better and batched tun interface

overlay/tun_darwin.go

@@ -6,7 +6,6 @@ package overlay
 import (
 	"errors"
 	"fmt"
-	"io"
 	"log/slog"
 	"net/netip"
 	"os"
@@ -16,6 +15,7 @@ import (
 
 	"github.com/gaissmai/bart"
 	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
 	netroute "golang.org/x/net/route"
@@ -31,6 +31,9 @@ type tun struct {
 	routeTree   atomic.Pointer[bart.Table[routing.Gateways]]
 	linkAddr    *netroute.LinkAddr
 	l           *slog.Logger
+
+	readBuf  []byte
+	batchRet [1][]byte
 }
 
 type ifReq struct {
@@ -126,6 +129,7 @@ func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, _ bool) (*t
 		vpnNetworks: vpnNetworks,
 		DefaultMTU:  c.GetInt("tun.mtu", DefaultMTU),
 		l:           l,
+		readBuf:     make([]byte, defaultBatchBufSize),
 	}
 
 	err = t.reload(c, true)
@@ -515,9 +519,9 @@ func tunWritev(fd int, iovecs []unix.Iovec) (n int, err error)
 //go:noescape
 func tunReadv(fd int, iovecs []unix.Iovec) (n int, err error)
 
-// Read pulls one IP packet off the utun device, scattering the 4 byte protocol header away from
+// readOne pulls one IP packet off the utun device, scattering the 4 byte protocol header away from
 // the packet so the payload lands directly in to.
-func (t *tun) Read(to []byte) (int, error) {
+func (t *tun) readOne(to []byte) (int, error) {
 	var head [4]byte
 
 	rc, err := t.f.SyscallConn()
@@ -550,7 +554,16 @@ func (t *tun) Read(to []byte) (int, error) {
 	return n - 4, nil
 }
 
-// Write pushes one IP packet onto the utun device.
+func (t *tun) Read() ([][]byte, error) {
+	n, err := t.readOne(t.readBuf)
+	if err != nil {
+		return nil, err
+	}
+	t.batchRet[0] = t.readBuf[:n]
+	return t.batchRet[:], nil
+}
+
+// Write pushes one IP packet onto the utun device. Only valid for single threaded use.
 func (t *tun) Write(from []byte) (int, error) {
 	if len(from) == 0 {
 		return 0, syscall.EIO
@@ -610,6 +623,10 @@ func (t *tun) SupportsMultiqueue() bool {
 	return false
 }
 
-func (t *tun) NewMultiQueueReader() (io.ReadWriteCloser, error) {
-	return nil, fmt.Errorf("TODO: multiqueue not implemented for darwin")
+func (t *tun) NewMultiQueueReader() error {
+	return fmt.Errorf("TODO: multiqueue not implemented for darwin")
+}
+
+func (t *tun) Readers() []tio.Queue {
+	return []tio.Queue{t}
 }