tun: linux: netpoll is broken for tun's epoll

So this mostly reverts the switch to Sysconn for Linux.

Issue: https://github.com/golang/go/issues/30426

tun/tun_linux.go

@@ -19,7 +19,6 @@ import (
 	"os"
 	"strconv"
 	"sync"
-	"syscall"
 	"time"
 	"unsafe"
 )
@@ -31,6 +30,8 @@ const (
 
 type nativeTun struct {
 	tunFile                 *os.File
+	fd                      uintptr
+	fdCancel                *rwcancel.RWCancel
 	index                   int32         // if index
 	name                    string        // name of interface
 	errors                  chan error    // async error handling
@@ -51,11 +52,9 @@ func (tun *nativeTun) routineHackListener() {
 	/* This is needed for the detection to work across network namespaces
 	 * If you are reading this and know a better method, please get in touch.
 	 */
+	fd := int(tun.fd)
 	for {
-		var err error
+		_, err := unix.Write(fd, nil)
-		tun.operateOnFd(func(fd uintptr) {
-			_, err = unix.Write(int(fd), nil)
-		})
 		switch err {
 		case unix.EINVAL:
 			tun.events <- TUNEventUp
@@ -257,15 +256,12 @@ func (tun *nativeTun) MTU() (int, error) {
 
 func (tun *nativeTun) Name() (string, error) {
 	var ifr [ifReqSize]byte
-	var errno syscall.Errno
+	_, _, errno := unix.Syscall(
-	tun.operateOnFd(func(fd uintptr) {
+		unix.SYS_IOCTL,
-		_, _, errno = unix.Syscall(
+		tun.fd,
-			unix.SYS_IOCTL,
+		uintptr(unix.TUNGETIFF),
-			fd,
+		uintptr(unsafe.Pointer(&ifr[0])),
-			uintptr(unix.TUNGETIFF),
+	)
-			uintptr(unsafe.Pointer(&ifr[0])),
-		)
-	})
 	if errno != 0 {
 		return "", errors.New("failed to get name of TUN device: " + strconv.FormatInt(int64(errno), 10))
 	}
@@ -306,7 +302,7 @@ func (tun *nativeTun) Write(buff []byte, offset int) (int, error) {
 	return tun.tunFile.Write(buff)
 }
 
-func (tun *nativeTun) Read(buff []byte, offset int) (int, error) {
+func (tun *nativeTun) doRead(buff []byte, offset int) (int, error) {
 	select {
 	case err := <-tun.errors:
 		return 0, err
@@ -324,6 +320,18 @@ func (tun *nativeTun) Read(buff []byte, offset int) (int, error) {
 	}
 }
 
+func (tun *nativeTun) Read(buff []byte, offset int) (int, error) {
+	for {
+		n, err := tun.doRead(buff, offset)
+		if err == nil || !rwcancel.RetryAfterError(err) {
+			return n, err
+		}
+		if !tun.fdCancel.ReadyRead() {
+			return 0, errors.New("tun device closed")
+		}
+	}
+}
+
 func (tun *nativeTun) Events() chan TUNEvent {
 	return tun.events
 }
@@ -339,20 +347,30 @@ func (tun *nativeTun) Close() error {
 		close(tun.events)
 	}
 	err2 := tun.tunFile.Close()
+	err3 := tun.fdCancel.Cancel()
 
 	if err1 != nil {
 		return err1
 	}
-	return err2
+	if err2 != nil {
+		return err2
+	}
+	return err3
 }
 
 func CreateTUN(name string, mtu int) (TUNDevice, error) {
-	tunFile, err := os.OpenFile(cloneDevicePath, os.O_RDWR, 0)
+	nfd, err := unix.Open(cloneDevicePath, os.O_RDWR, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	fd := os.NewFile(uintptr(nfd), cloneDevicePath)
 	if err != nil {
 		return nil, err
 	}
 
 	// create new device
+
 	var ifr [ifReqSize]byte
 	var flags uint16 = unix.IFF_TUN // | unix.IFF_NO_PI (disabled for TUN status hack)
 	nameBytes := []byte(name)
@@ -362,25 +380,23 @@ func CreateTUN(name string, mtu int) (TUNDevice, error) {
 	copy(ifr[:], nameBytes)
 	*(*uint16)(unsafe.Pointer(&ifr[unix.IFNAMSIZ])) = flags
 
-	var errno syscall.Errno
+	_, _, errno := unix.Syscall(
-	(&nativeTun{tunFile: tunFile}).operateOnFd(func(fd uintptr) {
+		unix.SYS_IOCTL,
-		_, _, errno = unix.Syscall(
+		fd.Fd(),
-			unix.SYS_IOCTL,
+		uintptr(unix.TUNSETIFF),
-			fd,
+		uintptr(unsafe.Pointer(&ifr[0])),
-			uintptr(unix.TUNSETIFF),
+	)
-			uintptr(unsafe.Pointer(&ifr[0])),
-		)
-	})
 	if errno != 0 {
 		return nil, errno
 	}
 
-	return CreateTUNFromFile(tunFile, mtu)
+	return CreateTUNFromFile(fd, mtu)
 }
 
 func CreateTUNFromFile(file *os.File, mtu int) (TUNDevice, error) {
 	tun := &nativeTun{
 		tunFile:                 file,
+		fd:                      file.Fd(),
 		events:                  make(chan TUNEvent, 5),
 		errors:                  make(chan error, 5),
 		statusListenersShutdown: make(chan struct{}),
@@ -388,6 +404,11 @@ func CreateTUNFromFile(file *os.File, mtu int) (TUNDevice, error) {
 	}
 	var err error
 
+	tun.fdCancel, err = rwcancel.NewRWCancel(int(tun.fd))
+	if err != nil {
+		return nil, err
+	}
+
 	_, err = tun.Name()
 	if err != nil {
 		return nil, err