Adding upstream version 1.21.8.upstream/1.21.8

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

1 files changed, 3272 insertions, 0 deletions

diff --git a/src/os/os_test.go b/src/os/os_test.go
new file mode 100644
index 0000000..94c3ad0
--- /dev/null
+++ b/src/os/os_test.go
@@ -0,0 +1,3272 @@
+// 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 os_test
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"io/fs"
+	. "os"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"runtime/debug"
+	"sort"
+	"strings"
+	"sync"
+	"syscall"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+func TestMain(m *testing.M) {
+	if Getenv("GO_OS_TEST_DRAIN_STDIN") == "1" {
+		Stdout.Close()
+		io.Copy(io.Discard, Stdin)
+		Exit(0)
+	}
+
+	Exit(m.Run())
+}
+
+var dot = []string{
+	"dir_unix.go",
+	"env.go",
+	"error.go",
+	"file.go",
+	"os_test.go",
+	"types.go",
+	"stat_darwin.go",
+	"stat_linux.go",
+}
+
+type sysDir struct {
+	name  string
+	files []string
+}
+
+var sysdir = func() *sysDir {
+	switch runtime.GOOS {
+	case "android":
+		return &sysDir{
+			"/system/lib",
+			[]string{
+				"libmedia.so",
+				"libpowermanager.so",
+			},
+		}
+	case "ios":
+		wd, err := syscall.Getwd()
+		if err != nil {
+			wd = err.Error()
+		}
+		sd := &sysDir{
+			filepath.Join(wd, "..", ".."),
+			[]string{
+				"ResourceRules.plist",
+				"Info.plist",
+			},
+		}
+		found := true
+		for _, f := range sd.files {
+			path := filepath.Join(sd.name, f)
+			if _, err := Stat(path); err != nil {
+				found = false
+				break
+			}
+		}
+		if found {
+			return sd
+		}
+		// In a self-hosted iOS build the above files might
+		// not exist. Look for system files instead below.
+	case "windows":
+		return &sysDir{
+			Getenv("SystemRoot") + "\\system32\\drivers\\etc",
+			[]string{
+				"networks",
+				"protocol",
+				"services",
+			},
+		}
+	case "plan9":
+		return &sysDir{
+			"/lib/ndb",
+			[]string{
+				"common",
+				"local",
+			},
+		}
+	case "wasip1":
+		// wasmtime has issues resolving symbolic links that are often present
+		// in directories like /etc/group below (e.g. private/etc/group on OSX).
+		// For this reason we use files in the Go source tree instead.
+		return &sysDir{
+			runtime.GOROOT(),
+			[]string{
+				"go.env",
+				"LICENSE",
+				"CONTRIBUTING.md",
+			},
+		}
+	}
+	return &sysDir{
+		"/etc",
+		[]string{
+			"group",
+			"hosts",
+			"passwd",
+		},
+	}
+}()
+
+func size(name string, t *testing.T) int64 {
+	file, err := Open(name)
+	if err != nil {
+		t.Fatal("open failed:", err)
+	}
+	defer func() {
+		if err := file.Close(); err != nil {
+			t.Error(err)
+		}
+	}()
+	n, err := io.Copy(io.Discard, file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return n
+}
+
+func equal(name1, name2 string) (r bool) {
+	switch runtime.GOOS {
+	case "windows":
+		r = strings.ToLower(name1) == strings.ToLower(name2)
+	default:
+		r = name1 == name2
+	}
+	return
+}
+
+// localTmp returns a local temporary directory not on NFS.
+func localTmp() string {
+	switch runtime.GOOS {
+	case "android", "ios", "windows":
+		return TempDir()
+	}
+	return "/tmp"
+}
+
+func newFile(testName string, t *testing.T) (f *File) {
+	f, err := CreateTemp(localTmp(), "_Go_"+testName)
+	if err != nil {
+		t.Fatalf("TempFile %s: %s", testName, err)
+	}
+	return
+}
+
+func newDir(testName string, t *testing.T) (name string) {
+	name, err := MkdirTemp(localTmp(), "_Go_"+testName)
+	if err != nil {
+		t.Fatalf("TempDir %s: %s", testName, err)
+	}
+	return
+}
+
+var sfdir = sysdir.name
+var sfname = sysdir.files[0]
+
+func TestStat(t *testing.T) {
+	t.Parallel()
+
+	path := sfdir + "/" + sfname
+	dir, err := Stat(path)
+	if err != nil {
+		t.Fatal("stat failed:", err)
+	}
+	if !equal(sfname, dir.Name()) {
+		t.Error("name should be ", sfname, "; is", dir.Name())
+	}
+	filesize := size(path, t)
+	if dir.Size() != filesize {
+		t.Error("size should be", filesize, "; is", dir.Size())
+	}
+}
+
+func TestStatError(t *testing.T) {
+	defer chtmpdir(t)()
+
+	path := "no-such-file"
+
+	fi, err := Stat(path)
+	if err == nil {
+		t.Fatal("got nil, want error")
+	}
+	if fi != nil {
+		t.Errorf("got %v, want nil", fi)
+	}
+	if perr, ok := err.(*PathError); !ok {
+		t.Errorf("got %T, want %T", err, perr)
+	}
+
+	testenv.MustHaveSymlink(t)
+
+	link := "symlink"
+	err = Symlink(path, link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fi, err = Stat(link)
+	if err == nil {
+		t.Fatal("got nil, want error")
+	}
+	if fi != nil {
+		t.Errorf("got %v, want nil", fi)
+	}
+	if perr, ok := err.(*PathError); !ok {
+		t.Errorf("got %T, want %T", err, perr)
+	}
+}
+
+func TestStatSymlinkLoop(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	defer chtmpdir(t)()
+
+	err := Symlink("x", "y")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove("y")
+
+	err = Symlink("y", "x")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer Remove("x")
+
+	_, err = Stat("x")
+	if _, ok := err.(*fs.PathError); !ok {
+		t.Errorf("expected *PathError, got %T: %v\n", err, err)
+	}
+}
+
+func TestFstat(t *testing.T) {
+	t.Parallel()
+
+	path := sfdir + "/" + sfname
+	file, err1 := Open(path)
+	if err1 != nil {
+		t.Fatal("open failed:", err1)
+	}
+	defer file.Close()
+	dir, err2 := file.Stat()
+	if err2 != nil {
+		t.Fatal("fstat failed:", err2)
+	}
+	if !equal(sfname, dir.Name()) {
+		t.Error("name should be ", sfname, "; is", dir.Name())
+	}
+	filesize := size(path, t)
+	if dir.Size() != filesize {
+		t.Error("size should be", filesize, "; is", dir.Size())
+	}
+}
+
+func TestLstat(t *testing.T) {
+	t.Parallel()
+
+	path := sfdir + "/" + sfname
+	dir, err := Lstat(path)
+	if err != nil {
+		t.Fatal("lstat failed:", err)
+	}
+	if !equal(sfname, dir.Name()) {
+		t.Error("name should be ", sfname, "; is", dir.Name())
+	}
+	if dir.Mode()&ModeSymlink == 0 {
+		filesize := size(path, t)
+		if dir.Size() != filesize {
+			t.Error("size should be", filesize, "; is", dir.Size())
+		}
+	}
+}
+
+// Read with length 0 should not return EOF.
+func TestRead0(t *testing.T) {
+	t.Parallel()
+
+	path := sfdir + "/" + sfname
+	f, err := Open(path)
+	if err != nil {
+		t.Fatal("open failed:", err)
+	}
+	defer f.Close()
+
+	b := make([]byte, 0)
+	n, err := f.Read(b)
+	if n != 0 || err != nil {
+		t.Errorf("Read(0) = %d, %v, want 0, nil", n, err)
+	}
+	b = make([]byte, 100)
+	n, err = f.Read(b)
+	if n <= 0 || err != nil {
+		t.Errorf("Read(100) = %d, %v, want >0, nil", n, err)
+	}
+}
+
+// Reading a closed file should return ErrClosed error
+func TestReadClosed(t *testing.T) {
+	t.Parallel()
+
+	path := sfdir + "/" + sfname
+	file, err := Open(path)
+	if err != nil {
+		t.Fatal("open failed:", err)
+	}
+	file.Close() // close immediately
+
+	b := make([]byte, 100)
+	_, err = file.Read(b)
+
+	e, ok := err.(*PathError)
+	if !ok || e.Err != ErrClosed {
+		t.Fatalf("Read: got %T(%v), want %T(%v)", err, err, e, ErrClosed)
+	}
+}
+
+func testReaddirnames(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		file, err := Open(dir)
+		if err != nil {
+			t.Fatalf("open %q failed: %v", dir, err)
+		}
+		defer file.Close()
+		s, err2 := file.Readdirnames(-1)
+		if err2 != nil {
+			t.Fatalf("Readdirnames %q failed: %v", dir, err2)
+		}
+		for _, m := range contents {
+			found := false
+			for _, n := range s {
+				if n == "." || n == ".." {
+					t.Errorf("got %q in directory", n)
+				}
+				if !equal(m, n) {
+					continue
+				}
+				if found {
+					t.Error("present twice:", m)
+				}
+				found = true
+			}
+			if !found {
+				t.Error("could not find", m)
+			}
+		}
+		if s == nil {
+			t.Error("Readdirnames returned nil instead of empty slice")
+		}
+	}
+}
+
+func testReaddir(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		file, err := Open(dir)
+		if err != nil {
+			t.Fatalf("open %q failed: %v", dir, err)
+		}
+		defer file.Close()
+		s, err2 := file.Readdir(-1)
+		if err2 != nil {
+			t.Fatalf("Readdir %q failed: %v", dir, err2)
+		}
+		for _, m := range contents {
+			found := false
+			for _, n := range s {
+				if n.Name() == "." || n.Name() == ".." {
+					t.Errorf("got %q in directory", n.Name())
+				}
+				if !equal(m, n.Name()) {
+					continue
+				}
+				if found {
+					t.Error("present twice:", m)
+				}
+				found = true
+			}
+			if !found {
+				t.Error("could not find", m)
+			}
+		}
+		if s == nil {
+			t.Error("Readdir returned nil instead of empty slice")
+		}
+	}
+}
+
+func testReadDir(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		file, err := Open(dir)
+		if err != nil {
+			t.Fatalf("open %q failed: %v", dir, err)
+		}
+		defer file.Close()
+		s, err2 := file.ReadDir(-1)
+		if err2 != nil {
+			t.Fatalf("ReadDir %q failed: %v", dir, err2)
+		}
+		for _, m := range contents {
+			found := false
+			for _, n := range s {
+				if n.Name() == "." || n.Name() == ".." {
+					t.Errorf("got %q in directory", n)
+				}
+				if !equal(m, n.Name()) {
+					continue
+				}
+				if found {
+					t.Error("present twice:", m)
+				}
+				found = true
+				lstat, err := Lstat(dir + "/" + m)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if n.IsDir() != lstat.IsDir() {
+					t.Errorf("%s: IsDir=%v, want %v", m, n.IsDir(), lstat.IsDir())
+				}
+				if n.Type() != lstat.Mode().Type() {
+					t.Errorf("%s: IsDir=%v, want %v", m, n.Type(), lstat.Mode().Type())
+				}
+				info, err := n.Info()
+				if err != nil {
+					t.Errorf("%s: Info: %v", m, err)
+					continue
+				}
+				if !SameFile(info, lstat) {
+					t.Errorf("%s: Info: SameFile(info, lstat) = false", m)
+				}
+			}
+			if !found {
+				t.Error("could not find", m)
+			}
+		}
+		if s == nil {
+			t.Error("ReadDir returned nil instead of empty slice")
+		}
+	}
+}
+
+func TestFileReaddirnames(t *testing.T) {
+	t.Parallel()
+
+	t.Run(".", testReaddirnames(".", dot))
+	t.Run("sysdir", testReaddirnames(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReaddirnames(t.TempDir(), nil))
+}
+
+func TestFileReaddir(t *testing.T) {
+	t.Parallel()
+
+	t.Run(".", testReaddir(".", dot))
+	t.Run("sysdir", testReaddir(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReaddir(t.TempDir(), nil))
+}
+
+func TestFileReadDir(t *testing.T) {
+	t.Parallel()
+
+	t.Run(".", testReadDir(".", dot))
+	t.Run("sysdir", testReadDir(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReadDir(t.TempDir(), nil))
+}
+
+func benchmarkReaddirname(path string, b *testing.B) {
+	var nentries int
+	for i := 0; i < b.N; i++ {
+		f, err := Open(path)
+		if err != nil {
+			b.Fatalf("open %q failed: %v", path, err)
+		}
+		ns, err := f.Readdirnames(-1)
+		f.Close()
+		if err != nil {
+			b.Fatalf("readdirnames %q failed: %v", path, err)
+		}
+		nentries = len(ns)
+	}
+	b.Logf("benchmarkReaddirname %q: %d entries", path, nentries)
+}
+
+func benchmarkReaddir(path string, b *testing.B) {
+	var nentries int
+	for i := 0; i < b.N; i++ {
+		f, err := Open(path)
+		if err != nil {
+			b.Fatalf("open %q failed: %v", path, err)
+		}
+		fs, err := f.Readdir(-1)
+		f.Close()
+		if err != nil {
+			b.Fatalf("readdir %q failed: %v", path, err)
+		}
+		nentries = len(fs)
+	}
+	b.Logf("benchmarkReaddir %q: %d entries", path, nentries)
+}
+
+func benchmarkReadDir(path string, b *testing.B) {
+	var nentries int
+	for i := 0; i < b.N; i++ {
+		f, err := Open(path)
+		if err != nil {
+			b.Fatalf("open %q failed: %v", path, err)
+		}
+		fs, err := f.ReadDir(-1)
+		f.Close()
+		if err != nil {
+			b.Fatalf("readdir %q failed: %v", path, err)
+		}
+		nentries = len(fs)
+	}
+	b.Logf("benchmarkReadDir %q: %d entries", path, nentries)
+}
+
+func BenchmarkReaddirname(b *testing.B) {
+	benchmarkReaddirname(".", b)
+}
+
+func BenchmarkReaddir(b *testing.B) {
+	benchmarkReaddir(".", b)
+}
+
+func BenchmarkReadDir(b *testing.B) {
+	benchmarkReadDir(".", b)
+}
+
+func benchmarkStat(b *testing.B, path string) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := Stat(path)
+		if err != nil {
+			b.Fatalf("Stat(%q) failed: %v", path, err)
+		}
+	}
+}
+
+func benchmarkLstat(b *testing.B, path string) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := Lstat(path)
+		if err != nil {
+			b.Fatalf("Lstat(%q) failed: %v", path, err)
+		}
+	}
+}
+
+func BenchmarkStatDot(b *testing.B) {
+	benchmarkStat(b, ".")
+}
+
+func BenchmarkStatFile(b *testing.B) {
+	benchmarkStat(b, filepath.Join(runtime.GOROOT(), "src/os/os_test.go"))
+}
+
+func BenchmarkStatDir(b *testing.B) {
+	benchmarkStat(b, filepath.Join(runtime.GOROOT(), "src/os"))
+}
+
+func BenchmarkLstatDot(b *testing.B) {
+	benchmarkLstat(b, ".")
+}
+
+func BenchmarkLstatFile(b *testing.B) {
+	benchmarkLstat(b, filepath.Join(runtime.GOROOT(), "src/os/os_test.go"))
+}
+
+func BenchmarkLstatDir(b *testing.B) {
+	benchmarkLstat(b, filepath.Join(runtime.GOROOT(), "src/os"))
+}
+
+// Read the directory one entry at a time.
+func smallReaddirnames(file *File, length int, t *testing.T) []string {
+	names := make([]string, length)
+	count := 0
+	for {
+		d, err := file.Readdirnames(1)
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("readdirnames %q failed: %v", file.Name(), err)
+		}
+		if len(d) == 0 {
+			t.Fatalf("readdirnames %q returned empty slice and no error", file.Name())
+		}
+		names[count] = d[0]
+		count++
+	}
+	return names[0:count]
+}
+
+// Check that reading a directory one entry at a time gives the same result
+// as reading it all at once.
+func TestReaddirnamesOneAtATime(t *testing.T) {
+	t.Parallel()
+
+	// big directory that doesn't change often.
+	dir := "/usr/bin"
+	switch runtime.GOOS {
+	case "android":
+		dir = "/system/bin"
+	case "ios", "wasip1":
+		wd, err := Getwd()
+		if err != nil {
+			t.Fatal(err)
+		}
+		dir = wd
+	case "plan9":
+		dir = "/bin"
+	case "windows":
+		dir = Getenv("SystemRoot") + "\\system32"
+	}
+	file, err := Open(dir)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", dir, err)
+	}
+	defer file.Close()
+	all, err1 := file.Readdirnames(-1)
+	if err1 != nil {
+		t.Fatalf("readdirnames %q failed: %v", dir, err1)
+	}
+	file1, err2 := Open(dir)
+	if err2 != nil {
+		t.Fatalf("open %q failed: %v", dir, err2)
+	}
+	defer file1.Close()
+	small := smallReaddirnames(file1, len(all)+100, t) // +100 in case we screw up
+	if len(small) < len(all) {
+		t.Fatalf("len(small) is %d, less than %d", len(small), len(all))
+	}
+	for i, n := range all {
+		if small[i] != n {
+			t.Errorf("small read %q mismatch: %v", small[i], n)
+		}
+	}
+}
+
+func TestReaddirNValues(t *testing.T) {
+	if testing.Short() {
+		t.Skip("test.short; skipping")
+	}
+	t.Parallel()
+
+	dir := t.TempDir()
+	for i := 1; i <= 105; i++ {
+		f, err := Create(filepath.Join(dir, fmt.Sprintf("%d", i)))
+		if err != nil {
+			t.Fatalf("Create: %v", err)
+		}
+		f.Write([]byte(strings.Repeat("X", i)))
+		f.Close()
+	}
+
+	var d *File
+	openDir := func() {
+		var err error
+		d, err = Open(dir)
+		if err != nil {
+			t.Fatalf("Open directory: %v", err)
+		}
+	}
+
+	readdirExpect := func(n, want int, wantErr error) {
+		t.Helper()
+		fi, err := d.Readdir(n)
+		if err != wantErr {
+			t.Fatalf("Readdir of %d got error %v, want %v", n, err, wantErr)
+		}
+		if g, e := len(fi), want; g != e {
+			t.Errorf("Readdir of %d got %d files, want %d", n, g, e)
+		}
+	}
+
+	readDirExpect := func(n, want int, wantErr error) {
+		t.Helper()
+		de, err := d.ReadDir(n)
+		if err != wantErr {
+			t.Fatalf("ReadDir of %d got error %v, want %v", n, err, wantErr)
+		}
+		if g, e := len(de), want; g != e {
+			t.Errorf("ReadDir of %d got %d files, want %d", n, g, e)
+		}
+	}
+
+	readdirnamesExpect := func(n, want int, wantErr error) {
+		t.Helper()
+		fi, err := d.Readdirnames(n)
+		if err != wantErr {
+			t.Fatalf("Readdirnames of %d got error %v, want %v", n, err, wantErr)
+		}
+		if g, e := len(fi), want; g != e {
+			t.Errorf("Readdirnames of %d got %d files, want %d", n, g, e)
+		}
+	}
+
+	for _, fn := range []func(int, int, error){readdirExpect, readdirnamesExpect, readDirExpect} {
+		// Test the slurp case
+		openDir()
+		fn(0, 105, nil)
+		fn(0, 0, nil)
+		d.Close()
+
+		// Slurp with -1 instead
+		openDir()
+		fn(-1, 105, nil)
+		fn(-2, 0, nil)
+		fn(0, 0, nil)
+		d.Close()
+
+		// Test the bounded case
+		openDir()
+		fn(1, 1, nil)
+		fn(2, 2, nil)
+		fn(105, 102, nil) // and tests buffer >100 case
+		fn(3, 0, io.EOF)
+		d.Close()
+	}
+}
+
+func touch(t *testing.T, name string) {
+	f, err := Create(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestReaddirStatFailures(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "plan9":
+		// Windows and Plan 9 already do this correctly,
+		// but are structured with different syscalls such
+		// that they don't use Lstat, so the hook below for
+		// testing it wouldn't work.
+		t.Skipf("skipping test on %v", runtime.GOOS)
+	}
+
+	var xerr error // error to return for x
+	*LstatP = func(path string) (FileInfo, error) {
+		if xerr != nil && strings.HasSuffix(path, "x") {
+			return nil, xerr
+		}
+		return Lstat(path)
+	}
+	defer func() { *LstatP = Lstat }()
+
+	dir := t.TempDir()
+	touch(t, filepath.Join(dir, "good1"))
+	touch(t, filepath.Join(dir, "x")) // will disappear or have an error
+	touch(t, filepath.Join(dir, "good2"))
+	readDir := func() ([]FileInfo, error) {
+		d, err := Open(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer d.Close()
+		return d.Readdir(-1)
+	}
+	mustReadDir := func(testName string) []FileInfo {
+		fis, err := readDir()
+		if err != nil {
+			t.Fatalf("%s: Readdir: %v", testName, err)
+		}
+		return fis
+	}
+	names := func(fis []FileInfo) []string {
+		s := make([]string, len(fis))
+		for i, fi := range fis {
+			s[i] = fi.Name()
+		}
+		sort.Strings(s)
+		return s
+	}
+
+	if got, want := names(mustReadDir("initial readdir")),
+		[]string{"good1", "good2", "x"}; !reflect.DeepEqual(got, want) {
+		t.Errorf("initial readdir got %q; want %q", got, want)
+	}
+
+	xerr = ErrNotExist
+	if got, want := names(mustReadDir("with x disappearing")),
+		[]string{"good1", "good2"}; !reflect.DeepEqual(got, want) {
+		t.Errorf("with x disappearing, got %q; want %q", got, want)
+	}
+
+	xerr = errors.New("some real error")
+	if _, err := readDir(); err != xerr {
+		t.Errorf("with a non-ErrNotExist error, got error %v; want %v", err, xerr)
+	}
+}
+
+// Readdir on a regular file should fail.
+func TestReaddirOfFile(t *testing.T) {
+	t.Parallel()
+
+	f, err := CreateTemp(t.TempDir(), "_Go_ReaddirOfFile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Write([]byte("foo"))
+	f.Close()
+	reg, err := Open(f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer reg.Close()
+
+	names, err := reg.Readdirnames(-1)
+	if err == nil {
+		t.Error("Readdirnames succeeded; want non-nil error")
+	}
+	var pe *PathError
+	if !errors.As(err, &pe) || pe.Path != f.Name() {
+		t.Errorf("Readdirnames returned %q; want a PathError with path %q", err, f.Name())
+	}
+	if len(names) > 0 {
+		t.Errorf("unexpected dir names in regular file: %q", names)
+	}
+}
+
+func TestHardLink(t *testing.T) {
+	testenv.MustHaveLink(t)
+
+	defer chtmpdir(t)()
+	from, to := "hardlinktestfrom", "hardlinktestto"
+	file, err := Create(to)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", to, err)
+	}
+	if err = file.Close(); err != nil {
+		t.Errorf("close %q failed: %v", to, err)
+	}
+	err = Link(to, from)
+	if err != nil {
+		t.Fatalf("link %q, %q failed: %v", to, from, err)
+	}
+
+	none := "hardlinktestnone"
+	err = Link(none, none)
+	// Check the returned error is well-formed.
+	if lerr, ok := err.(*LinkError); !ok || lerr.Error() == "" {
+		t.Errorf("link %q, %q failed to return a valid error", none, none)
+	}
+
+	tostat, err := Stat(to)
+	if err != nil {
+		t.Fatalf("stat %q failed: %v", to, err)
+	}
+	fromstat, err := Stat(from)
+	if err != nil {
+		t.Fatalf("stat %q failed: %v", from, err)
+	}
+	if !SameFile(tostat, fromstat) {
+		t.Errorf("link %q, %q did not create hard link", to, from)
+	}
+	// We should not be able to perform the same Link() a second time
+	err = Link(to, from)
+	switch err := err.(type) {
+	case *LinkError:
+		if err.Op != "link" {
+			t.Errorf("Link(%q, %q) err.Op = %q; want %q", to, from, err.Op, "link")
+		}
+		if err.Old != to {
+			t.Errorf("Link(%q, %q) err.Old = %q; want %q", to, from, err.Old, to)
+		}
+		if err.New != from {
+			t.Errorf("Link(%q, %q) err.New = %q; want %q", to, from, err.New, from)
+		}
+		if !IsExist(err.Err) {
+			t.Errorf("Link(%q, %q) err.Err = %q; want %q", to, from, err.Err, "file exists error")
+		}
+	case nil:
+		t.Errorf("link %q, %q: expected error, got nil", from, to)
+	default:
+		t.Errorf("link %q, %q: expected %T, got %T %v", from, to, new(LinkError), err, err)
+	}
+}
+
+// chtmpdir changes the working directory to a new temporary directory and
+// provides a cleanup function.
+func chtmpdir(t *testing.T) func() {
+	oldwd, err := Getwd()
+	if err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	d, err := MkdirTemp("", "test")
+	if err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	if err := Chdir(d); err != nil {
+		t.Fatalf("chtmpdir: %v", err)
+	}
+	return func() {
+		if err := Chdir(oldwd); err != nil {
+			t.Fatalf("chtmpdir: %v", err)
+		}
+		RemoveAll(d)
+	}
+}
+
+func TestSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	defer chtmpdir(t)()
+	from, to := "symlinktestfrom", "symlinktestto"
+	file, err := Create(to)
+	if err != nil {
+		t.Fatalf("Create(%q) failed: %v", to, err)
+	}
+	if err = file.Close(); err != nil {
+		t.Errorf("Close(%q) failed: %v", to, err)
+	}
+	err = Symlink(to, from)
+	if err != nil {
+		t.Fatalf("Symlink(%q, %q) failed: %v", to, from, err)
+	}
+	tostat, err := Lstat(to)
+	if err != nil {
+		t.Fatalf("Lstat(%q) failed: %v", to, err)
+	}
+	if tostat.Mode()&ModeSymlink != 0 {
+		t.Fatalf("Lstat(%q).Mode()&ModeSymlink = %v, want 0", to, tostat.Mode()&ModeSymlink)
+	}
+	fromstat, err := Stat(from)
+	if err != nil {
+		t.Fatalf("Stat(%q) failed: %v", from, err)
+	}
+	if !SameFile(tostat, fromstat) {
+		t.Errorf("Symlink(%q, %q) did not create symlink", to, from)
+	}
+	fromstat, err = Lstat(from)
+	if err != nil {
+		t.Fatalf("Lstat(%q) failed: %v", from, err)
+	}
+	if fromstat.Mode()&ModeSymlink == 0 {
+		t.Fatalf("Lstat(%q).Mode()&ModeSymlink = 0, want %v", from, ModeSymlink)
+	}
+	fromstat, err = Stat(from)
+	if err != nil {
+		t.Fatalf("Stat(%q) failed: %v", from, err)
+	}
+	if fromstat.Name() != from {
+		t.Errorf("Stat(%q).Name() = %q, want %q", from, fromstat.Name(), from)
+	}
+	if fromstat.Mode()&ModeSymlink != 0 {
+		t.Fatalf("Stat(%q).Mode()&ModeSymlink = %v, want 0", from, fromstat.Mode()&ModeSymlink)
+	}
+	s, err := Readlink(from)
+	if err != nil {
+		t.Fatalf("Readlink(%q) failed: %v", from, err)
+	}
+	if s != to {
+		t.Fatalf("Readlink(%q) = %q, want %q", from, s, to)
+	}
+	file, err = Open(from)
+	if err != nil {
+		t.Fatalf("Open(%q) failed: %v", from, err)
+	}
+	file.Close()
+}
+
+func TestLongSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	defer chtmpdir(t)()
+	s := "0123456789abcdef"
+	// Long, but not too long: a common limit is 255.
+	s = s + s + s + s + s + s + s + s + s + s + s + s + s + s + s
+	from := "longsymlinktestfrom"
+	err := Symlink(s, from)
+	if err != nil {
+		t.Fatalf("symlink %q, %q failed: %v", s, from, err)
+	}
+	r, err := Readlink(from)
+	if err != nil {
+		t.Fatalf("readlink %q failed: %v", from, err)
+	}
+	if r != s {
+		t.Fatalf("after symlink %q != %q", r, s)
+	}
+}
+
+func TestRename(t *testing.T) {
+	defer chtmpdir(t)()
+	from, to := "renamefrom", "renameto"
+
+	file, err := Create(from)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", from, err)
+	}
+	if err = file.Close(); err != nil {
+		t.Errorf("close %q failed: %v", from, err)
+	}
+	err = Rename(from, to)
+	if err != nil {
+		t.Fatalf("rename %q, %q failed: %v", to, from, err)
+	}
+	_, err = Stat(to)
+	if err != nil {
+		t.Errorf("stat %q failed: %v", to, err)
+	}
+}
+
+func TestRenameOverwriteDest(t *testing.T) {
+	defer chtmpdir(t)()
+	from, to := "renamefrom", "renameto"
+
+	toData := []byte("to")
+	fromData := []byte("from")
+
+	err := WriteFile(to, toData, 0777)
+	if err != nil {
+		t.Fatalf("write file %q failed: %v", to, err)
+	}
+
+	err = WriteFile(from, fromData, 0777)
+	if err != nil {
+		t.Fatalf("write file %q failed: %v", from, err)
+	}
+	err = Rename(from, to)
+	if err != nil {
+		t.Fatalf("rename %q, %q failed: %v", to, from, err)
+	}
+
+	_, err = Stat(from)
+	if err == nil {
+		t.Errorf("from file %q still exists", from)
+	}
+	if err != nil && !IsNotExist(err) {
+		t.Fatalf("stat from: %v", err)
+	}
+	toFi, err := Stat(to)
+	if err != nil {
+		t.Fatalf("stat %q failed: %v", to, err)
+	}
+	if toFi.Size() != int64(len(fromData)) {
+		t.Errorf(`"to" size = %d; want %d (old "from" size)`, toFi.Size(), len(fromData))
+	}
+}
+
+func TestRenameFailed(t *testing.T) {
+	defer chtmpdir(t)()
+	from, to := "renamefrom", "renameto"
+
+	err := Rename(from, to)
+	switch err := err.(type) {
+	case *LinkError:
+		if err.Op != "rename" {
+			t.Errorf("rename %q, %q: err.Op: want %q, got %q", from, to, "rename", err.Op)
+		}
+		if err.Old != from {
+			t.Errorf("rename %q, %q: err.Old: want %q, got %q", from, to, from, err.Old)
+		}
+		if err.New != to {
+			t.Errorf("rename %q, %q: err.New: want %q, got %q", from, to, to, err.New)
+		}
+	case nil:
+		t.Errorf("rename %q, %q: expected error, got nil", from, to)
+	default:
+		t.Errorf("rename %q, %q: expected %T, got %T %v", from, to, new(LinkError), err, err)
+	}
+}
+
+func TestRenameNotExisting(t *testing.T) {
+	defer chtmpdir(t)()
+	from, to := "doesnt-exist", "dest"
+
+	Mkdir(to, 0777)
+
+	if err := Rename(from, to); !IsNotExist(err) {
+		t.Errorf("Rename(%q, %q) = %v; want an IsNotExist error", from, to, err)
+	}
+}
+
+func TestRenameToDirFailed(t *testing.T) {
+	defer chtmpdir(t)()
+	from, to := "renamefrom", "renameto"
+
+	Mkdir(from, 0777)
+	Mkdir(to, 0777)
+
+	err := Rename(from, to)
+	switch err := err.(type) {
+	case *LinkError:
+		if err.Op != "rename" {
+			t.Errorf("rename %q, %q: err.Op: want %q, got %q", from, to, "rename", err.Op)
+		}
+		if err.Old != from {
+			t.Errorf("rename %q, %q: err.Old: want %q, got %q", from, to, from, err.Old)
+		}
+		if err.New != to {
+			t.Errorf("rename %q, %q: err.New: want %q, got %q", from, to, to, err.New)
+		}
+	case nil:
+		t.Errorf("rename %q, %q: expected error, got nil", from, to)
+	default:
+		t.Errorf("rename %q, %q: expected %T, got %T %v", from, to, new(LinkError), err, err)
+	}
+}
+
+func TestRenameCaseDifference(pt *testing.T) {
+	from, to := "renameFROM", "RENAMEfrom"
+	tests := []struct {
+		name   string
+		create func() error
+	}{
+		{"dir", func() error {
+			return Mkdir(from, 0777)
+		}},
+		{"file", func() error {
+			fd, err := Create(from)
+			if err != nil {
+				return err
+			}
+			return fd.Close()
+		}},
+	}
+
+	for _, test := range tests {
+		pt.Run(test.name, func(t *testing.T) {
+			defer chtmpdir(t)()
+
+			if err := test.create(); err != nil {
+				t.Fatalf("failed to create test file: %s", err)
+			}
+
+			if _, err := Stat(to); err != nil {
+				// Sanity check that the underlying filesystem is not case sensitive.
+				if IsNotExist(err) {
+					t.Skipf("case sensitive filesystem")
+				}
+				t.Fatalf("stat %q, got: %q", to, err)
+			}
+
+			if err := Rename(from, to); err != nil {
+				t.Fatalf("unexpected error when renaming from %q to %q: %s", from, to, err)
+			}
+
+			fd, err := Open(".")
+			if err != nil {
+				t.Fatalf("Open .: %s", err)
+			}
+
+			// Stat does not return the real case of the file (it returns what the called asked for)
+			// So we have to use readdir to get the real name of the file.
+			dirNames, err := fd.Readdirnames(-1)
+			if err != nil {
+				t.Fatalf("readdirnames: %s", err)
+			}
+
+			if dirNamesLen := len(dirNames); dirNamesLen != 1 {
+				t.Fatalf("unexpected dirNames len, got %q, want %q", dirNamesLen, 1)
+			}
+
+			if dirNames[0] != to {
+				t.Errorf("unexpected name, got %q, want %q", dirNames[0], to)
+			}
+		})
+	}
+}
+
+func testStartProcess(dir, cmd string, args []string, expect string) func(t *testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		r, w, err := Pipe()
+		if err != nil {
+			t.Fatalf("Pipe: %v", err)
+		}
+		defer r.Close()
+		attr := &ProcAttr{Dir: dir, Files: []*File{nil, w, Stderr}}
+		p, err := StartProcess(cmd, args, attr)
+		if err != nil {
+			t.Fatalf("StartProcess: %v", err)
+		}
+		w.Close()
+
+		var b strings.Builder
+		io.Copy(&b, r)
+		output := b.String()
+
+		fi1, _ := Stat(strings.TrimSpace(output))
+		fi2, _ := Stat(expect)
+		if !SameFile(fi1, fi2) {
+			t.Errorf("exec %q returned %q wanted %q",
+				strings.Join(append([]string{cmd}, args...), " "), output, expect)
+		}
+		p.Wait()
+	}
+}
+
+func TestStartProcess(t *testing.T) {
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	var dir, cmd string
+	var args []string
+	switch runtime.GOOS {
+	case "android":
+		t.Skip("android doesn't have /bin/pwd")
+	case "windows":
+		cmd = Getenv("COMSPEC")
+		dir = Getenv("SystemRoot")
+		args = []string{"/c", "cd"}
+	default:
+		var err error
+		cmd, err = exec.LookPath("pwd")
+		if err != nil {
+			t.Fatalf("Can't find pwd: %v", err)
+		}
+		dir = "/"
+		args = []string{}
+		t.Logf("Testing with %v", cmd)
+	}
+	cmddir, cmdbase := filepath.Split(cmd)
+	args = append([]string{cmdbase}, args...)
+	t.Run("absolute", testStartProcess(dir, cmd, args, dir))
+	t.Run("relative", testStartProcess(cmddir, cmdbase, args, cmddir))
+}
+
+func checkMode(t *testing.T, path string, mode FileMode) {
+	dir, err := Stat(path)
+	if err != nil {
+		t.Fatalf("Stat %q (looking for mode %#o): %s", path, mode, err)
+	}
+	if dir.Mode()&ModePerm != mode {
+		t.Errorf("Stat %q: mode %#o want %#o", path, dir.Mode(), mode)
+	}
+}
+
+func TestChmod(t *testing.T) {
+	// Chmod is not supported on wasip1.
+	if runtime.GOOS == "wasip1" {
+		t.Skip("Chmod is not supported on " + runtime.GOOS)
+	}
+	t.Parallel()
+
+	f := newFile("TestChmod", t)
+	defer Remove(f.Name())
+	defer f.Close()
+	// Creation mode is read write
+
+	fm := FileMode(0456)
+	if runtime.GOOS == "windows" {
+		fm = FileMode(0444) // read-only file
+	}
+	if err := Chmod(f.Name(), fm); err != nil {
+		t.Fatalf("chmod %s %#o: %s", f.Name(), fm, err)
+	}
+	checkMode(t, f.Name(), fm)
+
+	fm = FileMode(0123)
+	if runtime.GOOS == "windows" {
+		fm = FileMode(0666) // read-write file
+	}
+	if err := f.Chmod(fm); err != nil {
+		t.Fatalf("chmod %s %#o: %s", f.Name(), fm, err)
+	}
+	checkMode(t, f.Name(), fm)
+}
+
+func checkSize(t *testing.T, f *File, size int64) {
+	t.Helper()
+	dir, err := f.Stat()
+	if err != nil {
+		t.Fatalf("Stat %q (looking for size %d): %s", f.Name(), size, err)
+	}
+	if dir.Size() != size {
+		t.Errorf("Stat %q: size %d want %d", f.Name(), dir.Size(), size)
+	}
+}
+
+func TestFTruncate(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestFTruncate", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	checkSize(t, f, 0)
+	f.Write([]byte("hello, world\n"))
+	checkSize(t, f, 13)
+	f.Truncate(10)
+	checkSize(t, f, 10)
+	f.Truncate(1024)
+	checkSize(t, f, 1024)
+	f.Truncate(0)
+	checkSize(t, f, 0)
+	_, err := f.Write([]byte("surprise!"))
+	if err == nil {
+		checkSize(t, f, 13+9) // wrote at offset past where hello, world was.
+	}
+}
+
+func TestTruncate(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestTruncate", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	checkSize(t, f, 0)
+	f.Write([]byte("hello, world\n"))
+	checkSize(t, f, 13)
+	Truncate(f.Name(), 10)
+	checkSize(t, f, 10)
+	Truncate(f.Name(), 1024)
+	checkSize(t, f, 1024)
+	Truncate(f.Name(), 0)
+	checkSize(t, f, 0)
+	_, err := f.Write([]byte("surprise!"))
+	if err == nil {
+		checkSize(t, f, 13+9) // wrote at offset past where hello, world was.
+	}
+}
+
+func TestTruncateNonexistentFile(t *testing.T) {
+	t.Parallel()
+
+	assertPathError := func(t testing.TB, path string, err error) {
+		t.Helper()
+		if pe, ok := err.(*PathError); !ok || !IsNotExist(err) || pe.Path != path {
+			t.Errorf("got error: %v\nwant an ErrNotExist PathError with path %q", err, path)
+		}
+	}
+
+	path := filepath.Join(t.TempDir(), "nonexistent")
+
+	err := Truncate(path, 1)
+	assertPathError(t, path, err)
+
+	// Truncate shouldn't create any new file.
+	_, err = Stat(path)
+	assertPathError(t, path, err)
+}
+
+// Use TempDir (via newFile) to make sure we're on a local file system,
+// so that timings are not distorted by latency and caching.
+// On NFS, timings can be off due to caching of meta-data on
+// NFS servers (Issue 848).
+func TestChtimes(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestChtimes", t)
+	defer Remove(f.Name())
+
+	f.Write([]byte("hello, world\n"))
+	f.Close()
+
+	testChtimes(t, f.Name())
+}
+
+func TestChtimesWithZeroTimes(t *testing.T) {
+	file := newFile("chtimes-with-zero", t)
+	_, err := file.Write([]byte("hello, world\n"))
+	if err != nil {
+		t.Fatalf("Write: %s", err)
+	}
+	fName := file.Name()
+	defer Remove(file.Name())
+	err = file.Close()
+	if err != nil {
+		t.Errorf("%v", err)
+	}
+	fs, err := Stat(fName)
+	if err != nil {
+		t.Fatal(err)
+	}
+	startAtime := Atime(fs)
+	startMtime := fs.ModTime()
+	switch runtime.GOOS {
+	case "js":
+		startAtime = startAtime.Truncate(time.Second)
+		startMtime = startMtime.Truncate(time.Second)
+	}
+	at0 := startAtime
+	mt0 := startMtime
+	t0 := startMtime.Truncate(time.Second).Add(1 * time.Hour)
+
+	tests := []struct {
+		aTime     time.Time
+		mTime     time.Time
+		wantATime time.Time
+		wantMTime time.Time
+	}{
+		{
+			aTime:     time.Time{},
+			mTime:     time.Time{},
+			wantATime: startAtime,
+			wantMTime: startMtime,
+		},
+		{
+			aTime:     t0.Add(200 * time.Second),
+			mTime:     time.Time{},
+			wantATime: t0.Add(200 * time.Second),
+			wantMTime: startMtime,
+		},
+		{
+			aTime:     time.Time{},
+			mTime:     t0.Add(100 * time.Second),
+			wantATime: t0.Add(200 * time.Second),
+			wantMTime: t0.Add(100 * time.Second),
+		},
+		{
+			aTime:     t0.Add(300 * time.Second),
+			mTime:     t0.Add(100 * time.Second),
+			wantATime: t0.Add(300 * time.Second),
+			wantMTime: t0.Add(100 * time.Second),
+		},
+	}
+
+	for _, tt := range tests {
+		// Now change the times accordingly.
+		if err := Chtimes(fName, tt.aTime, tt.mTime); err != nil {
+			t.Error(err)
+		}
+
+		// Finally verify the expectations.
+		fs, err = Stat(fName)
+		if err != nil {
+			t.Error(err)
+		}
+		at0 = Atime(fs)
+		mt0 = fs.ModTime()
+
+		if got, want := at0, tt.wantATime; !got.Equal(want) {
+			errormsg := fmt.Sprintf("AccessTime mismatch with values ATime:%q-MTime:%q\ngot:  %q\nwant: %q", tt.aTime, tt.mTime, got, want)
+			switch runtime.GOOS {
+			case "plan9":
+				// Mtime is the time of the last change of
+				// content.  Similarly, atime is set whenever
+				// the contents are accessed; also, it is set
+				// whenever mtime is set.
+			case "windows":
+				t.Error(errormsg)
+			default: // unix's
+				if got, want := at0, tt.wantATime; !got.Equal(want) {
+					mounts, err := ReadFile("/bin/mounts")
+					if err != nil {
+						mounts, err = ReadFile("/etc/mtab")
+					}
+					if strings.Contains(string(mounts), "noatime") {
+						t.Log(errormsg)
+						t.Log("A filesystem is mounted with noatime; ignoring.")
+					} else {
+						switch runtime.GOOS {
+						case "netbsd", "dragonfly":
+							// On a 64-bit implementation, birth time is generally supported and cannot be changed.
+							// When supported, atime update is restricted and depends on the file system and on the
+							// OS configuration.
+							if strings.Contains(runtime.GOARCH, "64") {
+								t.Log(errormsg)
+								t.Log("Filesystem might not support atime changes; ignoring.")
+							}
+						default:
+							t.Error(errormsg)
+						}
+					}
+				}
+			}
+		}
+		if got, want := mt0, tt.wantMTime; !got.Equal(want) {
+			errormsg := fmt.Sprintf("ModTime mismatch with values ATime:%q-MTime:%q\ngot:  %q\nwant: %q", tt.aTime, tt.mTime, got, want)
+			switch runtime.GOOS {
+			case "dragonfly":
+				t.Log(errormsg)
+				t.Log("Mtime is always updated; ignoring.")
+			default:
+				t.Error(errormsg)
+			}
+		}
+	}
+}
+
+// Use TempDir (via newDir) to make sure we're on a local file system,
+// so that timings are not distorted by latency and caching.
+// On NFS, timings can be off due to caching of meta-data on
+// NFS servers (Issue 848).
+func TestChtimesDir(t *testing.T) {
+	t.Parallel()
+
+	name := newDir("TestChtimes", t)
+	defer RemoveAll(name)
+
+	testChtimes(t, name)
+}
+
+func testChtimes(t *testing.T, name string) {
+	st, err := Stat(name)
+	if err != nil {
+		t.Fatalf("Stat %s: %s", name, err)
+	}
+	preStat := st
+
+	// Move access and modification time back a second
+	at := Atime(preStat)
+	mt := preStat.ModTime()
+	err = Chtimes(name, at.Add(-time.Second), mt.Add(-time.Second))
+	if err != nil {
+		t.Fatalf("Chtimes %s: %s", name, err)
+	}
+
+	st, err = Stat(name)
+	if err != nil {
+		t.Fatalf("second Stat %s: %s", name, err)
+	}
+	postStat := st
+
+	pat := Atime(postStat)
+	pmt := postStat.ModTime()
+	if !pat.Before(at) {
+		switch runtime.GOOS {
+		case "plan9":
+			// Mtime is the time of the last change of
+			// content.  Similarly, atime is set whenever
+			// the contents are accessed; also, it is set
+			// whenever mtime is set.
+		case "netbsd":
+			mounts, _ := ReadFile("/proc/mounts")
+			if strings.Contains(string(mounts), "noatime") {
+				t.Logf("AccessTime didn't go backwards, but see a filesystem mounted noatime; ignoring. Issue 19293.")
+			} else {
+				t.Logf("AccessTime didn't go backwards; was=%v, after=%v (Ignoring on NetBSD, assuming noatime, Issue 19293)", at, pat)
+			}
+		default:
+			t.Errorf("AccessTime didn't go backwards; was=%v, after=%v", at, pat)
+		}
+	}
+
+	if !pmt.Before(mt) {
+		t.Errorf("ModTime didn't go backwards; was=%v, after=%v", mt, pmt)
+	}
+}
+
+func TestChtimesToUnixZero(t *testing.T) {
+	file := newFile("chtimes-to-unix-zero", t)
+	fn := file.Name()
+	defer Remove(fn)
+	if _, err := file.Write([]byte("hi")); err != nil {
+		t.Fatal(err)
+	}
+	if err := file.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	unixZero := time.Unix(0, 0)
+	if err := Chtimes(fn, unixZero, unixZero); err != nil {
+		t.Fatalf("Chtimes failed: %v", err)
+	}
+
+	st, err := Stat(fn)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if mt := st.ModTime(); mt != unixZero {
+		t.Errorf("mtime is %v, want %v", mt, unixZero)
+	}
+}
+
+func TestFileChdir(t *testing.T) {
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %s", err)
+	}
+	defer Chdir(wd)
+
+	fd, err := Open(".")
+	if err != nil {
+		t.Fatalf("Open .: %s", err)
+	}
+	defer fd.Close()
+
+	if err := Chdir("/"); err != nil {
+		t.Fatalf("Chdir /: %s", err)
+	}
+
+	if err := fd.Chdir(); err != nil {
+		t.Fatalf("fd.Chdir: %s", err)
+	}
+
+	wdNew, err := Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %s", err)
+	}
+	if !equal(wdNew, wd) {
+		t.Fatalf("fd.Chdir failed, got %s, want %s", wdNew, wd)
+	}
+}
+
+func TestChdirAndGetwd(t *testing.T) {
+	fd, err := Open(".")
+	if err != nil {
+		t.Fatalf("Open .: %s", err)
+	}
+	// These are chosen carefully not to be symlinks on a Mac
+	// (unlike, say, /var, /etc), except /tmp, which we handle below.
+	dirs := []string{"/", "/usr/bin", "/tmp"}
+	// /usr/bin does not usually exist on Plan 9 or Android.
+	switch runtime.GOOS {
+	case "android":
+		dirs = []string{"/system/bin"}
+	case "plan9":
+		dirs = []string{"/", "/usr"}
+	case "ios", "windows", "wasip1":
+		dirs = nil
+		for _, dir := range []string{t.TempDir(), t.TempDir()} {
+			// Expand symlinks so path equality tests work.
+			dir, err = filepath.EvalSymlinks(dir)
+			if err != nil {
+				t.Fatalf("EvalSymlinks: %v", err)
+			}
+			dirs = append(dirs, dir)
+		}
+	}
+	oldwd := Getenv("PWD")
+	for mode := 0; mode < 2; mode++ {
+		for _, d := range dirs {
+			if mode == 0 {
+				err = Chdir(d)
+			} else {
+				fd1, err1 := Open(d)
+				if err1 != nil {
+					t.Errorf("Open %s: %s", d, err1)
+					continue
+				}
+				err = fd1.Chdir()
+				fd1.Close()
+			}
+			if d == "/tmp" {
+				Setenv("PWD", "/tmp")
+			}
+			pwd, err1 := Getwd()
+			Setenv("PWD", oldwd)
+			err2 := fd.Chdir()
+			if err2 != nil {
+				// We changed the current directory and cannot go back.
+				// Don't let the tests continue; they'll scribble
+				// all over some other directory.
+				fmt.Fprintf(Stderr, "fchdir back to dot failed: %s\n", err2)
+				Exit(1)
+			}
+			if err != nil {
+				fd.Close()
+				t.Fatalf("Chdir %s: %s", d, err)
+			}
+			if err1 != nil {
+				fd.Close()
+				t.Fatalf("Getwd in %s: %s", d, err1)
+			}
+			if !equal(pwd, d) {
+				fd.Close()
+				t.Fatalf("Getwd returned %q want %q", pwd, d)
+			}
+		}
+	}
+	fd.Close()
+}
+
+// Test that Chdir+Getwd is program-wide.
+func TestProgWideChdir(t *testing.T) {
+	const N = 10
+	var wg sync.WaitGroup
+	hold := make(chan struct{})
+	done := make(chan struct{})
+
+	d := t.TempDir()
+	oldwd, err := Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %v", err)
+	}
+	defer func() {
+		if err := Chdir(oldwd); err != nil {
+			// It's not safe to continue with tests if we can't get back to
+			// the original working directory.
+			panic(err)
+		}
+	}()
+
+	// Note the deferred Wait must be called after the deferred close(done),
+	// to ensure the N goroutines have been released even if the main goroutine
+	// calls Fatalf. It must be called before the Chdir back to the original
+	// directory, and before the deferred deletion implied by TempDir,
+	// so as not to interfere while the N goroutines are still running.
+	defer wg.Wait()
+	defer close(done)
+
+	for i := 0; i < N; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			// Lock half the goroutines in their own operating system
+			// thread to exercise more scheduler possibilities.
+			if i%2 == 1 {
+				// On Plan 9, after calling LockOSThread, the goroutines
+				// run on different processes which don't share the working
+				// directory. This used to be an issue because Go expects
+				// the working directory to be program-wide.
+				// See issue 9428.
+				runtime.LockOSThread()
+			}
+			select {
+			case <-done:
+				return
+			case <-hold:
+			}
+			// Getwd might be wrong
+			f0, err := Stat(".")
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			pwd, err := Getwd()
+			if err != nil {
+				t.Errorf("Getwd: %v", err)
+				return
+			}
+			if pwd != d {
+				t.Errorf("Getwd() = %q, want %q", pwd, d)
+				return
+			}
+			f1, err := Stat(pwd)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			if !SameFile(f0, f1) {
+				t.Errorf(`Samefile(Stat("."), Getwd()) reports false (%s != %s)`, f0.Name(), f1.Name())
+				return
+			}
+		}(i)
+	}
+	if err = Chdir(d); err != nil {
+		t.Fatalf("Chdir: %v", err)
+	}
+	// OS X sets TMPDIR to a symbolic link.
+	// So we resolve our working directory again before the test.
+	d, err = Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %v", err)
+	}
+	close(hold)
+	wg.Wait()
+}
+
+func TestSeek(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestSeek", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	type test struct {
+		in     int64
+		whence int
+		out    int64
+	}
+	var tests = []test{
+		{0, io.SeekCurrent, int64(len(data))},
+		{0, io.SeekStart, 0},
+		{5, io.SeekStart, 5},
+		{0, io.SeekEnd, int64(len(data))},
+		{0, io.SeekStart, 0},
+		{-1, io.SeekEnd, int64(len(data)) - 1},
+		{1 << 33, io.SeekStart, 1 << 33},
+		{1 << 33, io.SeekEnd, 1<<33 + int64(len(data))},
+
+		// Issue 21681, Windows 4G-1, etc:
+		{1<<32 - 1, io.SeekStart, 1<<32 - 1},
+		{0, io.SeekCurrent, 1<<32 - 1},
+		{2<<32 - 1, io.SeekStart, 2<<32 - 1},
+		{0, io.SeekCurrent, 2<<32 - 1},
+	}
+	for i, tt := range tests {
+		off, err := f.Seek(tt.in, tt.whence)
+		if off != tt.out || err != nil {
+			if e, ok := err.(*PathError); ok && e.Err == syscall.EINVAL && tt.out > 1<<32 && runtime.GOOS == "linux" {
+				mounts, _ := ReadFile("/proc/mounts")
+				if strings.Contains(string(mounts), "reiserfs") {
+					// Reiserfs rejects the big seeks.
+					t.Skipf("skipping test known to fail on reiserfs; https://golang.org/issue/91")
+				}
+			}
+			t.Errorf("#%d: Seek(%v, %v) = %v, %v want %v, nil", i, tt.in, tt.whence, off, err, tt.out)
+		}
+	}
+}
+
+func TestSeekError(t *testing.T) {
+	switch runtime.GOOS {
+	case "js", "plan9", "wasip1":
+		t.Skipf("skipping test on %v", runtime.GOOS)
+	}
+	t.Parallel()
+
+	r, w, err := Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = r.Seek(0, 0)
+	if err == nil {
+		t.Fatal("Seek on pipe should fail")
+	}
+	if perr, ok := err.(*PathError); !ok || perr.Err != syscall.ESPIPE {
+		t.Errorf("Seek returned error %v, want &PathError{Err: syscall.ESPIPE}", err)
+	}
+	_, err = w.Seek(0, 0)
+	if err == nil {
+		t.Fatal("Seek on pipe should fail")
+	}
+	if perr, ok := err.(*PathError); !ok || perr.Err != syscall.ESPIPE {
+		t.Errorf("Seek returned error %v, want &PathError{Err: syscall.ESPIPE}", err)
+	}
+}
+
+type openErrorTest struct {
+	path  string
+	mode  int
+	error error
+}
+
+var openErrorTests = []openErrorTest{
+	{
+		sfdir + "/no-such-file",
+		O_RDONLY,
+		syscall.ENOENT,
+	},
+	{
+		sfdir,
+		O_WRONLY,
+		syscall.EISDIR,
+	},
+	{
+		sfdir + "/" + sfname + "/no-such-file",
+		O_WRONLY,
+		syscall.ENOTDIR,
+	},
+}
+
+func TestOpenError(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range openErrorTests {
+		f, err := OpenFile(tt.path, tt.mode, 0)
+		if err == nil {
+			t.Errorf("Open(%q, %d) succeeded", tt.path, tt.mode)
+			f.Close()
+			continue
+		}
+		perr, ok := err.(*PathError)
+		if !ok {
+			t.Errorf("Open(%q, %d) returns error of %T type; want *PathError", tt.path, tt.mode, err)
+		}
+		if perr.Err != tt.error {
+			if runtime.GOOS == "plan9" {
+				syscallErrStr := perr.Err.Error()
+				expectedErrStr := strings.Replace(tt.error.Error(), "file ", "", 1)
+				if !strings.HasSuffix(syscallErrStr, expectedErrStr) {
+					// Some Plan 9 file servers incorrectly return
+					// EACCES rather than EISDIR when a directory is
+					// opened for write.
+					if tt.error == syscall.EISDIR && strings.HasSuffix(syscallErrStr, syscall.EACCES.Error()) {
+						continue
+					}
+					t.Errorf("Open(%q, %d) = _, %q; want suffix %q", tt.path, tt.mode, syscallErrStr, expectedErrStr)
+				}
+				continue
+			}
+			if runtime.GOOS == "dragonfly" {
+				// DragonFly incorrectly returns EACCES rather
+				// EISDIR when a directory is opened for write.
+				if tt.error == syscall.EISDIR && perr.Err == syscall.EACCES {
+					continue
+				}
+			}
+			t.Errorf("Open(%q, %d) = _, %q; want %q", tt.path, tt.mode, perr.Err.Error(), tt.error.Error())
+		}
+	}
+}
+
+func TestOpenNoName(t *testing.T) {
+	f, err := Open("")
+	if err == nil {
+		f.Close()
+		t.Fatal(`Open("") succeeded`)
+	}
+}
+
+func runBinHostname(t *testing.T) string {
+	// Run /bin/hostname and collect output.
+	r, w, err := Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r.Close()
+
+	path, err := exec.LookPath("hostname")
+	if err != nil {
+		if errors.Is(err, exec.ErrNotFound) {
+			t.Skip("skipping test; test requires hostname but it does not exist")
+		}
+		t.Fatal(err)
+	}
+
+	argv := []string{"hostname"}
+	if runtime.GOOS == "aix" {
+		argv = []string{"hostname", "-s"}
+	}
+	p, err := StartProcess(path, argv, &ProcAttr{Files: []*File{nil, w, Stderr}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+
+	var b strings.Builder
+	io.Copy(&b, r)
+	_, err = p.Wait()
+	if err != nil {
+		t.Fatalf("run hostname Wait: %v", err)
+	}
+	err = p.Kill()
+	if err == nil {
+		t.Errorf("expected an error from Kill running 'hostname'")
+	}
+	output := b.String()
+	if n := len(output); n > 0 && output[n-1] == '\n' {
+		output = output[0 : n-1]
+	}
+	if output == "" {
+		t.Fatalf("/bin/hostname produced no output")
+	}
+
+	return output
+}
+
+func testWindowsHostname(t *testing.T, hostname string) {
+	cmd := testenv.Command(t, "hostname")
+	out, err := cmd.Output()
+	if err != nil {
+		t.Fatalf("Failed to execute hostname command: %v %s", err, out)
+	}
+	want := strings.Trim(string(out), "\r\n")
+	if hostname != want {
+		t.Fatalf("Hostname() = %q != system hostname of %q", hostname, want)
+	}
+}
+
+func TestHostname(t *testing.T) {
+	t.Parallel()
+
+	hostname, err := Hostname()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if hostname == "" {
+		t.Fatal("Hostname returned empty string and no error")
+	}
+	if strings.Contains(hostname, "\x00") {
+		t.Fatalf("unexpected zero byte in hostname: %q", hostname)
+	}
+
+	// There is no other way to fetch hostname on windows, but via winapi.
+	// On Plan 9 it can be taken from #c/sysname as Hostname() does.
+	switch runtime.GOOS {
+	case "android", "plan9":
+		// No /bin/hostname to verify against.
+		return
+	case "windows":
+		testWindowsHostname(t, hostname)
+		return
+	}
+
+	testenv.MustHaveExec(t)
+
+	// Check internal Hostname() against the output of /bin/hostname.
+	// Allow that the internal Hostname returns a Fully Qualified Domain Name
+	// and the /bin/hostname only returns the first component
+	want := runBinHostname(t)
+	if hostname != want {
+		host, _, ok := strings.Cut(hostname, ".")
+		if !ok || host != want {
+			t.Errorf("Hostname() = %q, want %q", hostname, want)
+		}
+	}
+}
+
+func TestReadAt(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestReadAt", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	b := make([]byte, 5)
+	n, err := f.ReadAt(b, 7)
+	if err != nil || n != len(b) {
+		t.Fatalf("ReadAt 7: %d, %v", n, err)
+	}
+	if string(b) != "world" {
+		t.Fatalf("ReadAt 7: have %q want %q", string(b), "world")
+	}
+}
+
+// Verify that ReadAt doesn't affect seek offset.
+// In the Plan 9 kernel, there used to be a bug in the implementation of
+// the pread syscall, where the channel offset was erroneously updated after
+// calling pread on a file.
+func TestReadAtOffset(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestReadAtOffset", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	f.Seek(0, 0)
+	b := make([]byte, 5)
+
+	n, err := f.ReadAt(b, 7)
+	if err != nil || n != len(b) {
+		t.Fatalf("ReadAt 7: %d, %v", n, err)
+	}
+	if string(b) != "world" {
+		t.Fatalf("ReadAt 7: have %q want %q", string(b), "world")
+	}
+
+	n, err = f.Read(b)
+	if err != nil || n != len(b) {
+		t.Fatalf("Read: %d, %v", n, err)
+	}
+	if string(b) != "hello" {
+		t.Fatalf("Read: have %q want %q", string(b), "hello")
+	}
+}
+
+// Verify that ReadAt doesn't allow negative offset.
+func TestReadAtNegativeOffset(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestReadAtNegativeOffset", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	f.Seek(0, 0)
+	b := make([]byte, 5)
+
+	n, err := f.ReadAt(b, -10)
+
+	const wantsub = "negative offset"
+	if !strings.Contains(fmt.Sprint(err), wantsub) || n != 0 {
+		t.Errorf("ReadAt(-10) = %v, %v; want 0, ...%q...", n, err, wantsub)
+	}
+}
+
+func TestWriteAt(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestWriteAt", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	n, err := f.WriteAt([]byte("WORLD"), 7)
+	if err != nil || n != 5 {
+		t.Fatalf("WriteAt 7: %d, %v", n, err)
+	}
+
+	b, err := ReadFile(f.Name())
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", f.Name(), err)
+	}
+	if string(b) != "hello, WORLD\n" {
+		t.Fatalf("after write: have %q want %q", string(b), "hello, WORLD\n")
+	}
+}
+
+// Verify that WriteAt doesn't allow negative offset.
+func TestWriteAtNegativeOffset(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestWriteAtNegativeOffset", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	n, err := f.WriteAt([]byte("WORLD"), -10)
+
+	const wantsub = "negative offset"
+	if !strings.Contains(fmt.Sprint(err), wantsub) || n != 0 {
+		t.Errorf("WriteAt(-10) = %v, %v; want 0, ...%q...", n, err, wantsub)
+	}
+}
+
+// Verify that WriteAt doesn't work in append mode.
+func TestWriteAtInAppendMode(t *testing.T) {
+	defer chtmpdir(t)()
+	f, err := OpenFile("write_at_in_append_mode.txt", O_APPEND|O_CREATE, 0666)
+	if err != nil {
+		t.Fatalf("OpenFile: %v", err)
+	}
+	defer f.Close()
+
+	_, err = f.WriteAt([]byte(""), 1)
+	if err != ErrWriteAtInAppendMode {
+		t.Fatalf("f.WriteAt returned %v, expected %v", err, ErrWriteAtInAppendMode)
+	}
+}
+
+func writeFile(t *testing.T, fname string, flag int, text string) string {
+	f, err := OpenFile(fname, flag, 0666)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	n, err := io.WriteString(f, text)
+	if err != nil {
+		t.Fatalf("WriteString: %d, %v", n, err)
+	}
+	f.Close()
+	data, err := ReadFile(fname)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	return string(data)
+}
+
+func TestAppend(t *testing.T) {
+	defer chtmpdir(t)()
+	const f = "append.txt"
+	s := writeFile(t, f, O_CREATE|O_TRUNC|O_RDWR, "new")
+	if s != "new" {
+		t.Fatalf("writeFile: have %q want %q", s, "new")
+	}
+	s = writeFile(t, f, O_APPEND|O_RDWR, "|append")
+	if s != "new|append" {
+		t.Fatalf("writeFile: have %q want %q", s, "new|append")
+	}
+	s = writeFile(t, f, O_CREATE|O_APPEND|O_RDWR, "|append")
+	if s != "new|append|append" {
+		t.Fatalf("writeFile: have %q want %q", s, "new|append|append")
+	}
+	err := Remove(f)
+	if err != nil {
+		t.Fatalf("Remove: %v", err)
+	}
+	s = writeFile(t, f, O_CREATE|O_APPEND|O_RDWR, "new&append")
+	if s != "new&append" {
+		t.Fatalf("writeFile: after append have %q want %q", s, "new&append")
+	}
+	s = writeFile(t, f, O_CREATE|O_RDWR, "old")
+	if s != "old&append" {
+		t.Fatalf("writeFile: after create have %q want %q", s, "old&append")
+	}
+	s = writeFile(t, f, O_CREATE|O_TRUNC|O_RDWR, "new")
+	if s != "new" {
+		t.Fatalf("writeFile: after truncate have %q want %q", s, "new")
+	}
+}
+
+func TestStatDirWithTrailingSlash(t *testing.T) {
+	t.Parallel()
+
+	// Create new temporary directory and arrange to clean it up.
+	path := t.TempDir()
+
+	// Stat of path should succeed.
+	if _, err := Stat(path); err != nil {
+		t.Fatalf("stat %s failed: %s", path, err)
+	}
+
+	// Stat of path+"/" should succeed too.
+	path += "/"
+	if _, err := Stat(path); err != nil {
+		t.Fatalf("stat %s failed: %s", path, err)
+	}
+}
+
+func TestNilProcessStateString(t *testing.T) {
+	var ps *ProcessState
+	s := ps.String()
+	if s != "<nil>" {
+		t.Errorf("(*ProcessState)(nil).String() = %q, want %q", s, "<nil>")
+	}
+}
+
+func TestSameFile(t *testing.T) {
+	defer chtmpdir(t)()
+	fa, err := Create("a")
+	if err != nil {
+		t.Fatalf("Create(a): %v", err)
+	}
+	fa.Close()
+	fb, err := Create("b")
+	if err != nil {
+		t.Fatalf("Create(b): %v", err)
+	}
+	fb.Close()
+
+	ia1, err := Stat("a")
+	if err != nil {
+		t.Fatalf("Stat(a): %v", err)
+	}
+	ia2, err := Stat("a")
+	if err != nil {
+		t.Fatalf("Stat(a): %v", err)
+	}
+	if !SameFile(ia1, ia2) {
+		t.Errorf("files should be same")
+	}
+
+	ib, err := Stat("b")
+	if err != nil {
+		t.Fatalf("Stat(b): %v", err)
+	}
+	if SameFile(ia1, ib) {
+		t.Errorf("files should be different")
+	}
+}
+
+func testDevNullFileInfo(t *testing.T, statname, devNullName string, fi FileInfo) {
+	pre := fmt.Sprintf("%s(%q): ", statname, devNullName)
+	if fi.Size() != 0 {
+		t.Errorf(pre+"wrong file size have %d want 0", fi.Size())
+	}
+	if fi.Mode()&ModeDevice == 0 {
+		t.Errorf(pre+"wrong file mode %q: ModeDevice is not set", fi.Mode())
+	}
+	if fi.Mode()&ModeCharDevice == 0 {
+		t.Errorf(pre+"wrong file mode %q: ModeCharDevice is not set", fi.Mode())
+	}
+	if fi.Mode().IsRegular() {
+		t.Errorf(pre+"wrong file mode %q: IsRegular returns true", fi.Mode())
+	}
+}
+
+func testDevNullFile(t *testing.T, devNullName string) {
+	f, err := Open(devNullName)
+	if err != nil {
+		t.Fatalf("Open(%s): %v", devNullName, err)
+	}
+	defer f.Close()
+
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatalf("Stat(%s): %v", devNullName, err)
+	}
+	testDevNullFileInfo(t, "f.Stat", devNullName, fi)
+
+	fi, err = Stat(devNullName)
+	if err != nil {
+		t.Fatalf("Stat(%s): %v", devNullName, err)
+	}
+	testDevNullFileInfo(t, "Stat", devNullName, fi)
+}
+
+func TestDevNullFile(t *testing.T) {
+	t.Parallel()
+
+	testDevNullFile(t, DevNull)
+	if runtime.GOOS == "windows" {
+		testDevNullFile(t, "./nul")
+		testDevNullFile(t, "//./nul")
+	}
+}
+
+var testLargeWrite = flag.Bool("large_write", false, "run TestLargeWriteToConsole test that floods console with output")
+
+func TestLargeWriteToConsole(t *testing.T) {
+	if !*testLargeWrite {
+		t.Skip("skipping console-flooding test; enable with -large_write")
+	}
+	b := make([]byte, 32000)
+	for i := range b {
+		b[i] = '.'
+	}
+	b[len(b)-1] = '\n'
+	n, err := Stdout.Write(b)
+	if err != nil {
+		t.Fatalf("Write to os.Stdout failed: %v", err)
+	}
+	if n != len(b) {
+		t.Errorf("Write to os.Stdout should return %d; got %d", len(b), n)
+	}
+	n, err = Stderr.Write(b)
+	if err != nil {
+		t.Fatalf("Write to os.Stderr failed: %v", err)
+	}
+	if n != len(b) {
+		t.Errorf("Write to os.Stderr should return %d; got %d", len(b), n)
+	}
+}
+
+func TestStatDirModeExec(t *testing.T) {
+	if runtime.GOOS == "wasip1" {
+		t.Skip("Chmod is not supported on " + runtime.GOOS)
+	}
+	t.Parallel()
+
+	const mode = 0111
+
+	path := t.TempDir()
+	if err := Chmod(path, 0777); err != nil {
+		t.Fatalf("Chmod %q 0777: %v", path, err)
+	}
+
+	dir, err := Stat(path)
+	if err != nil {
+		t.Fatalf("Stat %q (looking for mode %#o): %s", path, mode, err)
+	}
+	if dir.Mode()&mode != mode {
+		t.Errorf("Stat %q: mode %#o want %#o", path, dir.Mode()&mode, mode)
+	}
+}
+
+func TestStatStdin(t *testing.T) {
+	switch runtime.GOOS {
+	case "android", "plan9":
+		t.Skipf("%s doesn't have /bin/sh", runtime.GOOS)
+	}
+
+	if Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		st, err := Stdin.Stat()
+		if err != nil {
+			t.Fatalf("Stat failed: %v", err)
+		}
+		fmt.Println(st.Mode() & ModeNamedPipe)
+		Exit(0)
+	}
+
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	fi, err := Stdin.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	switch mode := fi.Mode(); {
+	case mode&ModeCharDevice != 0 && mode&ModeDevice != 0:
+	case mode&ModeNamedPipe != 0:
+	default:
+		t.Fatalf("unexpected Stdin mode (%v), want ModeCharDevice or ModeNamedPipe", mode)
+	}
+
+	var cmd *exec.Cmd
+	if runtime.GOOS == "windows" {
+		cmd = testenv.Command(t, "cmd", "/c", "echo output | "+Args[0]+" -test.run=TestStatStdin")
+	} else {
+		cmd = testenv.Command(t, "/bin/sh", "-c", "echo output | "+Args[0]+" -test.run=TestStatStdin")
+	}
+	cmd.Env = append(Environ(), "GO_WANT_HELPER_PROCESS=1")
+
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Failed to spawn child process: %v %q", err, string(output))
+	}
+
+	// result will be like "prw-rw-rw"
+	if len(output) < 1 || output[0] != 'p' {
+		t.Fatalf("Child process reports stdin is not pipe '%v'", string(output))
+	}
+}
+
+func TestStatRelativeSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+	t.Parallel()
+
+	tmpdir := t.TempDir()
+	target := filepath.Join(tmpdir, "target")
+	f, err := Create(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	st, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	link := filepath.Join(tmpdir, "link")
+	err = Symlink(filepath.Base(target), link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	st1, err := Stat(link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !SameFile(st, st1) {
+		t.Error("Stat doesn't follow relative symlink")
+	}
+
+	if runtime.GOOS == "windows" {
+		Remove(link)
+		err = Symlink(target[len(filepath.VolumeName(target)):], link)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		st1, err := Stat(link)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if !SameFile(st, st1) {
+			t.Error("Stat doesn't follow relative symlink")
+		}
+	}
+}
+
+func TestReadAtEOF(t *testing.T) {
+	t.Parallel()
+
+	f := newFile("TestReadAtEOF", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	_, err := f.ReadAt(make([]byte, 10), 0)
+	switch err {
+	case io.EOF:
+		// all good
+	case nil:
+		t.Fatalf("ReadAt succeeded")
+	default:
+		t.Fatalf("ReadAt failed: %s", err)
+	}
+}
+
+func TestLongPath(t *testing.T) {
+	t.Parallel()
+
+	tmpdir := newDir("TestLongPath", t)
+	defer func(d string) {
+		if err := RemoveAll(d); err != nil {
+			t.Fatalf("RemoveAll failed: %v", err)
+		}
+	}(tmpdir)
+
+	// Test the boundary of 247 and fewer bytes (normal) and 248 and more bytes (adjusted).
+	sizes := []int{247, 248, 249, 400}
+	for len(tmpdir) < 400 {
+		tmpdir += "/dir3456789"
+	}
+	for _, sz := range sizes {
+		t.Run(fmt.Sprintf("length=%d", sz), func(t *testing.T) {
+			sizedTempDir := tmpdir[:sz-1] + "x" // Ensure it does not end with a slash.
+
+			// The various sized runs are for this call to trigger the boundary
+			// condition.
+			if err := MkdirAll(sizedTempDir, 0755); err != nil {
+				t.Fatalf("MkdirAll failed: %v", err)
+			}
+			data := []byte("hello world\n")
+			if err := WriteFile(sizedTempDir+"/foo.txt", data, 0644); err != nil {
+				t.Fatalf("os.WriteFile() failed: %v", err)
+			}
+			if err := Rename(sizedTempDir+"/foo.txt", sizedTempDir+"/bar.txt"); err != nil {
+				t.Fatalf("Rename failed: %v", err)
+			}
+			mtime := time.Now().Truncate(time.Minute)
+			if err := Chtimes(sizedTempDir+"/bar.txt", mtime, mtime); err != nil {
+				t.Fatalf("Chtimes failed: %v", err)
+			}
+			names := []string{"bar.txt"}
+			if testenv.HasSymlink() {
+				if err := Symlink(sizedTempDir+"/bar.txt", sizedTempDir+"/symlink.txt"); err != nil {
+					t.Fatalf("Symlink failed: %v", err)
+				}
+				names = append(names, "symlink.txt")
+			}
+			if testenv.HasLink() {
+				if err := Link(sizedTempDir+"/bar.txt", sizedTempDir+"/link.txt"); err != nil {
+					t.Fatalf("Link failed: %v", err)
+				}
+				names = append(names, "link.txt")
+			}
+			for _, wantSize := range []int64{int64(len(data)), 0} {
+				for _, name := range names {
+					path := sizedTempDir + "/" + name
+					dir, err := Stat(path)
+					if err != nil {
+						t.Fatalf("Stat(%q) failed: %v", path, err)
+					}
+					filesize := size(path, t)
+					if dir.Size() != filesize || filesize != wantSize {
+						t.Errorf("Size(%q) is %d, len(ReadFile()) is %d, want %d", path, dir.Size(), filesize, wantSize)
+					}
+					if runtime.GOOS != "wasip1" { // Chmod is not supported on wasip1
+						err = Chmod(path, dir.Mode())
+						if err != nil {
+							t.Fatalf("Chmod(%q) failed: %v", path, err)
+						}
+					}
+				}
+				if err := Truncate(sizedTempDir+"/bar.txt", 0); err != nil {
+					t.Fatalf("Truncate failed: %v", err)
+				}
+			}
+		})
+	}
+}
+
+func testKillProcess(t *testing.T, processKiller func(p *Process)) {
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	// Re-exec the test binary to start a process that hangs until stdin is closed.
+	cmd := testenv.Command(t, Args[0])
+	cmd.Env = append(cmd.Environ(), "GO_OS_TEST_DRAIN_STDIN=1")
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = cmd.Start()
+	if err != nil {
+		t.Fatalf("Failed to start test process: %v", err)
+	}
+
+	defer func() {
+		if err := cmd.Wait(); err == nil {
+			t.Errorf("Test process succeeded, but expected to fail")
+		}
+		stdin.Close() // Keep stdin alive until the process has finished dying.
+	}()
+
+	// Wait for the process to be started.
+	// (It will close its stdout when it reaches TestMain.)
+	io.Copy(io.Discard, stdout)
+
+	processKiller(cmd.Process)
+}
+
+func TestKillStartProcess(t *testing.T) {
+	testKillProcess(t, func(p *Process) {
+		err := p.Kill()
+		if err != nil {
+			t.Fatalf("Failed to kill test process: %v", err)
+		}
+	})
+}
+
+func TestGetppid(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		// TODO: golang.org/issue/8206
+		t.Skipf("skipping test on plan9; see issue 8206")
+	}
+
+	if Getenv("GO_WANT_HELPER_PROCESS") == "1" {
+		fmt.Print(Getppid())
+		Exit(0)
+	}
+
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	cmd := testenv.Command(t, Args[0], "-test.run=TestGetppid")
+	cmd.Env = append(Environ(), "GO_WANT_HELPER_PROCESS=1")
+
+	// verify that Getppid() from the forked process reports our process id
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Failed to spawn child process: %v %q", err, string(output))
+	}
+
+	childPpid := string(output)
+	ourPid := fmt.Sprintf("%d", Getpid())
+	if childPpid != ourPid {
+		t.Fatalf("Child process reports parent process id '%v', expected '%v'", childPpid, ourPid)
+	}
+}
+
+func TestKillFindProcess(t *testing.T) {
+	testKillProcess(t, func(p *Process) {
+		p2, err := FindProcess(p.Pid)
+		if err != nil {
+			t.Fatalf("Failed to find test process: %v", err)
+		}
+		err = p2.Kill()
+		if err != nil {
+			t.Fatalf("Failed to kill test process: %v", err)
+		}
+	})
+}
+
+var nilFileMethodTests = []struct {
+	name string
+	f    func(*File) error
+}{
+	{"Chdir", func(f *File) error { return f.Chdir() }},
+	{"Close", func(f *File) error { return f.Close() }},
+	{"Chmod", func(f *File) error { return f.Chmod(0) }},
+	{"Chown", func(f *File) error { return f.Chown(0, 0) }},
+	{"Read", func(f *File) error { _, err := f.Read(make([]byte, 0)); return err }},
+	{"ReadAt", func(f *File) error { _, err := f.ReadAt(make([]byte, 0), 0); return err }},
+	{"Readdir", func(f *File) error { _, err := f.Readdir(1); return err }},
+	{"Readdirnames", func(f *File) error { _, err := f.Readdirnames(1); return err }},
+	{"Seek", func(f *File) error { _, err := f.Seek(0, io.SeekStart); return err }},
+	{"Stat", func(f *File) error { _, err := f.Stat(); return err }},
+	{"Sync", func(f *File) error { return f.Sync() }},
+	{"Truncate", func(f *File) error { return f.Truncate(0) }},
+	{"Write", func(f *File) error { _, err := f.Write(make([]byte, 0)); return err }},
+	{"WriteAt", func(f *File) error { _, err := f.WriteAt(make([]byte, 0), 0); return err }},
+	{"WriteString", func(f *File) error { _, err := f.WriteString(""); return err }},
+}
+
+// Test that all File methods give ErrInvalid if the receiver is nil.
+func TestNilFileMethods(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range nilFileMethodTests {
+		var file *File
+		got := tt.f(file)
+		if got != ErrInvalid {
+			t.Errorf("%v should fail when f is nil; got %v", tt.name, got)
+		}
+	}
+}
+
+func mkdirTree(t *testing.T, root string, level, max int) {
+	if level >= max {
+		return
+	}
+	level++
+	for i := 'a'; i < 'c'; i++ {
+		dir := filepath.Join(root, string(i))
+		if err := Mkdir(dir, 0700); err != nil {
+			t.Fatal(err)
+		}
+		mkdirTree(t, dir, level, max)
+	}
+}
+
+// Test that simultaneous RemoveAll do not report an error.
+// As long as it gets removed, we should be happy.
+func TestRemoveAllRace(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		// Windows has very strict rules about things like
+		// removing directories while someone else has
+		// them open. The racing doesn't work out nicely
+		// like it does on Unix.
+		t.Skip("skipping on windows")
+	}
+	if runtime.GOOS == "dragonfly" {
+		testenv.SkipFlaky(t, 52301)
+	}
+
+	n := runtime.GOMAXPROCS(16)
+	defer runtime.GOMAXPROCS(n)
+	root, err := MkdirTemp("", "issue")
+	if err != nil {
+		t.Fatal(err)
+	}
+	mkdirTree(t, root, 1, 6)
+	hold := make(chan struct{})
+	var wg sync.WaitGroup
+	for i := 0; i < 4; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			<-hold
+			err := RemoveAll(root)
+			if err != nil {
+				t.Errorf("unexpected error: %T, %q", err, err)
+			}
+		}()
+	}
+	close(hold) // let workers race to remove root
+	wg.Wait()
+}
+
+// Test that reading from a pipe doesn't use up a thread.
+func TestPipeThreads(t *testing.T) {
+	switch runtime.GOOS {
+	case "illumos", "solaris":
+		t.Skip("skipping on Solaris and illumos; issue 19111")
+	case "windows":
+		t.Skip("skipping on Windows; issue 19098")
+	case "plan9":
+		t.Skip("skipping on Plan 9; does not support runtime poller")
+	case "js":
+		t.Skip("skipping on js; no support for os.Pipe")
+	case "wasip1":
+		t.Skip("skipping on wasip1; no support for os.Pipe")
+	}
+
+	threads := 100
+
+	// OpenBSD has a low default for max number of files.
+	if runtime.GOOS == "openbsd" {
+		threads = 50
+	}
+
+	r := make([]*File, threads)
+	w := make([]*File, threads)
+	for i := 0; i < threads; i++ {
+		rp, wp, err := Pipe()
+		if err != nil {
+			for j := 0; j < i; j++ {
+				r[j].Close()
+				w[j].Close()
+			}
+			t.Fatal(err)
+		}
+		r[i] = rp
+		w[i] = wp
+	}
+
+	defer debug.SetMaxThreads(debug.SetMaxThreads(threads / 2))
+
+	creading := make(chan bool, threads)
+	cdone := make(chan bool, threads)
+	for i := 0; i < threads; i++ {
+		go func(i int) {
+			var b [1]byte
+			creading <- true
+			if _, err := r[i].Read(b[:]); err != nil {
+				t.Error(err)
+			}
+			if err := r[i].Close(); err != nil {
+				t.Error(err)
+			}
+			cdone <- true
+		}(i)
+	}
+
+	for i := 0; i < threads; i++ {
+		<-creading
+	}
+
+	// If we are still alive, it means that the 100 goroutines did
+	// not require 100 threads.
+
+	for i := 0; i < threads; i++ {
+		if _, err := w[i].Write([]byte{0}); err != nil {
+			t.Error(err)
+		}
+		if err := w[i].Close(); err != nil {
+			t.Error(err)
+		}
+		<-cdone
+	}
+}
+
+func testDoubleCloseError(path string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		file, err := Open(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := file.Close(); err != nil {
+			t.Fatalf("unexpected error from Close: %v", err)
+		}
+		if err := file.Close(); err == nil {
+			t.Error("second Close did not fail")
+		} else if pe, ok := err.(*PathError); !ok {
+			t.Errorf("second Close: got %T, want %T", err, pe)
+		} else if pe.Err != ErrClosed {
+			t.Errorf("second Close: got %q, want %q", pe.Err, ErrClosed)
+		} else {
+			t.Logf("second close returned expected error %q", err)
+		}
+	}
+}
+
+func TestDoubleCloseError(t *testing.T) {
+	t.Parallel()
+	t.Run("file", testDoubleCloseError(filepath.Join(sfdir, sfname)))
+	t.Run("dir", testDoubleCloseError(sfdir))
+}
+
+func TestUserHomeDir(t *testing.T) {
+	t.Parallel()
+
+	dir, err := UserHomeDir()
+	if dir == "" && err == nil {
+		t.Fatal("UserHomeDir returned an empty string but no error")
+	}
+	if err != nil {
+		// UserHomeDir may return a non-nil error if the environment variable
+		// for the home directory is empty or unset in the environment.
+		t.Skipf("skipping: %v", err)
+	}
+
+	fi, err := Stat(dir)
+	if err != nil {
+		if IsNotExist(err) {
+			// The user's home directory has a well-defined location, but does not
+			// exist. (Maybe nothing has written to it yet? That could happen, for
+			// example, on minimal VM images used for CI testing.)
+			t.Log(err)
+			return
+		}
+		t.Fatal(err)
+	}
+	if !fi.IsDir() {
+		t.Fatalf("dir %s is not directory; type = %v", dir, fi.Mode())
+	}
+}
+
+func TestDirSeek(t *testing.T) {
+	t.Parallel()
+
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	f, err := Open(wd)
+	if err != nil {
+		t.Fatal(err)
+	}
+	dirnames1, err := f.Readdirnames(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ret, err := f.Seek(0, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ret != 0 {
+		t.Fatalf("seek result not zero: %d", ret)
+	}
+
+	dirnames2, err := f.Readdirnames(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(dirnames1) != len(dirnames2) {
+		t.Fatalf("listings have different lengths: %d and %d\n", len(dirnames1), len(dirnames2))
+	}
+	for i, n1 := range dirnames1 {
+		n2 := dirnames2[i]
+		if n1 != n2 {
+			t.Fatalf("different name i=%d n1=%s n2=%s\n", i, n1, n2)
+		}
+	}
+}
+
+func TestReaddirSmallSeek(t *testing.T) {
+	// See issue 37161. Read only one entry from a directory,
+	// seek to the beginning, and read again. We should not see
+	// duplicate entries.
+	t.Parallel()
+
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	df, err := Open(filepath.Join(wd, "testdata", "issue37161"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	names1, err := df.Readdirnames(1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err = df.Seek(0, 0); err != nil {
+		t.Fatal(err)
+	}
+	names2, err := df.Readdirnames(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(names2) != 3 {
+		t.Fatalf("first names: %v, second names: %v", names1, names2)
+	}
+}
+
+// isDeadlineExceeded reports whether err is or wraps ErrDeadlineExceeded.
+// We also check that the error has a Timeout method that returns true.
+func isDeadlineExceeded(err error) bool {
+	if !IsTimeout(err) {
+		return false
+	}
+	if !errors.Is(err, ErrDeadlineExceeded) {
+		return false
+	}
+	return true
+}
+
+// Test that opening a file does not change its permissions.  Issue 38225.
+func TestOpenFileKeepsPermissions(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	name := filepath.Join(dir, "x")
+	f, err := Create(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Error(err)
+	}
+	f, err = OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi, err := f.Stat(); err != nil {
+		t.Error(err)
+	} else if fi.Mode()&0222 == 0 {
+		t.Errorf("f.Stat.Mode after OpenFile is %v, should be writable", fi.Mode())
+	}
+	if err := f.Close(); err != nil {
+		t.Error(err)
+	}
+	if fi, err := Stat(name); err != nil {
+		t.Error(err)
+	} else if fi.Mode()&0222 == 0 {
+		t.Errorf("Stat after OpenFile is %v, should be writable", fi.Mode())
+	}
+}
+
+func TestDirFS(t *testing.T) {
+	t.Parallel()
+
+	// On Windows, we force the MFT to update by reading the actual metadata from GetFileInformationByHandle and then
+	// explicitly setting that. Otherwise it might get out of sync with FindFirstFile. See golang.org/issues/42637.
+	if runtime.GOOS == "windows" {
+		if err := filepath.WalkDir("./testdata/dirfs", func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				t.Fatal(err)
+			}
+			info, err := d.Info()
+			if err != nil {
+				t.Fatal(err)
+			}
+			stat, err := Stat(path) // This uses GetFileInformationByHandle internally.
+			if err != nil {
+				t.Fatal(err)
+			}
+			if stat.ModTime() == info.ModTime() {
+				return nil
+			}
+			if err := Chtimes(path, stat.ModTime(), stat.ModTime()); err != nil {
+				t.Log(err) // We only log, not die, in case the test directory is not writable.
+			}
+			return nil
+		}); err != nil {
+			t.Fatal(err)
+		}
+	}
+	fsys := DirFS("./testdata/dirfs")
+	if err := fstest.TestFS(fsys, "a", "b", "dir/x"); err != nil {
+		t.Fatal(err)
+	}
+
+	rdfs, ok := fsys.(fs.ReadDirFS)
+	if !ok {
+		t.Error("expected DirFS result to implement fs.ReadDirFS")
+	}
+	if _, err := rdfs.ReadDir("nonexistent"); err == nil {
+		t.Error("fs.ReadDir of nonexistent directory succeeded")
+	}
+
+	// Test that the error message does not contain a backslash,
+	// and does not contain the DirFS argument.
+	const nonesuch = "dir/nonesuch"
+	_, err := fsys.Open(nonesuch)
+	if err == nil {
+		t.Error("fs.Open of nonexistent file succeeded")
+	} else {
+		if !strings.Contains(err.Error(), nonesuch) {
+			t.Errorf("error %q does not contain %q", err, nonesuch)
+		}
+		if strings.Contains(err.(*PathError).Path, "testdata") {
+			t.Errorf("error %q contains %q", err, "testdata")
+		}
+	}
+
+	// Test that Open does not accept backslash as separator.
+	d := DirFS(".")
+	_, err = d.Open(`testdata\dirfs`)
+	if err == nil {
+		t.Fatalf(`Open testdata\dirfs succeeded`)
+	}
+
+	// Test that Open does not open Windows device files.
+	_, err = d.Open(`NUL`)
+	if err == nil {
+		t.Errorf(`Open NUL succeeded`)
+	}
+}
+
+func TestDirFSRootDir(t *testing.T) {
+	t.Parallel()
+
+	cwd, err := Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cwd = cwd[len(filepath.VolumeName(cwd)):] // trim volume prefix (C:) on Windows
+	cwd = filepath.ToSlash(cwd)               // convert \ to /
+	cwd = strings.TrimPrefix(cwd, "/")        // trim leading /
+
+	// Test that Open can open a path starting at /.
+	d := DirFS("/")
+	f, err := d.Open(cwd + "/testdata/dirfs/a")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+}
+
+func TestDirFSEmptyDir(t *testing.T) {
+	t.Parallel()
+
+	d := DirFS("")
+	cwd, _ := Getwd()
+	for _, path := range []string{
+		"testdata/dirfs/a",                          // not DirFS(".")
+		filepath.ToSlash(cwd) + "/testdata/dirfs/a", // not DirFS("/")
+	} {
+		_, err := d.Open(path)
+		if err == nil {
+			t.Fatalf(`DirFS("").Open(%q) succeeded`, path)
+		}
+	}
+}
+
+func TestDirFSPathsValid(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("skipping on Windows")
+	}
+	t.Parallel()
+
+	d := t.TempDir()
+	if err := WriteFile(filepath.Join(d, "control.txt"), []byte(string("Hello, world!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := WriteFile(filepath.Join(d, `e:xperi\ment.txt`), []byte(string("Hello, colon and backslash!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	fsys := DirFS(d)
+	err := fs.WalkDir(fsys, ".", func(path string, e fs.DirEntry, err error) error {
+		if fs.ValidPath(e.Name()) {
+			t.Logf("%q ok", e.Name())
+		} else {
+			t.Errorf("%q INVALID", e.Name())
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestReadFileProc(t *testing.T) {
+	t.Parallel()
+
+	// Linux files in /proc report 0 size,
+	// but then if ReadFile reads just a single byte at offset 0,
+	// the read at offset 1 returns EOF instead of more data.
+	// ReadFile has a minimum read size of 512 to work around this,
+	// but test explicitly that it's working.
+	name := "/proc/sys/fs/pipe-max-size"
+	if _, err := Stat(name); err != nil {
+		t.Skip(err)
+	}
+	data, err := ReadFile(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) == 0 || data[len(data)-1] != '\n' {
+		t.Fatalf("read %s: not newline-terminated: %q", name, data)
+	}
+}
+
+func TestDirFSReadFileProc(t *testing.T) {
+	t.Parallel()
+
+	fsys := DirFS("/")
+	name := "proc/sys/fs/pipe-max-size"
+	if _, err := fs.Stat(fsys, name); err != nil {
+		t.Skip()
+	}
+	data, err := fs.ReadFile(fsys, name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) == 0 || data[len(data)-1] != '\n' {
+		t.Fatalf("read %s: not newline-terminated: %q", name, data)
+	}
+}
+
+func TestWriteStringAlloc(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skip("js allocates a lot during File.WriteString")
+	}
+	d := t.TempDir()
+	f, err := Create(filepath.Join(d, "whiteboard.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	allocs := testing.AllocsPerRun(100, func() {
+		f.WriteString("I will not allocate when passed a string longer than 32 bytes.\n")
+	})
+	if allocs != 0 {
+		t.Errorf("expected 0 allocs for File.WriteString, got %v", allocs)
+	}
+}
+
+// Test that it's OK to have parallel I/O and Close on a pipe.
+func TestPipeIOCloseRace(t *testing.T) {
+	// Skip on wasm, which doesn't have pipes.
+	if runtime.GOOS == "js" || runtime.GOOS == "wasip1" {
+		t.Skipf("skipping on %s: no pipes", runtime.GOOS)
+	}
+	t.Parallel()
+
+	r, w, err := Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(3)
+
+	go func() {
+		defer wg.Done()
+		for {
+			n, err := w.Write([]byte("hi"))
+			if err != nil {
+				// We look at error strings as the
+				// expected errors are OS-specific.
+				switch {
+				case errors.Is(err, ErrClosed),
+					strings.Contains(err.Error(), "broken pipe"),
+					strings.Contains(err.Error(), "pipe is being closed"),
+					strings.Contains(err.Error(), "hungup channel"):
+					// Ignore an expected error.
+				default:
+					// Unexpected error.
+					t.Error(err)
+				}
+				return
+			}
+			if n != 2 {
+				t.Errorf("wrote %d bytes, expected 2", n)
+				return
+			}
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+		for {
+			var buf [2]byte
+			n, err := r.Read(buf[:])
+			if err != nil {
+				if err != io.EOF && !errors.Is(err, ErrClosed) {
+					t.Error(err)
+				}
+				return
+			}
+			if n != 2 {
+				t.Errorf("read %d bytes, want 2", n)
+			}
+		}
+	}()
+
+	go func() {
+		defer wg.Done()
+
+		// Let the other goroutines start. This is just to get
+		// a better test, the test will still pass if they
+		// don't start.
+		time.Sleep(time.Millisecond)
+
+		if err := r.Close(); err != nil {
+			t.Error(err)
+		}
+		if err := w.Close(); err != nil {
+			t.Error(err)
+		}
+	}()
+
+	wg.Wait()
+}
+
+// Test that it's OK to call Close concurrently on a pipe.
+func TestPipeCloseRace(t *testing.T) {
+	// Skip on wasm, which doesn't have pipes.
+	if runtime.GOOS == "js" || runtime.GOOS == "wasip1" {
+		t.Skipf("skipping on %s: no pipes", runtime.GOOS)
+	}
+	t.Parallel()
+
+	r, w, err := Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	var wg sync.WaitGroup
+	c := make(chan error, 4)
+	f := func() {
+		defer wg.Done()
+		c <- r.Close()
+		c <- w.Close()
+	}
+	wg.Add(2)
+	go f()
+	go f()
+	nils, errs := 0, 0
+	for i := 0; i < 4; i++ {
+		err := <-c
+		if err == nil {
+			nils++
+		} else {
+			errs++
+		}
+	}
+	if nils != 2 || errs != 2 {
+		t.Errorf("got nils %d errs %d, want 2 2", nils, errs)
+	}
+}