fix the closing of the linux udp reading loop using Shutdown

udp/udp_linux.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net"
 	"net/netip"
+	"sync"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
@@ -25,6 +26,7 @@ import (
 type StdConn struct {
 	sysFd  int
 	closed atomic.Bool
+	wg     *sync.WaitGroup
 	isV4   bool
 	l      *logrus.Logger
 	batch  int
@@ -81,7 +83,14 @@ func NewListener(l *logrus.Logger, ip netip.Addr, port int, multi bool, batch in
 	//v, err := unix.GetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_INCOMING_CPU)
 	//l.Println(v, err)
 
-	return &StdConn{sysFd: fd, isV4: ip.Is4(), l: l, batch: batch}, err
+	return &StdConn{
+		sysFd:  fd,
+		closed: atomic.Bool{},
+		wg:     &sync.WaitGroup{},
+		isV4:   ip.Is4(),
+		l:      l,
+		batch:  batch,
+	}, err
 }
 
 func (u *StdConn) Rebind() error {
@@ -123,6 +132,15 @@ func (u *StdConn) LocalAddr() (netip.AddrPort, error) {
 }
 
 func (u *StdConn) ListenOut(r EncReader, lhf LightHouseHandlerFunc, cache *firewall.ConntrackCacheTicker, q int) {
+
+	u.wg.Add(1)
+	defer func() {
+		u.wg.Done()
+	}()
+	if u.closed.Load() {
+		return
+	}
+
 	plaintext := make([]byte, MTU)
 	h := &header.H{}
 	fwPacket := &firewall.Packet{}
@@ -144,7 +162,7 @@ func (u *StdConn) ListenOut(r EncReader, lhf LightHouseHandlerFunc, cache *firew
 			return
 		}
 
-		if u.closed {
+		if u.closed.Load() {
 			u.l.Debug("flag for closing connection is set, exiting read loop")
 			return
 		}
@@ -321,11 +339,20 @@ func (u *StdConn) getMemInfo(meminfo *[unix.SK_MEMINFO_VARS]uint32) error {
 }
 
 func (u *StdConn) Close() error {
-	//TODO: this will not interrupt the read loop
-	if u.closed {
+	if !u.closed.CompareAndSwap(false, true) {
+		// already closed by e.g. other thread
 		return nil
 	}
-	u.closed = true
+	err := syscall.Shutdown(u.sysFd, syscall.SHUT_RDWR)
+	if err != nil {
+		errno, ok := err.(syscall.Errno)
+		// connection might have been terminated by remote before
+		wasDisconnected := ok && (errno == syscall.ENOTCONN)
+		if !wasDisconnected {
+			panic(fmt.Sprintf("error while shutdown of UDP socket: %v", err))
+		}
+	}
+	u.wg.Wait()
 	return syscall.Close(u.sysFd)
 }