src/internal/poll/fd_wasip1.go - go - Git at Google

 // Copyright 2023 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package poll

 import (
 	"sync/atomic"
 	"syscall"
 	"unsafe"
 )

 type SysFile struct {
 	// RefCountPtr is a pointer to the reference count of Sysfd.
 	//
 	// WASI preview 1 lacks a dup(2) system call. When the os and net packages
 	// need to share a file/socket, instead of duplicating the underlying file
 	// descriptor, we instead provide a way to copy FD instances and manage the
 	// underlying file descriptor with reference counting.
 	RefCountPtr *int32

 	// RefCount is the reference count of Sysfd. When a copy of an FD is made,
 	// it points to the reference count of the original FD instance.
 	RefCount int32

 	// Cache for the file type, lazily initialized when Seek is called.
 	Filetype uint32

 	// If the file represents a directory, this field contains the current
 	// readdir position. It is reset to zero if the program calls Seek(0, 0).
 	Dircookie uint64

 	// Absolute path of the file, as returned by syscall.PathOpen;
 	// this is used by Fchdir to emulate setting the current directory
 	// to an open file descriptor.
 	Path string

 	// TODO(achille): it could be meaningful to move isFile from FD to a method
 	// on this struct type, and expose it as `IsFile() bool` which derives the
 	// result from the Filetype field. We would need to ensure that Filetype is
 	// always set instead of being lazily initialized.
 }

 func (s *SysFile) init() {
 	if s.RefCountPtr == nil {
 		s.RefCount = 1
 		s.RefCountPtr = &s.RefCount
 	}
 }

 func (s *SysFile) ref() SysFile {
 	atomic.AddInt32(s.RefCountPtr, +1)
 	return SysFile{RefCountPtr: s.RefCountPtr}
 }

 func (s *SysFile) destroy(fd int) error {
 	if s.RefCountPtr != nil && atomic.AddInt32(s.RefCountPtr, -1) > 0 {
 		return nil
 	}

 	// We don't use ignoringEINTR here because POSIX does not define
 	// whether the descriptor is closed if close returns EINTR.
 	// If the descriptor is indeed closed, using a loop would race
 	// with some other goroutine opening a new descriptor.
 	// (The Linux kernel guarantees that it is closed on an EINTR error.)
 	return CloseFunc(fd)
 }

 // Copy creates a copy of the FD.
 //
 // The FD instance points to the same underlying file descriptor. The file
 // descriptor isn't closed until all FD instances that refer to it have been
 // closed/destroyed.
 func (fd *FD) Copy() FD {
 	return FD{
 		Sysfd:         fd.Sysfd,
 		SysFile:       fd.SysFile.ref(),
 		IsStream:      fd.IsStream,
 		ZeroReadIsEOF: fd.ZeroReadIsEOF,
 		isBlocking:    fd.isBlocking,
 		isFile:        fd.isFile,
 	}
 }

 // dupCloseOnExecOld always errors on wasip1 because there is no mechanism to
 // duplicate file descriptors.
 func dupCloseOnExecOld(fd int) (int, string, error) {
 	return -1, "dup", syscall.ENOSYS
 }

 // Fchdir wraps syscall.Fchdir.
 func (fd *FD) Fchdir() error {
 	if err := fd.incref(); err != nil {
 		return err
 	}
 	defer fd.decref()
 	return syscall.Chdir(fd.Path)
 }

 // ReadDir wraps syscall.ReadDir.
 // We treat this like an ordinary system call rather than a call
 // that tries to fill the buffer.
 func (fd *FD) ReadDir(buf []byte, cookie syscall.Dircookie) (int, error) {
 	if err := fd.incref(); err != nil {
 		return 0, err
 	}
 	defer fd.decref()
 	for {
 		n, err := syscall.ReadDir(fd.Sysfd, buf, cookie)
 		if err != nil {
 			n = 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		// Do not call eofError; caller does not expect to see io.EOF.
 		return n, err
 	}
 }

 func (fd *FD) ReadDirent(buf []byte) (int, error) {
 	n, err := fd.ReadDir(buf, fd.Dircookie)
 	if err != nil {
 		return 0, err
 	}
 	if n <= 0 {
 		return n, nil // EOF
 	}

 	// We assume that the caller of ReadDirent will consume the entire buffer
 	// up to the last full entry, so we scan through the buffer looking for the
 	// value of the last next cookie.
 	b := buf[:n]

 	for len(b) > 0 {
 		next, ok := direntNext(b)
 		if !ok {
 			break
 		}
 		size, ok := direntReclen(b)
 		if !ok {
 			break
 		}
 		if size > uint64(len(b)) {
 			break
 		}
 		fd.Dircookie = syscall.Dircookie(next)
 		b = b[size:]
 	}

 	// Trim a potentially incomplete trailing entry; this is necessary because
 	// the code in src/os/dir_unix.go does not deal well with partial values in
 	// calls to direntReclen, etc... and ends up causing an early EOF before all
 	// directory entries were consumed. ReadDirent is called with a large enough
 	// buffer (8 KiB) that at least one entry should always fit, tho this seems
 	// a bit brittle but cannot be addressed without a large change of the
 	// algorithm in the os.(*File).readdir method.
 	return n - len(b), nil
 }

 // Seek wraps syscall.Seek.
 func (fd *FD) Seek(offset int64, whence int) (int64, error) {
 	if err := fd.incref(); err != nil {
 		return 0, err
 	}
 	defer fd.decref()
 	// syscall.Filetype is a uint8 but we store it as a uint32 in SysFile in
 	// order to use atomic load/store on the field, which is why we have to
 	// perform this type conversion.
 	fileType := syscall.Filetype(atomic.LoadUint32(&fd.Filetype))

 	if fileType == syscall.FILETYPE_UNKNOWN {
 		var stat syscall.Stat_t
 		if err := fd.Fstat(&stat); err != nil {
 			return 0, err
 		}
 		fileType = stat.Filetype
 		atomic.StoreUint32(&fd.Filetype, uint32(fileType))
 	}

 	if fileType == syscall.FILETYPE_DIRECTORY {
 		// If the file descriptor is opened on a directory, we reset the readdir
 		// cookie when seeking back to the beginning to allow reusing the file
 		// descriptor to scan the directory again.
 		if offset == 0 && whence == 0 {
 			fd.Dircookie = 0
 			return 0, nil
 		} else {
 			return 0, syscall.EINVAL
 		}
 	}

 	return syscall.Seek(fd.Sysfd, offset, whence)
 }

 // https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#-dirent-record
 const sizeOfDirent = 24

 func direntReclen(buf []byte) (uint64, bool) {
 	namelen, ok := direntNamlen(buf)
 	return sizeOfDirent + namelen, ok
 }

 func direntNamlen(buf []byte) (uint64, bool) {
 	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Namlen), unsafe.Sizeof(syscall.Dirent{}.Namlen))
 }

 func direntNext(buf []byte) (uint64, bool) {
 	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Next), unsafe.Sizeof(syscall.Dirent{}.Next))
 }

 // readInt returns the size-bytes unsigned integer in native byte order at offset off.
 func readInt(b []byte, off, size uintptr) (u uint64, ok bool) {
 	if len(b) < int(off+size) {
 		return 0, false
 	}
 	return readIntLE(b[off:], size), true
 }

 func readIntLE(b []byte, size uintptr) uint64 {
 	switch size {
 	case 1:
 		return uint64(b[0])
 	case 2:
 		_ = b[1] // bounds check hint to compiler; see golang.org/issue/14808
 		return uint64(b[0]) | uint64(b[1])<<8
 	case 4:
 		_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
 		return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24
 	case 8:
 		_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
 		return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
 			uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
 	default:
 		panic("internal/poll: readInt with unsupported size")
 	}
 }
	// Copyright 2023 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package poll

	import (
	"sync/atomic"
	"syscall"
	"unsafe"
	)

	type SysFile struct {
	// RefCountPtr is a pointer to the reference count of Sysfd.
	//
	// WASI preview 1 lacks a dup(2) system call. When the os and net packages
	// need to share a file/socket, instead of duplicating the underlying file
	// descriptor, we instead provide a way to copy FD instances and manage the
	// underlying file descriptor with reference counting.
	RefCountPtr *int32

	// RefCount is the reference count of Sysfd. When a copy of an FD is made,
	// it points to the reference count of the original FD instance.
	RefCount int32

	// Cache for the file type, lazily initialized when Seek is called.
	Filetype uint32

	// If the file represents a directory, this field contains the current
	// readdir position. It is reset to zero if the program calls Seek(0, 0).
	Dircookie uint64

	// Absolute path of the file, as returned by syscall.PathOpen;
	// this is used by Fchdir to emulate setting the current directory
	// to an open file descriptor.
	Path string

	// TODO(achille): it could be meaningful to move isFile from FD to a method
	// on this struct type, and expose it as `IsFile() bool` which derives the
	// result from the Filetype field. We would need to ensure that Filetype is
	// always set instead of being lazily initialized.
	}

	func (s *SysFile) init() {
	if s.RefCountPtr == nil {
	s.RefCount = 1
	s.RefCountPtr = &s.RefCount
	}
	}

	func (s *SysFile) ref() SysFile {
	atomic.AddInt32(s.RefCountPtr, +1)
	return SysFile{RefCountPtr: s.RefCountPtr}
	}

	func (s *SysFile) destroy(fd int) error {
	if s.RefCountPtr != nil && atomic.AddInt32(s.RefCountPtr, -1) > 0 {
	return nil
	}

	// We don't use ignoringEINTR here because POSIX does not define
	// whether the descriptor is closed if close returns EINTR.
	// If the descriptor is indeed closed, using a loop would race
	// with some other goroutine opening a new descriptor.
	// (The Linux kernel guarantees that it is closed on an EINTR error.)
	return CloseFunc(fd)
	}

	// Copy creates a copy of the FD.
	//
	// The FD instance points to the same underlying file descriptor. The file
	// descriptor isn't closed until all FD instances that refer to it have been
	// closed/destroyed.
	func (fd *FD) Copy() FD {
	return FD{
	Sysfd: fd.Sysfd,
	SysFile: fd.SysFile.ref(),
	IsStream: fd.IsStream,
	ZeroReadIsEOF: fd.ZeroReadIsEOF,
	isBlocking: fd.isBlocking,
	isFile: fd.isFile,
	}
	}

	// dupCloseOnExecOld always errors on wasip1 because there is no mechanism to
	// duplicate file descriptors.
	func dupCloseOnExecOld(fd int) (int, string, error) {
	return -1, "dup", syscall.ENOSYS
	}

	// Fchdir wraps syscall.Fchdir.
	func (fd *FD) Fchdir() error {
	if err := fd.incref(); err != nil {
	return err
	}
	defer fd.decref()
	return syscall.Chdir(fd.Path)
	}

	// ReadDir wraps syscall.ReadDir.
	// We treat this like an ordinary system call rather than a call
	// that tries to fill the buffer.
	func (fd *FD) ReadDir(buf []byte, cookie syscall.Dircookie) (int, error) {
	if err := fd.incref(); err != nil {
	return 0, err
	}
	defer fd.decref()
	for {
	n, err := syscall.ReadDir(fd.Sysfd, buf, cookie)
	if err != nil {
	n = 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	// Do not call eofError; caller does not expect to see io.EOF.
	return n, err
	}
	}

	func (fd *FD) ReadDirent(buf []byte) (int, error) {
	n, err := fd.ReadDir(buf, fd.Dircookie)
	if err != nil {
	return 0, err
	}
	if n <= 0 {
	return n, nil // EOF
	}

	// We assume that the caller of ReadDirent will consume the entire buffer
	// up to the last full entry, so we scan through the buffer looking for the
	// value of the last next cookie.
	b := buf[:n]

	for len(b) > 0 {
	next, ok := direntNext(b)
	if !ok {
	break
	}
	size, ok := direntReclen(b)
	if !ok {
	break
	}
	if size > uint64(len(b)) {
	break
	}
	fd.Dircookie = syscall.Dircookie(next)
	b = b[size:]
	}

	// Trim a potentially incomplete trailing entry; this is necessary because
	// the code in src/os/dir_unix.go does not deal well with partial values in
	// calls to direntReclen, etc... and ends up causing an early EOF before all
	// directory entries were consumed. ReadDirent is called with a large enough
	// buffer (8 KiB) that at least one entry should always fit, tho this seems
	// a bit brittle but cannot be addressed without a large change of the
	// algorithm in the os.(*File).readdir method.
	return n - len(b), nil
	}

	// Seek wraps syscall.Seek.
	func (fd *FD) Seek(offset int64, whence int) (int64, error) {
	if err := fd.incref(); err != nil {
	return 0, err
	}
	defer fd.decref()
	// syscall.Filetype is a uint8 but we store it as a uint32 in SysFile in
	// order to use atomic load/store on the field, which is why we have to
	// perform this type conversion.
	fileType := syscall.Filetype(atomic.LoadUint32(&fd.Filetype))

	if fileType == syscall.FILETYPE_UNKNOWN {
	var stat syscall.Stat_t
	if err := fd.Fstat(&stat); err != nil {
	return 0, err
	}
	fileType = stat.Filetype
	atomic.StoreUint32(&fd.Filetype, uint32(fileType))
	}

	if fileType == syscall.FILETYPE_DIRECTORY {
	// If the file descriptor is opened on a directory, we reset the readdir
	// cookie when seeking back to the beginning to allow reusing the file
	// descriptor to scan the directory again.
	if offset == 0 && whence == 0 {
	fd.Dircookie = 0
	return 0, nil
	} else {
	return 0, syscall.EINVAL
	}
	}

	return syscall.Seek(fd.Sysfd, offset, whence)
	}

	// https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#-dirent-record
	const sizeOfDirent = 24

	func direntReclen(buf []byte) (uint64, bool) {
	namelen, ok := direntNamlen(buf)
	return sizeOfDirent + namelen, ok
	}

	func direntNamlen(buf []byte) (uint64, bool) {
	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Namlen), unsafe.Sizeof(syscall.Dirent{}.Namlen))
	}

	func direntNext(buf []byte) (uint64, bool) {
	return readInt(buf, unsafe.Offsetof(syscall.Dirent{}.Next), unsafe.Sizeof(syscall.Dirent{}.Next))
	}

	// readInt returns the size-bytes unsigned integer in native byte order at offset off.
	func readInt(b []byte, off, size uintptr) (u uint64, ok bool) {
	if len(b) < int(off+size) {
	return 0, false
	}
	return readIntLE(b[off:], size), true
	}

	func readIntLE(b []byte, size uintptr) uint64 {
	switch size {
	case 1:
	return uint64(b[0])
	case 2:
	_ = b[1] // bounds check hint to compiler; see golang.org/issue/14808
	return uint64(b[0]) \| uint64(b[1])<<8
	case 4:
	_ = b[3] // bounds check hint to compiler; see golang.org/issue/14808
	return uint64(b[0]) \| uint64(b[1])<<8 \| uint64(b[2])<<16 \| uint64(b[3])<<24
	case 8:
	_ = b[7] // bounds check hint to compiler; see golang.org/issue/14808
	return uint64(b[0]) \| uint64(b[1])<<8 \| uint64(b[2])<<16 \| uint64(b[3])<<24 \|
	uint64(b[4])<<32 \| uint64(b[5])<<40 \| uint64(b[6])<<48 \| uint64(b[7])<<56
	default:
	panic("internal/poll: readInt with unsupported size")
	}
	}