only allocate datagram receive queue when receiving datagrams (#3702)

datagram_queue.go

@@ -1,6 +1,8 @@
 package quic
 
 import (
+	"sync"
+
 	"github.com/quic-go/quic-go/internal/protocol"
 	"github.com/quic-go/quic-go/internal/utils"
 	"github.com/quic-go/quic-go/internal/wire"
@@ -9,7 +11,10 @@ import (
 type datagramQueue struct {
 	sendQueue chan *wire.DatagramFrame
 	nextFrame *wire.DatagramFrame
-	rcvQueue  chan []byte
+
+	rcvMx    sync.Mutex
+	rcvQueue [][]byte
+	rcvd     chan struct{} // used to notify Receive that a new datagram was received
 
 	closeErr error
 	closed   chan struct{}
@@ -25,7 +30,7 @@ func newDatagramQueue(hasData func(), logger utils.Logger) *datagramQueue {
 	return &datagramQueue{
 		hasData:   hasData,
 		sendQueue: make(chan *wire.DatagramFrame, 1),
-		rcvQueue:  make(chan []byte, protocol.DatagramRcvQueueLen),
+		rcvd:      make(chan struct{}, 1),
 		dequeued:  make(chan struct{}),
 		closed:    make(chan struct{}),
 		logger:    logger,
@@ -76,22 +81,41 @@ func (h *datagramQueue) Pop() {
 func (h *datagramQueue) HandleDatagramFrame(f *wire.DatagramFrame) {
 	data := make([]byte, len(f.Data))
 	copy(data, f.Data)
+	var queued bool
+	h.rcvMx.Lock()
+	if len(h.rcvQueue) < protocol.DatagramRcvQueueLen {
+		h.rcvQueue = append(h.rcvQueue, data)
+		queued = true
 		select {
-	case h.rcvQueue <- data:
+		case h.rcvd <- struct{}{}:
 		default:
+		}
+	}
+	h.rcvMx.Unlock()
+	if !queued && h.logger.Debug() {
 		h.logger.Debugf("Discarding DATAGRAM frame (%d bytes payload)", len(f.Data))
 	}
 }
 
 // Receive gets a received DATAGRAM frame.
 func (h *datagramQueue) Receive() ([]byte, error) {
-	select {
-	case data := <-h.rcvQueue:
+	for {
+		h.rcvMx.Lock()
+		if len(h.rcvQueue) > 0 {
+			data := h.rcvQueue[0]
+			h.rcvQueue = h.rcvQueue[1:]
+			h.rcvMx.Unlock()
 			return data, nil
+		}
+		h.rcvMx.Unlock()
+		select {
+		case <-h.rcvd:
+			continue
 		case <-h.closed:
 			return nil, h.closeErr
 		}
 	}
+}
 
 func (h *datagramQueue) CloseWithError(e error) {
 	h.closeErr = e