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

 // Copyright 2017 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.

 //go:build aix || darwin || dragonfly || freebsd || (js && wasm) || linux || netbsd || openbsd || solaris
 // +build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris

 package poll

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

 // FD is a file descriptor. The net and os packages use this type as a
 // field of a larger type representing a network connection or OS file.
 type FD struct {
 	// Lock sysfd and serialize access to Read and Write methods.
 	fdmu fdMutex

 	// System file descriptor. Immutable until Close.
 	Sysfd int

 	// I/O poller.
 	pd pollDesc

 	// Writev cache.
 	iovecs *[]syscall.Iovec

 	// Semaphore signaled when file is closed.
 	csema uint32

 	// Non-zero if this file has been set to blocking mode.
 	isBlocking uint32

 	// Whether this is a streaming descriptor, as opposed to a
 	// packet-based descriptor like a UDP socket. Immutable.
 	IsStream bool

 	// Whether a zero byte read indicates EOF. This is false for a
 	// message based socket connection.
 	ZeroReadIsEOF bool

 	// Whether this is a file rather than a network socket.
 	isFile bool
 }

 // Init initializes the FD. The Sysfd field should already be set.
 // This can be called multiple times on a single FD.
 // The net argument is a network name from the net package (e.g., "tcp"),
 // or "file".
 // Set pollable to true if fd should be managed by runtime netpoll.
 func (fd *FD) Init(net string, pollable bool) error {
 	// We don't actually care about the various network types.
 	if net == "file" {
 		fd.isFile = true
 	}
 	if !pollable {
 		fd.isBlocking = 1
 		return nil
 	}
 	err := fd.pd.init(fd)
 	if err != nil {
 		// If we could not initialize the runtime poller,
 		// assume we are using blocking mode.
 		fd.isBlocking = 1
 	}
 	return err
 }

 // Destroy closes the file descriptor. This is called when there are
 // no remaining references.
 func (fd *FD) destroy() error {
 	// Poller may want to unregister fd in readiness notification mechanism,
 	// so this must be executed before CloseFunc.
 	fd.pd.close()

 	// 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.)
 	err := CloseFunc(fd.Sysfd)

 	fd.Sysfd = -1
 	runtime_Semrelease(&fd.csema)
 	return err
 }

 // Close closes the FD. The underlying file descriptor is closed by the
 // destroy method when there are no remaining references.
 func (fd *FD) Close() error {
 	if !fd.fdmu.increfAndClose() {
 		return errClosing(fd.isFile)
 	}

 	// Unblock any I/O.  Once it all unblocks and returns,
 	// so that it cannot be referring to fd.sysfd anymore,
 	// the final decref will close fd.sysfd. This should happen
 	// fairly quickly, since all the I/O is non-blocking, and any
 	// attempts to block in the pollDesc will return errClosing(fd.isFile).
 	fd.pd.evict()

 	// The call to decref will call destroy if there are no other
 	// references.
 	err := fd.decref()

 	// Wait until the descriptor is closed. If this was the only
 	// reference, it is already closed. Only wait if the file has
 	// not been set to blocking mode, as otherwise any current I/O
 	// may be blocking, and that would block the Close.
 	// No need for an atomic read of isBlocking, increfAndClose means
 	// we have exclusive access to fd.
 	if fd.isBlocking == 0 {
 		runtime_Semacquire(&fd.csema)
 	}

 	return err
 }

 // SetBlocking puts the file into blocking mode.
 func (fd *FD) SetBlocking() error {
 	if err := fd.incref(); err != nil {
 		return err
 	}
 	defer fd.decref()
 	// Atomic store so that concurrent calls to SetBlocking
 	// do not cause a race condition. isBlocking only ever goes
 	// from 0 to 1 so there is no real race here.
 	atomic.StoreUint32(&fd.isBlocking, 1)
 	return syscall.SetNonblock(fd.Sysfd, false)
 }

 // Darwin and FreeBSD can't read or write 2GB+ files at a time,
 // even on 64-bit systems.
 // The same is true of socket implementations on many systems.
 // See golang.org/issue/7812 and golang.org/issue/16266.
 // Use 1GB instead of, say, 2GB-1, to keep subsequent reads aligned.
 const maxRW = 1 << 30

 // Read implements io.Reader.
 func (fd *FD) Read(p []byte) (int, error) {
 	if err := fd.readLock(); err != nil {
 		return 0, err
 	}
 	defer fd.readUnlock()
 	if len(p) == 0 {
 		// If the caller wanted a zero byte read, return immediately
 		// without trying (but after acquiring the readLock).
 		// Otherwise syscall.Read returns 0, nil which looks like
 		// io.EOF.
 		// TODO(bradfitz): make it wait for readability? (Issue 15735)
 		return 0, nil
 	}
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return 0, err
 	}
 	if fd.IsStream && len(p) > maxRW {
 		p = p[:maxRW]
 	}
 	for {
 		n, err := ignoringEINTRIO(syscall.Read, fd.Sysfd, p)
 		if err != nil {
 			n = 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		err = fd.eofError(n, err)
 		return n, err
 	}
 }

 // Pread wraps the pread system call.
 func (fd *FD) Pread(p []byte, off int64) (int, error) {
 	// Call incref, not readLock, because since pread specifies the
 	// offset it is independent from other reads.
 	// Similarly, using the poller doesn't make sense for pread.
 	if err := fd.incref(); err != nil {
 		return 0, err
 	}
 	if fd.IsStream && len(p) > maxRW {
 		p = p[:maxRW]
 	}
 	var (
 		n   int
 		err error
 	)
 	for {
 		n, err = syscall.Pread(fd.Sysfd, p, off)
 		if err != syscall.EINTR {
 			break
 		}
 	}
 	if err != nil {
 		n = 0
 	}
 	fd.decref()
 	err = fd.eofError(n, err)
 	return n, err
 }

 // ReadFrom wraps the recvfrom network call.
 func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
 	if err := fd.readLock(); err != nil {
 		return 0, nil, err
 	}
 	defer fd.readUnlock()
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return 0, nil, err
 	}
 	for {
 		n, sa, err := syscall.Recvfrom(fd.Sysfd, p, 0)
 		if err != nil {
 			if err == syscall.EINTR {
 				continue
 			}
 			n = 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		err = fd.eofError(n, err)
 		return n, sa, err
 	}
 }

 // ReadFrom wraps the recvfrom network call for IPv4.
 func (fd *FD) ReadFromInet4(p []byte, from *syscall.SockaddrInet4) (int, error) {
 	if err := fd.readLock(); err != nil {
 		return 0, err
 	}
 	defer fd.readUnlock()
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return 0, err
 	}
 	for {
 		n, err := syscall.RecvfromInet4(fd.Sysfd, p, 0, from)
 		if err != nil {
 			if err == syscall.EINTR {
 				continue
 			}
 			n = 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		err = fd.eofError(n, err)
 		return n, err
 	}
 }

 // ReadFrom wraps the recvfrom network call for IPv6.
 func (fd *FD) ReadFromInet6(p []byte, from *syscall.SockaddrInet6) (int, error) {
 	if err := fd.readLock(); err != nil {
 		return 0, err
 	}
 	defer fd.readUnlock()
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return 0, err
 	}
 	for {
 		n, err := syscall.RecvfromInet6(fd.Sysfd, p, 0, from)
 		if err != nil {
 			if err == syscall.EINTR {
 				continue
 			}
 			n = 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		err = fd.eofError(n, err)
 		return n, err
 	}
 }

 // ReadMsg wraps the recvmsg network call.
 func (fd *FD) ReadMsg(p []byte, oob []byte, flags int) (int, int, int, syscall.Sockaddr, error) {
 	if err := fd.readLock(); err != nil {
 		return 0, 0, 0, nil, err
 	}
 	defer fd.readUnlock()
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return 0, 0, 0, nil, err
 	}
 	for {
 		n, oobn, sysflags, sa, err := syscall.Recvmsg(fd.Sysfd, p, oob, flags)
 		if err != nil {
 			if err == syscall.EINTR {
 				continue
 			}
 			// TODO(dfc) should n and oobn be set to 0
 			if err == syscall.EAGAIN && fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		}
 		err = fd.eofError(n, err)
 		return n, oobn, sysflags, sa, err
 	}
 }

 // Write implements io.Writer.
 func (fd *FD) Write(p []byte) (int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return 0, err
 	}
 	var nn int
 	for {
 		max := len(p)
 		if fd.IsStream && max-nn > maxRW {
 			max = nn + maxRW
 		}
 		n, err := ignoringEINTRIO(syscall.Write, fd.Sysfd, p[nn:max])
 		if n > 0 {
 			nn += n
 		}
 		if nn == len(p) {
 			return nn, err
 		}
 		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
 		}
 		if err != nil {
 			return nn, err
 		}
 		if n == 0 {
 			return nn, io.ErrUnexpectedEOF
 		}
 	}
 }

 // Pwrite wraps the pwrite system call.
 func (fd *FD) Pwrite(p []byte, off int64) (int, error) {
 	// Call incref, not writeLock, because since pwrite specifies the
 	// offset it is independent from other writes.
 	// Similarly, using the poller doesn't make sense for pwrite.
 	if err := fd.incref(); err != nil {
 		return 0, err
 	}
 	defer fd.decref()
 	var nn int
 	for {
 		max := len(p)
 		if fd.IsStream && max-nn > maxRW {
 			max = nn + maxRW
 		}
 		n, err := syscall.Pwrite(fd.Sysfd, p[nn:max], off+int64(nn))
 		if err == syscall.EINTR {
 			continue
 		}
 		if n > 0 {
 			nn += n
 		}
 		if nn == len(p) {
 			return nn, err
 		}
 		if err != nil {
 			return nn, err
 		}
 		if n == 0 {
 			return nn, io.ErrUnexpectedEOF
 		}
 	}
 }

 // WriteToInet4 wraps the sendto network call for IPv4 addresses.
 func (fd *FD) WriteToInet4(p []byte, sa syscall.SockaddrInet4) (int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return 0, err
 	}
 	for {
 		err := syscall.SendtoInet4(fd.Sysfd, p, 0, sa)
 		if err == syscall.EINTR {
 			continue
 		}
 		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
 		}
 		if err != nil {
 			return 0, err
 		}
 		return len(p), nil
 	}
 }

 // WriteToInet6 wraps the sendto network call for IPv6 addresses.
 func (fd *FD) WriteToInet6(p []byte, sa syscall.SockaddrInet6) (int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return 0, err
 	}
 	for {
 		err := syscall.SendtoInet6(fd.Sysfd, p, 0, sa)
 		if err == syscall.EINTR {
 			continue
 		}
 		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
 		}
 		if err != nil {
 			return 0, err
 		}
 		return len(p), nil
 	}
 }

 // WriteTo wraps the sendto network call.
 func (fd *FD) WriteTo(p []byte, sa syscall.Sockaddr) (int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return 0, err
 	}
 	for {
 		err := syscall.Sendto(fd.Sysfd, p, 0, sa)
 		if err == syscall.EINTR {
 			continue
 		}
 		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
 		}
 		if err != nil {
 			return 0, err
 		}
 		return len(p), nil
 	}
 }

 // WriteMsg wraps the sendmsg network call.
 func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, 0, err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return 0, 0, err
 	}
 	for {
 		n, err := syscall.SendmsgN(fd.Sysfd, p, oob, sa, 0)
 		if err == syscall.EINTR {
 			continue
 		}
 		if err == syscall.EAGAIN && fd.pd.pollable() {
 			if err = fd.pd.waitWrite(fd.isFile); err == nil {
 				continue
 			}
 		}
 		if err != nil {
 			return n, 0, err
 		}
 		return n, len(oob), err
 	}
 }

 // Accept wraps the accept network call.
 func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
 	if err := fd.readLock(); err != nil {
 		return -1, nil, "", err
 	}
 	defer fd.readUnlock()

 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return -1, nil, "", err
 	}
 	for {
 		s, rsa, errcall, err := accept(fd.Sysfd)
 		if err == nil {
 			return s, rsa, "", err
 		}
 		switch err {
 		case syscall.EINTR:
 			continue
 		case syscall.EAGAIN:
 			if fd.pd.pollable() {
 				if err = fd.pd.waitRead(fd.isFile); err == nil {
 					continue
 				}
 			}
 		case syscall.ECONNABORTED:
 			// This means that a socket on the listen
 			// queue was closed before we Accept()ed it;
 			// it's a silly error, so try again.
 			continue
 		}
 		return -1, nil, errcall, err
 	}
 }

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

 // ReadDirent wraps syscall.ReadDirent.
 // We treat this like an ordinary system call rather than a call
 // that tries to fill the buffer.
 func (fd *FD) ReadDirent(buf []byte) (int, error) {
 	if err := fd.incref(); err != nil {
 		return 0, err
 	}
 	defer fd.decref()
 	for {
 		n, err := ignoringEINTRIO(syscall.ReadDirent, fd.Sysfd, buf)
 		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
 	}
 }

 // Fchmod wraps syscall.Fchmod.
 func (fd *FD) Fchmod(mode uint32) error {
 	if err := fd.incref(); err != nil {
 		return err
 	}
 	defer fd.decref()
 	return ignoringEINTR(func() error {
 		return syscall.Fchmod(fd.Sysfd, mode)
 	})
 }

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

 // Fstat wraps syscall.Fstat
 func (fd *FD) Fstat(s *syscall.Stat_t) error {
 	if err := fd.incref(); err != nil {
 		return err
 	}
 	defer fd.decref()
 	return ignoringEINTR(func() error {
 		return syscall.Fstat(fd.Sysfd, s)
 	})
 }

 // tryDupCloexec indicates whether F_DUPFD_CLOEXEC should be used.
 // If the kernel doesn't support it, this is set to 0.
 var tryDupCloexec = int32(1)

 // DupCloseOnExec dups fd and marks it close-on-exec.
 func DupCloseOnExec(fd int) (int, string, error) {
 	if syscall.F_DUPFD_CLOEXEC != 0 && atomic.LoadInt32(&tryDupCloexec) == 1 {
 		r0, e1 := fcntl(fd, syscall.F_DUPFD_CLOEXEC, 0)
 		if e1 == nil {
 			return r0, "", nil
 		}
 		switch e1.(syscall.Errno) {
 		case syscall.EINVAL, syscall.ENOSYS:
 			// Old kernel, or js/wasm (which returns
 			// ENOSYS). Fall back to the portable way from
 			// now on.
 			atomic.StoreInt32(&tryDupCloexec, 0)
 		default:
 			return -1, "fcntl", e1
 		}
 	}
 	return dupCloseOnExecOld(fd)
 }

 // dupCloseOnExecOld is the traditional way to dup an fd and
 // set its O_CLOEXEC bit, using two system calls.
 func dupCloseOnExecOld(fd int) (int, string, error) {
 	syscall.ForkLock.RLock()
 	defer syscall.ForkLock.RUnlock()
 	newfd, err := syscall.Dup(fd)
 	if err != nil {
 		return -1, "dup", err
 	}
 	syscall.CloseOnExec(newfd)
 	return newfd, "", nil
 }

 // Dup duplicates the file descriptor.
 func (fd *FD) Dup() (int, string, error) {
 	if err := fd.incref(); err != nil {
 		return -1, "", err
 	}
 	defer fd.decref()
 	return DupCloseOnExec(fd.Sysfd)
 }

 // On Unix variants only, expose the IO event for the net code.

 // WaitWrite waits until data can be read from fd.
 func (fd *FD) WaitWrite() error {
 	return fd.pd.waitWrite(fd.isFile)
 }

 // WriteOnce is for testing only. It makes a single write call.
 func (fd *FD) WriteOnce(p []byte) (int, error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
 	defer fd.writeUnlock()
 	return ignoringEINTRIO(syscall.Write, fd.Sysfd, p)
 }

 // RawRead invokes the user-defined function f for a read operation.
 func (fd *FD) RawRead(f func(uintptr) bool) error {
 	if err := fd.readLock(); err != nil {
 		return err
 	}
 	defer fd.readUnlock()
 	if err := fd.pd.prepareRead(fd.isFile); err != nil {
 		return err
 	}
 	for {
 		if f(uintptr(fd.Sysfd)) {
 			return nil
 		}
 		if err := fd.pd.waitRead(fd.isFile); err != nil {
 			return err
 		}
 	}
 }

 // RawWrite invokes the user-defined function f for a write operation.
 func (fd *FD) RawWrite(f func(uintptr) bool) error {
 	if err := fd.writeLock(); err != nil {
 		return err
 	}
 	defer fd.writeUnlock()
 	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
 		return err
 	}
 	for {
 		if f(uintptr(fd.Sysfd)) {
 			return nil
 		}
 		if err := fd.pd.waitWrite(fd.isFile); err != nil {
 			return err
 		}
 	}
 }

 // ignoringEINTRIO is like ignoringEINTR, but just for IO calls.
 func ignoringEINTRIO(fn func(fd int, p []byte) (int, error), fd int, p []byte) (int, error) {
 	for {
 		n, err := fn(fd, p)
 		if err != syscall.EINTR {
 			return n, err
 		}
 	}
 }
	// Copyright 2017 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.

	//go:build aix \|\| darwin \|\| dragonfly \|\| freebsd \|\| (js && wasm) \|\| linux \|\| netbsd \|\| openbsd \|\| solaris
	// +build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris

	package poll

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

	// FD is a file descriptor. The net and os packages use this type as a
	// field of a larger type representing a network connection or OS file.
	type FD struct {
	// Lock sysfd and serialize access to Read and Write methods.
	fdmu fdMutex

	// System file descriptor. Immutable until Close.
	Sysfd int

	// I/O poller.
	pd pollDesc

	// Writev cache.
	iovecs *[]syscall.Iovec

	// Semaphore signaled when file is closed.
	csema uint32

	// Non-zero if this file has been set to blocking mode.
	isBlocking uint32

	// Whether this is a streaming descriptor, as opposed to a
	// packet-based descriptor like a UDP socket. Immutable.
	IsStream bool

	// Whether a zero byte read indicates EOF. This is false for a
	// message based socket connection.
	ZeroReadIsEOF bool

	// Whether this is a file rather than a network socket.
	isFile bool
	}

	// Init initializes the FD. The Sysfd field should already be set.
	// This can be called multiple times on a single FD.
	// The net argument is a network name from the net package (e.g., "tcp"),
	// or "file".
	// Set pollable to true if fd should be managed by runtime netpoll.
	func (fd *FD) Init(net string, pollable bool) error {
	// We don't actually care about the various network types.
	if net == "file" {
	fd.isFile = true
	}
	if !pollable {
	fd.isBlocking = 1
	return nil
	}
	err := fd.pd.init(fd)
	if err != nil {
	// If we could not initialize the runtime poller,
	// assume we are using blocking mode.
	fd.isBlocking = 1
	}
	return err
	}

	// Destroy closes the file descriptor. This is called when there are
	// no remaining references.
	func (fd *FD) destroy() error {
	// Poller may want to unregister fd in readiness notification mechanism,
	// so this must be executed before CloseFunc.
	fd.pd.close()

	// 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.)
	err := CloseFunc(fd.Sysfd)

	fd.Sysfd = -1
	runtime_Semrelease(&fd.csema)
	return err
	}

	// Close closes the FD. The underlying file descriptor is closed by the
	// destroy method when there are no remaining references.
	func (fd *FD) Close() error {
	if !fd.fdmu.increfAndClose() {
	return errClosing(fd.isFile)
	}

	// Unblock any I/O. Once it all unblocks and returns,
	// so that it cannot be referring to fd.sysfd anymore,
	// the final decref will close fd.sysfd. This should happen
	// fairly quickly, since all the I/O is non-blocking, and any
	// attempts to block in the pollDesc will return errClosing(fd.isFile).
	fd.pd.evict()

	// The call to decref will call destroy if there are no other
	// references.
	err := fd.decref()

	// Wait until the descriptor is closed. If this was the only
	// reference, it is already closed. Only wait if the file has
	// not been set to blocking mode, as otherwise any current I/O
	// may be blocking, and that would block the Close.
	// No need for an atomic read of isBlocking, increfAndClose means
	// we have exclusive access to fd.
	if fd.isBlocking == 0 {
	runtime_Semacquire(&fd.csema)
	}

	return err
	}

	// SetBlocking puts the file into blocking mode.
	func (fd *FD) SetBlocking() error {
	if err := fd.incref(); err != nil {
	return err
	}
	defer fd.decref()
	// Atomic store so that concurrent calls to SetBlocking
	// do not cause a race condition. isBlocking only ever goes
	// from 0 to 1 so there is no real race here.
	atomic.StoreUint32(&fd.isBlocking, 1)
	return syscall.SetNonblock(fd.Sysfd, false)
	}

	// Darwin and FreeBSD can't read or write 2GB+ files at a time,
	// even on 64-bit systems.
	// The same is true of socket implementations on many systems.
	// See golang.org/issue/7812 and golang.org/issue/16266.
	// Use 1GB instead of, say, 2GB-1, to keep subsequent reads aligned.
	const maxRW = 1 << 30

	// Read implements io.Reader.
	func (fd *FD) Read(p []byte) (int, error) {
	if err := fd.readLock(); err != nil {
	return 0, err
	}
	defer fd.readUnlock()
	if len(p) == 0 {
	// If the caller wanted a zero byte read, return immediately
	// without trying (but after acquiring the readLock).
	// Otherwise syscall.Read returns 0, nil which looks like
	// io.EOF.
	// TODO(bradfitz): make it wait for readability? (Issue 15735)
	return 0, nil
	}
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return 0, err
	}
	if fd.IsStream && len(p) > maxRW {
	p = p[:maxRW]
	}
	for {
	n, err := ignoringEINTRIO(syscall.Read, fd.Sysfd, p)
	if err != nil {
	n = 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	err = fd.eofError(n, err)
	return n, err
	}
	}

	// Pread wraps the pread system call.
	func (fd *FD) Pread(p []byte, off int64) (int, error) {
	// Call incref, not readLock, because since pread specifies the
	// offset it is independent from other reads.
	// Similarly, using the poller doesn't make sense for pread.
	if err := fd.incref(); err != nil {
	return 0, err
	}
	if fd.IsStream && len(p) > maxRW {
	p = p[:maxRW]
	}
	var (
	n int
	err error
	)
	for {
	n, err = syscall.Pread(fd.Sysfd, p, off)
	if err != syscall.EINTR {
	break
	}
	}
	if err != nil {
	n = 0
	}
	fd.decref()
	err = fd.eofError(n, err)
	return n, err
	}

	// ReadFrom wraps the recvfrom network call.
	func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
	if err := fd.readLock(); err != nil {
	return 0, nil, err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return 0, nil, err
	}
	for {
	n, sa, err := syscall.Recvfrom(fd.Sysfd, p, 0)
	if err != nil {
	if err == syscall.EINTR {
	continue
	}
	n = 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	err = fd.eofError(n, err)
	return n, sa, err
	}
	}

	// ReadFrom wraps the recvfrom network call for IPv4.
	func (fd FD) ReadFromInet4(p []byte, from syscall.SockaddrInet4) (int, error) {
	if err := fd.readLock(); err != nil {
	return 0, err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return 0, err
	}
	for {
	n, err := syscall.RecvfromInet4(fd.Sysfd, p, 0, from)
	if err != nil {
	if err == syscall.EINTR {
	continue
	}
	n = 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	err = fd.eofError(n, err)
	return n, err
	}
	}

	// ReadFrom wraps the recvfrom network call for IPv6.
	func (fd FD) ReadFromInet6(p []byte, from syscall.SockaddrInet6) (int, error) {
	if err := fd.readLock(); err != nil {
	return 0, err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return 0, err
	}
	for {
	n, err := syscall.RecvfromInet6(fd.Sysfd, p, 0, from)
	if err != nil {
	if err == syscall.EINTR {
	continue
	}
	n = 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	err = fd.eofError(n, err)
	return n, err
	}
	}

	// ReadMsg wraps the recvmsg network call.
	func (fd *FD) ReadMsg(p []byte, oob []byte, flags int) (int, int, int, syscall.Sockaddr, error) {
	if err := fd.readLock(); err != nil {
	return 0, 0, 0, nil, err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return 0, 0, 0, nil, err
	}
	for {
	n, oobn, sysflags, sa, err := syscall.Recvmsg(fd.Sysfd, p, oob, flags)
	if err != nil {
	if err == syscall.EINTR {
	continue
	}
	// TODO(dfc) should n and oobn be set to 0
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	}
	err = fd.eofError(n, err)
	return n, oobn, sysflags, sa, err
	}
	}

	// Write implements io.Writer.
	func (fd *FD) Write(p []byte) (int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return 0, err
	}
	var nn int
	for {
	max := len(p)
	if fd.IsStream && max-nn > maxRW {
	max = nn + maxRW
	}
	n, err := ignoringEINTRIO(syscall.Write, fd.Sysfd, p[nn:max])
	if n > 0 {
	nn += n
	}
	if nn == len(p) {
	return nn, err
	}
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitWrite(fd.isFile); err == nil {
	continue
	}
	}
	if err != nil {
	return nn, err
	}
	if n == 0 {
	return nn, io.ErrUnexpectedEOF
	}
	}
	}

	// Pwrite wraps the pwrite system call.
	func (fd *FD) Pwrite(p []byte, off int64) (int, error) {
	// Call incref, not writeLock, because since pwrite specifies the
	// offset it is independent from other writes.
	// Similarly, using the poller doesn't make sense for pwrite.
	if err := fd.incref(); err != nil {
	return 0, err
	}
	defer fd.decref()
	var nn int
	for {
	max := len(p)
	if fd.IsStream && max-nn > maxRW {
	max = nn + maxRW
	}
	n, err := syscall.Pwrite(fd.Sysfd, p[nn:max], off+int64(nn))
	if err == syscall.EINTR {
	continue
	}
	if n > 0 {
	nn += n
	}
	if nn == len(p) {
	return nn, err
	}
	if err != nil {
	return nn, err
	}
	if n == 0 {
	return nn, io.ErrUnexpectedEOF
	}
	}
	}

	// WriteToInet4 wraps the sendto network call for IPv4 addresses.
	func (fd *FD) WriteToInet4(p []byte, sa syscall.SockaddrInet4) (int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return 0, err
	}
	for {
	err := syscall.SendtoInet4(fd.Sysfd, p, 0, sa)
	if err == syscall.EINTR {
	continue
	}
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitWrite(fd.isFile); err == nil {
	continue
	}
	}
	if err != nil {
	return 0, err
	}
	return len(p), nil
	}
	}

	// WriteToInet6 wraps the sendto network call for IPv6 addresses.
	func (fd *FD) WriteToInet6(p []byte, sa syscall.SockaddrInet6) (int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return 0, err
	}
	for {
	err := syscall.SendtoInet6(fd.Sysfd, p, 0, sa)
	if err == syscall.EINTR {
	continue
	}
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitWrite(fd.isFile); err == nil {
	continue
	}
	}
	if err != nil {
	return 0, err
	}
	return len(p), nil
	}
	}

	// WriteTo wraps the sendto network call.
	func (fd *FD) WriteTo(p []byte, sa syscall.Sockaddr) (int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return 0, err
	}
	for {
	err := syscall.Sendto(fd.Sysfd, p, 0, sa)
	if err == syscall.EINTR {
	continue
	}
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitWrite(fd.isFile); err == nil {
	continue
	}
	}
	if err != nil {
	return 0, err
	}
	return len(p), nil
	}
	}

	// WriteMsg wraps the sendmsg network call.
	func (fd *FD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (int, int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return 0, 0, err
	}
	for {
	n, err := syscall.SendmsgN(fd.Sysfd, p, oob, sa, 0)
	if err == syscall.EINTR {
	continue
	}
	if err == syscall.EAGAIN && fd.pd.pollable() {
	if err = fd.pd.waitWrite(fd.isFile); err == nil {
	continue
	}
	}
	if err != nil {
	return n, 0, err
	}
	return n, len(oob), err
	}
	}

	// Accept wraps the accept network call.
	func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
	if err := fd.readLock(); err != nil {
	return -1, nil, "", err
	}
	defer fd.readUnlock()

	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return -1, nil, "", err
	}
	for {
	s, rsa, errcall, err := accept(fd.Sysfd)
	if err == nil {
	return s, rsa, "", err
	}
	switch err {
	case syscall.EINTR:
	continue
	case syscall.EAGAIN:
	if fd.pd.pollable() {
	if err = fd.pd.waitRead(fd.isFile); err == nil {
	continue
	}
	}
	case syscall.ECONNABORTED:
	// This means that a socket on the listen
	// queue was closed before we Accept()ed it;
	// it's a silly error, so try again.
	continue
	}
	return -1, nil, errcall, err
	}
	}

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

	// ReadDirent wraps syscall.ReadDirent.
	// We treat this like an ordinary system call rather than a call
	// that tries to fill the buffer.
	func (fd *FD) ReadDirent(buf []byte) (int, error) {
	if err := fd.incref(); err != nil {
	return 0, err
	}
	defer fd.decref()
	for {
	n, err := ignoringEINTRIO(syscall.ReadDirent, fd.Sysfd, buf)
	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
	}
	}

	// Fchmod wraps syscall.Fchmod.
	func (fd *FD) Fchmod(mode uint32) error {
	if err := fd.incref(); err != nil {
	return err
	}
	defer fd.decref()
	return ignoringEINTR(func() error {
	return syscall.Fchmod(fd.Sysfd, mode)
	})
	}

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

	// Fstat wraps syscall.Fstat
	func (fd FD) Fstat(s syscall.Stat_t) error {
	if err := fd.incref(); err != nil {
	return err
	}
	defer fd.decref()
	return ignoringEINTR(func() error {
	return syscall.Fstat(fd.Sysfd, s)
	})
	}

	// tryDupCloexec indicates whether F_DUPFD_CLOEXEC should be used.
	// If the kernel doesn't support it, this is set to 0.
	var tryDupCloexec = int32(1)

	// DupCloseOnExec dups fd and marks it close-on-exec.
	func DupCloseOnExec(fd int) (int, string, error) {
	if syscall.F_DUPFD_CLOEXEC != 0 && atomic.LoadInt32(&tryDupCloexec) == 1 {
	r0, e1 := fcntl(fd, syscall.F_DUPFD_CLOEXEC, 0)
	if e1 == nil {
	return r0, "", nil
	}
	switch e1.(syscall.Errno) {
	case syscall.EINVAL, syscall.ENOSYS:
	// Old kernel, or js/wasm (which returns
	// ENOSYS). Fall back to the portable way from
	// now on.
	atomic.StoreInt32(&tryDupCloexec, 0)
	default:
	return -1, "fcntl", e1
	}
	}
	return dupCloseOnExecOld(fd)
	}

	// dupCloseOnExecOld is the traditional way to dup an fd and
	// set its O_CLOEXEC bit, using two system calls.
	func dupCloseOnExecOld(fd int) (int, string, error) {
	syscall.ForkLock.RLock()
	defer syscall.ForkLock.RUnlock()
	newfd, err := syscall.Dup(fd)
	if err != nil {
	return -1, "dup", err
	}
	syscall.CloseOnExec(newfd)
	return newfd, "", nil
	}

	// Dup duplicates the file descriptor.
	func (fd *FD) Dup() (int, string, error) {
	if err := fd.incref(); err != nil {
	return -1, "", err
	}
	defer fd.decref()
	return DupCloseOnExec(fd.Sysfd)
	}

	// On Unix variants only, expose the IO event for the net code.

	// WaitWrite waits until data can be read from fd.
	func (fd *FD) WaitWrite() error {
	return fd.pd.waitWrite(fd.isFile)
	}

	// WriteOnce is for testing only. It makes a single write call.
	func (fd *FD) WriteOnce(p []byte) (int, error) {
	if err := fd.writeLock(); err != nil {
	return 0, err
	}
	defer fd.writeUnlock()
	return ignoringEINTRIO(syscall.Write, fd.Sysfd, p)
	}

	// RawRead invokes the user-defined function f for a read operation.
	func (fd *FD) RawRead(f func(uintptr) bool) error {
	if err := fd.readLock(); err != nil {
	return err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
	return err
	}
	for {
	if f(uintptr(fd.Sysfd)) {
	return nil
	}
	if err := fd.pd.waitRead(fd.isFile); err != nil {
	return err
	}
	}
	}

	// RawWrite invokes the user-defined function f for a write operation.
	func (fd *FD) RawWrite(f func(uintptr) bool) error {
	if err := fd.writeLock(); err != nil {
	return err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
	return err
	}
	for {
	if f(uintptr(fd.Sysfd)) {
	return nil
	}
	if err := fd.pd.waitWrite(fd.isFile); err != nil {
	return err
	}
	}
	}

	// ignoringEINTRIO is like ignoringEINTR, but just for IO calls.
	func ignoringEINTRIO(fn func(fd int, p []byte) (int, error), fd int, p []byte) (int, error) {
	for {
	n, err := fn(fd, p)
	if err != syscall.EINTR {
	return n, err
	}
	}
	}