Fix UDP conn stuck on sniff

This change only avoids permanent hangs. We need to implement read deadlines for UDP conns in 1.10 for server inbounds.

route/router.go

@@ -951,12 +951,34 @@ func (r *Router) RoutePacketConnection(ctx context.Context, conn N.PacketConn, m
 	}*/
 
 	if metadata.InboundOptions.SniffEnabled || metadata.Destination.Addr.IsUnspecified() {
-		buffer := buf.NewPacket()
-		destination, err := conn.ReadPacket(buffer)
+		var (
+			buffer      = buf.NewPacket()
+			destination M.Socksaddr
+			done        = make(chan struct{})
+			err         error
+		)
+		go func() {
+			sniffTimeout := C.ReadPayloadTimeout
+			if metadata.InboundOptions.SniffTimeout > 0 {
+				sniffTimeout = time.Duration(metadata.InboundOptions.SniffTimeout)
+			}
+			conn.SetReadDeadline(time.Now().Add(sniffTimeout))
+			destination, err = conn.ReadPacket(buffer)
+			conn.SetReadDeadline(time.Time{})
+			close(done)
+		}()
+		select {
+		case <-done:
+		case <-ctx.Done():
+			conn.Close()
+			return ctx.Err()
+		}
 		if err != nil {
 			buffer.Release()
+			if !errors.Is(err, os.ErrDeadlineExceeded) {
 				return err
 			}
+		} else {
 			if metadata.Destination.Addr.IsUnspecified() {
 				metadata.Destination = destination
 			}
@@ -980,6 +1002,7 @@ func (r *Router) RoutePacketConnection(ctx context.Context, conn N.PacketConn, m
 			}
 			conn = bufio.NewCachedPacketConn(conn, buffer, destination)
 		}
+	}
 	if r.dnsReverseMapping != nil && metadata.Domain == "" {
 		domain, loaded := r.dnsReverseMapping.Query(metadata.Destination.Addr)
 		if loaded {