conn, device, tun: implement vectorized I/O plumbing

Accept packet vectors for reading and writing in the tun.Device and
conn.Bind interfaces, so that the internal plumbing between these
interfaces now passes a vector of packets. Vectors move untouched
between these interfaces, i.e. if 128 packets are received from
conn.Bind.Read(), 128 packets are passed to tun.Device.Write(). There is
no internal buffering.

Currently, existing implementations are only adjusted to have vectors
of length one. Subsequent patches will improve that.

Also, as a related fixup, use the unix and windows packages rather than
the syscall package when possible.

Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: James Tucker <james@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/tun_windows.go

@@ -15,7 +15,6 @@ import (
 	_ "unsafe"
 
 	"golang.org/x/sys/windows"
-
 	"golang.zx2c4.com/wintun"
 )
 
@@ -44,6 +43,7 @@ type NativeTun struct {
 	closeOnce sync.Once
 	close     atomic.Bool
 	forcedMTU int
+	outSizes  []int
 }
 
 var (
@@ -134,9 +134,14 @@ func (tun *NativeTun) ForceMTU(mtu int) {
 	}
 }
 
+func (tun *NativeTun) BatchSize() int {
+	// TODO: implement batching with wintun
+	return 1
+}
+
 // Note: Read() and Write() assume the caller comes only from a single thread; there's no locking.
 
-func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
+func (tun *NativeTun) Read(buffs [][]byte, sizes []int, offset int) (int, error) {
 	tun.running.Add(1)
 	defer tun.running.Done()
 retry:
@@ -153,10 +158,11 @@ retry:
 		switch err {
 		case nil:
 			packetSize := len(packet)
-			copy(buff[offset:], packet)
+			copy(buffs[0][offset:], packet)
+			sizes[0] = packetSize
 			tun.session.ReleaseReceivePacket(packet)
 			tun.rate.update(uint64(packetSize))
-			return packetSize, nil
+			return 1, nil
 		case windows.ERROR_NO_MORE_ITEMS:
 			if !shouldSpin || uint64(nanotime()-start) >= spinloopDuration {
 				windows.WaitForSingleObject(tun.readWait, windows.INFINITE)
@@ -173,33 +179,33 @@ retry:
 	}
 }
 
-func (tun *NativeTun) Flush() error {
-	return nil
-}
-
-func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
+func (tun *NativeTun) Write(buffs [][]byte, offset int) (int, error) {
 	tun.running.Add(1)
 	defer tun.running.Done()
 	if tun.close.Load() {
 		return 0, os.ErrClosed
 	}
 
-	packetSize := len(buff) - offset
-	tun.rate.update(uint64(packetSize))
+	for i, buff := range buffs {
+		packetSize := len(buff) - offset
+		tun.rate.update(uint64(packetSize))
 
-	packet, err := tun.session.AllocateSendPacket(packetSize)
-	if err == nil {
-		copy(packet, buff[offset:])
-		tun.session.SendPacket(packet)
-		return packetSize, nil
+		packet, err := tun.session.AllocateSendPacket(packetSize)
+		switch err {
+		case nil:
+			// TODO: Explore options to eliminate this copy.
+			copy(packet, buff[offset:])
+			tun.session.SendPacket(packet)
+			continue
+		case windows.ERROR_HANDLE_EOF:
+			return i, os.ErrClosed
+		case windows.ERROR_BUFFER_OVERFLOW:
+			continue // Dropping when ring is full.
+		default:
+			return i, fmt.Errorf("Write failed: %w", err)
+		}
 	}
-	switch err {
-	case windows.ERROR_HANDLE_EOF:
-		return 0, os.ErrClosed
-	case windows.ERROR_BUFFER_OVERFLOW:
-		return 0, nil // Dropping when ring is full.
-	}
-	return 0, fmt.Errorf("Write failed: %w", err)
+	return len(buffs), nil
 }
 
 // LUID returns Windows interface instance ID.