17156
--- /dev/null
+++ b/src/io/fs/glob.go
@@ -0,0 +1,129 @@
+// 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 (
+	"path"
+)
+
+// A GlobFS is a file system with a Glob method.
+type GlobFS interface {
+	FS
+
+	// Glob returns the names of all files matching pattern,
+	// providing an implementation of the top-level
+	// Glob function.
+	Glob(pattern string) ([]string, error)
+}
+
+// Glob returns the names of all files matching pattern or nil
+// if there is no matching file. The syntax of patterns is the same
+// as in [path.Match]. The pattern may describe hierarchical names such as
+// usr/*/bin/ed.
+//
+// Glob ignores file system errors such as I/O errors reading directories.
+// The only possible returned error is [path.ErrBadPattern], reporting that
+// the pattern is malformed.
+//
+// If fs implements [GlobFS], Glob calls fs.Glob.
+// Otherwise, Glob uses [ReadDir] to traverse the directory tree
+// and look for matches for the pattern.
+func Glob(fsys FS, pattern string) (matches []string, err error) {
+	return globWithLimit(fsys, pattern, 0)
+}
+
+func globWithLimit(fsys FS, pattern string, depth int) (matches []string, err error) {
+	// This limit is added to prevent stack exhaustion issues. See
+	// CVE-2022-30630.
+	const pathSeparatorsLimit = 10000
+	if depth > pathSeparatorsLimit {
+		return nil, path.ErrBadPattern
+	}
+	if fsys, ok := fsys.(GlobFS); ok {
+		return fsys.Glob(pattern)
+	}
+
+	// Check pattern is well-formed.
+	if _, err := path.Match(pattern, ""); err != nil {
+		return nil, err
+	}
+	if !hasMeta(pattern) {
+		if _, err = Stat(fsys, pattern); err != nil {
+			return nil, nil
+		}
+		return []string{pattern}, nil
+	}
+
+	dir, file := path.Split(pattern)
+	dir = cleanGlobPath(dir)
+
+	if !hasMeta(dir) {
+		return glob(fsys, dir, file, nil)
+	}
+
+	// Prevent infinite recursion. See issue 15879.
+	if dir == pattern {
+		return nil, path.ErrBadPattern
+	}
+
+	var m []string
+	m, err = globWithLimit(fsys, dir, depth+1)
+	if err != nil {
+		return nil, err
+	}
+	for _, d := range m {
+		matches, err = glob(fsys, d, file, matches)
+		if err != nil {
+			return
+		}
+	}
+	return
+}
+
+// cleanGlobPath prepares path for glob matching.
+func cleanGlobPath(path string) string {
+	switch path {
+	case "":
+		return "."
+	default:
+		return path[0 : len(path)-1] // chop off trailing separator
+	}
+}
+
+// glob searches for files matching pattern in the directory dir
+// and appends them to matches, returning the updated slice.
+// If the directory cannot be opened, glob returns the existing matches.
+// New matches are added in lexicographical order.
+func glob(fs FS, dir, pattern string, matches []string) (m []string, e error) {
+	m = matches
+	infos, err := ReadDir(fs, dir)
+	if err != nil {
+		return // ignore I/O error
+	}
+
+	for _, info := range infos {
+		n := info.Name()
+		matched, err := path.Match(pattern, n)
+		if err != nil {
+			return m, err
+		}
+		if matched {
+			m = append(m, path.Join(dir, n))
+		}
+	}
+	return
+}
+
+// hasMeta reports whether path contains any of the magic characters
+// recognized by path.Match.
+func hasMeta(path string) bool {
+	for i := 0; i < len(path); i++ {
+		switch path[i] {
+		case '*', '?', '[', '\\':
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/io/fs/glob_test.go b/src/io/fs/glob_test.go
new file mode 100644
index 0000000..d052eab
--- /dev/null
+++ b/src/io/fs/glob_test.go
@@ -0,0 +1,97 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"os"
+	"path"
+	"strings"
+	"testing"
+)
+
+var globTests = []struct {
+	fs              FS
+	pattern, result string
+}{
+	{os.DirFS("."), "glob.go", "glob.go"},
+	{os.DirFS("."), "gl?b.go", "glob.go"},
+	{os.DirFS("."), `gl\ob.go`, "glob.go"},
+	{os.DirFS("."), "*", "glob.go"},
+	{os.DirFS(".."), "*/glob.go", "fs/glob.go"},
+}
+
+func TestGlob(t *testing.T) {
+	for _, tt := range globTests {
+		matches, err := Glob(tt.fs, tt.pattern)
+		if err != nil {
+			t.Errorf("Glob error for %q: %s", tt.pattern, err)
+			continue
+		}
+		if !contains(matches, tt.result) {
+			t.Errorf("Glob(%#q) = %#v want %v", tt.pattern, matches, tt.result)
+		}
+	}
+	for _, pattern := range []string{"no_match", "../*/no_match", `\*`} {
+		matches, err := Glob(os.DirFS("."), pattern)
+		if err != nil {
+			t.Errorf("Glob error for %q: %s", pattern, err)
+			continue
+		}
+		if len(matches) != 0 {
+			t.Errorf("Glob(%#q) = %#v want []", pattern, matches)
+		}
+	}
+}
+
+func TestGlobError(t *testing.T) {
+	bad := []string{`[]`, `nonexist/[]`}
+	for _, pattern := range bad {
+		_, err := Glob(os.DirFS("."), pattern)
+		if err != path.ErrBadPattern {
+			t.Errorf("Glob(fs, %#q) returned err=%v, want path.ErrBadPattern", pattern, err)
+		}
+	}
+}
+
+func TestCVE202230630(t *testing.T) {
+	// Prior to CVE-2022-30630, a stack exhaustion would occur given a large
+	// number of separators. There is now a limit of 10,000.
+	_, err := Glob(os.DirFS("."), "/*"+strings.Repeat("/", 10001))
+	if err != path.ErrBadPattern {
+		t.Fatalf("Glob returned err=%v, want %v", err, path.ErrBadPattern)
+	}
+}
+
+// contains reports whether vector contains the string s.
+func contains(vector []string, s string) bool {
+	for _, elem := range vector {
+		if elem == s {
+			return true
+		}
+	}
+	return false
+}
+
+type globOnly struct{ GlobFS }
+
+func (globOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestGlobMethod(t *testing.T) {
+	check := func(desc string, names []string, err error) {
+		t.Helper()
+		if err != nil || len(names) != 1 || names[0] != "hello.txt" {
+			t.Errorf("Glob(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt"})
+		}
+	}
+
+	// Test that ReadDir uses the method when present.
+	names, err := Glob(globOnly{testFsys}, "*.txt")
+	check("readDirOnly", names, err)
+
+	// Test that ReadDir uses Open when the method is not present.
+	names, err = Glob(openOnly{testFsys}, "*.txt")
+	check("openOnly", names, err)
+}
diff --git a/src/io/fs/readdir.go b/src/io/fs/readdir.go
new file mode 100644
index 0000000..22ced48
--- /dev/null
+++ b/src/io/fs/readdir.go
@@ -0,0 +1,81 @@
+// 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"
+	"sort"
+)
+
+// ReadDirFS is the interface implemented by a file system
+// that provides an optimized implementation of [ReadDir].
+type ReadDirFS interface {
+	FS
+
+	// ReadDir reads the named directory
+	// and returns a list of directory entries sorted by filename.
+	ReadDir(name string) ([]DirEntry, error)
+}
+
+// ReadDir reads the named directory
+// and returns a list of directory entries sorted by filename.
+//
+// If fs implements [ReadDirFS], ReadDir calls fs.ReadDir.
+// Otherwise ReadDir calls fs.Open and uses ReadDir and Close
+// on the returned file.
+func ReadDir(fsys FS, name string) ([]DirEntry, error) {
+	if fsys, ok := fsys.(ReadDirFS); ok {
+		return fsys.ReadDir(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	dir, ok := file.(ReadDirFile)
+	if !ok {
+		return nil, &PathError{Op: "readdir", Path: name, Err: errors.New("not implemented")}
+	}
+
+	list, err := dir.ReadDir(-1)
+	sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
+	return list, err
+}
+
+// dirInfo is a DirEntry based on a FileInfo.
+type dirInfo struct {
+	fileInfo FileInfo
+}
+
+func (di dirInfo) IsDir() bool {
+	return di.fileInfo.IsDir()
+}
+
+func (di dirInfo) Type() FileMode {
+	return di.fileInfo.Mode().Type()
+}
+
+func (di dirInfo) Info() (FileInfo, error) {
+	return di.fileInfo, nil
+}
+
+func (di dirInfo) Name() string {
+	return di.fileInfo.Name()
+}
+
+func (di dirInfo) String() string {
+	return FormatDirEntry(di)
+}
+
+// FileInfoToDirEntry returns a [DirEntry] that returns information from info.
+// If info is nil, FileInfoToDirEntry returns nil.
+func FileInfoToDirEntry(info FileInfo) DirEntry {
+	if info == nil {
+		return nil
+	}
+	return dirInfo{fileInfo: info}
+}
diff --git a/src/io/fs/readdir_test.go b/src/io/fs/readdir_test.go
new file mode 100644
index 0000000..4c409ae
--- /dev/null
+++ b/src/io/fs/readdir_test.go
@@ -0,0 +1,111 @@
+// 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_test
+
+import (
+	"errors"
+	. "io/fs"
+	"os"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+type readDirOnly struct{ ReadDirFS }
+
+func (readDirOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestReadDir(t *testing.T) {
+	check := func(desc string, dirs []DirEntry, err error) {
+		t.Helper()
+		if err != nil || len(dirs) != 2 || dirs[0].Name() != "hello.txt" || dirs[1].Name() != "sub" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf("ReadDir(%s) = %v, %v, want %v, nil", desc, names, err, []string{"hello.txt", "sub"})
+		}
+	}
+
+	// Test that ReadDir uses the method when present.
+	dirs, err := ReadDir(readDirOnly{testFsys}, ".")
+	check("readDirOnly", dirs, err)
+
+	// Test that ReadDir uses Open when the method is not present.
+	dirs, err = ReadDir(openOnly{testFsys}, ".")
+	check("openOnly", dirs, err)
+
+	// Test that ReadDir on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dirs, err = ReadDir(sub, ".")
+	check("sub(.)", dirs, err)
+}
+
+func TestFileInfoToDirEntry(t *testing.T) {
+	testFs := fstest.MapFS{
+		"notadir.txt": {
+			Data:    []byte("hello, world"),
+			Mode:    0,
+			ModTime: time.Now(),
+			Sys:     &sysValue,
+		},
+		"adir": {
+			Data:    nil,
+			Mode:    os.ModeDir,
+			ModTime: time.Now(),
+			Sys:     &sysValue,
+		},
+	}
+
+	tests := []struct {
+		path     string
+		wantMode FileMode
+		wantDir  bool
+	}{
+		{path: "notadir.txt", wantMode: 0, wantDir: false},
+		{path: "adir", wantMode: os.ModeDir, wantDir: true},
+	}
+
+	for _, test := range tests {
+		test := test
+		t.Run(test.path, func(t *testing.T) {
+			fi, err := Stat(testFs, test.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			dirEntry := FileInfoToDirEntry(fi)
+			if g, w := dirEntry.Type(), test.wantMode; g != w {
+				t.Errorf("FileMode mismatch: got=%v, want=%v", g, w)
+			}
+			if g, w := dirEntry.Name(), test.path; g != w {
+				t.Errorf("Name mismatch: got=%v, want=%v", g, w)
+			}
+			if g, w := dirEntry.IsDir(), test.wantDir; g != w {
+				t.Errorf("IsDir mismatch: got=%v, want=%v", g, w)
+			}
+		})
+	}
+}
+
+func errorPath(err error) string {
+	var perr *PathError
+	if !errors.As(err, &perr) {
+		return ""
+	}
+	return perr.Path
+}
+
+func TestReadDirPath(t *testing.T) {
+	fsys := os.DirFS(t.TempDir())
+	_, err1 := ReadDir(fsys, "non-existent")
+	_, err2 := ReadDir(struct{ FS }{fsys}, "non-existent")
+	if s1, s2 := errorPath(err1), errorPath(err2); s1 != s2 {
+		t.Fatalf("s1: %s != s2: %s", s1, s2)
+	}
+}
diff --git a/src/io/fs/readfile.go b/src/io/fs/readfile.go
new file mode 100644
index 0000000..41ca5bf
--- /dev/null
+++ b/src/io/fs/readfile.go
@@ -0,0 +1,66 @@
+// 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 "io"
+
+// ReadFileFS is the interface implemented by a file system
+// that provides an optimized implementation of [ReadFile].
+type ReadFileFS interface {
+	FS
+
+	// ReadFile reads the named file and returns its contents.
+	// A successful call returns a nil error, not io.EOF.
+	// (Because ReadFile reads the whole file, the expected EOF
+	// from the final Read is not treated as an error to be reported.)
+	//
+	// The caller is permitted to modify the returned byte slice.
+	// This method should return a copy of the underlying data.
+	ReadFile(name string) ([]byte, error)
+}
+
+// ReadFile reads the named file from the file system fs and returns its contents.
+// A successful call returns a nil error, not [io.EOF].
+// (Because ReadFile reads the whole file, the expected EOF
+// from the final Read is not treated as an error to be reported.)
+//
+// If fs implements [ReadFileFS], ReadFile calls fs.ReadFile.
+// Otherwise ReadFile calls fs.Open and uses Read and Close
+// on the returned [File].
+func ReadFile(fsys FS, name string) ([]byte, error) {
+	if fsys, ok := fsys.(ReadFileFS); ok {
+		return fsys.ReadFile(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	var size int
+	if info, err := file.Stat(); err == nil {
+		size64 := info.Size()
+		if int64(int(size64)) == size64 {
+			size = int(size64)
+		}
+	}
+
+	data := make([]byte, 0, size+1)
+	for {
+		if len(data) >= cap(data) {
+			d := append(data[:cap(data)], 0)
+			data = d[:len(data)]
+		}
+		n, err := file.Read(data[len(data):cap(data)])
+		data = data[:len(data)+n]
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+			return data, err
+		}
+	}
+}
diff --git a/src/io/fs/readfile_test.go b/src/io/fs/readfile_test.go
new file mode 100644
index 0000000..3c521f6
--- /dev/null
+++ b/src/io/fs/readfile_test.go
@@ -0,0 +1,69 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"os"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+var testFsys = fstest.MapFS{
+	"hello.txt": {
+		Data:    []byte("hello, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+	"sub/goodbye.txt": {
+		Data:    []byte("goodbye, world"),
+		Mode:    0456,
+		ModTime: time.Now(),
+		Sys:     &sysValue,
+	},
+}
+
+var sysValue int
+
+type readFileOnly struct{ ReadFileFS }
+
+func (readFileOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+type openOnly struct{ FS }
+
+func TestReadFile(t *testing.T) {
+	// Test that ReadFile uses the method when present.
+	data, err := ReadFile(readFileOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(readFileOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile uses Open when the method is not present.
+	data, err = ReadFile(openOnly{testFsys}, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(openOnly, "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+
+	// Test that ReadFile on Sub of . works (sub_test checks non-trivial subs).
+	sub, err := Sub(testFsys, ".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err = ReadFile(sub, "hello.txt")
+	if string(data) != "hello, world" || err != nil {
+		t.Fatalf(`ReadFile(sub(.), "hello.txt") = %q, %v, want %q, nil`, data, err, "hello, world")
+	}
+}
+
+func TestReadFilePath(t *testing.T) {
+	fsys := os.DirFS(t.TempDir())
+	_, err1 := ReadFile(fsys, "non-existent")
+	_, err2 := ReadFile(struct{ FS }{fsys}, "non-existent")
+	if s1, s2 := errorPath(err1), errorPath(err2); s1 != s2 {
+		t.Fatalf("s1: %s != s2: %s", s1, s2)
+	}
+}
diff --git a/src/io/fs/stat.go b/src/io/fs/stat.go
new file mode 100644
index 0000000..bbb91c2
--- /dev/null
+++ b/src/io/fs/stat.go
@@ -0,0 +1,31 @@
+// 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
+
+// A StatFS is a file system with a Stat method.
+type StatFS interface {
+	FS
+
+	// Stat returns a FileInfo describing the file.
+	// If there is an error, it should be of type *PathError.
+	Stat(name string) (FileInfo, error)
+}
+
+// Stat returns a [FileInfo] describing the named file from the file system.
+//
+// If fs implements [StatFS], Stat calls fs.Stat.
+// Otherwise, Stat opens the [File] to stat it.
+func Stat(fsys FS, name string) (FileInfo, error) {
+	if fsys, ok := fsys.(StatFS); ok {
+		return fsys.Stat(name)
+	}
+
+	file, err := fsys.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+	return file.Stat()
+}
diff --git a/src/io/fs/stat_test.go b/src/io/fs/stat_test.go
new file mode 100644
index 0000000..e312b6f
--- /dev/null
+++ b/src/io/fs/stat_test.go
@@ -0,0 +1,36 @@
+// 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_test
+
+import (
+	"fmt"
+	. "io/fs"
+	"testing"
+)
+
+type statOnly struct{ StatFS }
+
+func (statOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestStat(t *testing.T) {
+	check := func(desc string, info FileInfo, err error) {
+		t.Helper()
+		if err != nil || info == nil || info.Mode() != 0456 {
+			infoStr := "<nil>"
+			if info != nil {
+				infoStr = fmt.Sprintf("FileInfo(Mode: %#o)", info.Mode())
+			}
+			t.Fatalf("Stat(%s) = %v, %v, want Mode:0456, nil", desc, infoStr, err)
+		}
+	}
+
+	// Test that Stat uses the method when present.
+	info, err := Stat(statOnly{testFsys}, "hello.txt")
+	check("statOnly", info, err)
+
+	// Test that Stat uses Open when the method is not present.
+	info, err = Stat(openOnly{testFsys}, "hello.txt")
+	check("openOnly", info, err)
+}
diff --git a/src/io/fs/sub.go b/src/io/fs/sub.go
new file mode 100644
index 0000000..9999e63
--- /dev/null
+++ b/src/io/fs/sub.go
@@ -0,0 +1,138 @@
+// 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
+}
diff --git a/src/io/fs/sub_test.go b/src/io/fs/sub_test.go
new file mode 100644
index 0000000..451b0ef
--- /dev/null
+++ b/src/io/fs/sub_test.go
@@ -0,0 +1,57 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"testing"
+)
+
+type subOnly struct{ SubFS }
+
+func (subOnly) Open(name string) (File, error) { return nil, ErrNotExist }
+
+func TestSub(t *testing.T) {
+	check := func(desc string, sub FS, err error) {
+		t.Helper()
+		if err != nil {
+			t.Errorf("Sub(sub): %v", err)
+			return
+		}
+		data, err := ReadFile(sub, "goodbye.txt")
+		if string(data) != "goodbye, world" || err != nil {
+			t.Errorf(`ReadFile(%s, "goodbye.txt" = %q, %v, want %q, nil`, desc, string(data), err, "goodbye, world")
+		}
+
+		dirs, err := ReadDir(sub, ".")
+		if err != nil || len(dirs) != 1 || dirs[0].Name() != "goodbye.txt" {
+			var names []string
+			for _, d := range dirs {
+				names = append(names, d.Name())
+			}
+			t.Errorf(`ReadDir(%s, ".") = %v, %v, want %v, nil`, desc, names, err, []string{"goodbye.txt"})
+		}
+	}
+
+	// Test that Sub uses the method when present.
+	sub, err := Sub(subOnly{testFsys}, "sub")
+	check("subOnly", sub, err)
+
+	// Test that Sub uses Open when the method is not present.
+	sub, err = Sub(openOnly{testFsys}, "sub")
+	check("openOnly", sub, err)
+
+	_, err = sub.Open("nonexist")
+	if err == nil {
+		t.Fatal("Open(nonexist): succeeded")
+	}
+	pe, ok := err.(*PathError)
+	if !ok {
+		t.Fatalf("Open(nonexist): error is %T, want *PathError", err)
+	}
+	if pe.Path != "nonexist" {
+		t.Fatalf("Open(nonexist): err.Path = %q, want %q", pe.Path, "nonexist")
+	}
+}
diff --git a/src/io/fs/walk.go b/src/io/fs/walk.go
new file mode 100644
index 0000000..2e8a8db
--- /dev/null
+++ b/src/io/fs/walk.go
@@ -0,0 +1,128 @@
+// 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"
+)
+
+// SkipDir is used as a return value from [WalkDirFunc] to indicate that
+// the directory named in the call is to be skipped. It is not returned
+// as an error by any function.
+var SkipDir = errors.New("skip this directory")
+
+// SkipAll is used as a return value from [WalkDirFunc] to indicate that
+// all remaining files and directories are to be skipped. It is not returned
+// as an error by any function.
+var SkipAll = errors.New("skip everything and stop the walk")
+
+// WalkDirFunc is the type of the function called by [WalkDir] to visit
+// each file or directory.
+//
+// The path argument contains the argument to [WalkDir] as a prefix.
+// That is, if WalkDir is called with root argument "dir" and finds a file
+// named "a" in that directory, the walk function will be called with
+// argument "dir/a".
+//
+// The d argument is the [DirEntry] for the named path.
+//
+// The error result returned by the function controls how [WalkDir]
+// continues. If the function returns the special value [SkipDir], WalkDir
+// skips the current directory (path if d.IsDir() is true, otherwise
+// path's parent directory). If the function returns the special value
+// [SkipAll], WalkDir skips all remaining files and directories. Otherwise,
+// if the function returns a non-nil error, WalkDir stops entirely and
+// returns that error.
+//
+// The err argument reports an error related to path, signaling that
+// [WalkDir] will not walk into that directory. The function can decide how
+// to handle that error; as described earlier, returning the error will
+// cause WalkDir to stop walking the entire tree.
+//
+// [WalkDir] calls the function with a non-nil err argument in two cases.
+//
+// First, if the initial [Stat] on the root directory fails, WalkDir
+// calls the function with path set to root, d set to nil, and err set to
+// the error from [fs.Stat].
+//
+// Second, if a directory's ReadDir method (see [ReadDirFile]) fails, WalkDir calls the
+// function with path set to the directory's path, d set to an
+// [DirEntry] describing the directory, and err set to the error from
+// ReadDir. In this second case, the function is called twice with the
+// path of the directory: the first call is before the directory read is
+// attempted and has err set to nil, giving the function a chance to
+// return [SkipDir] or [SkipAll] and avoid the ReadDir entirely. The second call
+// is after a failed ReadDir and reports the error from ReadDir.
+// (If ReadDir succeeds, there is no second call.)
+//
+// The differences between WalkDirFunc compared to [path/filepath.WalkFunc] are:
+//
+//   - The second argument has type [DirEntry] instead of [FileInfo].
+//   - The function is called before reading a directory, to allow [SkipDir]
+//     or [SkipAll] to bypass the directory read entirely or skip all remaining
+//     files and directories respectively.
+//   - If a directory read fails, the function is called a second time
+//     for that directory to report the error.
+type WalkDirFunc func(path string, d DirEntry, err error) error
+
+// walkDir recursively descends path, calling walkDirFn.
+func walkDir(fsys FS, name string, d DirEntry, walkDirFn WalkDirFunc) error {
+	if err := walkDirFn(name, d, nil); err != nil || !d.IsDir() {
+		if err == SkipDir && d.IsDir() {
+			// Successfully skipped directory.
+			err = nil
+		}
+		return err
+	}
+
+	dirs, err := ReadDir(fsys, name)
+	if err != nil {
+		// Second call, to report ReadDir error.
+		err = walkDirFn(name, d, err)
+		if err != nil {
+			if err == SkipDir && d.IsDir() {
+				err = nil
+			}
+			return err
+		}
+	}
+
+	for _, d1 := range dirs {
+		name1 := path.Join(name, d1.Name())
+		if err := walkDir(fsys, name1, d1, walkDirFn); err != nil {
+			if err == SkipDir {
+				break
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+// WalkDir walks the file tree rooted at root, calling fn for each file or
+// directory in the tree, including root.
+//
+// All errors that arise visiting files and directories are filtered by fn:
+// see the [fs.WalkDirFunc] documentation for details.
+//
+// The files are walked in lexical order, which makes the output deterministic
+// but requires WalkDir to read an entire directory into memory before proceeding
+// to walk that directory.
+//
+// WalkDir does not follow symbolic links found in directories,
+// but if root itself is a symbolic link, its target will be walked.
+func WalkDir(fsys FS, root string, fn WalkDirFunc) error {
+	info, err := Stat(fsys, root)
+	if err != nil {
+		err = fn(root, nil, err)
+	} else {
+		err = walkDir(fsys, root, FileInfoToDirEntry(info), fn)
+	}
+	if err == SkipDir || err == SkipAll {
+		return nil
+	}
+	return err
+}
diff --git a/src/io/fs/walk_test.go b/src/io/fs/walk_test.go
new file mode 100644
index 0000000..40f4e1a
--- /dev/null
+++ b/src/io/fs/walk_test.go
@@ -0,0 +1,151 @@
+// 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_test
+
+import (
+	. "io/fs"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"reflect"
+	"testing"
+	"testing/fstest"
+)
+
+type Node struct {
+	name    string
+	entries []*Node // nil if the entry is a file
+	mark    int
+}
+
+var tree = &Node{
+	"testdata",
+	[]*Node{
+		{"a", nil, 0},
+		{"b", []*Node{}, 0},
+		{"c", nil, 0},
+		{
+			"d",
+			[]*Node{
+				{"x", nil, 0},
+				{"y", []*Node{}, 0},
+				{
+					"z",
+					[]*Node{
+						{"u", nil, 0},
+						{"v", nil, 0},
+					},
+					0,
+				},
+			},
+			0,
+		},
+	},
+	0,
+}
+
+func walkTree(n *Node, path string, f func(path string, n *Node)) {
+	f(path, n)
+	for _, e := range n.entries {
+		walkTree(e, pathpkg.Join(path, e.name), f)
+	}
+}
+
+func makeTree() FS {
+	fsys := fstest.MapFS{}
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.entries == nil {
+			fsys[path] = &fstest.MapFile{}
+		} else {
+			fsys[path] = &fstest.MapFile{Mode: ModeDir}
+		}
+	})
+	return fsys
+}
+
+// Assumes that each node name is unique. Good enough for a test.
+// If clear is true, any incoming error is cleared before return. The errors
+// are always accumulated, though.
+func mark(entry DirEntry, err error, errors *[]error, clear bool) error {
+	name := entry.Name()
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.name == name {
+			n.mark++
+		}
+	})
+	if err != nil {
+		*errors = append(*errors, err)
+		if clear {
+			return nil
+		}
+		return err
+	}
+	return nil
+}
+
+func TestWalkDir(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal("finding working dir:", err)
+	}
+	if err = os.Chdir(tmpDir); err != nil {
+		t.Fatal("entering temp dir:", err)
+	}
+	defer os.Chdir(origDir)
+
+	fsys := makeTree()
+	errors := make([]error, 0, 10)
+	clear := true
+	markFn := func(path string, entry DirEntry, err error) error {
+		return mark(entry, err, &errors, clear)
+	}
+	// Expect no errors.
+	err = WalkDir(fsys, ".", markFn)
+	if err != nil {
+		t.Fatalf("no error expected, found: %s", err)
+	}
+	if len(errors) != 0 {
+		t.Fatalf("unexpected errors: %s", errors)
+	}
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.mark != 1 {
+			t.Errorf("node %s mark = %d; expected 1", path, n.mark)
+		}
+		n.mark = 0
+	})
+}
+
+func TestIssue51617(t *testing.T) {
+	dir := t.TempDir()
+	for _, sub := range []string{"a", filepath.Join("a", "bad"), filepath.Join("a", "next")} {
+		if err := os.Mkdir(filepath.Join(dir, sub), 0755); err != nil {
+			t.Fatal(err)
+		}
+	}
+	bad := filepath.Join(dir, "a", "bad")
+	if err := os.Chmod(bad, 0); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chmod(bad, 0700) // avoid errors on cleanup
+	var saw []string
+	err := WalkDir(os.DirFS(dir), ".", func(path string, d DirEntry, err error) error {
+		if err != nil {
+			return filepath.SkipDir
+		}
+		if d.IsDir() {
+			saw = append(saw, path)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []string{".", "a", "a/bad", "a/next"}
+	if !reflect.DeepEqual(saw, want) {
+		t.Errorf("got directories %v, want %v", saw, want)
+	}
+}
diff --git a/src/io/io.go b/src/io/io.go
new file mode 100644
index 0000000..7f16e18
--- /dev/null
+++ b/src/io/io.go
@@ -0,0 +1,726 @@
+// Copyright 2009 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 io provides basic interfaces to I/O primitives.
+// Its primary job is to wrap existing implementations of such primitives,
+// such as those in package os, into shared public interfaces that
+// abstract the functionality, plus some other related primitives.
+//
+// Because these interfaces and primitives wrap lower-level operations with
+// various implementations, unless otherwise informed clients should not
+// assume they are safe for parallel execution.
+package io
+
+import (
+	"errors"
+	"sync"
+)
+
+// Seek whence values.
+const (
+	SeekStart   = 0 // seek relative to the origin of the file
+	SeekCurrent = 1 // seek relative to the current offset
+	SeekEnd     = 2 // seek relative to the end
+)
+
+// ErrShortWrite means that a write accepted fewer bytes than requested
+// but failed to return an explicit error.
+var ErrShortWrite = errors.New("short write")
+
+// errInvalidWrite means that a write returned an impossible count.
+var errInvalidWrite = errors.New("invalid write result")
+
+// ErrShortBuffer means that a read required a longer buffer than was provided.
+var ErrShortBuffer = errors.New("short buffer")
+
+// EOF is the error returned by Read when no more input is available.
+// (Read must return EOF itself, not an error wrapping EOF,
+// because callers will test for EOF using ==.)
+// Functions should return EOF only to signal a graceful end of input.
+// If the EOF occurs unexpectedly in a structured data stream,
+// the appropriate error is either [ErrUnexpectedEOF] or some other error
+// giving more detail.
+var EOF = errors.New("EOF")
+
+// ErrUnexpectedEOF means that EOF was encountered in the
+// middle of reading a fixed-size block or data structure.
+var ErrUnexpectedEOF = errors.New("unexpected EOF")
+
+// ErrNoProgress is returned by some clients of a [Reader] when
+// many calls to Read have failed to return any data or error,
+// usually the sign of a broken [Reader] implementation.
+var ErrNoProgress = errors.New("multiple Read calls return no data or error")
+
+// Reader is the interface that wraps the basic Read method.
+//
+// Read reads up to len(p) bytes into p. It returns the number of bytes
+// read (0 <= n <= len(p)) and any error encountered. Even if Read
+// returns n < len(p), it may use all of p as scratch space during the call.
+// If some data is available but not len(p) bytes, Read conventionally
+// returns what is available instead of waiting for more.
+//
+// When Read encounters an error or end-of-file condition after
+// successfully reading n > 0 bytes, it returns the number of
+// bytes read. It may return the (non-nil) error from the same call
+// or return the error (and n == 0) from a subsequent call.
+// An instance of this general case is that a Reader returning
+// a non-zero number of bytes at the end of the input stream may
+// return either err == EOF or err == nil. The next Read should
+// return 0, EOF.
+//
+// Callers should always process the n > 0 bytes returned before
+// considering the error err. Doing so correctly handles I/O errors
+// that happen after reading some bytes and also both of the
+// allowed EOF behaviors.
+//
+// If len(p) == 0, Read should always return n == 0. It may return a
+// non-nil error if some error condition is known, such as EOF.
+//
+// Implementations of Read are discouraged from returning a
+// zero byte count with a nil error, except when len(p) == 0.
+// Callers should treat a return of 0 and nil as indicating that
+// nothing happened; in particular it does not indicate EOF.
+//
+// Implementations must not retain p.
+type Reader interface {
+	Read(p []byte) (n int, err error)
+}
+
+// Writer is the interface that wraps the basic Write method.
+//
+// Write writes len(p) bytes from p to the underlying data stream.
+// It returns the number of bytes written from p (0 <= n <= len(p))
+// and any error encountered that caused the write to stop early.
+// Write must return a non-nil error if it returns n < len(p).
+// Write must not modify the slice data, even temporarily.
+//
+// Implementations must not retain p.
+type Writer interface {
+	Write(p []byte) (n int, err error)
+}
+
+// Closer is the interface that wraps the basic Close method.
+//
+// The behavior of Close after the first call is undefined.
+// Specific implementations may document their own behavior.
+type Closer interface {
+	Close() error
+}
+
+// Seeker is the interface that wraps the basic Seek method.
+//
+// Seek sets the offset for the next Read or Write to offset,
+// interpreted according to whence:
+// [SeekStart] means relative to the start of the file,
+// [SeekCurrent] means relative to the current offset, and
+// [SeekEnd] means relative to the end
+// (for example, offset = -2 specifies the penultimate byte of the file).
+// Seek returns the new offset relative to the start of the
+// file or an error, if any.
+//
+// Seeking to an offset before the start of the file is an error.
+// Seeking to any positive offset may be allowed, but if the new offset exceeds
+// the size of the underlying object the behavior of subsequent I/O operations
+// is implementation-dependent.
+type Seeker interface {
+	Seek(offset int64, whence int) (int64, error)
+}
+
+// ReadWriter is the interface that groups the basic Read and Write methods.
+type ReadWriter interface {
+	Reader
+	Writer
+}
+
+// ReadCloser is the interface that groups the basic Read and Close methods.
+type ReadCloser interface {
+	Reader
+	Closer
+}
+
+// WriteCloser is the interface that groups the basic Write and Close methods.
+type WriteCloser interface {
+	Writer
+	Closer
+}
+
+// ReadWriteCloser is the interface that groups the basic Read, Write and Close methods.
+type ReadWriteCloser interface {
+	Reader
+	Writer
+	Closer
+}
+
+// ReadSeeker is the interface that groups the basic Read and Seek methods.
+type ReadSeeker interface {
+	Reader
+	Seeker
+}
+
+// ReadSeekCloser is the interface that groups the basic Read, Seek and Close
+// methods.
+type ReadSeekCloser interface {
+	Reader
+	Seeker
+	Closer
+}
+
+// WriteSeeker is the interface that groups the basic Write and Seek methods.
+type WriteSeeker interface {
+	Writer
+	Seeker
+}
+
+// ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.
+type ReadWriteSeeker interface {
+	Reader
+	Writer
+	Seeker
+}
+
+// ReaderFrom is the interface that wraps the ReadFrom method.
+//
+// ReadFrom reads data from r until EOF or error.
+// The return value n is the number of bytes read.
+// Any error except EOF encountered during the read is also returned.
+//
+// The [Copy] function uses [ReaderFrom] if available.
+type ReaderFrom interface {
+	ReadFrom(r Reader) (n int64, err error)
+}
+
+// WriterTo is the interface that wraps the WriteTo method.
+//
+// WriteTo writes data to w until there's no more data to write or
+// when an error occurs. The return value n is the number of bytes
+// written. Any error encountered during the write is also returned.
+//
+// The Copy function uses WriterTo if available.
+type WriterTo interface {
+	WriteTo(w Writer) (n int64, err error)
+}
+
+// ReaderAt is the interface that wraps the basic ReadAt method.
+//
+// ReadAt reads len(p) bytes into p starting at offset off in the
+// underlying input source. It returns the number of bytes
+// read (0 <= n <= len(p)) and any error encountered.
+//
+// When ReadAt returns n < len(p), it returns a non-nil error
+// explaining why more bytes were not returned. In this respect,
+// ReadAt is stricter than Read.
+//
+// Even if ReadAt returns n < len(p), it may use all of p as scratch
+// space during the call. If some data is available but not len(p) bytes,
+// ReadAt blocks until either all the data is available or an error occurs.
+// In this respect ReadAt is different from Read.
+//
+// If the n = len(p) bytes returned by ReadAt are at the end of the
+// input source, ReadAt may return either err == EOF or err == nil.
+//
+// If ReadAt is reading from an input source with a seek offset,
+// ReadAt should not affect nor be affected by the underlying
+// seek offset.
+//
+// Clients of ReadAt can execute parallel ReadAt calls on the
+// same input source.
+//
+// Implementations must not retain p.
+type ReaderAt interface {
+	ReadAt(p []byte, off int64) (n int, err error)
+}
+
+// WriterAt is the interface that wraps the basic WriteAt method.
+//
+// WriteAt writes len(p) bytes from p to the underlying data stream
+// at offset off. It returns the number of bytes written from p (0 <= n <= len(p))
+// and any error encountered that caused the write to stop early.
+// WriteAt must return a non-nil error if it returns n < len(p).
+//
+// If WriteAt is writing to a destination with a seek offset,
+// WriteAt should not affect nor be affected by the underlying
+// seek offset.
+//
+// Clients of WriteAt can execute parallel WriteAt calls on the same
+// destination if the ranges do not overlap.
+//
+// Implementations must not retain p.
+type WriterAt interface {
+	WriteAt(p []byte, off int64) (n int, err error)
+}
+
+// ByteReader is the interface that wraps the ReadByte method.
+//
+// ReadByte reads and returns the next byte from the input or
+// any error encountered. If ReadByte returns an error, no input
+// byte was consumed, and the returned byte value is undefined.
+//
+// ReadByte provides an efficient interface for byte-at-time
+// processing. A [Reader] that does not implement  ByteReader
+// can be wrapped using bufio.NewReader to add this method.
+type ByteReader interface {
+	ReadByte() (byte, error)
+}
+
+// ByteScanner is the interface that adds the UnreadByte method to the
+// basic ReadByte method.
+//
+// UnreadByte causes the next call to ReadByte to return the last byte read.
+// If the last operation was not a successful call to ReadByte, UnreadByte may
+// return an error, unread the last byte read (or the byte prior to the
+// last-unread byte), or (in implementations that support the [Seeker] interface)
+// seek to one byte before the current offset.
+type ByteScanner interface {
+	ByteReader
+	UnreadByte() error
+}
+
+// ByteWriter is the interface that wraps the WriteByte method.
+type ByteWriter interface {
+	WriteByte(c byte) error
+}
+
+// RuneReader is the interface that wraps the ReadRune method.
+//
+// ReadRune reads a single encoded Unicode character
+// and returns the rune and its size in bytes. If no character is
+// available, err will be set.
+type RuneReader interface {
+	ReadRune() (r rune, size int, err error)
+}
+
+// RuneScanner is the interface that adds the UnreadRune method to the
+// basic ReadRune method.
+//
+// UnreadRune causes the next call to ReadRune to return the last rune read.
+// If the last operation was not a successful call to ReadRune, UnreadRune may
+// return an error, unread the last rune read (or the rune prior to the
+// last-unread rune), or (in implementations that support the [Seeker] interface)
+// seek to the start of the rune before the current offset.
+type RuneScanner interface {
+	RuneReader
+	UnreadRune() error
+}
+
+// StringWriter is the interface that wraps the WriteString method.
+type StringWriter interface {
+	WriteString(s string) (n int, err error)
+}
+
+// WriteString writes the contents of the string s to w, which accepts a slice of bytes.
+// If w implements [StringWriter], [StringWriter.WriteString] is invoked directly.
+// Otherwise, [Writer.Write] is called exactly once.
+func WriteString(w Writer, s string) (n int, err error) {
+	if sw, ok := w.(StringWriter); ok {
+		return sw.WriteString(s)
+	}
+	return w.Write([]byte(s))
+}
+
+// ReadAtLeast reads from r into buf until it has read at least min bytes.
+// It returns the number of bytes copied and an error if fewer bytes were read.
+// The error is EOF only if no bytes were read.
+// If an EOF happens after reading fewer than min bytes,
+// ReadAtLeast returns [ErrUnexpectedEOF].
+// If min is greater than the length of buf, ReadAtLeast returns [ErrShortBuffer].
+// On return, n >= min if and only if err == nil.
+// If r returns an error having read at least min bytes, the error is dropped.
+func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
+	if len(buf) < min {
+		return 0, ErrShortBuffer
+	}
+	for n < min && err == nil {
+		var nn int
+		nn, err = r.Read(buf[n:])
+		n += nn
+	}
+	if n >= min {
+		err = nil
+	} else if n > 0 && err == EOF {
+		err = ErrUnexpectedEOF
+	}
+	return
+}
+
+// ReadFull reads exactly len(buf) bytes from r into buf.
+// It returns the number of bytes copied and an error if fewer bytes were read.
+// The error is EOF only if no bytes were read.
+// If an EOF happens after reading some but not all the bytes,
+// ReadFull returns [ErrUnexpectedEOF].
+// On return, n == len(buf) if and only if err == nil.
+// If r returns an error having read at least len(buf) bytes, the error is dropped.
+func ReadFull(r Reader, buf []byte) (n int, err error) {
+	return ReadAtLeast(r, buf, len(buf))
+}
+
+// CopyN copies n bytes (or until an error) from src to dst.
+// It returns the number of bytes copied and the earliest
+// error encountered while copying.
+// On return, written == n if and only if err == nil.
+//
+// If dst implements [ReaderFrom], the copy is implemented using it.
+func CopyN(dst Writer, src Reader, n int64) (written int64, err error) {
+	written, err = Copy(dst, LimitReader(src, n))
+	if written == n {
+		return n, nil
+	}
+	if written < n && err == nil {
+		// src stopped early; must have been EOF.
+		err = EOF
+	}
+	return
+}
+
+// Copy copies from src to dst until either EOF is reached
+// on src or an error occurs. It returns the number of bytes
+// copied and the first error encountered while copying, if any.
+//
+// A successful Copy returns err == nil, not err == EOF.
+// Because Copy is defined to read from src until EOF, it does
+// not treat an EOF from Read as an error to be reported.
+//
+// If src implements [WriterTo],
+// the copy is implemented by calling src.WriteTo(dst).
+// Otherwise, if dst implements [ReaderFrom],
+// the copy is implemented by calling dst.ReadFrom(src).
+func Copy(dst Writer, src Reader) (written int64, err error) {
+	return copyBuffer(dst, src, nil)
+}
+
+// CopyBuffer is identical to Copy except that it stages through the
+// provided buffer (if one is required) rather than allocating a
+// temporary one. If buf is nil, one is allocated; otherwise if it has
+// zero length, CopyBuffer panics.
+//
+// If either src implements [WriterTo] or dst implements [ReaderFrom],
+// buf will not be used to perform the copy.
+func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
+	if buf != nil && len(buf) == 0 {
+		panic("empty buffer in CopyBuffer")
+	}
+	return copyBuffer(dst, src, buf)
+}
+
+// copyBuffer is the actual implementation of Copy and CopyBuffer.
+// if buf is nil, one is allocated.
+func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
+	// If the reader has a WriteTo method, use it to do the copy.
+	// Avoids an allocation and a copy.
+	if wt, ok := src.(WriterTo); ok {
+		return wt.WriteTo(dst)
+	}
+	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
+	if rt, ok := dst.(ReaderFrom); ok {
+		return rt.ReadFrom(src)
+	}
+	if buf == nil {
+		size := 32 * 1024
+		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
+			if l.N < 1 {
+				size = 1
+			} else {
+				size = int(l.N)
+			}
+		}
+		buf = make([]byte, size)
+	}
+	for {
+		nr, er := src.Read(buf)
+		if nr > 0 {
+			nw, ew := dst.Write(buf[0:nr])
+			if nw < 0 || nr < nw {
+				nw = 0
+				if ew == nil {
+					ew = errInvalidWrite
+				}
+			}
+			written += int64(nw)
+			if ew != nil {
+				err = ew
+				break
+			}
+			if nr != nw {
+				err = ErrShortWrite
+				break
+			}
+		}
+		if er != nil {
+			if er != EOF {
+				err = er
+			}
+			break
+		}
+	}
+	return written, err
+}
+
+// LimitReader returns a Reader that reads from r
+// but stops with EOF after n bytes.
+// The underlying implementation is a *LimitedReader.
+func LimitReader(r Reader, n int64) Reader { return &LimitedReader{r, n} }
+
+// A LimitedReader reads from R but limits the amount of
+// data returned to just N bytes. Each call to Read
+// updates N to reflect the new amount remaining.
+// Read returns EOF when N <= 0 or when the underlying R returns EOF.
+type LimitedReader struct {
+	R Reader // underlying reader
+	N int64  // max bytes remaining
+}
+
+func (l *LimitedReader) Read(p []byte) (n int, err error) {
+	if l.N <= 0 {
+		return 0, EOF
+	}
+	if int64(len(p)) > l.N {
+		p = p[0:l.N]
+	}
+	n, err = l.R.Read(p)
+	l.N -= int64(n)
+	return
+}
+
+// NewSectionReader returns a [SectionReader] that reads from r
+// starting at offset off and stops with EOF after n bytes.
+func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader {
+	var remaining int64
+	const maxint64 = 1<<63 - 1
+	if off <= maxint64-n {
+		remaining = n + off
+	} else {
+		// Overflow, with no way to return error.
+		// Assume we can read up to an offset of 1<<63 - 1.
+		remaining = maxint64
+	}
+	return &SectionReader{r, off, off, remaining, n}
+}
+
+// SectionReader implements Read, Seek, and ReadAt on a section
+// of an underlying [ReaderAt].
+type SectionReader struct {
+	r     ReaderAt // constant after creation
+	base  int64    // constant after creation
+	off   int64
+	limit int64 // constant after creation
+	n     int64 // constant after creation
+}
+
+func (s *SectionReader) Read(p []byte) (n int, err error) {
+	if s.off >= s.limit {
+		return 0, EOF
+	}
+	if max := s.limit - s.off; int64(len(p)) > max {
+		p = p[0:max]
+	}
+	n, err = s.r.ReadAt(p, s.off)
+	s.off += int64(n)
+	return
+}
+
+var errWhence = errors.New("Seek: invalid whence")
+var errOffset = errors.New("Seek: invalid offset")
+
+func (s *SectionReader) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	default:
+		return 0, errWhence
+	case SeekStart:
+		offset += s.base
+	case SeekCurrent:
+		offset += s.off
+	case SeekEnd:
+		offset += s.limit
+	}
+	if offset < s.base {
+		return 0, errOffset
+	}
+	s.off = offset
+	return offset - s.base, nil
+}
+
+func (s *SectionReader) ReadAt(p []byte, off int64) (n int, err error) {
+	if off < 0 || off >= s.Size() {
+		return 0, EOF
+	}
+	off += s.base
+	if max := s.limit - off; int64(len(p)) > max {
+		p = p[0:max]
+		n, err = s.r.ReadAt(p, off)
+		if err == nil {
+			err = EOF
+		}
+		return n, err
+	}
+	return s.r.ReadAt(p, off)
+}
+
+// Size returns the size of the section in bytes.
+func (s *SectionReader) Size() int64 { return s.limit - s.base }
+
+// Outer returns the underlying [ReaderAt] and offsets for the section.
+//
+// The returned values are the same that were passed to [NewSectionReader]
+// when the [SectionReader] was created.
+func (s *SectionReader) Outer() (r ReaderAt, off int64, n int64) {
+	return s.r, s.base, s.n
+}
+
+// An OffsetWriter maps writes at offset base to offset base+off in the underlying writer.
+type OffsetWriter struct {
+	w    WriterAt
+	base int64 // the original offset
+	off  int64 // the current offset
+}
+
+// NewOffsetWriter returns an [OffsetWriter] that writes to w
+// starting at offset off.
+func NewOffsetWriter(w WriterAt, off int64) *OffsetWriter {
+	return &OffsetWriter{w, off, off}
+}
+
+func (o *OffsetWriter) Write(p []byte) (n int, err error) {
+	n, err = o.w.WriteAt(p, o.off)
+	o.off += int64(n)
+	return
+}
+
+func (o *OffsetWriter) WriteAt(p []byte, off int64) (n int, err error) {
+	if off < 0 {
+		return 0, errOffset
+	}
+
+	off += o.base
+	return o.w.WriteAt(p, off)
+}
+
+func (o *OffsetWriter) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	default:
+		return 0, errWhence
+	case SeekStart:
+		offset += o.base
+	case SeekCurrent:
+		offset += o.off
+	}
+	if offset < o.base {
+		return 0, errOffset
+	}
+	o.off = offset
+	return offset - o.base, nil
+}
+
+// TeeReader returns a [Reader] that writes to w what it reads from r.
+// All reads from r performed through it are matched with
+// corresponding writes to w. There is no internal buffering -
+// the write must complete before the read completes.
+// Any error encountered while writing is reported as a read error.
+func TeeReader(r Reader, w Writer) Reader {
+	return &teeReader{r, w}
+}
+
+type teeReader struct {
+	r Reader
+	w Writer
+}
+
+func (t *teeReader) Read(p []byte) (n int, err error) {
+	n, err = t.r.Read(p)
+	if n > 0 {
+		if n, err := t.w.Write(p[:n]); err != nil {
+			return n, err
+		}
+	}
+	return
+}
+
+// Discard is a [Writer] on which all Write calls succeed
+// without doing anything.
+var Discard Writer = discard{}
+
+type discard struct{}
+
+// discard implements ReaderFrom as an optimization so Copy to
+// io.Discard can avoid doing unnecessary work.
+var _ ReaderFrom = discard{}
+
+func (discard) Write(p []byte) (int, error) {
+	return len(p), nil
+}
+
+func (discard) WriteString(s string) (int, error) {
+	return len(s), nil
+}
+
+var blackHolePool = sync.Pool{
+	New: func() any {
+		b := make([]byte, 8192)
+		return &b
+	},
+}
+
+func (discard) ReadFrom(r Reader) (n int64, err error) {
+	bufp := blackHolePool.Get().(*[]byte)
+	readSize := 0
+	for {
+		readSize, err = r.Read(*bufp)
+		n += int64(readSize)
+		if err != nil {
+			blackHolePool.Put(bufp)
+			if err == EOF {
+				return n, nil
+			}
+			return
+		}
+	}
+}
+
+// NopCloser returns a [ReadCloser] with a no-op Close method wrapping
+// the provided [Reader] r.
+// If r implements [WriterTo], the returned [ReadCloser] will implement [WriterTo]
+// by forwarding calls to r.
+func NopCloser(r Reader) ReadCloser {
+	if _, ok := r.(WriterTo); ok {
+		return nopCloserWriterTo{r}
+	}
+	return nopCloser{r}
+}
+
+type nopCloser struct {
+	Reader
+}
+
+func (nopCloser) Close() error { return nil }
+
+type nopCloserWriterTo struct {
+	Reader
+}
+
+func (nopCloserWriterTo) Close() error { return nil }
+
+func (c nopCloserWriterTo) WriteTo(w Writer) (n int64, err error) {
+	return c.Reader.(WriterTo).WriteTo(w)
+}
+
+// ReadAll reads from r until an error or EOF and returns the data it read.
+// A successful call returns err == nil, not err == EOF. Because ReadAll is
+// defined to read from src until EOF, it does not treat an EOF from Read
+// as an error to be reported.
+func ReadAll(r Reader) ([]byte, error) {
+	b := make([]byte, 0, 512)
+	for {
+		n, err := r.Read(b[len(b):cap(b)])
+		b = b[:len(b)+n]
+		if err != nil {
+			if err == EOF {
+				err = nil
+			}
+			return b, err
+		}
+
+		if len(b) == cap(b) {
+			// Add more capacity (let append pick how much).
+			b = append(b, 0)[:len(b)]
+		}
+	}
+}
diff --git a/src/io/io_test.go b/src/io/io_test.go
new file mode 100644
index 0000000..9491ffa
--- /dev/null
+++ b/src/io/io_test.go
@@ -0,0 +1,697 @@
+// Copyright 2009 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 io_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	. "io"
+	"os"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+// A version of bytes.Buffer without ReadFrom and WriteTo
+type Buffer struct {
+	bytes.Buffer
+	ReaderFrom // conflicts with and hides bytes.Buffer's ReaderFrom.
+	WriterTo   // conflicts with and hides bytes.Buffer's WriterTo.
+}
+
+// Simple tests, primarily to verify the ReadFrom and WriteTo callouts inside Copy, CopyBuffer and CopyN.
+
+func TestCopy(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	Copy(wb, rb)
+	if wb.String() != "hello, world." {
+		t.Errorf("Copy did not work properly")
+	}
+}
+
+func TestCopyNegative(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello")
+	Copy(wb, &LimitedReader{R: rb, N: -1})
+	if wb.String() != "" {
+		t.Errorf("Copy on LimitedReader with N<0 copied data")
+	}
+
+	CopyN(wb, rb, -1)
+	if wb.String() != "" {
+		t.Errorf("CopyN with N<0 copied data")
+	}
+}
+
+func TestCopyBuffer(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	CopyBuffer(wb, rb, make([]byte, 1)) // Tiny buffer to keep it honest.
+	if wb.String() != "hello, world." {
+		t.Errorf("CopyBuffer did not work properly")
+	}
+}
+
+func TestCopyBufferNil(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	CopyBuffer(wb, rb, nil) // Should allocate a buffer.
+	if wb.String() != "hello, world." {
+		t.Errorf("CopyBuffer did not work properly")
+	}
+}
+
+func TestCopyReadFrom(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(bytes.Buffer) // implements ReadFrom.
+	rb.WriteString("hello, world.")
+	Copy(wb, rb)
+	if wb.String() != "hello, world." {
+		t.Errorf("Copy did not work properly")
+	}
+}
+
+func TestCopyWriteTo(t *testing.T) {
+	rb := new(bytes.Buffer) // implements WriteTo.
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	Copy(wb, rb)
+	if wb.String() != "hello, world." {
+		t.Errorf("Copy did not work properly")
+	}
+}
+
+// Version of bytes.Buffer that checks whether WriteTo was called or not
+type writeToChecker struct {
+	bytes.Buffer
+	writeToCalled bool
+}
+
+func (wt *writeToChecker) WriteTo(w Writer) (int64, error) {
+	wt.writeToCalled = true
+	return wt.Buffer.WriteTo(w)
+}
+
+// It's preferable to choose WriterTo over ReaderFrom, since a WriterTo can issue one large write,
+// while the ReaderFrom must read until EOF, potentially allocating when running out of buffer.
+// Make sure that we choose WriterTo when both are implemented.
+func TestCopyPriority(t *testing.T) {
+	rb := new(writeToChecker)
+	wb := new(bytes.Buffer)
+	rb.WriteString("hello, world.")
+	Copy(wb, rb)
+	if wb.String() != "hello, world." {
+		t.Errorf("Copy did not work properly")
+	} else if !rb.writeToCalled {
+		t.Errorf("WriteTo was not prioritized over ReadFrom")
+	}
+}
+
+type zeroErrReader struct {
+	err error
+}
+
+func (r zeroErrReader) Read(p []byte) (int, error) {
+	return copy(p, []byte{0}), r.err
+}
+
+type errWriter struct {
+	err error
+}
+
+func (w errWriter) Write([]byte) (int, error) {
+	return 0, w.err
+}
+
+// In case a Read results in an error with non-zero bytes read, and
+// the subsequent Write also results in an error, the error from Write
+// is returned, as it is the one that prevented progressing further.
+func TestCopyReadErrWriteErr(t *testing.T) {
+	er, ew := errors.New("readError"), errors.New("writeError")
+	r, w := zeroErrReader{err: er}, errWriter{err: ew}
+	n, err := Copy(w, r)
+	if n != 0 || err != ew {
+		t.Errorf("Copy(zeroErrReader, errWriter) = %d, %v; want 0, writeError", n, err)
+	}
+}
+
+func TestCopyN(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	CopyN(wb, rb, 5)
+	if wb.String() != "hello" {
+		t.Errorf("CopyN did not work properly")
+	}
+}
+
+func TestCopyNReadFrom(t *testing.T) {
+	rb := new(Buffer)
+	wb := new(bytes.Buffer) // implements ReadFrom.
+	rb.WriteString("hello")
+	CopyN(wb, rb, 5)
+	if wb.String() != "hello" {
+		t.Errorf("CopyN did not work properly")
+	}
+}
+
+func TestCopyNWriteTo(t *testing.T) {
+	rb := new(bytes.Buffer) // implements WriteTo.
+	wb := new(Buffer)
+	rb.WriteString("hello, world.")
+	CopyN(wb, rb, 5)
+	if wb.String() != "hello" {
+		t.Errorf("CopyN did not work properly")
+	}
+}
+
+func BenchmarkCopyNSmall(b *testing.B) {
+	bs := bytes.Repeat([]byte{0}, 512+1)
+	rd := bytes.NewReader(bs)
+	buf := new(Buffer)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		CopyN(buf, rd, 512)
+		rd.Reset(bs)
+	}
+}
+
+func BenchmarkCopyNLarge(b *testing.B) {
+	bs := bytes.Repeat([]byte{0}, (32*1024)+1)
+	rd := bytes.NewReader(bs)
+	buf := new(Buffer)
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		CopyN(buf, rd, 32*1024)
+		rd.Reset(bs)
+	}
+}
+
+type noReadFrom struct {
+	w Writer
+}
+
+func (w *noReadFrom) Write(p []byte) (n int, err error) {
+	return w.w.Write(p)
+}
+
+type wantedAndErrReader struct{}
+
+func (wantedAndErrReader) Read(p []byte) (int, error) {
+	return len(p), errors.New("wantedAndErrReader error")
+}
+
+func TestCopyNEOF(t *testing.T) {
+	// Test that EOF behavior is the same regardless of whether
+	// argument to CopyN has ReadFrom.
+
+	b := new(bytes.Buffer)
+
+	n, err := CopyN(&noReadFrom{b}, strings.NewReader("foo"), 3)
+	if n != 3 || err != nil {
+		t.Errorf("CopyN(noReadFrom, foo, 3) = %d, %v; want 3, nil", n, err)
+	}
+
+	n, err = CopyN(&noReadFrom{b}, strings.NewReader("foo"), 4)
+	if n != 3 || err != EOF {
+		t.Errorf("CopyN(noReadFrom, foo, 4) = %d, %v; want 3, EOF", n, err)
+	}
+
+	n, err = CopyN(b, strings.NewReader("foo"), 3) // b has read from
+	if n != 3 || err != nil {
+		t.Errorf("CopyN(bytes.Buffer, foo, 3) = %d, %v; want 3, nil", n, err)
+	}
+
+	n, err = CopyN(b, strings.NewReader("foo"), 4) // b has read from
+	if n != 3 || err != EOF {
+		t.Errorf("CopyN(bytes.Buffer, foo, 4) = %d, %v; want 3, EOF", n, err)
+	}
+
+	n, err = CopyN(b, wantedAndErrReader{}, 5)
+	if n != 5 || err != nil {
+		t.Errorf("CopyN(bytes.Buffer, wantedAndErrReader, 5) = %d, %v; want 5, nil", n, err)
+	}
+
+	n, err = CopyN(&noReadFrom{b}, wantedAndErrReader{}, 5)
+	if n != 5 || err != nil {
+		t.Errorf("CopyN(noReadFrom, wantedAndErrReader, 5) = %d, %v; want 5, nil", n, err)
+	}
+}
+
+func TestReadAtLeast(t *testing.T) {
+	var rb bytes.Buffer
+	testReadAtLeast(t, &rb)
+}
+
+// A version of bytes.Buffer that returns n > 0, err on Read
+// when the input is exhausted.
+type dataAndErrorBuffer struct {
+	err error
+	bytes.Buffer
+}
+
+func (r *dataAndErrorBuffer) Read(p []byte) (n int, err error) {
+	n, err = r.Buffer.Read(p)
+	if n > 0 && r.Buffer.Len() == 0 && err == nil {
+		err = r.err
+	}
+	return
+}
+
+func TestReadAtLeastWithDataAndEOF(t *testing.T) {
+	var rb dataAndErrorBuffer
+	rb.err = EOF
+	testReadAtLeast(t, &rb)
+}
+
+func TestReadAtLeastWithDataAndError(t *testing.T) {
+	var rb dataAndErrorBuffer
+	rb.err = fmt.Errorf("fake error")
+	testReadAtLeast(t, &rb)
+}
+
+func testReadAtLeast(t *testing.T, rb ReadWriter) {
+	rb.Write([]byte("0123"))
+	buf := make([]byte, 2)
+	n, err := ReadAtLeast(rb, buf, 2)
+	if err != nil {
+		t.Error(err)
+	}
+	if n != 2 {
+		t.Errorf("expected to have read 2 bytes, got %v", n)
+	}
+	n, err = ReadAtLeast(rb, buf, 4)
+	if err != ErrShortBuffer {
+		t.Errorf("expected ErrShortBuffer got %v", err)
+	}
+	if n != 0 {
+		t.Errorf("expected to have read 0 bytes, got %v", n)
+	}
+	n, err = ReadAtLeast(rb, buf, 1)
+	if err != nil {
+		t.Error(err)
+	}
+	if n != 2 {
+		t.Errorf("expected to have read 2 bytes, got %v", n)
+	}
+	n, err = ReadAtLeast(rb, buf, 2)
+	if err != EOF {
+		t.Errorf("expected EOF, got %v", err)
+	}
+	if n != 0 {
+		t.Errorf("expected to have read 0 bytes, got %v", n)
+	}
+	rb.Write([]byte("4"))
+	n, err = ReadAtLeast(rb, buf, 2)
+	want := ErrUnexpectedEOF
+	if rb, ok := rb.(*dataAndErrorBuffer); ok && rb.err != EOF {
+		want = rb.err
+	}
+	if err != want {
+		t.Errorf("expected %v, got %v", want, err)
+	}
+	if n != 1 {
+		t.Errorf("expected to have read 1 bytes, got %v", n)
+	}
+}
+
+func TestTeeReader(t *testing.T) {
+	src := []byte("hello, world")
+	dst := make([]byte, len(src))
+	rb := bytes.NewBuffer(src)
+	wb := new(bytes.Buffer)
+	r := TeeReader(rb, wb)
+	if n, err := ReadFull(r, dst); err != nil || n != len(src) {
+		t.Fatalf("ReadFull(r, dst) = %d, %v; want %d, nil", n, err, len(src))
+	}
+	if !bytes.Equal(dst, src) {
+		t.Errorf("bytes read = %q want %q", dst, src)
+	}
+	if !bytes.Equal(wb.Bytes(), src) {
+		t.Errorf("bytes written = %q want %q", wb.Bytes(), src)
+	}
+	if n, err := r.Read(dst); n != 0 || err != EOF {
+		t.Errorf("r.Read at EOF = %d, %v want 0, EOF", n, err)
+	}
+	rb = bytes.NewBuffer(src)
+	pr, pw := Pipe()
+	pr.Close()
+	r = TeeReader(rb, pw)
+	if n, err := ReadFull(r, dst); n != 0 || err != ErrClosedPipe {
+		t.Errorf("closed tee: ReadFull(r, dst) = %d, %v; want 0, EPIPE", n, err)
+	}
+}
+
+func TestSectionReader_ReadAt(t *testing.T) {
+	dat := "a long sample data, 1234567890"
+	tests := []struct {
+		data   string
+		off    int
+		n      int
+		bufLen int
+		at     int
+		exp    string
+		err    error
+	}{
+		{data: "", off: 0, n: 10, bufLen: 2, at: 0, exp: "", err: EOF},
+		{data: dat, off: 0, n: len(dat), bufLen: 0, at: 0, exp: "", err: nil},
+		{data: dat, off: len(dat), n: 1, bufLen: 1, at: 0, exp: "", err: EOF},
+		{data: dat, off: 0, n: len(dat) + 2, bufLen: len(dat), at: 0, exp: dat, err: nil},
+		{data: dat, off: 0, n: len(dat), bufLen: len(dat) / 2, at: 0, exp: dat[:len(dat)/2], err: nil},
+		{data: dat, off: 0, n: len(dat), bufLen: len(dat), at: 0, exp: dat, err: nil},
+		{data: dat, off: 0, n: len(dat), bufLen: len(dat) / 2, at: 2, exp: dat[2 : 2+len(dat)/2], err: nil},
+		{data: dat, off: 3, n: len(dat), bufLen: len(dat) / 2, at: 2, exp: dat[5 : 5+len(dat)/2], err: nil},
+		{data: dat, off: 3, n: len(dat) / 2, bufLen: len(dat)/2 - 2, at: 2, exp: dat[5 : 5+len(dat)/2-2], err: nil},
+		{data: dat, off: 3, n: len(dat) / 2, bufLen: len(dat)/2 + 2, at: 2, exp: dat[5 : 5+len(dat)/2-2], err: EOF},
+		{data: dat, off: 0, n: 0, bufLen: 0, at: -1, exp: "", err: EOF},
+		{data: dat, off: 0, n: 0, bufLen: 0, at: 1, exp: "", err: EOF},
+	}
+	for i, tt := range tests {
+		r := strings.NewReader(tt.data)
+		s := NewSectionReader(r, int64(tt.off), int64(tt.n))
+		buf := make([]byte, tt.bufLen)
+		if n, err := s.ReadAt(buf, int64(tt.at)); n != len(tt.exp) || string(buf[:n]) != tt.exp || err != tt.err {
+			t.Fatalf("%d: ReadAt(%d) = %q, %v; expected %q, %v", i, tt.at, buf[:n], err, tt.exp, tt.err)
+		}
+		if _r, off, n := s.Outer(); _r != r || off != int64(tt.off) || n != int64(tt.n) {
+			t.Fatalf("%d: Outer() = %v, %d, %d; expected %v, %d, %d", i, _r, off, n, r, tt.off, tt.n)
+		}
+	}
+}
+
+func TestSectionReader_Seek(t *testing.T) {
+	// Verifies that NewSectionReader's Seeker behaves like bytes.NewReader (which is like strings.NewReader)
+	br := bytes.NewReader([]byte("foo"))
+	sr := NewSectionReader(br, 0, int64(len("foo")))
+
+	for _, whence := range []int{SeekStart, SeekCurrent, SeekEnd} {
+		for offset := int64(-3); offset <= 4; offset++ {
+			brOff, brErr := br.Seek(offset, whence)
+			srOff, srErr := sr.Seek(offset, whence)
+			if (brErr != nil) != (srErr != nil) || brOff != srOff {
+				t.Errorf("For whence %d, offset %d: bytes.Reader.Seek = (%v, %v) != SectionReader.Seek = (%v, %v)",
+					whence, offset, brOff, brErr, srErr, srOff)
+			}
+		}
+	}
+
+	// And verify we can just seek past the end and get an EOF
+	got, err := sr.Seek(100, SeekStart)
+	if err != nil || got != 100 {
+		t.Errorf("Seek = %v, %v; want 100, nil", got, err)
+	}
+
+	n, err := sr.Read(make([]byte, 10))
+	if n != 0 || err != EOF {
+		t.Errorf("Read = %v, %v; want 0, EOF", n, err)
+	}
+}
+
+func TestSectionReader_Size(t *testing.T) {
+	tests := []struct {
+		data string
+		want int64
+	}{
+		{"a long sample data, 1234567890", 30},
+		{"", 0},
+	}
+
+	for _, tt := range tests {
+		r := strings.NewReader(tt.data)
+		sr := NewSectionReader(r, 0, int64(len(tt.data)))
+		if got := sr.Size(); got != tt.want {
+			t.Errorf("Size = %v; want %v", got, tt.want)
+		}
+	}
+}
+
+func TestSectionReader_Max(t *testing.T) {
+	r := strings.NewReader("abcdef")
+	const maxint64 = 1<<63 - 1
+	sr := NewSectionReader(r, 3, maxint64)
+	n, err := sr.Read(make([]byte, 3))
+	if n != 3 || err != nil {
+		t.Errorf("Read = %v %v, want 3, nil", n, err)
+	}
+	n, err = sr.Read(make([]byte, 3))
+	if n != 0 || err != EOF {
+		t.Errorf("Read = %v, %v, want 0, EOF", n, err)
+	}
+	if _r, off, n := sr.Outer(); _r != r || off != 3 || n != maxint64 {
+		t.Fatalf("Outer = %v, %d, %d; expected %v, %d, %d", _r, off, n, r, 3, int64(maxint64))
+	}
+}
+
+// largeWriter returns an invalid count that is larger than the number
+// of bytes provided (issue 39978).
+type largeWriter struct {
+	err error
+}
+
+func (w largeWriter) Write(p []byte) (int, error) {
+	return len(p) + 1, w.err
+}
+
+func TestCopyLargeWriter(t *testing.T) {
+	want := ErrInvalidWrite
+	rb := new(Buffer)
+	wb := largeWriter{}
+	rb.WriteString("hello, world.")
+	if _, err := Copy(wb, rb); err != want {
+		t.Errorf("Copy error: got %v, want %v", err, want)
+	}
+
+	want = errors.New("largeWriterError")
+	rb = new(Buffer)
+	wb = largeWriter{err: want}
+	rb.WriteString("hello, world.")
+	if _, err := Copy(wb, rb); err != want {
+		t.Errorf("Copy error: got %v, want %v", err, want)
+	}
+}
+
+func TestNopCloserWriterToForwarding(t *testing.T) {
+	for _, tc := range [...]struct {
+		Name string
+		r    Reader
+	}{
+		{"not a WriterTo", Reader(nil)},
+		{"a WriterTo", struct {
+			Reader
+			WriterTo
+		}{}},
+	} {
+		nc := NopCloser(tc.r)
+
+		_, expected := tc.r.(WriterTo)
+		_, got := nc.(WriterTo)
+		if expected != got {
+			t.Errorf("NopCloser incorrectly forwards WriterTo for %s, got %t want %t", tc.Name, got, expected)
+		}
+	}
+}
+
+func TestOffsetWriter_Seek(t *testing.T) {
+	tmpfilename := "TestOffsetWriter_Seek"
+	tmpfile, err := os.CreateTemp(t.TempDir(), tmpfilename)
+	if err != nil || tmpfile == nil {
+		t.Fatalf("CreateTemp(%s) failed: %v", tmpfilename, err)
+	}
+	defer tmpfile.Close()
+	w := NewOffsetWriter(tmpfile, 0)
+
+	// Should throw error errWhence if whence is not valid
+	t.Run("errWhence", func(t *testing.T) {
+		for _, whence := range []int{-3, -2, -1, 3, 4, 5} {
+			var offset int64 = 0
+			gotOff, gotErr := w.Seek(offset, whence)
+			if gotOff != 0 || gotErr != ErrWhence {
+				t.Errorf("For whence %d, offset %d, OffsetWriter.Seek got: (%d, %v), want: (%d, %v)",
+					whence, offset, gotOff, gotErr, 0, ErrWhence)
+			}
+		}
+	})
+
+	// Should throw error errOffset if offset is negative
+	t.Run("errOffset", func(t *testing.T) {
+		for _, whence := range []int{SeekStart, SeekCurrent} {
+			for offset := int64(-3); offset < 0; offset++ {
+				gotOff, gotErr := w.Seek(offset, whence)
+				if gotOff != 0 || gotErr != ErrOffset {
+					t.Errorf("For whence %d, offset %d, OffsetWriter.Seek got: (%d, %v), want: (%d, %v)",
+						whence, offset, gotOff, gotErr, 0, ErrOffset)
+				}
+			}
+		}
+	})
+
+	// Normal tests
+	t.Run("normal", func(t *testing.T) {
+		tests := []struct {
+			offset    int64
+			whence    int
+			returnOff int64
+		}{
+			// keep in order
+			{whence: SeekStart, offset: 1, returnOff: 1},
+			{whence: SeekStart, offset: 2, returnOff: 2},
+			{whence: SeekStart, offset: 3, returnOff: 3},
+			{whence: SeekCurrent, offset: 1, returnOff: 4},
+			{whence: SeekCurrent, offset: 2, returnOff: 6},
+			{whence: SeekCurrent, offset: 3, returnOff: 9},
+		}
+		for idx, tt := range tests {
+			gotOff, gotErr := w.Seek(tt.offset, tt.whence)
+			if gotOff != tt.returnOff || gotErr != nil {
+				t.Errorf("%d:: For whence %d, offset %d, OffsetWriter.Seek got: (%d, %v), want: (%d, <nil>)",
+					idx+1, tt.whence, tt.offset, gotOff, gotErr, tt.returnOff)
+			}
+		}
+	})
+}
+
+func TestOffsetWriter_WriteAt(t *testing.T) {
+	const content = "0123456789ABCDEF"
+	contentSize := int64(len(content))
+	tmpdir, err := os.MkdirTemp(t.TempDir(), "TestOffsetWriter_WriteAt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	work := func(off, at int64) {
+		position := fmt.Sprintf("off_%d_at_%d", off, at)
+		tmpfile, err := os.CreateTemp(tmpdir, position)
+		if err != nil || tmpfile == nil {
+			t.Fatalf("CreateTemp(%s) failed: %v", position, err)
+		}
+		defer tmpfile.Close()
+
+		var writeN int64
+		var wg sync.WaitGroup
+		// Concurrent writes, one byte at a time
+		for step, value := range []byte(content) {
+			wg.Add(1)
+			go func(wg *sync.WaitGroup, tmpfile *os.File, value byte, off, at int64, step int) {
+				defer wg.Done()
+
+				w := NewOffsetWriter(tmpfile, off)
+				n, e := w.WriteAt([]byte{value}, at+int64(step))
+				if e != nil {
+					t.Errorf("WriteAt failed. off: %d, at: %d, step: %d\n error: %v", off, at, step, e)
+				}
+				atomic.AddInt64(&writeN, int64(n))
+			}(&wg, tmpfile, value, off, at, step)
+		}
+		wg.Wait()
+
+		// Read one more byte to reach EOF
+		buf := make([]byte, contentSize+1)
+		readN, err := tmpfile.ReadAt(buf, off+at)
+		if err != EOF {
+			t.Fatalf("ReadAt failed: %v", err)
+		}
+		readContent := string(buf[:contentSize])
+		if writeN != int64(readN) || writeN != contentSize || readContent != content {
+			t.Fatalf("%s:: WriteAt(%s, %d) error. \ngot n: %v, content: %s \nexpected n: %v, content: %v",
+				position, content, at, readN, readContent, contentSize, content)
+		}
+	}
+	for off := int64(0); off < 2; off++ {
+		for at := int64(0); at < 2; at++ {
+			work(off, at)
+		}
+	}
+}
+
+func TestWriteAt_PositionPriorToBase(t *testing.T) {
+	tmpdir := t.TempDir()
+	tmpfilename := "TestOffsetWriter_WriteAt"
+	tmpfile, err := os.CreateTemp(tmpdir, tmpfilename)
+	if err != nil {
+		t.Fatalf("CreateTemp(%s) failed: %v", tmpfilename, err)
+	}
+	defer tmpfile.Close()
+
+	// start writing position in OffsetWriter
+	offset := int64(10)
+	// position we want to write to the tmpfile
+	at := int64(-1)
+	w := NewOffsetWriter(tmpfile, offset)
+	_, e := w.WriteAt([]byte("hello"), at)
+	if e == nil {
+		t.Errorf("error expected to be not nil")
+	}
+}
+
+func TestOffsetWriter_Write(t *testing.T) {
+	const content = "0123456789ABCDEF"
+	contentSize := len(content)
+	tmpdir := t.TempDir()
+
+	makeOffsetWriter := func(name string) (*OffsetWriter, *os.File) {
+		tmpfilename := "TestOffsetWriter_Write_" + name
+		tmpfile, err := os.CreateTemp(tmpdir, tmpfilename)
+		if err != nil || tmpfile == nil {
+			t.Fatalf("CreateTemp(%s) failed: %v", tmpfilename, err)
+		}
+		return NewOffsetWriter(tmpfile, 0), tmpfile
+	}
+	checkContent := func(name string, f *os.File) {
+		// Read one more byte to reach EOF
+		buf := make([]byte, contentSize+1)
+		readN, err := f.ReadAt(buf, 0)
+		if err != EOF {
+			t.Fatalf("ReadAt failed, err: %v", err)
+		}
+		readContent := string(buf[:contentSize])
+		if readN != contentSize || readContent != content {
+			t.Fatalf("%s error. \ngot n: %v, content: %s \nexpected n: %v, content: %v",
+				name, readN, readContent, contentSize, content)
+		}
+	}
+
+	var name string
+	name = "Write"
+	t.Run(name, func(t *testing.T) {
+		// Write directly (off: 0, at: 0)
+		// Write content to file
+		w, f := makeOffsetWriter(name)
+		defer f.Close()
+		for _, value := range []byte(content) {
+			n, err := w.Write([]byte{value})
+			if err != nil {
+				t.Fatalf("Write failed, n: %d, err: %v", n, err)
+			}
+		}
+		checkContent(name, f)
+
+		// Copy -> Write
+		// Copy file f to file f2
+		name = "Copy"
+		w2, f2 := makeOffsetWriter(name)
+		defer f2.Close()
+		Copy(w2, f)
+		checkContent(name, f2)
+	})
+
+	// Copy -> WriteTo -> Write
+	// Note: strings.Reader implements the io.WriterTo interface.
+	name = "Write_Of_Copy_WriteTo"
+	t.Run(name, func(t *testing.T) {
+		w, f := makeOffsetWriter(name)
+		defer f.Close()
+		Copy(w, strings.NewReader(content))
+		checkContent(name, f)
+	})
+}
diff --git a/src/io/ioutil/example_test.go b/src/io/ioutil/example_test.go
new file mode 100644
index 0000000..78b0730
--- /dev/null
+++ b/src/io/ioutil/example_test.go
@@ -0,0 +1,132 @@
+// Copyright 2015 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 ioutil_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func ExampleReadAll() {
+	r := strings.NewReader("Go is a general-purpose language designed with systems programming in mind.")
+
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s", b)
+
+	// Output:
+	// Go is a general-purpose language designed with systems programming in mind.
+}
+
+func ExampleReadDir() {
+	files, err := ioutil.ReadDir(".")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, file := range files {
+		fmt.Println(file.Name())
+	}
+}
+
+func ExampleTempDir() {
+	content := []byte("temporary file's content")
+	dir, err := ioutil.TempDir("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	defer os.RemoveAll(dir) // clean up
+
+	tmpfn := filepath.Join(dir, "tmpfile")
+	if err := ioutil.WriteFile(tmpfn, content, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleTempDir_suffix() {
+	parentDir := os.TempDir()
+	logsDir, err := ioutil.TempDir(parentDir, "*-logs")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(logsDir) // clean up
+
+	// Logs can be cleaned out earlier if needed by searching
+	// for all directories whose suffix ends in *-logs.
+	globPattern := filepath.Join(parentDir, "*-logs")
+	matches, err := filepath.Glob(globPattern)
+	if err != nil {
+		log.Fatalf("Failed to match %q: %v", globPattern, err)
+	}
+
+	for _, match := range matches {
+		if err := os.RemoveAll(match); err != nil {
+			log.Printf("Failed to remove %q: %v", match, err)
+		}
+	}
+}
+
+func ExampleTempFile() {
+	content := []byte("temporary file's content")
+	tmpfile, err := ioutil.TempFile("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	defer os.Remove(tmpfile.Name()) // clean up
+
+	if _, err := tmpfile.Write(content); err != nil {
+		log.Fatal(err)
+	}
+	if err := tmpfile.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleTempFile_suffix() {
+	content := []byte("temporary file's content")
+	tmpfile, err := ioutil.TempFile("", "example.*.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	defer os.Remove(tmpfile.Name()) // clean up
+
+	if _, err := tmpfile.Write(content); err != nil {
+		tmpfile.Close()
+		log.Fatal(err)
+	}
+	if err := tmpfile.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleReadFile() {
+	content, err := ioutil.ReadFile("testdata/hello")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("File contents: %s", content)
+
+	// Output:
+	// File contents: Hello, Gophers!
+}
+
+func ExampleWriteFile() {
+	message := []byte("Hello, Gophers!")
+	err := ioutil.WriteFile("hello", message, 0644)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/src/io/ioutil/ioutil.go b/src/io/ioutil/ioutil.go
new file mode 100644
index 0000000..67768e5
--- /dev/null
+++ b/src/io/ioutil/ioutil.go
@@ -0,0 +1,95 @@
+// Copyright 2009 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 ioutil implements some I/O utility functions.
+//
+// Deprecated: As of Go 1.16, the same functionality is now provided
+// by package [io] or package [os], and those implementations
+// should be preferred in new code.
+// See the specific function documentation for details.
+package ioutil
+
+import (
+	"io"
+	"io/fs"
+	"os"
+	"sort"
+)
+
+// ReadAll reads from r until an error or EOF and returns the data it read.
+// A successful call returns err == nil, not err == EOF. Because ReadAll is
+// defined to read from src until EOF, it does not treat an EOF from Read
+// as an error to be reported.
+//
+// Deprecated: As of Go 1.16, this function simply calls [io.ReadAll].
+func ReadAll(r io.Reader) ([]byte, error) {
+	return io.ReadAll(r)
+}
+
+// ReadFile reads the file named by filename and returns the contents.
+// A successful call returns err == nil, not err == EOF. Because ReadFile
+// reads the whole file, it does not treat an EOF from Read as an error
+// to be reported.
+//
+// Deprecated: As of Go 1.16, this function simply calls [os.ReadFile].
+func ReadFile(filename string) ([]byte, error) {
+	return os.ReadFile(filename)
+}
+
+// WriteFile writes data to a file named by filename.
+// If the file does not exist, WriteFile creates it with permissions perm
+// (before umask); otherwise WriteFile truncates it before writing, without changing permissions.
+//
+// Deprecated: As of Go 1.16, this function simply calls [os.WriteFile].
+func WriteFile(filename string, data []byte, perm fs.FileMode) error {
+	return os.WriteFile(filename, data, perm)
+}
+
+// ReadDir reads the directory named by dirname and returns
+// a list of fs.FileInfo for the directory's contents,
+// sorted by filename. If an error occurs reading the directory,
+// ReadDir returns no directory entries along with the error.
+//
+// Deprecated: As of Go 1.16, [os.ReadDir] is a more efficient and correct choice:
+// it returns a list of [fs.DirEntry] instead of [fs.FileInfo],
+// and it returns partial results in the case of an error
+// midway through reading a directory.
+//
+// If you must continue obtaining a list of [fs.FileInfo], you still can:
+//
+//	entries, err := os.ReadDir(dirname)
+//	if err != nil { ... }
+//	infos := make([]fs.FileInfo, 0, len(entries))
+//	for _, entry := range entries {
+//		info, err := entry.Info()
+//		if err != nil { ... }
+//		infos = append(infos, info)
+//	}
+func ReadDir(dirname string) ([]fs.FileInfo, error) {
+	f, err := os.Open(dirname)
+	if err != nil {
+		return nil, err
+	}
+	list, err := f.Readdir(-1)
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+	sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
+	return list, nil
+}
+
+// NopCloser returns a ReadCloser with a no-op Close method wrapping
+// the provided Reader r.
+//
+// Deprecated: As of Go 1.16, this function simply calls [io.NopCloser].
+func NopCloser(r io.Reader) io.ReadCloser {
+	return io.NopCloser(r)
+}
+
+// Discard is an io.Writer on which all Write calls succeed
+// without doing anything.
+//
+// Deprecated: As of Go 1.16, this value is simply [io.Discard].
+var Discard io.Writer = io.Discard
diff --git a/src/io/ioutil/ioutil_test.go b/src/io/ioutil/ioutil_test.go
new file mode 100644
index 0000000..6bff8c6
--- /dev/null
+++ b/src/io/ioutil/ioutil_test.go
@@ -0,0 +1,134 @@
+// Copyright 2009 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 ioutil_test
+
+import (
+	"bytes"
+	. "io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+func checkSize(t *testing.T, path string, size int64) {
+	dir, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("Stat %q (looking for size %d): %s", path, size, err)
+	}
+	if dir.Size() != size {
+		t.Errorf("Stat %q: size %d want %d", path, dir.Size(), size)
+	}
+}
+
+func TestReadFile(t *testing.T) {
+	filename := "rumpelstilzchen"
+	contents, err := ReadFile(filename)
+	if err == nil {
+		t.Fatalf("ReadFile %s: error expected, none found", filename)
+	}
+
+	filename = "ioutil_test.go"
+	contents, err = ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+
+	checkSize(t, filename, int64(len(contents)))
+}
+
+func TestWriteFile(t *testing.T) {
+	f, err := TempFile("", "ioutil-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	filename := f.Name()
+	data := "Programming today is a race between software engineers striving to " +
+		"build bigger and better idiot-proof programs, and the Universe trying " +
+		"to produce bigger and better idiots. So far, the Universe is winning."
+
+	if err := WriteFile(filename, []byte(data), 0644); err != nil {
+		t.Fatalf("WriteFile %s: %v", filename, err)
+	}
+
+	contents, err := ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+
+	if string(contents) != data {
+		t.Fatalf("contents = %q\nexpected = %q", string(contents), data)
+	}
+
+	// cleanup
+	f.Close()
+	os.Remove(filename) // ignore error
+}
+
+func TestReadOnlyWriteFile(t *testing.T) {
+	if os.Getuid() == 0 {
+		t.Skipf("Root can write to read-only files anyway, so skip the read-only test.")
+	}
+	if runtime.GOOS == "wasip1" {
+		t.Skip("file permissions are not supported by wasip1")
+	}
+
+	// We don't want to use TempFile directly, since that opens a file for us as 0600.
+	tempDir, err := TempDir("", t.Name())
+	if err != nil {
+		t.Fatalf("TempDir %s: %v", t.Name(), err)
+	}
+	defer os.RemoveAll(tempDir)
+	filename := filepath.Join(tempDir, "blurp.txt")
+
+	shmorp := []byte("shmorp")
+	florp := []byte("florp")
+	err = WriteFile(filename, shmorp, 0444)
+	if err != nil {
+		t.Fatalf("WriteFile %s: %v", filename, err)
+	}
+	err = WriteFile(filename, florp, 0444)
+	if err == nil {
+		t.Fatalf("Expected an error when writing to read-only file %s", filename)
+	}
+	got, err := ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+	if !bytes.Equal(got, shmorp) {
+		t.Fatalf("want %s, got %s", shmorp, got)
+	}
+}
+
+func TestReadDir(t *testing.T) {
+	dirname := "rumpelstilzchen"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error expected, none found", dirname)
+	}
+
+	dirname = ".."
+	list, err := ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("ReadDir %s: %v", dirname, err)
+	}
+
+	foundFile := false
+	foundSubDir := false
+	for _, dir := range list {
+		switch {
+		case !dir.IsDir() && dir.Name() == "io_test.go":
+			foundFile = true
+		case dir.IsDir() && dir.Name() == "ioutil":
+			foundSubDir = true
+		}
+	}
+	if !foundFile {
+		t.Fatalf("ReadDir %s: io_test.go file not found", dirname)
+	}
+	if !foundSubDir {
+		t.Fatalf("ReadDir %s: ioutil directory not found", dirname)
+	}
+}
diff --git a/src/io/ioutil/tempfile.go b/src/io/ioutil/tempfile.go
new file mode 100644
index 0000000..47b2e40
--- /dev/null
+++ b/src/io/ioutil/tempfile.go
@@ -0,0 +1,41 @@
+// Copyright 2010 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 ioutil
+
+import (
+	"os"
+)
+
+// TempFile creates a new temporary file in the directory dir,
+// opens the file for reading and writing, and returns the resulting *[os.File].
+// The filename is generated by taking pattern and adding a random
+// string to the end. If pattern includes a "*", the random string
+// replaces the last "*".
+// If dir is the empty string, TempFile uses the default directory
+// for temporary files (see [os.TempDir]).
+// Multiple programs calling TempFile simultaneously
+// will not choose the same file. The caller can use f.Name()
+// to find the pathname of the file. It is the caller's responsibility
+// to remove the file when no longer needed.
+//
+// Deprecated: As of Go 1.17, this function simply calls [os.CreateTemp].
+func TempFile(dir, pattern string) (f *os.File, err error) {
+	return os.CreateTemp(dir, pattern)
+}
+
+// TempDir creates a new temporary directory in the directory dir.
+// The directory name is generated by taking pattern and applying a
+// random string to the end. If pattern includes a "*", the random string
+// replaces the last "*". TempDir returns the name of the new directory.
+// If dir is the empty string, TempDir uses the
+// default directory for temporary files (see [os.TempDir]).
+// Multiple programs calling TempDir simultaneously
+// will not choose the same directory. It is the caller's responsibility
+// to remove the directory when no longer needed.
+//
+// Deprecated: As of Go 1.17, this function simply calls [os.MkdirTemp].
+func TempDir(dir, pattern string) (name string, err error) {
+	return os.MkdirTemp(dir, pattern)
+}
diff --git a/src/io/ioutil/tempfile_test.go b/src/io/ioutil/tempfile_test.go
new file mode 100644
index 0000000..818fcda
--- /dev/null
+++ b/src/io/ioutil/tempfile_test.go
@@ -0,0 +1,196 @@
+// Copyright 2010 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 ioutil_test
+
+import (
+	"io/fs"
+	. "io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestTempFile(t *testing.T) {
+	dir, err := TempDir("", "TestTempFile_BadDir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	nonexistentDir := filepath.Join(dir, "_not_exists_")
+	f, err := TempFile(nonexistentDir, "foo")
+	if f != nil || err == nil {
+		t.Errorf("TempFile(%q, `foo`) = %v, %v", nonexistentDir, f, err)
+	}
+}
+
+func TestTempFile_pattern(t *testing.T) {
+	tests := []struct{ pattern, prefix, suffix string }{
+		{"ioutil_test", "ioutil_test", ""},
+		{"ioutil_test*", "ioutil_test", ""},
+		{"ioutil_test*xyz", "ioutil_test", "xyz"},
+	}
+	for _, test := range tests {
+		f, err := TempFile("", test.pattern)
+		if err != nil {
+			t.Errorf("TempFile(..., %q) error: %v", test.pattern, err)
+			continue
+		}
+		defer os.Remove(f.Name())
+		base := filepath.Base(f.Name())
+		f.Close()
+		if !(strings.HasPrefix(base, test.prefix) && strings.HasSuffix(base, test.suffix)) {
+			t.Errorf("TempFile pattern %q created bad name %q; want prefix %q & suffix %q",
+				test.pattern, base, test.prefix, test.suffix)
+		}
+	}
+}
+
+// This string is from os.errPatternHasSeparator.
+const patternHasSeparator = "pattern contains path separator"
+
+func TestTempFile_BadPattern(t *testing.T) {
+	tmpDir, err := TempDir("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	const sep = string(os.PathSeparator)
+	tests := []struct {
+		pattern string
+		wantErr bool
+	}{
+		{"ioutil*test", false},
+		{"ioutil_test*foo", false},
+		{"ioutil_test" + sep + "foo", true},
+		{"ioutil_test*" + sep + "foo", true},
+		{"ioutil_test" + sep + "*foo", true},
+		{sep + "ioutil_test" + sep + "*foo", true},
+		{"ioutil_test*foo" + sep, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			tmpfile, err := TempFile(tmpDir, tt.pattern)
+			defer func() {
+				if tmpfile != nil {
+					tmpfile.Close()
+				}
+			}()
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("Expected an error for pattern %q", tt.pattern)
+				} else if !strings.Contains(err.Error(), patternHasSeparator) {
+					t.Errorf("Error mismatch: got %#v, want %q for pattern %q", err, patternHasSeparator, tt.pattern)
+				}
+			} else if err != nil {
+				t.Errorf("Unexpected error %v for pattern %q", err, tt.pattern)
+			}
+		})
+	}
+}
+
+func TestTempDir(t *testing.T) {
+	name, err := TempDir("/_not_exists_", "foo")
+	if name != "" || err == nil {
+		t.Errorf("TempDir(`/_not_exists_`, `foo`) = %v, %v", name, err)
+	}
+
+	tests := []struct {
+		pattern                string
+		wantPrefix, wantSuffix string
+	}{
+		{"ioutil_test", "ioutil_test", ""},
+		{"ioutil_test*", "ioutil_test", ""},
+		{"ioutil_test*xyz", "ioutil_test", "xyz"},
+	}
+
+	dir := os.TempDir()
+
+	runTestTempDir := func(t *testing.T, pattern, wantRePat string) {
+		name, err := TempDir(dir, pattern)
+		if name == "" || err != nil {
+			t.Fatalf("TempDir(dir, `ioutil_test`) = %v, %v", name, err)
+		}
+		defer os.Remove(name)
+
+		re := regexp.MustCompile(wantRePat)
+		if !re.MatchString(name) {
+			t.Errorf("TempDir(%q, %q) created bad name\n\t%q\ndid not match pattern\n\t%q", dir, pattern, name, wantRePat)
+		}
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			wantRePat := "^" + regexp.QuoteMeta(filepath.Join(dir, tt.wantPrefix)) + "[0-9]+" + regexp.QuoteMeta(tt.wantSuffix) + "$"
+			runTestTempDir(t, tt.pattern, wantRePat)
+		})
+	}
+
+	// Separately testing "*xyz" (which has no prefix). That is when constructing the
+	// pattern to assert on, as in the previous loop, using filepath.Join for an empty
+	// prefix filepath.Join(dir, ""), produces the pattern:
+	//     ^<DIR>[0-9]+xyz$
+	// yet we just want to match
+	//     "^<DIR>/[0-9]+xyz"
+	t.Run("*xyz", func(t *testing.T) {
+		wantRePat := "^" + regexp.QuoteMeta(filepath.Join(dir)) + regexp.QuoteMeta(string(filepath.Separator)) + "[0-9]+xyz$"
+		runTestTempDir(t, "*xyz", wantRePat)
+	})
+}
+
+// test that we return a nice error message if the dir argument to TempDir doesn't
+// exist (or that it's empty and os.TempDir doesn't exist)
+func TestTempDir_BadDir(t *testing.T) {
+	dir, err := TempDir("", "TestTempDir_BadDir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	badDir := filepath.Join(dir, "not-exist")
+	_, err = TempDir(badDir, "foo")
+	if pe, ok := err.(*fs.PathError); !ok || !os.IsNotExist(err) || pe.Path != badDir {
+		t.Errorf("TempDir error = %#v; want PathError for path %q satisfying os.IsNotExist", err, badDir)
+	}
+}
+
+func TestTempDir_BadPattern(t *testing.T) {
+	tmpDir, err := TempDir("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	const sep = string(os.PathSeparator)
+	tests := []struct {
+		pattern string
+		wantErr bool
+	}{
+		{"ioutil*test", false},
+		{"ioutil_test*foo", false},
+		{"ioutil_test" + sep + "foo", true},
+		{"ioutil_test*" + sep + "foo", true},
+		{"ioutil_test" + sep + "*foo", true},
+		{sep + "ioutil_test" + sep + "*foo", true},
+		{"ioutil_test*foo" + sep, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			_, err := TempDir(tmpDir, tt.pattern)
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("Expected an error for pattern %q", tt.pattern)
+				} else if !strings.Contains(err.Error(), patternHasSeparator) {
+					t.Errorf("Error mismatch: got %#v, want %q for pattern %q", err, patternHasSeparator, tt.pattern)
+				}
+			} else if err != nil {
+				t.Errorf("Unexpected error %v for pattern %q", err, tt.pattern)
+			}
+		})
+	}
+}
diff --git a/src/io/ioutil/testdata/hello b/src/io/ioutil/testdata/hello
new file mode 100644
index 0000000..e47c092
--- /dev/null
+++ b/src/io/ioutil/testdata/hello
@@ -0,0 +1 @@
+Hello, Gophers!
diff --git a/src/io/multi.go b/src/io/multi.go
new file mode 100644
index 0000000..07a9aff
--- /dev/null
+++ b/src/io/multi.go
@@ -0,0 +1,137 @@
+// Copyright 2010 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 io
+
+type eofReader struct{}
+
+func (eofReader) Read([]byte) (int, error) {
+	return 0, EOF
+}
+
+type multiReader struct {
+	readers []Reader
+}
+
+func (mr *multiReader) Read(p []byte) (n int, err error) {
+	for len(mr.readers) > 0 {
+		// Optimization to flatten nested multiReaders (Issue 13558).
+		if len(mr.readers) == 1 {
+			if r, ok := mr.readers[0].(*multiReader); ok {
+				mr.readers = r.readers
+				continue
+			}
+		}
+		n, err = mr.readers[0].Read(p)
+		if err == EOF {
+			// Use eofReader instead of nil to avoid nil panic
+			// after performing flatten (Issue 18232).
+			mr.readers[0] = eofReader{} // permit earlier GC
+			mr.readers = mr.readers[1:]
+		}
+		if n > 0 || err != EOF {
+			if err == EOF && len(mr.readers) > 0 {
+				// Don't return EOF yet. More readers remain.
+				err = nil
+			}
+			return
+		}
+	}
+	return 0, EOF
+}
+
+func (mr *multiReader) WriteTo(w Writer) (sum int64, err error) {
+	return mr.writeToWithBuffer(w, make([]byte, 1024*32))
+}
+
+func (mr *multiReader) writeToWithBuffer(w Writer, buf []byte) (sum int64, err error) {
+	for i, r := range mr.readers {
+		var n int64
+		if subMr, ok := r.(*multiReader); ok { // reuse buffer with nested multiReaders
+			n, err = subMr.writeToWithBuffer(w, buf)
+		} else {
+			n, err = copyBuffer(w, r, buf)
+		}
+		sum += n
+		if err != nil {
+			mr.readers = mr.readers[i:] // permit resume / retry after error
+			return sum, err
+		}
+		mr.readers[i] = nil // permit early GC
+	}
+	mr.readers = nil
+	return sum, nil
+}
+
+var _ WriterTo = (*multiReader)(nil)
+
+// MultiReader returns a Reader that's the logical concatenation of
+// the provided input readers. They're read sequentially. Once all
+// inputs have returned EOF, Read will return EOF.  If any of the readers
+// return a non-nil, non-EOF error, Read will return that error.
+func MultiReader(readers ...Reader) Reader {
+	r := make([]Reader, len(readers))
+	copy(r, readers)
+	return &multiReader{r}
+}
+
+type multiWriter struct {
+	writers []Writer
+}
+
+func (t *multiWriter) Write(p []byte) (n int, err error) {
+	for _, w := range t.writers {
+		n, err = w.Write(p)
+		if err != nil {
+			return
+		}
+		if n != len(p) {
+			err = ErrShortWrite
+			return
+		}
+	}
+	return len(p), nil
+}
+
+var _ StringWriter = (*multiWriter)(nil)
+
+func (t *multiWriter) WriteString(s string) (n int, err error) {
+	var p []byte // lazily initialized if/when needed
+	for _, w := range t.writers {
+		if sw, ok := w.(StringWriter); ok {
+			n, err = sw.WriteString(s)
+		} else {
+			if p == nil {
+				p = []byte(s)
+			}
+			n, err = w.Write(p)
+		}
+		if err != nil {
+			return
+		}
+		if n != len(s) {
+			err = ErrShortWrite
+			return
+		}
+	}
+	return len(s), nil
+}
+
+// MultiWriter creates a writer that duplicates its writes to all the
+// provided writers, similar to the Unix tee(1) command.
+//
+// Each write is written to each listed writer, one at a time.
+// If a listed writer returns an error, that overall write operation
+// stops and returns the error; it does not continue down the list.
+func MultiWriter(writers ...Writer) Writer {
+	allWriters := make([]Writer, 0, len(writers))
+	for _, w := range writers {
+		if mw, ok := w.(*multiWriter); ok {
+			allWriters = append(allWriters, mw.writers...)
+		} else {
+			allWriters = append(allWriters, w)
+		}
+	}
+	return &multiWriter{allWriters}
+}
diff --git a/src/io/multi_test.go b/src/io/multi_test.go
new file mode 100644
index 0000000..7a24a8a
--- /dev/null
+++ b/src/io/multi_test.go
@@ -0,0 +1,379 @@
+// Copyright 2010 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 io_test
+
+import (
+	"bytes"
+	"crypto/sha1"
+	"errors"
+	"fmt"
+	. "io"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestMultiReader(t *testing.T) {
+	var mr Reader
+	var buf []byte
+	nread := 0
+	withFooBar := func(tests func()) {
+		r1 := strings.NewReader("foo ")
+		r2 := strings.NewReader("")
+		r3 := strings.NewReader("bar")
+		mr = MultiReader(r1, r2, r3)
+		buf = make([]byte, 20)
+		tests()
+	}
+	expectRead := func(size int, expected string, eerr error) {
+		nread++
+		n, gerr := mr.Read(buf[0:size])
+		if n != len(expected) {
+			t.Errorf("#%d, expected %d bytes; got %d",
+				nread, len(expected), n)
+		}
+		got := string(buf[0:n])
+		if got != expected {
+			t.Errorf("#%d, expected %q; got %q",
+				nread, expected, got)
+		}
+		if gerr != eerr {
+			t.Errorf("#%d, expected error %v; got %v",
+				nread, eerr, gerr)
+		}
+		buf = buf[n:]
+	}
+	withFooBar(func() {
+		expectRead(2, "fo", nil)
+		expectRead(5, "o ", nil)
+		expectRead(5, "bar", nil)
+		expectRead(5, "", EOF)
+	})
+	withFooBar(func() {
+		expectRead(4, "foo ", nil)
+		expectRead(1, "b", nil)
+		expectRead(3, "ar", nil)
+		expectRead(1, "", EOF)
+	})
+	withFooBar(func() {
+		expectRead(5, "foo ", nil)
+	})
+}
+
+func TestMultiReaderAsWriterTo(t *testing.T) {
+	mr := MultiReader(
+		strings.NewReader("foo "),
+		MultiReader( // Tickle the buffer reusing codepath
+			strings.NewReader(""),
+			strings.NewReader("bar"),
+		),
+	)
+	mrAsWriterTo, ok := mr.(WriterTo)
+	if !ok {
+		t.Fatalf("expected cast to WriterTo to succeed")
+	}
+	sink := &strings.Builder{}
+	n, err := mrAsWriterTo.WriteTo(sink)
+	if err != nil {
+		t.Fatalf("expected no error; got %v", err)
+	}
+	if n != 7 {
+		t.Errorf("expected read 7 bytes; got %d", n)
+	}
+	if result := sink.String(); result != "foo bar" {
+		t.Errorf(`expected "foo bar"; got %q`, result)
+	}
+}
+
+func TestMultiWriter(t *testing.T) {
+	sink := new(bytes.Buffer)
+	// Hide bytes.Buffer's WriteString method:
+	testMultiWriter(t, struct {
+		Writer
+		fmt.Stringer
+	}{sink, sink})
+}
+
+func TestMultiWriter_String(t *testing.T) {
+	testMultiWriter(t, new(bytes.Buffer))
+}
+
+// Test that a multiWriter.WriteString calls results in at most 1 allocation,
+// even if multiple targets don't support WriteString.
+func TestMultiWriter_WriteStringSingleAlloc(t *testing.T) {
+	var sink1, sink2 bytes.Buffer
+	type simpleWriter struct { // hide bytes.Buffer's WriteString
+		Writer
+	}
+	mw := MultiWriter(simpleWriter{&sink1}, simpleWriter{&sink2})
+	allocs := int(testing.AllocsPerRun(1000, func() {
+		WriteString(mw, "foo")
+	}))
+	if allocs != 1 {
+		t.Errorf("num allocations = %d; want 1", allocs)
+	}
+}
+
+type writeStringChecker struct{ called bool }
+
+func (c *writeStringChecker) WriteString(s string) (n int, err error) {
+	c.called = true
+	return len(s), nil
+}
+
+func (c *writeStringChecker) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+func TestMultiWriter_StringCheckCall(t *testing.T) {
+	var c writeStringChecker
+	mw := MultiWriter(&c)
+	WriteString(mw, "foo")
+	if !c.called {
+		t.Error("did not see WriteString call to writeStringChecker")
+	}
+}
+
+func testMultiWriter(t *testing.T, sink interface {
+	Writer
+	fmt.Stringer
+}) {
+	sha1 := sha1.New()
+	mw := MultiWriter(sha1, sink)
+
+	sourceString := "My input text."
+	source := strings.NewReader(sourceString)
+	written, err := Copy(mw, source)
+
+	if written != int64(len(sourceString)) {
+		t.Errorf("short write of %d, not %d", written, len(sourceString))
+	}
+
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	sha1hex := fmt.Sprintf("%x", sha1.Sum(nil))
+	if sha1hex != "01cb303fa8c30a64123067c5aa6284ba7ec2d31b" {
+		t.Error("incorrect sha1 value")
+	}
+
+	if sink.String() != sourceString {
+		t.Errorf("expected %q; got %q", sourceString, sink.String())
+	}
+}
+
+// writerFunc is a Writer implemented by the underlying func.
+type writerFunc func(p []byte) (int, error)
+
+func (f writerFunc) Write(p []byte) (int, error) {
+	return f(p)
+}
+
+// Test that MultiWriter properly flattens chained multiWriters.
+func TestMultiWriterSingleChainFlatten(t *testing.T) {
+	pc := make([]uintptr, 1000) // 1000 should fit the full stack
+	n := runtime.Callers(0, pc)
+	var myDepth = callDepth(pc[:n])
+	var writeDepth int // will contain the depth from which writerFunc.Writer was called
+	var w Writer = MultiWriter(writerFunc(func(p []byte) (int, error) {
+		n := runtime.Callers(1, pc)
+		writeDepth += callDepth(pc[:n])
+		return 0, nil
+	}))
+
+	mw := w
+	// chain a bunch of multiWriters
+	for i := 0; i < 100; i++ {
+		mw = MultiWriter(w)
+	}
+
+	mw = MultiWriter(w, mw, w, mw)
+	mw.Write(nil) // don't care about errors, just want to check the call-depth for Write
+
+	if writeDepth != 4*(myDepth+2) { // 2 should be multiWriter.Write and writerFunc.Write
+		t.Errorf("multiWriter did not flatten chained multiWriters: expected writeDepth %d, got %d",
+			4*(myDepth+2), writeDepth)
+	}
+}
+
+func TestMultiWriterError(t *testing.T) {
+	f1 := writerFunc(func(p []byte) (int, error) {
+		return len(p) / 2, ErrShortWrite
+	})
+	f2 := writerFunc(func(p []byte) (int, error) {
+		t.Errorf("MultiWriter called f2.Write")
+		return len(p), nil
+	})
+	w := MultiWriter(f1, f2)
+	n, err := w.Write(make([]byte, 100))
+	if n != 50 || err != ErrShortWrite {
+		t.Errorf("Write = %d, %v, want 50, ErrShortWrite", n, err)
+	}
+}
+
+// Test that MultiReader copies the input slice and is insulated from future modification.
+func TestMultiReaderCopy(t *testing.T) {
+	slice := []Reader{strings.NewReader("hello world")}
+	r := MultiReader(slice...)
+	slice[0] = nil
+	data, err := ReadAll(r)
+	if err != nil || string(data) != "hello world" {
+		t.Errorf("ReadAll() = %q, %v, want %q, nil", data, err, "hello world")
+	}
+}
+
+// Test that MultiWriter copies the input slice and is insulated from future modification.
+func TestMultiWriterCopy(t *testing.T) {
+	var buf strings.Builder
+	slice := []Writer{&buf}
+	w := MultiWriter(slice...)
+	slice[0] = nil
+	n, err := w.Write([]byte("hello world"))
+	if err != nil || n != 11 {
+		t.Errorf("Write(`hello world`) = %d, %v, want 11, nil", n, err)
+	}
+	if buf.String() != "hello world" {
+		t.Errorf("buf.String() = %q, want %q", buf.String(), "hello world")
+	}
+}
+
+// readerFunc is a Reader implemented by the underlying func.
+type readerFunc func(p []byte) (int, error)
+
+func (f readerFunc) Read(p []byte) (int, error) {
+	return f(p)
+}
+
+// callDepth returns the logical call depth for the given PCs.
+func callDepth(callers []uintptr) (depth int) {
+	frames := runtime.CallersFrames(callers)
+	more := true
+	for more {
+		_, more = frames.Next()
+		depth++
+	}
+	return
+}
+
+// Test that MultiReader properly flattens chained multiReaders when Read is called
+func TestMultiReaderFlatten(t *testing.T) {
+	pc := make([]uintptr, 1000) // 1000 should fit the full stack
+	n := runtime.Callers(0, pc)
+	var myDepth = callDepth(pc[:n])
+	var readDepth int // will contain the depth from which fakeReader.Read was called
+	var r Reader = MultiReader(readerFunc(func(p []byte) (int, error) {
+		n := runtime.Callers(1, pc)
+		readDepth = callDepth(pc[:n])
+		return 0, errors.New("irrelevant")
+	}))
+
+	// chain a bunch of multiReaders
+	for i := 0; i < 100; i++ {
+		r = MultiReader(r)
+	}
+
+	r.Read(nil) // don't care about errors, just want to check the call-depth for Read
+
+	if readDepth != myDepth+2 { // 2 should be multiReader.Read and fakeReader.Read
+		t.Errorf("multiReader did not flatten chained multiReaders: expected readDepth %d, got %d",
+			myDepth+2, readDepth)
+	}
+}
+
+// byteAndEOFReader is a Reader which reads one byte (the underlying
+// byte) and EOF at once in its Read call.
+type byteAndEOFReader byte
+
+func (b byteAndEOFReader) Read(p []byte) (n int, err error) {
+	if len(p) == 0 {
+		// Read(0 bytes) is useless. We expect no such useless
+		// calls in this test.
+		panic("unexpected call")
+	}
+	p[0] = byte(b)
+	return 1, EOF
+}
+
+// This used to yield bytes forever; issue 16795.
+func TestMultiReaderSingleByteWithEOF(t *testing.T) {
+	got, err := ReadAll(LimitReader(MultiReader(byteAndEOFReader('a'), byteAndEOFReader('b')), 10))
+	if err != nil {
+		t.Fatal(err)
+	}
+	const want = "ab"
+	if string(got) != want {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}
+
+// Test that a reader returning (n, EOF) at the end of a MultiReader
+// chain continues to return EOF on its final read, rather than
+// yielding a (0, EOF).
+func TestMultiReaderFinalEOF(t *testing.T) {
+	r := MultiReader(bytes.NewReader(nil), byteAndEOFReader('a'))
+	buf := make([]byte, 2)
+	n, err := r.Read(buf)
+	if n != 1 || err != EOF {
+		t.Errorf("got %v, %v; want 1, EOF", n, err)
+	}
+}
+
+func TestMultiReaderFreesExhaustedReaders(t *testing.T) {
+	var mr Reader
+	closed := make(chan struct{})
+	// The closure ensures that we don't have a live reference to buf1
+	// on our stack after MultiReader is inlined (Issue 18819).  This
+	// is a work around for a limitation in liveness analysis.
+	func() {
+		buf1 := bytes.NewReader([]byte("foo"))
+		buf2 := bytes.NewReader([]byte("bar"))
+		mr = MultiReader(buf1, buf2)
+		runtime.SetFinalizer(buf1, func(*bytes.Reader) {
+			close(closed)
+		})
+	}()
+
+	buf := make([]byte, 4)
+	if n, err := ReadFull(mr, buf); err != nil || string(buf) != "foob" {
+		t.Fatalf(`ReadFull = %d (%q), %v; want 3, "foo", nil`, n, buf[:n], err)
+	}
+
+	runtime.GC()
+	select {
+	case <-closed:
+	case <-time.After(5 * time.Second):
+		t.Fatal("timeout waiting for collection of buf1")
+	}
+
+	if n, err := ReadFull(mr, buf[:2]); err != nil || string(buf[:2]) != "ar" {
+		t.Fatalf(`ReadFull = %d (%q), %v; want 2, "ar", nil`, n, buf[:n], err)
+	}
+}
+
+func TestInterleavedMultiReader(t *testing.T) {
+	r1 := strings.NewReader("123")
+	r2 := strings.NewReader("45678")
+
+	mr1 := MultiReader(r1, r2)
+	mr2 := MultiReader(mr1)
+
+	buf := make([]byte, 4)
+
+	// Have mr2 use mr1's []Readers.
+	// Consume r1 (and clear it for GC to handle) and consume part of r2.
+	n, err := ReadFull(mr2, buf)
+	if got := string(buf[:n]); got != "1234" || err != nil {
+		t.Errorf(`ReadFull(mr2) = (%q, %v), want ("1234", nil)`, got, err)
+	}
+
+	// Consume the rest of r2 via mr1.
+	// This should not panic even though mr2 cleared r1.
+	n, err = ReadFull(mr1, buf)
+	if got := string(buf[:n]); got != "5678" || err != nil {
+		t.Errorf(`ReadFull(mr1) = (%q, %v), want ("5678", nil)`, got, err)
+	}
+}
diff --git a/src/io/pipe.go b/src/io/pipe.go
new file mode 100644
index 0000000..f34cf25
--- /dev/null
+++ b/src/io/pipe.go
@@ -0,0 +1,202 @@
+// Copyright 2009 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.
+
+// Pipe adapter to connect code expecting an io.Reader
+// with code expecting an io.Writer.
+
+package io
+
+import (
+	"errors"
+	"sync"
+)
+
+// onceError is an object that will only store an error once.
+type onceError struct {
+	sync.Mutex // guards following
+	err        error
+}
+
+func (a *onceError) Store(err error) {
+	a.Lock()
+	defer a.Unlock()
+	if a.err != nil {
+		return
+	}
+	a.err = err
+}
+func (a *onceError) Load() error {
+	a.Lock()
+	defer a.Unlock()
+	return a.err
+}
+
+// ErrClosedPipe is the error used for read or write operations on a closed pipe.
+var ErrClosedPipe = errors.New("io: read/write on closed pipe")
+
+// A pipe is the shared pipe structure underlying PipeReader and PipeWriter.
+type pipe struct {
+	wrMu sync.Mutex // Serializes Write operations
+	wrCh chan []byte
+	rdCh chan int
+
+	once sync.Once // Protects closing done
+	done chan struct{}
+	rerr onceError
+	werr onceError
+}
+
+func (p *pipe) read(b []byte) (n int, err error) {
+	select {
+	case <-p.done:
+		return 0, p.readCloseError()
+	default:
+	}
+
+	select {
+	case bw := <-p.wrCh:
+		nr := copy(b, bw)
+		p.rdCh <- nr
+		return nr, nil
+	case <-p.done:
+		return 0, p.readCloseError()
+	}
+}
+
+func (p *pipe) closeRead(err error) error {
+	if err == nil {
+		err = ErrClosedPipe
+	}
+	p.rerr.Store(err)
+	p.once.Do(func() { close(p.done) })
+	return nil
+}
+
+func (p *pipe) write(b []byte) (n int, err error) {
+	select {
+	case <-p.done:
+		return 0, p.writeCloseError()
+	default:
+		p.wrMu.Lock()
+		defer p.wrMu.Unlock()
+	}
+
+	for once := true; once || len(b) > 0; once = false {
+		select {
+		case p.wrCh <- b:
+			nw := <-p.rdCh
+			b = b[nw:]
+			n += nw
+		case <-p.done:
+			return n, p.writeCloseError()
+		}
+	}
+	return n, nil
+}
+
+func (p *pipe) closeWrite(err error) error {
+	if err == nil {
+		err = EOF
+	}
+	p.werr.Store(err)
+	p.once.Do(func() { close(p.done) })
+	return nil
+}
+
+// readCloseError is considered internal to the pipe type.
+func (p *pipe) readCloseError() error {
+	rerr := p.rerr.Load()
+	if werr := p.werr.Load(); rerr == nil && werr != nil {
+		return werr
+	}
+	return ErrClosedPipe
+}
+
+// writeCloseError is considered internal to the pipe type.
+func (p *pipe) writeCloseError() error {
+	werr := p.werr.Load()
+	if rerr := p.rerr.Load(); werr == nil && rerr != nil {
+		return rerr
+	}
+	return ErrClosedPipe
+}
+
+// A PipeReader is the read half of a pipe.
+type PipeReader struct{ pipe }
+
+// Read implements the standard Read interface:
+// it reads data from the pipe, blocking until a writer
+// arrives or the write end is closed.
+// If the write end is closed with an error, that error is
+// returned as err; otherwise err is EOF.
+func (r *PipeReader) Read(data []byte) (n int, err error) {
+	return r.pipe.read(data)
+}
+
+// Close closes the reader; subsequent writes to the
+// write half of the pipe will return the error [ErrClosedPipe].
+func (r *PipeReader) Close() error {
+	return r.CloseWithError(nil)
+}
+
+// CloseWithError closes the reader; subsequent writes
+// to the write half of the pipe will return the error err.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (r *PipeReader) CloseWithError(err error) error {
+	return r.pipe.closeRead(err)
+}
+
+// A PipeWriter is the write half of a pipe.
+type PipeWriter struct{ r PipeReader }
+
+// Write implements the standard Write interface:
+// it writes data to the pipe, blocking until one or more readers
+// have consumed all the data or the read end is closed.
+// If the read end is closed with an error, that err is
+// returned as err; otherwise err is [ErrClosedPipe].
+func (w *PipeWriter) Write(data []byte) (n int, err error) {
+	return w.r.pipe.write(data)
+}
+
+// Close closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and EOF.
+func (w *PipeWriter) Close() error {
+	return w.CloseWithError(nil)
+}
+
+// CloseWithError closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and the error err,
+// or EOF if err is nil.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (w *PipeWriter) CloseWithError(err error) error {
+	return w.r.pipe.closeWrite(err)
+}
+
+// Pipe creates a synchronous in-memory pipe.
+// It can be used to connect code expecting an [io.Reader]
+// with code expecting an [io.Writer].
+//
+// Reads and Writes on the pipe are matched one to one
+// except when multiple Reads are needed to consume a single Write.
+// That is, each Write to the [PipeWriter] blocks until it has satisfied
+// one or more Reads from the [PipeReader] that fully consume
+// the written data.
+// The data is copied directly from the Write to the corresponding
+// Read (or Reads); there is no internal buffering.
+//
+// It is safe to call Read and Write in parallel with each other or with Close.
+// Parallel calls to Read and parallel calls to Write are also safe:
+// the individual calls will be gated sequentially.
+func Pipe() (*PipeReader, *PipeWriter) {
+	pw := &PipeWriter{r: PipeReader{pipe: pipe{
+		wrCh: make(chan []byte),
+		rdCh: make(chan int),
+		done: make(chan struct{}),
+	}}}
+	return &pw.r, pw
+}
diff --git a/src/io/pipe_test.go b/src/io/pipe_test.go
new file mode 100644
index 0000000..8973360
--- /dev/null
+++ b/src/io/pipe_test.go
@@ -0,0 +1,423 @@
+// Copyright 2009 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 io_test
+
+import (
+	"bytes"
+	"fmt"
+	. "io"
+	"sort"
+	"strings"
+	"testing"
+	"time"
+)
+
+func checkWrite(t *testing.T, w Writer, data []byte, c chan int) {
+	n, err := w.Write(data)
+	if err != nil {
+		t.Errorf("write: %v", err)
+	}
+	if n != len(data) {
+		t.Errorf("short write: %d != %d", n, len(data))
+	}
+	c <- 0
+}
+
+// Test a single read/write pair.
+func TestPipe1(t *testing.T) {
+	c := make(chan int)
+	r, w := Pipe()
+	var buf = make([]byte, 64)
+	go checkWrite(t, w, []byte("hello, world"), c)
+	n, err := r.Read(buf)
+	if err != nil {
+		t.Errorf("read: %v", err)
+	} else if n != 12 || string(buf[0:12]) != "hello, world" {
+		t.Errorf("bad read: got %q", buf[0:n])
+	}
+	<-c
+	r.Close()
+	w.Close()
+}
+
+func reader(t *testing.T, r Reader, c chan int) {
+	var buf = make([]byte, 64)
+	for {
+		n, err := r.Read(buf)
+		if err == EOF {
+			c <- 0
+			break
+		}
+		if err != nil {
+			t.Errorf("read: %v", err)
+		}
+		c <- n
+	}
+}
+
+// Test a sequence of read/write pairs.
+func TestPipe2(t *testing.T) {
+	c := make(chan int)
+	r, w := Pipe()
+	go reader(t, r, c)
+	var buf = make([]byte, 64)
+	for i := 0; i < 5; i++ {
+		p := buf[0 : 5+i*10]
+		n, err := w.Write(p)
+		if n != len(p) {
+			t.Errorf("wrote %d, got %d", len(p), n)
+		}
+		if err != nil {
+			t.Errorf("write: %v", err)
+		}
+		nn := <-c
+		if nn != n {
+			t.Errorf("wrote %d, read got %d", n, nn)
+		}
+	}
+	w.Close()
+	nn := <-c
+	if nn != 0 {
+		t.Errorf("final read got %d", nn)
+	}
+}
+
+type pipeReturn struct {
+	n   int
+	err error
+}
+
+// Test a large write that requires multiple reads to satisfy.
+func writer(w WriteCloser, buf []byte, c chan pipeReturn) {
+	n, err := w.Write(buf)
+	w.Close()
+	c <- pipeReturn{n, err}
+}
+
+func TestPipe3(t *testing.T) {
+	c := make(chan pipeReturn)
+	r, w := Pipe()
+	var wdat = make([]byte, 128)
+	for i := 0; i < len(wdat); i++ {
+		wdat[i] = byte(i)
+	}
+	go writer(w, wdat, c)
+	var rdat = make([]byte, 1024)
+	tot := 0
+	for n := 1; n <= 256; n *= 2 {
+		nn, err := r.Read(rdat[tot : tot+n])
+		if err != nil && err != EOF {
+			t.Fatalf("read: %v", err)
+		}
+
+		// only final two reads should be short - 1 byte, then 0
+		expect := n
+		if n == 128 {
+			expect = 1
+		} else if n == 256 {
+			expect = 0
+			if err != EOF {
+				t.Fatalf("read at end: %v", err)
+			}
+		}
+		if nn != expect {
+			t.Fatalf("read %d, expected %d, got %d", n, expect, nn)
+		}
+		tot += nn
+	}
+	pr := <-c
+	if pr.n != 128 || pr.err != nil {
+		t.Fatalf("write 128: %d, %v", pr.n, pr.err)
+	}
+	if tot != 128 {
+		t.Fatalf("total read %d != 128", tot)
+	}
+	for i := 0; i < 128; i++ {
+		if rdat[i] != byte(i) {
+			t.Fatalf("rdat[%d] = %d", i, rdat[i])
+		}
+	}
+}
+
+// Test read after/before writer close.
+
+type closer interface {
+	CloseWithError(error) error
+	Close() error
+}
+
+type pipeTest struct {
+	async          bool
+	err            error
+	closeWithError bool
+}
+
+func (p pipeTest) String() string {
+	return fmt.Sprintf("async=%v err=%v closeWithError=%v", p.async, p.err, p.closeWithError)
+}
+
+var pipeTests = []pipeTest{
+	{true, nil, false},
+	{true, nil, true},
+	{true, ErrShortWrite, true},
+	{false, nil, false},
+	{false, nil, true},
+	{false, ErrShortWrite, true},
+}
+
+func delayClose(t *testing.T, cl closer, ch chan int, tt pipeTest) {
+	time.Sleep(1 * time.Millisecond)
+	var err error
+	if tt.closeWithError {
+		err = cl.CloseWithError(tt.err)
+	} else {
+		err = cl.Close()
+	}
+	if err != nil {
+		t.Errorf("delayClose: %v", err)
+	}
+	ch <- 0
+}
+
+func TestPipeReadClose(t *testing.T) {
+	for _, tt := range pipeTests {
+		c := make(chan int, 1)
+		r, w := Pipe()
+		if tt.async {
+			go delayClose(t, w, c, tt)
+		} else {
+			delayClose(t, w, c, tt)
+		}
+		var buf = make([]byte, 64)
+		n, err := r.Read(buf)
+		<-c
+		want := tt.err
+		if want == nil {
+			want = EOF
+		}
+		if err != want {
+			t.Errorf("read from closed pipe: %v want %v", err, want)
+		}
+		if n != 0 {
+			t.Errorf("read on closed pipe returned %d", n)
+		}
+		if err = r.Close(); err != nil {
+			t.Errorf("r.Close: %v", err)
+		}
+	}
+}
+
+// Test close on Read side during Read.
+func TestPipeReadClose2(t *testing.T) {
+	c := make(chan int, 1)
+	r, _ := Pipe()
+	go delayClose(t, r, c, pipeTest{})
+	n, err := r.Read(make([]byte, 64))
+	<-c
+	if n != 0 || err != ErrClosedPipe {
+		t.Errorf("read from closed pipe: %v, %v want %v, %v", n, err, 0, ErrClosedPipe)
+	}
+}
+
+// Test write after/before reader close.
+
+func TestPipeWriteClose(t *testing.T) {
+	for _, tt := range pipeTests {
+		c := make(chan int, 1)
+		r, w := Pipe()
+		if tt.async {
+			go delayClose(t, r, c, tt)
+		} else {
+			delayClose(t, r, c, tt)
+		}
+		n, err := WriteString(w, "hello, world")
+		<-c
+		expect := tt.err
+		if expect == nil {
+			expect = ErrClosedPipe
+		}
+		if err != expect {
+			t.Errorf("write on closed pipe: %v want %v", err, expect)
+		}
+		if n != 0 {
+			t.Errorf("write on closed pipe returned %d", n)
+		}
+		if err = w.Close(); err != nil {
+			t.Errorf("w.Close: %v", err)
+		}
+	}
+}
+
+// Test close on Write side during Write.
+func TestPipeWriteClose2(t *testing.T) {
+	c := make(chan int, 1)
+	_, w := Pipe()
+	go delayClose(t, w, c, pipeTest{})
+	n, err := w.Write(make([]byte, 64))
+	<-c
+	if n != 0 || err != ErrClosedPipe {
+		t.Errorf("write to closed pipe: %v, %v want %v, %v", n, err, 0, ErrClosedPipe)
+	}
+}
+
+func TestWriteEmpty(t *testing.T) {
+	r, w := Pipe()
+	go func() {
+		w.Write([]byte{})
+		w.Close()
+	}()
+	var b [2]byte
+	ReadFull(r, b[0:2])
+	r.Close()
+}
+
+func TestWriteNil(t *testing.T) {
+	r, w := Pipe()
+	go func() {
+		w.Write(nil)
+		w.Close()
+	}()
+	var b [2]byte
+	ReadFull(r, b[0:2])
+	r.Close()
+}
+
+func TestWriteAfterWriterClose(t *testing.T) {
+	r, w := Pipe()
+
+	done := make(chan bool)
+	var writeErr error
+	go func() {
+		_, err := w.Write([]byte("hello"))
+		if err != nil {
+			t.Errorf("got error: %q; expected none", err)
+		}
+		w.Close()
+		_, writeErr = w.Write([]byte("world"))
+		done <- true
+	}()
+
+	buf := make([]byte, 100)
+	var result string
+	n, err := ReadFull(r, buf)
+	if err != nil && err != ErrUnexpectedEOF {
+		t.Fatalf("got: %q; want: %q", err, ErrUnexpectedEOF)
+	}
+	result = string(buf[0:n])
+	<-done
+
+	if result != "hello" {
+		t.Errorf("got: %q; want: %q", result, "hello")
+	}
+	if writeErr != ErrClosedPipe {
+		t.Errorf("got: %q; want: %q", writeErr, ErrClosedPipe)
+	}
+}
+
+func TestPipeCloseError(t *testing.T) {
+	type testError1 struct{ error }
+	type testError2 struct{ error }
+
+	r, w := Pipe()
+	r.CloseWithError(testError1{})
+	if _, err := w.Write(nil); err != (testError1{}) {
+		t.Errorf("Write error: got %T, want testError1", err)
+	}
+	r.CloseWithError(testError2{})
+	if _, err := w.Write(nil); err != (testError1{}) {
+		t.Errorf("Write error: got %T, want testError1", err)
+	}
+
+	r, w = Pipe()
+	w.CloseWithError(testError1{})
+	if _, err := r.Read(nil); err != (testError1{}) {
+		t.Errorf("Read error: got %T, want testError1", err)
+	}
+	w.CloseWithError(testError2{})
+	if _, err := r.Read(nil); err != (testError1{}) {
+		t.Errorf("Read error: got %T, want testError1", err)
+	}
+}
+
+func TestPipeConcurrent(t *testing.T) {
+	const (
+		input    = "0123456789abcdef"
+		count    = 8
+		readSize = 2
+	)
+
+	t.Run("Write", func(t *testing.T) {
+		r, w := Pipe()
+
+		for i := 0; i < count; i++ {
+			go func() {
+				time.Sleep(time.Millisecond) // Increase probability of race
+				if n, err := w.Write([]byte(input)); n != len(input) || err != nil {
+					t.Errorf("Write() = (%d, %v); want (%d, nil)", n, err, len(input))
+				}
+			}()
+		}
+
+		buf := make([]byte, count*len(input))
+		for i := 0; i < len(buf); i += readSize {
+			if n, err := r.Read(buf[i : i+readSize]); n != readSize || err != nil {
+				t.Errorf("Read() = (%d, %v); want (%d, nil)", n, err, readSize)
+			}
+		}
+
+		// Since each Write is fully gated, if multiple Read calls were needed,
+		// the contents of Write should still appear together in the output.
+		got := string(buf)
+		want := strings.Repeat(input, count)
+		if got != want {
+			t.Errorf("got: %q; want: %q", got, want)
+		}
+	})
+
+	t.Run("Read", func(t *testing.T) {
+		r, w := Pipe()
+
+		c := make(chan []byte, count*len(input)/readSize)
+		for i := 0; i < cap(c); i++ {
+			go func() {
+				time.Sleep(time.Millisecond) // Increase probability of race
+				buf := make([]byte, readSize)
+				if n, err := r.Read(buf); n != readSize || err != nil {
+					t.Errorf("Read() = (%d, %v); want (%d, nil)", n, err, readSize)
+				}
+				c <- buf
+			}()
+		}
+
+		for i := 0; i < count; i++ {
+			if n, err := w.Write([]byte(input)); n != len(input) || err != nil {
+				t.Errorf("Write() = (%d, %v); want (%d, nil)", n, err, len(input))
+			}
+		}
+
+		// Since each read is independent, the only guarantee about the output
+		// is that it is a permutation of the input in readSized groups.
+		got := make([]byte, 0, count*len(input))
+		for i := 0; i < cap(c); i++ {
+			got = append(got, (<-c)...)
+		}
+		got = sortBytesInGroups(got, readSize)
+		want := bytes.Repeat([]byte(input), count)
+		want = sortBytesInGroups(want, readSize)
+		if string(got) != string(want) {
+			t.Errorf("got: %q; want: %q", got, want)
+		}
+	})
+}
+
+func sortBytesInGroups(b []byte, n int) []byte {
+	var groups [][]byte
+	for len(b) > 0 {
+		groups = append(groups, b[:n])
+		b = b[n:]
+	}
+	sort.Slice(groups, func(i, j int) bool { return bytes.Compare(groups[i], groups[j]) < 0 })
+	return bytes.Join(groups, nil)
+}