Take ownership of buffer in QueuePacketConn.WriteTo.

Like the corresponding change to QueuePacketConn.QueueIncoming, this is
more efficient when the caller does not reuse the buffer.

Before:
	$ go test -bench=BenchmarkWriteTo -benchtime 2s -benchmem
	BenchmarkWriteTo-4       3777459               627 ns/op            1024 B/op          2 allocs/op
After:
	$ go test -bench=BenchmarkWriteTo -benchtime 2s -benchmem
	BenchmarkWriteTo-4       6702324               373 ns/op             512 B/op          1 allocs/op

common/turbotunnel/queuepacketconn.go

@@ -82,22 +82,20 @@ func (c *QueuePacketConn) ReadFrom(p []byte) (int, net.Addr, error) {
 }
 
 // WriteTo queues an outgoing packet for the given address. The queue can later
-// be retrieved using the OutgoingQueue method.
+// be retrieved using the OutgoingQueue method. This function takes ownership of
+// p and the caller must not reuse it.
 func (c *QueuePacketConn) WriteTo(p []byte, addr net.Addr) (int, error) {
 	select {
 	case <-c.closed:
 		return 0, &net.OpError{Op: "write", Net: c.LocalAddr().Network(), Addr: c.LocalAddr(), Err: c.err.Load().(error)}
 	default:
 	}
-	// Copy the slice so that the caller may reuse it.
-	buf := make([]byte, len(p))
-	copy(buf, p)
 	select {
-	case c.clients.SendQueue(addr) <- buf:
-		return len(buf), nil
+	case c.clients.SendQueue(addr) <- p:
+		return len(p), nil
 	default:
 		// Drop the outgoing packet if the send queue is full.
-		return len(buf), nil
+		return len(p), nil
 	}
 }
 

common/turbotunnel/queuepacketconn_test.go

@@ -25,3 +25,19 @@ func BenchmarkQueueIncoming(b *testing.B) {
 	}
 	b.StopTimer()
 }
+
+// BenchmarkWriteTo benchmarks the QueuePacketConn.WriteTo function.
+func BenchmarkWriteTo(b *testing.B) {
+	conn := NewQueuePacketConn(emptyAddr{}, 1*time.Hour)
+	defer conn.Close()
+
+	b.ResetTimer()
+	s := 500
+	for i := 0; i < b.N; i++ {
+		// Use a variable for the length to stop the compiler from
+		// optimizing out the allocation.
+		p := make([]byte, s)
+		conn.WriteTo(p, emptyAddr{})
+	}
+	b.StopTimer()
+}