fix: prevent data corruption in multiPacketListener by avoiding buffer resizing (#213)

* Add regression test that covers the issue.

* Fix the test.

* Use consistent comments.

Author: sbruens

service/listeners.go

@@ -288,10 +288,10 @@ func (m *multiPacketListener) Acquire() (net.PacketConn, error) {
 			buffer := make([]byte, serverUDPBufferSize)
 			for {
 				n, addr, err := m.pc.ReadFrom(buffer)
-				buffer = buffer[:n]
+				pkt := buffer[:n]
 				select {
 				case req := <-m.readCh:
-					n := copy(req.buffer, buffer)
+					n := copy(req.buffer, pkt)
 					req.respCh <- struct {
 						n    int
 						addr net.Addr

service/listeners_test.go

@@ -142,6 +142,33 @@ func TestListenerManagerPacketListenerCreatesListenerOnDemand(t *testing.T) {
 	<-done
 }
 
+func TestMultiPacketListener_SequentialReads(t *testing.T) {
+	m := NewListenerManager()
+	conn, err := m.ListenPacket("127.0.0.1:0")
+	require.NoError(t, err)
+	udpConn, err := net.Dial("udp", conn.LocalAddr().String())
+	require.NoError(t, err)
+
+	// Send and receive the first packet.
+	data1 := []byte("hello")
+	_, err = udpConn.Write(data1)
+	require.NoError(t, err)
+	received1 := make([]byte, serverUDPBufferSize)
+	n1, _, err := conn.ReadFrom(received1)
+	require.NoError(t, err)
+
+	// Send and receive a second larger packet.
+	data2 := []byte("a longer message than the first one")
+	_, err = udpConn.Write(data2)
+	require.NoError(t, err)
+	received2 := make([]byte, serverUDPBufferSize)
+	n2, _, err := conn.ReadFrom(received2)
+	require.NoError(t, err)
+
+	require.Equal(t, string(data1), string(received1[:n1]))
+	require.Equal(t, string(data2), string(received2[:n2]))
+}
+
 func BenchmarkMultiStreamListener_Acquire(b *testing.B) {
 	lm := NewListenerManager()