fix(ssu): prevent goroutine/connection leak in UDP session cache

The ssuHandler's connMap cached UDP forwarding connections but spawned
goroutines had no context cancellation mechanism. When relayPacketUDP
exited (e.g. on SS UDP session timeout), orphaned goroutines blocked
indefinitely on ReadFrom, preventing connMap cleanup and leaking both
goroutines and connections.

Fix: derive a cancellable context in relayPacketUDP so all per-session
goroutines are cancelled on exit. Add SetReadDeadline with 30s periodic
timeout so goroutines can check context cancellation instead of blocking
forever. Store cancel functions in a cancels sync.Map for cleanup.

Fixes #821

handler/ss/udp/handler.go

@@ -36,6 +36,7 @@ type ssuHandler struct {
 	md        metadata
 	options   handler.Options
 	recorder  recorder.RecorderObject
+	cancels   sync.Map // session hash -> context.CancelFunc
 }
 
 func NewHandler(opts ...handler.Option) handler.Handler {
@@ -174,6 +175,9 @@ func (h *ssuHandler) Handle(ctx context.Context, conn net.Conn, opts ...handler.
 }
 
 func (h *ssuHandler) relayPacketUDP(ctx context.Context, src net.PacketConn, ro *xrecorder.HandlerRecorderObject, log logger.Logger) error {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel() // cancel all per-session goroutines on exit
+
 	bufferSize := h.md.udpBufferSize
 	if bufferSize <= 0 {
 		bufferSize = defaultBufferSize
@@ -248,16 +252,22 @@ func (h *ssuHandler) packetConnForSession(ctx context.Context, src net.PacketCon
 		return nil, err
 	}
 
-	actual, loaded := h.connMap.LoadOrStore(session.Hash(), cc)
-	if loaded {
+	if actual, loaded := h.connMap.LoadOrStore(session.Hash(), cc); loaded {
 		c.Close()
 		return actual.(net.PacketConn), nil
 	}
 
+	// Create a per-session cancel so the goroutine can be cleaned up
+	// when relayPacketUDP exits (parent context cancelled) or on error.
+	sessionCtx, sessionCancel := context.WithCancel(ctx)
+	h.cancels.Store(session.Hash(), sessionCancel)
+
 	go func() {
 		defer func() {
+			sessionCancel()
 			cc.Close()
 			h.connMap.Delete(session.Hash())
+			h.cancels.Delete(session.Hash())
 		}()
 
 		bufSize := h.md.udpBufferSize
@@ -267,17 +277,32 @@ func (h *ssuHandler) packetConnForSession(ctx context.Context, src net.PacketCon
 		b := make([]byte, bufSize)
 
 		for {
+			// Use a read deadline so the goroutine periodically checks
+			// whether the parent context has been cancelled, instead of
+			// blocking indefinitely on ReadFrom.
+			cc.SetReadDeadline(time.Now().Add(30 * time.Second))
 			n, raddr, err := cc.ReadFrom(b)
 			if err != nil {
+				// If the parent context is done, exit silently.
+				if sessionCtx.Err() != nil {
+					return
+				}
+				// Timeout is expected — check context again before retrying.
+				if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
+					continue
+				}
 				log.Warnf("failed to read response: %v", err)
 				return
 			}
 
-			if h.options.Bypass != nil && h.options.Bypass.Contains(ctx, "udp", raddr.String(), bypass.WithService(h.options.Service)) {
+			if h.options.Bypass != nil && h.options.Bypass.Contains(sessionCtx, "udp", raddr.String(), bypass.WithService(h.options.Service)) {
 				log.Warn("bypass: ", raddr)
 				return
 			}
 
+			if sessionCtx.Err() != nil {
+				return
+			}
 			addr := core.NewUDPServerPacketAddr(raddr, net.UDPAddrFromAddrPort(session.ClientAddr()), session)
 			if _, err = src.WriteTo(b[:n], addr); err != nil {
 				log.Warnf("failed to write response to %v: %v", session.ClientAddr(), err)
@@ -302,3 +327,20 @@ func (h *ssuHandler) checkRateLimit(addr net.Addr) bool {
 
 	return true
 }
+
+// Close cancels all per-session goroutines and closes cached connections.
+func (h *ssuHandler) Close() error {
+	h.cancels.Range(func(key, value any) bool {
+		if cancel, ok := value.(context.CancelFunc); ok {
+			cancel()
+		}
+		return true
+	})
+	h.connMap.Range(func(key, value any) bool {
+		if cc, ok := value.(net.PacketConn); ok {
+			cc.Close()
+		}
+		return true
+	})
+	return nil
+}