os: make File.Readdir et al concurrency-safe

Before, all methods of File (including Close) were
safe for concurrent use (I checked), except the three
variants of ReadDir.

This change makes the ReadDir operations
atomic too, and documents explicitly that all methods
of File have this property, which was already implied
by the package documentation.

Fixes #66498

Change-Id: I05c88b4e60b44c702062e99ed8f4a32e7945927a
Reviewed-on: https://go-review.googlesource.com/c/go/+/578322
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/dir_darwin.go

@@ -25,16 +25,24 @@ func (d *dirInfo) close() {
 }
 
 func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
-	if f.dirinfo == nil {
+	// If this file has no dirinfo, create one.
+	var d *dirInfo
+	for {
+		d = f.dirinfo.Load()
+		if d != nil {
+			break
+		}
 		dir, call, errno := f.pfd.OpenDir()
 		if errno != nil {
 			return nil, nil, nil, &PathError{Op: call, Path: f.name, Err: errno}
 		}
-		f.dirinfo = &dirInfo{
-			dir: dir,
+		d = &dirInfo{dir: dir}
+		if f.dirinfo.CompareAndSwap(nil, d) {
+			break
 		}
+		// We lost the race: try again.
+		d.close()
 	}
-	d := f.dirinfo
 
 	size := n
 	if size <= 0 {

src/os/dir_plan9.go

@@ -12,10 +12,14 @@ import (
 
 func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
 	// If this file has no dirinfo, create one.
-	if file.dirinfo == nil {
-		file.dirinfo = new(dirInfo)
+	d := file.dirinfo.Load()
+	if d == nil {
+		d = new(dirInfo)
+		file.dirinfo.Store(d)
 	}
-	d := file.dirinfo
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
 	size := n
 	if size <= 0 {
 		size = 100

src/os/dir_unix.go

@@ -17,6 +17,7 @@ import (
 
 // Auxiliary information if the File describes a directory
 type dirInfo struct {
+	mu   sync.Mutex
 	buf  *[]byte // buffer for directory I/O
 	nbuf int     // length of buf; return value from Getdirentries
 	bufp int     // location of next record in buf.
@@ -43,13 +44,16 @@ func (d *dirInfo) close() {
 }
 
 func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
-	// If this file has no dirinfo, create one.
-	if f.dirinfo == nil {
-		f.dirinfo = new(dirInfo)
+	// If this file has no dirInfo, create one.
+	d := f.dirinfo.Load()
+	if d == nil {
+		d = new(dirInfo)
+		f.dirinfo.Store(d)
 	}
-	d := f.dirinfo
+	d.mu.Lock()
+	defer d.mu.Unlock()
 	if d.buf == nil {
-		f.dirinfo.buf = dirBufPool.Get().(*[]byte)
+		d.buf = dirBufPool.Get().(*[]byte)
 	}
 
 	// Change the meaning of n for the implementation below.

src/os/dir_windows.go

@@ -16,6 +16,7 @@ import (
 
 // Auxiliary information if the File describes a directory
 type dirInfo struct {
+	mu sync.Mutex
 	// buf is a slice pointer so the slice header
 	// does not escape to the heap when returning
 	// buf to dirBufPool.
@@ -93,14 +94,27 @@ func (d *dirInfo) init(h syscall.Handle) {
 }
 
 func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
-	if file.dirinfo == nil {
-		file.dirinfo = new(dirInfo)
-		file.dirinfo.init(file.pfd.Sysfd)
+	// If this file has no dirInfo, create one.
+	var d *dirInfo
+	for {
+		d = file.dirinfo.Load()
+		if d != nil {
+			break
 		}
-	d := file.dirinfo
+		d = new(dirInfo)
+		d.init(file.pfd.Sysfd)
+		if file.dirinfo.CompareAndSwap(nil, d) {
+			break
+		}
+		// We lost the race: try again.
+		d.close()
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
 	if d.buf == nil {
 		d.buf = dirBufPool.Get().(*[]byte)
 	}
+
 	wantAll := n <= 0
 	if wantAll {
 		n = -1

src/os/file.go

@@ -34,9 +34,13 @@
 //	}
 //	fmt.Printf("read %d bytes: %q\n", count, data[:count])
 //
-// Note: The maximum number of concurrent operations on a File may be limited by
-// the OS or the system. The number should be high, but exceeding it may degrade
-// performance or cause other issues.
+// # Concurrency
+//
+// The methods of [File] correspond to file system operations. All are
+// safe for concurrent use. The maximum number of concurrent
+// operations on a File may be limited by the OS or the system. The
+// number should be high, but exceeding it may degrade performance or
+// cause other issues.
 package os
 
 import (
@@ -53,6 +57,8 @@ import (
 )
 
 // Name returns the name of the file as presented to Open.
+//
+// It is safe to call Name after [Close].
 func (f *File) Name() string { return f.name }
 
 // Stdin, Stdout, and Stderr are open Files pointing to the standard input,
@@ -279,7 +285,7 @@ func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 		return 0, err
 	}
 	r, e := f.seek(offset, whence)
-	if e == nil && f.dirinfo != nil && r != 0 {
+	if e == nil && f.dirinfo.Load() != nil && r != 0 {
 		e = syscall.EISDIR
 	}
 	if e != nil {

src/os/file_plan9.go

@@ -9,6 +9,8 @@ import (
 	"internal/poll"
 	"io"
 	"runtime"
+	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 )
@@ -26,7 +28,7 @@ type file struct {
 	fdmu       poll.FDMutex
 	fd         int
 	name       string
-	dirinfo    *dirInfo // nil unless directory being read
+	dirinfo    atomic.Pointer[dirInfo] // nil unless directory being read
 	appendMode bool                    // whether file is opened for appending
 }
 
@@ -60,6 +62,7 @@ func NewFile(fd uintptr, name string) *File {
 
 // Auxiliary information if the File describes a directory
 type dirInfo struct {
+	mu   sync.Mutex
 	buf  [syscall.STATMAX]byte // buffer for directory I/O
 	nbuf int                   // length of buf; return value from Read
 	bufp int                   // location of next record in buf.
@@ -349,11 +352,9 @@ func (f *File) seek(offset int64, whence int) (ret int64, err error) {
 		return 0, err
 	}
 	defer f.decref()
-	if f.dirinfo != nil {
 	// Free cached dirinfo, so we allocate a new one if we
 	// access this file as a directory again. See #35767 and #37161.
-		f.dirinfo = nil
-	}
+	f.dirinfo.Store(nil)
 	return syscall.Seek(f.fd, offset, whence)
 }
 

src/os/file_unix.go

@@ -11,6 +11,7 @@ import (
 	"internal/syscall/unix"
 	"io/fs"
 	"runtime"
+	"sync/atomic"
 	"syscall"
 	_ "unsafe" // for go:linkname
 )
@@ -58,7 +59,7 @@ func rename(oldname, newname string) error {
 type file struct {
 	pfd         poll.FD
 	name        string
-	dirinfo     *dirInfo // nil unless directory being read
+	dirinfo     atomic.Pointer[dirInfo] // nil unless directory being read
 	nonblock    bool                    // whether we set nonblocking mode
 	stdoutOrErr bool                    // whether this is stdout or stderr
 	appendMode  bool                    // whether file is opened for appending
@@ -325,9 +326,8 @@ func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL
 	}
-	if file.dirinfo != nil {
-		file.dirinfo.close()
-		file.dirinfo = nil
+	if info := file.dirinfo.Swap(nil); info != nil {
+		info.close()
 	}
 	var err error
 	if e := file.pfd.Close(); e != nil {
@@ -347,11 +347,10 @@ func (file *file) close() error {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) seek(offset int64, whence int) (ret int64, err error) {
-	if f.dirinfo != nil {
+	if info := f.dirinfo.Swap(nil); info != nil {
 		// Free cached dirinfo, so we allocate a new one if we
 		// access this file as a directory again. See #35767 and #37161.
-		f.dirinfo.close()
-		f.dirinfo = nil
+		info.close()
 	}
 	ret, err = f.pfd.Seek(offset, whence)
 	runtime.KeepAlive(f)

src/os/file_windows.go

@@ -11,6 +11,7 @@ import (
 	"internal/syscall/windows"
 	"runtime"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"unsafe"
 )
@@ -25,7 +26,7 @@ const _UTIME_OMIT = -1
 type file struct {
 	pfd        poll.FD
 	name       string
-	dirinfo    *dirInfo // nil unless directory being read
+	dirinfo    atomic.Pointer[dirInfo] // nil unless directory being read
 	appendMode bool                    // whether file is opened for appending
 }
 
@@ -127,9 +128,8 @@ func (file *file) close() error {
 	if file == nil {
 		return syscall.EINVAL
 	}
-	if file.dirinfo != nil {
-		file.dirinfo.close()
-		file.dirinfo = nil
+	if info := file.dirinfo.Swap(nil); info != nil {
+		info.close()
 	}
 	var err error
 	if e := file.pfd.Close(); e != nil {
@@ -149,11 +149,10 @@ func (file *file) close() error {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) seek(offset int64, whence int) (ret int64, err error) {
-	if f.dirinfo != nil {
+	if info := f.dirinfo.Swap(nil); info != nil {
 		// Free cached dirinfo, so we allocate a new one if we
 		// access this file as a directory again. See #35767 and #37161.
-		f.dirinfo.close()
-		f.dirinfo = nil
+		info.close()
 	}
 	ret, err = f.pfd.Seek(offset, whence)
 	runtime.KeepAlive(f)

src/os/types.go

@@ -13,6 +13,8 @@ import (
 func Getpagesize() int { return syscall.Getpagesize() }
 
 // File represents an open file descriptor.
+//
+// The methods of File are safe for concurrent use.
 type File struct {
 	*file // os specific
 }