udpnat2: Add purge expire ticker

common/udpnat2/service.go

@@ -3,6 +3,7 @@ package udpnat
 import (
 	"context"
 	"net/netip"
+	"sync"
 	"time"
 
 	"github.com/sagernet/sing/common"
@@ -18,6 +19,10 @@ type Service struct {
 	handler N.UDPConnectionHandlerEx
 	prepare PrepareFunc
 	metrics Metrics
+
+	timeout   time.Duration
+	closeOnce sync.Once
+	doneChan  chan struct{}
 }
 
 type PrepareFunc func(source M.Socksaddr, destination M.Socksaddr, userData any) (bool, context.Context, N.PacketWriter, N.CloseHandlerFunc)
@@ -50,12 +55,38 @@ func New(handler N.UDPConnectionHandlerEx, prepare PrepareFunc, timeout time.Dur
 		conn.Close()
 	})
 	return &Service{
-		cache:   cache,
-		handler: handler,
-		prepare: prepare,
+		cache:    cache,
+		handler:  handler,
+		prepare:  prepare,
+		timeout:  timeout,
+		doneChan: make(chan struct{}),
 	}
 }
 
+func (s *Service) Start() error {
+	ticker := time.NewTicker(s.timeout)
+	go func() {
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				s.PurgeExpired()
+			case <-s.doneChan:
+				s.Purge()
+				return
+			}
+		}
+	}()
+	return nil
+}
+
+func (s *Service) Close() error {
+	s.closeOnce.Do(func() {
+		close(s.doneChan)
+	})
+	return nil
+}
+
 func (s *Service) NewPacket(bufferSlices [][]byte, source M.Socksaddr, destination M.Socksaddr, userData any) {
 	conn, loaded := s.cache.Get(source.AddrPort())
 	if !loaded {