libgo: update to Go1.16beta1 release

This does not yet include support for the //go:embed directive added
in this release.

	* Makefile.am (check-runtime): Don't create check-runtime-dir.
	(mostlyclean-local): Don't remove check-runtime-dir.
	(check-go-tool, check-vet): Copy in go.mod and modules.txt.
	(check-cgo-test, check-carchive-test): Add go.mod file.
	* Makefile.in: Regenerate.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/280172

21 files changed, 1445 insertions, 120 deletions

diff --git a/libgo/go/io/example_test.go b/libgo/go/io/example_test.go
index 465eed4d5e4..6d338acd140 100644
--- a/libgo/go/io/example_test.go
+++ b/libgo/go/io/example_test.go
@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
 	"strings"
@@ -141,7 +140,7 @@ func ExampleTeeReader() {
 	r = io.TeeReader(r, os.Stdout)
 
 	// Everything read from r will be copied to stdout.
-	ioutil.ReadAll(r)
+	io.ReadAll(r)
 
 	// Output:
 	// some io.Reader stream to be read
@@ -241,3 +240,17 @@ func ExamplePipe() {
 	// 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/libgo/go/io/export_test.go b/libgo/go/io/export_test.go
new file mode 100644
index 00000000000..fa3e8e76f61
--- /dev/null
+++ b/libgo/go/io/export_test.go
@@ -0,0 +1,8 @@
+// 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
diff --git a/libgo/go/io/fs/fs.go b/libgo/go/io/fs/fs.go
new file mode 100644
index 00000000000..b691a860498
--- /dev/null
+++ b/libgo/go/io/fs/fs.go
@@ -0,0 +1,251 @@
+// 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.
+package fs
+
+import (
+	"internal/oserror"
+	"time"
+)
+
+// 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.
+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 unrooted, slash-separated
+// sequences of path elements, like “x/y/z”.
+// Path names must not contain a “.” or “..” or empty element,
+// except for the special case that the root directory is named “.”.
+//
+// Paths are slash-separated on all systems, even Windows.
+// Backslashes must not appear in path names.
+func ValidPath(name string) bool {
+	if name == "." {
+		// special case
+		return true
+	}
+
+	// Iterate over elements in name, checking each.
+	for {
+		i := 0
+		for i < len(name) && name[i] != '/' {
+			if name[i] == '\\' {
+				return false
+			}
+			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.
+// A file may implement additional interfaces, such as
+// ReadDirFile, ReaderAt, or Seeker, to provide additional or optimized functionality.
+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.
+	//
+	// 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() interface{}   // 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/libgo/go/io/fs/fs_test.go b/libgo/go/io/fs/fs_test.go
new file mode 100644
index 00000000000..8d395fc0db1
--- /dev/null
+++ b/libgo/go/io/fs/fs_test.go
@@ -0,0 +1,48 @@
+// 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\`, false},
+	{`x\y`, false},
+	{`\x`, false},
+}
+
+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/libgo/go/io/fs/glob.go b/libgo/go/io/fs/glob.go
new file mode 100644
index 00000000000..cde6c49f3db
--- /dev/null
+++ b/libgo/go/io/fs/glob.go
@@ -0,0 +1,120 @@
+// 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 fs
+
+import (
+	"path"
+	"runtime"
+)
+
+// 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 (assuming the Separator is '/').
+//
+// 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) {
+	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 = Glob(fsys, dir)
+	if err != nil {
+		return
+	}
+	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++ {
+		c := path[i]
+		if c == '*' || c == '?' || c == '[' || runtime.GOOS == "windows" && c == '\\' {
+			return true
+		}
+	}
+	return false
+}
diff --git a/libgo/go/io/fs/glob_test.go b/libgo/go/io/fs/glob_test.go
new file mode 100644
index 00000000000..08fc3e059a0
--- /dev/null
+++ b/libgo/go/io/fs/glob_test.go
@@ -0,0 +1,88 @@
+// 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 fs_test
+
+import (
+	. "io/fs"
+	"os"
+	"path"
+	"testing"
+)
+
+var globTests = []struct {
+	fs              FS
+	pattern, result string
+}{
+	{os.DirFS("."), "glob.go", "glob.go"},
+	{os.DirFS("."), "gl?b.go", "glob.go"},
+	{os.DirFS("."), "*", "glob.go"},
+	// This test fails on gofrontend because the directory structure
+	// is different.
+	// {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)
+		}
+	}
+}
+
+// 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/libgo/go/io/fs/readdir.go b/libgo/go/io/fs/readdir.go
new file mode 100644
index 00000000000..3a5aa6d86a6
--- /dev/null
+++ b/libgo/go/io/fs/readdir.go
@@ -0,0 +1,47 @@
+// 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
+}
diff --git a/libgo/go/io/fs/readdir_test.go b/libgo/go/io/fs/readdir_test.go
new file mode 100644
index 00000000000..405bfa67ca4
--- /dev/null
+++ b/libgo/go/io/fs/readdir_test.go
@@ -0,0 +1,43 @@
+// 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 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)
+}
diff --git a/libgo/go/io/fs/readfile.go b/libgo/go/io/fs/readfile.go
new file mode 100644
index 00000000000..7ee9eadac4d
--- /dev/null
+++ b/libgo/go/io/fs/readfile.go
@@ -0,0 +1,63 @@
+// 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.)
+	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/libgo/go/io/fs/readfile_test.go b/libgo/go/io/fs/readfile_test.go
new file mode 100644
index 00000000000..07219c14458
--- /dev/null
+++ b/libgo/go/io/fs/readfile_test.go
@@ -0,0 +1,59 @@
+// 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"
+	"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")
+	}
+}
diff --git a/libgo/go/io/fs/stat.go b/libgo/go/io/fs/stat.go
new file mode 100644
index 00000000000..735a6e3281c
--- /dev/null
+++ b/libgo/go/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/libgo/go/io/fs/stat_test.go b/libgo/go/io/fs/stat_test.go
new file mode 100644
index 00000000000..e312b6fbd91
--- /dev/null
+++ b/libgo/go/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/libgo/go/io/fs/sub.go b/libgo/go/io/fs/sub.go
new file mode 100644
index 00000000000..64cdffe6dec
--- /dev/null
+++ b/libgo/go/io/fs/sub.go
@@ -0,0 +1,127 @@
+// 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 fs implements SubFS, Sub calls returns fsys.Sub(dir).
+// Otherwise, if dir is ".", Sub returns fsys unchanged.
+// Otherwise, Sub returns a new FS implementation sub that,
+// in effect, implements sub.Open(dir) as fsys.Open(path.Join(dir, name)).
+// The implementation also translates calls to ReadDir, ReadFile, and Glob appropriately.
+//
+// Note that Sub(os.DirFS("/"), "prefix") is equivalent to os.DirFS("/prefix")
+// and that neither of them guarantees to avoid operating system
+// accesses outside "/prefix", because the implementation of os.DirFS
+// does not check for symbolic links inside "/prefix" that point to
+// other directories. That is, os.DirFS is not a general substitute for a
+// chroot-style security mechanism, and Sub does not change that fact.
+func Sub(fsys FS, dir string) (FS, error) {
+	if !ValidPath(dir) {
+		return nil, &PathError{Op: "sub", Path: dir, Err: errors.New("invalid name")}
+	}
+	if dir == "." {
+		return fsys, nil
+	}
+	if fsys, ok := fsys.(SubFS); ok {
+		return fsys.Sub(dir)
+	}
+	return &subFS{fsys, dir}, nil
+}
+
+type subFS struct {
+	fsys FS
+	dir  string
+}
+
+// fullName maps name to the fully-qualified name dir/name.
+func (f *subFS) fullName(op string, name string) (string, error) {
+	if !ValidPath(name) {
+		return "", &PathError{Op: op, Path: name, Err: errors.New("invalid name")}
+	}
+	return path.Join(f.dir, name), nil
+}
+
+// shorten maps name, which should start with f.dir, back to the suffix after f.dir.
+func (f *subFS) shorten(name string) (rel string, ok bool) {
+	if name == f.dir {
+		return ".", true
+	}
+	if len(name) >= len(f.dir)+2 && name[len(f.dir)] == '/' && name[:len(f.dir)] == f.dir {
+		return name[len(f.dir)+1:], true
+	}
+	return "", false
+}
+
+// fixErr shortens any reported names in PathErrors by stripping dir.
+func (f *subFS) fixErr(err error) error {
+	if e, ok := err.(*PathError); ok {
+		if short, ok := f.shorten(e.Path); ok {
+			e.Path = short
+		}
+	}
+	return err
+}
+
+func (f *subFS) Open(name string) (File, error) {
+	full, err := f.fullName("open", name)
+	if err != nil {
+		return nil, err
+	}
+	file, err := f.fsys.Open(full)
+	return file, f.fixErr(err)
+}
+
+func (f *subFS) ReadDir(name string) ([]DirEntry, error) {
+	full, err := f.fullName("read", name)
+	if err != nil {
+		return nil, err
+	}
+	dir, err := ReadDir(f.fsys, full)
+	return dir, f.fixErr(err)
+}
+
+func (f *subFS) ReadFile(name string) ([]byte, error) {
+	full, err := f.fullName("read", name)
+	if err != nil {
+		return nil, err
+	}
+	data, err := ReadFile(f.fsys, full)
+	return data, f.fixErr(err)
+}
+
+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)
+}
diff --git a/libgo/go/io/fs/sub_test.go b/libgo/go/io/fs/sub_test.go
new file mode 100644
index 00000000000..451b0efb02f
--- /dev/null
+++ b/libgo/go/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/libgo/go/io/fs/walk.go b/libgo/go/io/fs/walk.go
new file mode 100644
index 00000000000..06d0b1769c5
--- /dev/null
+++ b/libgo/go/io/fs/walk.go
@@ -0,0 +1,132 @@
+// 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 WalkDirFuncs 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")
+
+// WalkDirFunc is the type of the function called by WalkDir to visit
+// each each file or directory.
+//
+// The path argument contains the argument to Walk as a prefix.
+// That is, if Walk 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 directory and file are joined with Join, which may clean the
+// directory name: if Walk is called with the root argument "x/../dir"
+// and finds a file named "a" in that directory, the walk function will
+// be called with argument "dir/a", not "x/../dir/a".
+//
+// The d argument is the fs.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). 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 os.Lstat 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 os.Lstat.
+//
+// Second, if a directory's ReadDir method fails, WalkDir calls the
+// function with path set to the directory's path, d set to an
+// fs.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 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 filepath.WalkFunc are:
+//
+//   - The second argument has type fs.DirEntry instead of fs.FileInfo.
+//   - The function is called before reading a directory, to allow SkipDir
+//     to bypass the directory read entirely.
+//   - 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 {
+			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, &statDirEntry{info}, fn)
+	}
+	if err == SkipDir {
+		return nil
+	}
+	return err
+}
+
+type statDirEntry struct {
+	info FileInfo
+}
+
+func (d *statDirEntry) Name() string            { return d.info.Name() }
+func (d *statDirEntry) IsDir() bool             { return d.info.IsDir() }
+func (d *statDirEntry) Type() FileMode          { return d.info.Mode().Type() }
+func (d *statDirEntry) Info() (FileInfo, error) { return d.info, nil }
diff --git a/libgo/go/io/fs/walk_test.go b/libgo/go/io/fs/walk_test.go
new file mode 100644
index 00000000000..395471e2e8a
--- /dev/null
+++ b/libgo/go/io/fs/walk_test.go
@@ -0,0 +1,155 @@
+// 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"
+	"io/ioutil"
+	"os"
+	pathpkg "path"
+	"runtime"
+	"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(t *testing.T) 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
+}
+
+func markTree(n *Node) { walkTree(n, "", func(path string, n *Node) { n.mark++ }) }
+
+func checkMarks(t *testing.T, report bool) {
+	walkTree(tree, tree.name, func(path string, n *Node) {
+		if n.mark != 1 && report {
+			t.Errorf("node %s mark = %d; expected 1", path, n.mark)
+		}
+		n.mark = 0
+	})
+}
+
+// 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 chtmpdir(t *testing.T) (restore func()) {
+	oldwd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	d, err := ioutil.TempDir("", "test")
+	if err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	if err := os.Chdir(d); err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	return func() {
+		if err := os.Chdir(oldwd); err != nil {
+			t.Fatalf("chtmpdir: %v", err)
+		}
+		os.RemoveAll(d)
+	}
+}
+
+func TestWalkDir(t *testing.T) {
+	if runtime.GOOS == "darwin" && runtime.GOARCH == "arm64" {
+		restore := chtmpdir(t)
+		defer restore()
+	}
+
+	tmpDir, err := ioutil.TempDir("", "TestWalk")
+	if err != nil {
+		t.Fatal("creating temp dir:", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	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(t)
+	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)
+	}
+	checkMarks(t, true)
+}
diff --git a/libgo/go/io/io.go b/libgo/go/io/io.go
index 3dea70b947b..ffd3cedc254 100644
--- a/libgo/go/io/io.go
+++ b/libgo/go/io/io.go
@@ -14,6 +14,7 @@ package io
 
 import (
 	"errors"
+	"sync"
 )
 
 // Seek whence values.
@@ -27,10 +28,15 @@ const (
 // 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
@@ -41,9 +47,9 @@ var EOF = errors.New("EOF")
 // middle of reading a fixed-size block or data structure.
 var ErrUnexpectedEOF = errors.New("unexpected EOF")
 
-// ErrNoProgress is returned by some clients of an io.Reader when
+// ErrNoProgress is returned by some clients of an Reader when
 // many calls to Read have failed to return any data or error,
-// usually the sign of a broken io.Reader implementation.
+// 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.
@@ -147,6 +153,14 @@ type ReadSeeker interface {
 	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
@@ -164,7 +178,7 @@ type ReadWriteSeeker interface {
 //
 // ReadFrom reads data from r until EOF or error.
 // The return value n is the number of bytes read.
-// Any error except io.EOF encountered during the read is also returned.
+// Any error except EOF encountered during the read is also returned.
 //
 // The Copy function uses ReaderFrom if available.
 type ReaderFrom interface {
@@ -377,7 +391,7 @@ func Copy(dst Writer, src Reader) (written int64, err error) {
 // 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 io.CopyBuffer")
+		panic("empty buffer in CopyBuffer")
 	}
 	return copyBuffer(dst, src, buf)
 }
@@ -409,9 +423,13 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		nr, er := src.Read(buf)
 		if nr > 0 {
 			nw, ew := dst.Write(buf[0:nr])
-			if nw > 0 {
-				written += int64(nw)
+			if nw < 0 || nr < nw {
+				nw = 0
+				if ew == nil {
+					ew = errInvalidWrite
+				}
 			}
+			written += int64(nw)
 			if ew != nil {
 				err = ew
 				break
@@ -547,3 +565,78 @@ func (t *teeReader) Read(p []byte) (n int, err error) {
 	}
 	return
 }
+
+// Discard is an 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() interface{} {
+		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.
+func NopCloser(r Reader) ReadCloser {
+	return nopCloser{r}
+}
+
+type nopCloser struct {
+	Reader
+}
+
+func (nopCloser) Close() error { return nil }
+
+// 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 {
+		if len(b) == cap(b) {
+			// Add more capacity (let append pick how much).
+			b = append(b, 0)[:len(b)]
+		}
+		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
+		}
+	}
+}
diff --git a/libgo/go/io/io_test.go b/libgo/go/io/io_test.go
index 170513dcc0d..5b355e8c55b 100644
--- a/libgo/go/io/io_test.go
+++ b/libgo/go/io/io_test.go
@@ -429,3 +429,31 @@ func TestSectionReader_Size(t *testing.T) {
 		}
 	}
 }
+
+// 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)
+	}
+}
diff --git a/libgo/go/io/ioutil/ioutil.go b/libgo/go/io/ioutil/ioutil.go
index acc6ec3a40a..45682b89c99 100644
--- a/libgo/go/io/ioutil/ioutil.go
+++ b/libgo/go/io/ioutil/ioutil.go
@@ -3,94 +3,59 @@
 // license that can be found in the LICENSE file.
 
 // Package ioutil implements some I/O utility functions.
+//
+// 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 (
-	"bytes"
 	"io"
+	"io/fs"
 	"os"
 	"sort"
-	"sync"
 )
 
-// readAll reads from r until an error or EOF and returns the data it read
-// from the internal buffer allocated with a specified capacity.
-func readAll(r io.Reader, capacity int64) (b []byte, err error) {
-	var buf bytes.Buffer
-	// If the buffer overflows, we will get bytes.ErrTooLarge.
-	// Return that as an error. Any other panic remains.
-	defer func() {
-		e := recover()
-		if e == nil {
-			return
-		}
-		if panicErr, ok := e.(error); ok && panicErr == bytes.ErrTooLarge {
-			err = panicErr
-		} else {
-			panic(e)
-		}
-	}()
-	if int64(int(capacity)) == capacity {
-		buf.Grow(int(capacity))
-	}
-	_, err = buf.ReadFrom(r)
-	return buf.Bytes(), err
-}
-
 // 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.
+//
+// As of Go 1.16, this function simply calls io.ReadAll.
 func ReadAll(r io.Reader) ([]byte, error) {
-	return readAll(r, bytes.MinRead)
+	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.
+//
+// As of Go 1.16, this function simply calls os.ReadFile.
 func ReadFile(filename string) ([]byte, error) {
-	f, err := os.Open(filename)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	// It's a good but not certain bet that FileInfo will tell us exactly how much to
-	// read, so let's try it but be prepared for the answer to be wrong.
-	var n int64 = bytes.MinRead
-
-	if fi, err := f.Stat(); err == nil {
-		// As initial capacity for readAll, use Size + a little extra in case Size
-		// is zero, and to avoid another allocation after Read has filled the
-		// buffer. The readAll call will read into its allocated internal buffer
-		// cheaply. If the size was wrong, we'll either waste some space off the end
-		// or reallocate as needed, but in the overwhelmingly common case we'll get
-		// it just right.
-		if size := fi.Size() + bytes.MinRead; size > n {
-			n = size
-		}
-	}
-	return readAll(f, n)
+	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.
-func WriteFile(filename string, data []byte, perm os.FileMode) error {
-	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
-	if err != nil {
-		return err
-	}
-	_, err = f.Write(data)
-	if err1 := f.Close(); err == nil {
-		err = err1
-	}
-	return err
+//
+// 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 directory entries sorted by filename.
-func ReadDir(dirname string) ([]os.FileInfo, error) {
+// 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.
+//
+// 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.
+func ReadDir(dirname string) ([]fs.FileInfo, error) {
 	f, err := os.Open(dirname)
 	if err != nil {
 		return nil, err
@@ -104,55 +69,16 @@ func ReadDir(dirname string) ([]os.FileInfo, error) {
 	return list, nil
 }
 
-type nopCloser struct {
-	io.Reader
-}
-
-func (nopCloser) Close() error { return nil }
-
 // NopCloser returns a ReadCloser with a no-op Close method wrapping
 // the provided Reader r.
+//
+// As of Go 1.16, this function simply calls io.NopCloser.
 func NopCloser(r io.Reader) io.ReadCloser {
-	return nopCloser{r}
-}
-
-type devNull int
-
-// devNull implements ReaderFrom as an optimization so io.Copy to
-// ioutil.Discard can avoid doing unnecessary work.
-var _ io.ReaderFrom = devNull(0)
-
-func (devNull) Write(p []byte) (int, error) {
-	return len(p), nil
-}
-
-func (devNull) WriteString(s string) (int, error) {
-	return len(s), nil
-}
-
-var blackHolePool = sync.Pool{
-	New: func() interface{} {
-		b := make([]byte, 8192)
-		return &b
-	},
-}
-
-func (devNull) ReadFrom(r io.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 == io.EOF {
-				return n, nil
-			}
-			return
-		}
-	}
+	return io.NopCloser(r)
 }
 
 // Discard is an io.Writer on which all Write calls succeed
 // without doing anything.
-var Discard io.Writer = devNull(0)
+//
+// As of Go 1.16, this value is simply io.Discard.
+var Discard io.Writer = io.Discard
diff --git a/libgo/go/io/ioutil/tempfile_test.go b/libgo/go/io/ioutil/tempfile_test.go
index fcc5101fcc8..440c7cffc67 100644
--- a/libgo/go/io/ioutil/tempfile_test.go
+++ b/libgo/go/io/ioutil/tempfile_test.go
@@ -5,6 +5,7 @@
 package ioutil_test
 
 import (
+	"io/fs"
 	. "io/ioutil"
 	"os"
 	"path/filepath"
@@ -151,7 +152,7 @@ func TestTempDir_BadDir(t *testing.T) {
 
 	badDir := filepath.Join(dir, "not-exist")
 	_, err = TempDir(badDir, "foo")
-	if pe, ok := err.(*os.PathError); !ok || !os.IsNotExist(err) || pe.Path != badDir {
+	if pe, ok := err.(*fs.PathError); !ok || !os.IsNotExist(err) || pe.Path != badDir {
 		t.Errorf("TempDir error = %#v; want PathError for path %q satisifying os.IsNotExist", err, badDir)
 	}
 }
diff --git a/libgo/go/io/multi_test.go b/libgo/go/io/multi_test.go
index bc6b0ab7546..c059fa67dc6 100644
--- a/libgo/go/io/multi_test.go
+++ b/libgo/go/io/multi_test.go
@@ -10,7 +10,6 @@ import (
 	"errors"
 	"fmt"
 	. "io"
-	"io/ioutil"
 	"runtime"
 	"strings"
 	"testing"
@@ -143,7 +142,7 @@ func testMultiWriter(t *testing.T, sink interface {
 	}
 }
 
-// writerFunc is an io.Writer implemented by the underlying func.
+// writerFunc is an Writer implemented by the underlying func.
 type writerFunc func(p []byte) (int, error)
 
 func (f writerFunc) Write(p []byte) (int, error) {
@@ -197,7 +196,7 @@ func TestMultiReaderCopy(t *testing.T) {
 	slice := []Reader{strings.NewReader("hello world")}
 	r := MultiReader(slice...)
 	slice[0] = nil
-	data, err := ioutil.ReadAll(r)
+	data, err := ReadAll(r)
 	if err != nil || string(data) != "hello world" {
 		t.Errorf("ReadAll() = %q, %v, want %q, nil", data, err, "hello world")
 	}
@@ -218,7 +217,7 @@ func TestMultiWriterCopy(t *testing.T) {
 	}
 }
 
-// readerFunc is an io.Reader implemented by the underlying func.
+// readerFunc is an Reader implemented by the underlying func.
 type readerFunc func(p []byte) (int, error)
 
 func (f readerFunc) Read(p []byte) (int, error) {
@@ -262,7 +261,7 @@ func TestMultiReaderFlatten(t *testing.T) {
 }
 
 // byteAndEOFReader is a Reader which reads one byte (the underlying
-// byte) and io.EOF at once in its Read call.
+// byte) and EOF at once in its Read call.
 type byteAndEOFReader byte
 
 func (b byteAndEOFReader) Read(p []byte) (n int, err error) {
@@ -277,7 +276,7 @@ func (b byteAndEOFReader) Read(p []byte) (n int, err error) {
 
 // This used to yield bytes forever; issue 16795.
 func TestMultiReaderSingleByteWithEOF(t *testing.T) {
-	got, err := ioutil.ReadAll(LimitReader(MultiReader(byteAndEOFReader('a'), byteAndEOFReader('b')), 10))
+	got, err := ReadAll(LimitReader(MultiReader(byteAndEOFReader('a'), byteAndEOFReader('b')), 10))
 	if err != nil {
 		t.Fatal(err)
 	}