os: add ReadFile, WriteFile, CreateTemp (was TempFile), MkdirTemp (was TempDir) from io/ioutil

io/ioutil was a poorly defined collection of helpers.
Proposal #40025 moved out the generic I/O helpers to io.
This CL for proposal #42026 moves the OS-specific helpers to os,
making the entire io/ioutil package deprecated.

For #42026.

Change-Id: I018bcb2115ef2ff1bc7ca36a9247eda429af21ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/266364
Trust: Russ Cox <rsc@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

10 files changed, 644 insertions, 7 deletions

diff --git a/src/os/dir.go b/src/os/dir.go
index 1d90b970e7..5306bcb3ba 100644
--- a/src/os/dir.go
+++ b/src/os/dir.go
@@ -4,7 +4,10 @@
 
 package os
 
-import "io/fs"
+import (
+	"io/fs"
+	"sort"
+)
 
 type readdirMode int
 
@@ -103,3 +106,20 @@ func (f *File) ReadDir(n int) ([]DirEntry, error) {
 // testingForceReadDirLstat forces ReadDir to call Lstat, for testing that code path.
 // This can be difficult to provoke on some Unix systems otherwise.
 var testingForceReadDirLstat bool
+
+// ReadDir reads the named directory,
+// returning all its directory entries sorted by filename.
+// If an error occurs reading the directory,
+// ReadDir returns the entries it was able to read before the error,
+// along with the error.
+func ReadDir(name string) ([]DirEntry, error) {
+	f, err := Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	dirs, err := f.ReadDir(-1)
+	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
+	return dirs, err
+}
diff --git a/src/os/example_test.go b/src/os/example_test.go
index fbb277b6f1..3adce51784 100644
--- a/src/os/example_test.go
+++ b/src/os/example_test.go
@@ -9,6 +9,7 @@ import (
 	"io/fs"
 	"log"
 	"os"
+	"path/filepath"
 	"time"
 )
 
@@ -144,3 +145,98 @@ func ExampleUnsetenv() {
 	os.Setenv("TMPDIR", "/my/tmp")
 	defer os.Unsetenv("TMPDIR")
 }
+
+func ExampleReadDir() {
+	files, err := os.ReadDir(".")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, file := range files {
+		fmt.Println(file.Name())
+	}
+}
+
+func ExampleMkdirTemp() {
+	dir, err := os.MkdirTemp("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(dir) // clean up
+
+	file := filepath.Join(dir, "tmpfile")
+	if err := os.WriteFile(file, []byte("content"), 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleMkdirTemp_suffix() {
+	logsDir, err := os.MkdirTemp("", "*-logs")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(logsDir) // clean up
+
+	// Logs can be cleaned out earlier if needed by searching
+	// for all directories whose suffix ends in *-logs.
+	globPattern := filepath.Join(os.TempDir(), "*-logs")
+	matches, err := filepath.Glob(globPattern)
+	if err != nil {
+		log.Fatalf("Failed to match %q: %v", globPattern, err)
+	}
+
+	for _, match := range matches {
+		if err := os.RemoveAll(match); err != nil {
+			log.Printf("Failed to remove %q: %v", match, err)
+		}
+	}
+}
+
+func ExampleCreateTemp() {
+	f, err := os.CreateTemp("", "example")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.Remove(f.Name()) // clean up
+
+	if _, err := f.Write([]byte("content")); err != nil {
+		log.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleCreateTemp_suffix() {
+	f, err := os.CreateTemp("", "example.*.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.Remove(f.Name()) // clean up
+
+	if _, err := f.Write([]byte("content")); err != nil {
+		f.Close()
+		log.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExampleReadFile() {
+	data, err := os.ReadFile("testdata/hello")
+	if err != nil {
+		log.Fatal(err)
+	}
+	os.Stdout.Write(data)
+
+	// Output:
+	// Hello, Gophers!
+}
+
+func ExampleWriteFile() {
+	err := os.WriteFile("testdata/hello", []byte("Hello, Gophers!"), 0666)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/src/os/export_test.go b/src/os/export_test.go
index d66264a68f..f3cb1a2bef 100644
--- a/src/os/export_test.go
+++ b/src/os/export_test.go
@@ -10,3 +10,4 @@ var Atime = atime
 var LstatP = &lstat
 var ErrWriteAtInAppendMode = errWriteAtInAppendMode
 var TestingForceReadDirLstat = &testingForceReadDirLstat
+var ErrPatternHasSeparator = errPatternHasSeparator
diff --git a/src/os/file.go b/src/os/file.go
index 420e62ef2c..304b055dbe 100644
--- a/src/os/file.go
+++ b/src/os/file.go
@@ -625,3 +625,63 @@ func (dir dirFS) Open(name string) (fs.File, error) {
 	}
 	return f, nil
 }
+
+// ReadFile reads the named file 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.
+func ReadFile(name string) ([]byte, error) {
+	f, err := Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var size int
+	if info, err := f.Stat(); err == nil {
+		size64 := info.Size()
+		if int64(int(size64)) == size64 {
+			size = int(size64)
+		}
+	}
+	size++ // one byte for final read at EOF
+
+	// If a file claims a small size, read at least 512 bytes.
+	// In particular, files in Linux's /proc claim size 0 but
+	// then do not work right if read in small pieces,
+	// so an initial read of 1 byte would not work correctly.
+	if size < 512 {
+		size = 512
+	}
+
+	data := make([]byte, 0, size)
+	for {
+		if len(data) >= cap(data) {
+			d := append(data[:cap(data)], 0)
+			data = d[:len(data)]
+		}
+		n, err := f.Read(data[len(data):cap(data)])
+		data = data[:len(data)+n]
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+			return data, err
+		}
+	}
+}
+
+// WriteFile writes data to the named file, creating it if necessary.
+// 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(name string, data []byte, perm FileMode) error {
+	f, err := OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	_, err = f.Write(data)
+	if err1 := f.Close(); err1 != nil && err == nil {
+		err = err1
+	}
+	return err
+}
diff --git a/src/os/os_test.go b/src/os/os_test.go
index a1c0578887..c5e5cbbb1b 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -419,19 +419,19 @@ func testReadDir(dir string, contents []string, t *testing.T) {
 	}
 }
 
-func TestReaddirnames(t *testing.T) {
+func TestFileReaddirnames(t *testing.T) {
 	testReaddirnames(".", dot, t)
 	testReaddirnames(sysdir.name, sysdir.files, t)
 	testReaddirnames(t.TempDir(), nil, t)
 }
 
-func TestReaddir(t *testing.T) {
+func TestFileReaddir(t *testing.T) {
 	testReaddir(".", dot, t)
 	testReaddir(sysdir.name, sysdir.files, t)
 	testReaddir(t.TempDir(), nil, t)
 }
 
-func TestReadDir(t *testing.T) {
+func TestFileReadDir(t *testing.T) {
 	testReadDir(".", dot, t)
 	testReadDir(sysdir.name, sysdir.files, t)
 	testReadDir(t.TempDir(), nil, t)
@@ -1235,6 +1235,7 @@ func TestChmod(t *testing.T) {
 }
 
 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)
@@ -2690,3 +2691,22 @@ func TestDirFS(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestReadFileProc(t *testing.T) {
+	// 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)
+	}
+}
diff --git a/src/os/read_test.go b/src/os/read_test.go
new file mode 100644
index 0000000000..5c58d7d7df
--- /dev/null
+++ b/src/os/read_test.go
@@ -0,0 +1,127 @@
+// 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 (
+	"bytes"
+	. "os"
+	"path/filepath"
+	"testing"
+)
+
+func checkNamedSize(t *testing.T, path string, size int64) {
+	dir, err := Stat(path)
+	if err != nil {
+		t.Fatalf("Stat %q (looking for size %d): %s", path, size, err)
+	}
+	if dir.Size() != size {
+		t.Errorf("Stat %q: size %d want %d", path, dir.Size(), size)
+	}
+}
+
+func TestReadFile(t *testing.T) {
+	filename := "rumpelstilzchen"
+	contents, err := ReadFile(filename)
+	if err == nil {
+		t.Fatalf("ReadFile %s: error expected, none found", filename)
+	}
+
+	filename = "read_test.go"
+	contents, err = ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+
+	checkNamedSize(t, filename, int64(len(contents)))
+}
+
+func TestWriteFile(t *testing.T) {
+	f, err := CreateTemp("", "ioutil-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	defer Remove(f.Name())
+
+	msg := "Programming today is a race between software engineers striving to " +
+		"build bigger and better idiot-proof programs, and the Universe trying " +
+		"to produce bigger and better idiots. So far, the Universe is winning."
+
+	if err := WriteFile(f.Name(), []byte(msg), 0644); err != nil {
+		t.Fatalf("WriteFile %s: %v", f.Name(), err)
+	}
+
+	data, err := ReadFile(f.Name())
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", f.Name(), err)
+	}
+
+	if string(data) != msg {
+		t.Fatalf("ReadFile: wrong data:\nhave %q\nwant %q", string(data), msg)
+	}
+}
+
+func TestReadOnlyWriteFile(t *testing.T) {
+	if Getuid() == 0 {
+		t.Skipf("Root can write to read-only files anyway, so skip the read-only test.")
+	}
+
+	// We don't want to use CreateTemp directly, since that opens a file for us as 0600.
+	tempDir, err := MkdirTemp("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tempDir)
+	filename := filepath.Join(tempDir, "blurp.txt")
+
+	shmorp := []byte("shmorp")
+	florp := []byte("florp")
+	err = WriteFile(filename, shmorp, 0444)
+	if err != nil {
+		t.Fatalf("WriteFile %s: %v", filename, err)
+	}
+	err = WriteFile(filename, florp, 0444)
+	if err == nil {
+		t.Fatalf("Expected an error when writing to read-only file %s", filename)
+	}
+	got, err := ReadFile(filename)
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", filename, err)
+	}
+	if !bytes.Equal(got, shmorp) {
+		t.Fatalf("want %s, got %s", shmorp, got)
+	}
+}
+
+func TestReadDir(t *testing.T) {
+	dirname := "rumpelstilzchen"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error expected, none found", dirname)
+	}
+
+	dirname = "."
+	list, err := ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("ReadDir %s: %v", dirname, err)
+	}
+
+	foundFile := false
+	foundSubDir := false
+	for _, dir := range list {
+		switch {
+		case !dir.IsDir() && dir.Name() == "read_test.go":
+			foundFile = true
+		case dir.IsDir() && dir.Name() == "exec":
+			foundSubDir = true
+		}
+	}
+	if !foundFile {
+		t.Fatalf("ReadDir %s: read_test.go file not found", dirname)
+	}
+	if !foundSubDir {
+		t.Fatalf("ReadDir %s: exec directory not found", dirname)
+	}
+}
diff --git a/src/os/removeall_test.go b/src/os/removeall_test.go
index bc9c468ce3..90efa313ea 100644
--- a/src/os/removeall_test.go
+++ b/src/os/removeall_test.go
@@ -355,11 +355,12 @@ func TestRemoveAllButReadOnlyAndPathError(t *testing.T) {
 	// The error should be of type *PathError.
 	// see issue 30491 for details.
 	if pathErr, ok := err.(*PathError); ok {
-		if g, w := pathErr.Path, filepath.Join(tempDir, "b", "y"); g != w {
-			t.Errorf("got %q, expected pathErr.path %q", g, w)
+		want := filepath.Join(tempDir, "b", "y")
+		if pathErr.Path != want {
+			t.Errorf("RemoveAll(%q): err.Path=%q, want %q", tempDir, pathErr.Path, want)
 		}
 	} else {
-		t.Errorf("got %T, expected *fs.PathError", err)
+		t.Errorf("RemoveAll(%q): error has type %T, want *fs.PathError", tempDir, err)
 	}
 
 	for _, dir := range dirs {
diff --git a/src/os/tempfile.go b/src/os/tempfile.go
new file mode 100644
index 0000000000..2728485c32
--- /dev/null
+++ b/src/os/tempfile.go
@@ -0,0 +1,118 @@
+// 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 os
+
+import (
+	"errors"
+	"strings"
+)
+
+// fastrand provided by runtime.
+// We generate random temporary file names so that there's a good
+// chance the file doesn't exist yet - keeps the number of tries in
+// TempFile to a minimum.
+func fastrand() uint32
+
+func nextRandom() string {
+	return uitoa(uint(fastrand()))
+}
+
+// CreateTemp creates a new temporary file in the directory dir,
+// opens the file for reading and writing, and returns the resulting file.
+// The filename is generated by taking pattern and adding a random string to the end.
+// If pattern includes a "*", the random string replaces the last "*".
+// If dir is the empty string, TempFile uses the default directory for temporary files, as returned by TempDir.
+// Multiple programs or goroutines calling CreateTemp simultaneously will not choose the same file.
+// The caller can use the file's Name method to find the pathname of the file.
+// It is the caller's responsibility to remove the file when it is no longer needed.
+func CreateTemp(dir, pattern string) (*File, error) {
+	if dir == "" {
+		dir = TempDir()
+	}
+
+	prefix, suffix, err := prefixAndSuffix(pattern)
+	if err != nil {
+		return nil, &PathError{Op: "createtemp", Path: pattern, Err: err}
+	}
+	prefix = joinPath(dir, prefix)
+
+	try := 0
+	for {
+		name := prefix + nextRandom() + suffix
+		f, err := OpenFile(name, O_RDWR|O_CREATE|O_EXCL, 0600)
+		if IsExist(err) {
+			if try++; try < 10000 {
+				continue
+			}
+			return nil, &PathError{Op: "createtemp", Path: dir + string(PathSeparator) + prefix + "*" + suffix, Err: ErrExist}
+		}
+		return f, err
+	}
+}
+
+var errPatternHasSeparator = errors.New("pattern contains path separator")
+
+// prefixAndSuffix splits pattern by the last wildcard "*", if applicable,
+// returning prefix as the part before "*" and suffix as the part after "*".
+func prefixAndSuffix(pattern string) (prefix, suffix string, err error) {
+	for i := 0; i < len(pattern); i++ {
+		if IsPathSeparator(pattern[i]) {
+			return "", "", errPatternHasSeparator
+		}
+	}
+	if pos := strings.LastIndex(pattern, "*"); pos != -1 {
+		prefix, suffix = pattern[:pos], pattern[pos+1:]
+	} else {
+		prefix = pattern
+	}
+	return prefix, suffix, nil
+}
+
+// MkdirTemp creates a new temporary directory in the directory dir
+// and returns the pathname of the new directory.
+// The new directory's name is generated by adding a random string to the end of pattern.
+// If pattern includes a "*", the random string replaces the last "*" instead.
+// If dir is the empty string, TempFile uses the default directory for temporary files, as returned by TempDir.
+// Multiple programs or goroutines calling MkdirTemp simultaneously will not choose the same directory.
+// It is the caller's responsibility to remove the directory when it is no longer needed.
+func MkdirTemp(dir, pattern string) (string, error) {
+	if dir == "" {
+		dir = TempDir()
+	}
+
+	prefix, suffix, err := prefixAndSuffix(pattern)
+	if err != nil {
+		return "", &PathError{Op: "mkdirtemp", Path: pattern, Err: err}
+	}
+	prefix = joinPath(dir, prefix)
+
+	try := 0
+	for {
+		name := prefix + nextRandom() + suffix
+		err := Mkdir(name, 0700)
+		if err == nil {
+			return name, nil
+		}
+		if IsExist(err) {
+			if try++; try < 10000 {
+				continue
+			}
+			return "", &PathError{Op: "mkdirtemp", Path: dir + string(PathSeparator) + prefix + "*" + suffix, Err: ErrExist}
+		}
+		if IsNotExist(err) {
+			if _, err := Stat(dir); IsNotExist(err) {
+				return "", err
+			}
+		}
+		return "", err
+	}
+}
+
+func joinPath(dir, name string) string {
+	if len(dir) > 0 && IsPathSeparator(dir[len(dir)-1]) {
+		return dir + name
+	}
+	return dir + string(PathSeparator) + name
+}
diff --git a/src/os/tempfile_test.go b/src/os/tempfile_test.go
new file mode 100644
index 0000000000..e71a2444c9
--- /dev/null
+++ b/src/os/tempfile_test.go
@@ -0,0 +1,193 @@
+// 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 os_test
+
+import (
+	"errors"
+	"io/fs"
+	. "os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestCreateTemp(t *testing.T) {
+	dir, err := MkdirTemp("", "TestCreateTempBadDir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(dir)
+
+	nonexistentDir := filepath.Join(dir, "_not_exists_")
+	f, err := CreateTemp(nonexistentDir, "foo")
+	if f != nil || err == nil {
+		t.Errorf("CreateTemp(%q, `foo`) = %v, %v", nonexistentDir, f, err)
+	}
+}
+
+func TestCreateTempPattern(t *testing.T) {
+	tests := []struct{ pattern, prefix, suffix string }{
+		{"tempfile_test", "tempfile_test", ""},
+		{"tempfile_test*", "tempfile_test", ""},
+		{"tempfile_test*xyz", "tempfile_test", "xyz"},
+	}
+	for _, test := range tests {
+		f, err := CreateTemp("", test.pattern)
+		if err != nil {
+			t.Errorf("CreateTemp(..., %q) error: %v", test.pattern, err)
+			continue
+		}
+		defer Remove(f.Name())
+		base := filepath.Base(f.Name())
+		f.Close()
+		if !(strings.HasPrefix(base, test.prefix) && strings.HasSuffix(base, test.suffix)) {
+			t.Errorf("CreateTemp pattern %q created bad name %q; want prefix %q & suffix %q",
+				test.pattern, base, test.prefix, test.suffix)
+		}
+	}
+}
+
+func TestCreateTempBadPattern(t *testing.T) {
+	tmpDir, err := MkdirTemp("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tmpDir)
+
+	const sep = string(PathSeparator)
+	tests := []struct {
+		pattern string
+		wantErr bool
+	}{
+		{"ioutil*test", false},
+		{"tempfile_test*foo", false},
+		{"tempfile_test" + sep + "foo", true},
+		{"tempfile_test*" + sep + "foo", true},
+		{"tempfile_test" + sep + "*foo", true},
+		{sep + "tempfile_test" + sep + "*foo", true},
+		{"tempfile_test*foo" + sep, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			tmpfile, err := CreateTemp(tmpDir, tt.pattern)
+			if tmpfile != nil {
+				defer tmpfile.Close()
+			}
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("CreateTemp(..., %#q) succeeded, expected error", tt.pattern)
+				}
+				if !errors.Is(err, ErrPatternHasSeparator) {
+					t.Errorf("CreateTemp(..., %#q): %v, expected ErrPatternHasSeparator", tt.pattern, err)
+				}
+			} else if err != nil {
+				t.Errorf("CreateTemp(..., %#q): %v", tt.pattern, err)
+			}
+		})
+	}
+}
+
+func TestMkdirTemp(t *testing.T) {
+	name, err := MkdirTemp("/_not_exists_", "foo")
+	if name != "" || err == nil {
+		t.Errorf("MkdirTemp(`/_not_exists_`, `foo`) = %v, %v", name, err)
+	}
+
+	tests := []struct {
+		pattern                string
+		wantPrefix, wantSuffix string
+	}{
+		{"tempfile_test", "tempfile_test", ""},
+		{"tempfile_test*", "tempfile_test", ""},
+		{"tempfile_test*xyz", "tempfile_test", "xyz"},
+	}
+
+	dir := filepath.Clean(TempDir())
+
+	runTestMkdirTemp := func(t *testing.T, pattern, wantRePat string) {
+		name, err := MkdirTemp(dir, pattern)
+		if name == "" || err != nil {
+			t.Fatalf("MkdirTemp(dir, `tempfile_test`) = %v, %v", name, err)
+		}
+		defer Remove(name)
+
+		re := regexp.MustCompile(wantRePat)
+		if !re.MatchString(name) {
+			t.Errorf("MkdirTemp(%q, %q) created bad name\n\t%q\ndid not match pattern\n\t%q", dir, pattern, name, wantRePat)
+		}
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			wantRePat := "^" + regexp.QuoteMeta(filepath.Join(dir, tt.wantPrefix)) + "[0-9]+" + regexp.QuoteMeta(tt.wantSuffix) + "$"
+			runTestMkdirTemp(t, tt.pattern, wantRePat)
+		})
+	}
+
+	// Separately testing "*xyz" (which has no prefix). That is when constructing the
+	// pattern to assert on, as in the previous loop, using filepath.Join for an empty
+	// prefix filepath.Join(dir, ""), produces the pattern:
+	//     ^<DIR>[0-9]+xyz$
+	// yet we just want to match
+	//     "^<DIR>/[0-9]+xyz"
+	t.Run("*xyz", func(t *testing.T) {
+		wantRePat := "^" + regexp.QuoteMeta(filepath.Join(dir)) + regexp.QuoteMeta(string(filepath.Separator)) + "[0-9]+xyz$"
+		runTestMkdirTemp(t, "*xyz", wantRePat)
+	})
+}
+
+// test that we return a nice error message if the dir argument to TempDir doesn't
+// exist (or that it's empty and TempDir doesn't exist)
+func TestMkdirTempBadDir(t *testing.T) {
+	dir, err := MkdirTemp("", "MkdirTempBadDir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(dir)
+
+	badDir := filepath.Join(dir, "not-exist")
+	_, err = MkdirTemp(badDir, "foo")
+	if pe, ok := err.(*fs.PathError); !ok || !IsNotExist(err) || pe.Path != badDir {
+		t.Errorf("TempDir error = %#v; want PathError for path %q satisifying IsNotExist", err, badDir)
+	}
+}
+
+func TestMkdirTempBadPattern(t *testing.T) {
+	tmpDir, err := MkdirTemp("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(tmpDir)
+
+	const sep = string(PathSeparator)
+	tests := []struct {
+		pattern string
+		wantErr bool
+	}{
+		{"ioutil*test", false},
+		{"tempfile_test*foo", false},
+		{"tempfile_test" + sep + "foo", true},
+		{"tempfile_test*" + sep + "foo", true},
+		{"tempfile_test" + sep + "*foo", true},
+		{sep + "tempfile_test" + sep + "*foo", true},
+		{"tempfile_test*foo" + sep, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			_, err := MkdirTemp(tmpDir, tt.pattern)
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("MkdirTemp(..., %#q) succeeded, expected error", tt.pattern)
+				}
+				if !errors.Is(err, ErrPatternHasSeparator) {
+					t.Errorf("MkdirTemp(..., %#q): %v, expected ErrPatternHasSeparator", tt.pattern, err)
+				}
+			} else if err != nil {
+				t.Errorf("MkdirTemp(..., %#q): %v", tt.pattern, err)
+			}
+		})
+	}
+}
diff --git a/src/os/testdata/hello b/src/os/testdata/hello
new file mode 100644
index 0000000000..e47c092a51
--- /dev/null
+++ b/src/os/testdata/hello
@@ -0,0 +1 @@
+Hello, Gophers!