lnwire: add new TestSerializedSize method

This uses all the interfaces and implementations added in the prior test.

lnwire/pong.go

@@ -39,7 +39,8 @@ func NewPong(pongBytes []byte) *Pong {
 // A compile time check to ensure Pong implements the lnwire.Message interface.
 var _ Message = (*Pong)(nil)
 
-// A compile time check to ensure Pong implements the lnwire.SizeableMessage interface.
+// A compile time check to ensure Pong implements the lnwire.SizeableMessage
+// interface.
 var _ SizeableMessage = (*Pong)(nil)
 
 // Decode deserializes a serialized Pong message stored in the passed io.Reader

lnwire/serialized_size_test.go

@@ -0,0 +1,68 @@
+package lnwire
+
+import (
+	"bytes"
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"pgregory.net/rapid"
+)
+
+// TestSerializedSize uses property-based testing to verify that
+// SerializedSize returns the correct value for randomly generated messages.
+func TestSerializedSize(t *testing.T) {
+	t.Parallel()
+
+	rapid.Check(t, func(t *rapid.T) {
+		// Pick a random message type.
+		msgType := rapid.Custom(func(t *rapid.T) MessageType {
+			return MessageType(
+				rapid.IntRange(
+					0, int(math.MaxUint16),
+				).Draw(t, "msgType"),
+			)
+		}).Draw(t, "msgType")
+
+		// Create an empty message of the given type.
+		m, err := MakeEmptyMessage(msgType)
+
+		// An error means this isn't a valid message type, so we skip
+		// it.
+		if err != nil {
+			return
+		}
+
+		testMsg, ok := m.(TestMessage)
+		require.True(
+			t, ok, "message type %s does not "+
+				"implement TestMessage", msgType,
+		)
+
+		// Use the testMsg to make a new random message.
+		msg := testMsg.RandTestMessage(t)
+
+		// Type assertion to ensure the message implements
+		// SizeableMessage.
+		sizeMsg, ok := msg.(SizeableMessage)
+		require.True(
+			t, ok, "message type %s does not "+
+				"implement SizeableMessage", msgType,
+		)
+
+		// Get the size using SerializedSize.
+		size, err := sizeMsg.SerializedSize()
+		require.NoError(t, err, "SerializedSize error")
+
+		// Get the size by actually serializing the message.
+		var buf bytes.Buffer
+		writtenBytes, err := WriteMessage(&buf, msg, 0)
+		require.NoError(t, err, "WriteMessage error")
+
+		// The SerializedSize should match the number of bytes written.
+		require.Equal(t, uint32(writtenBytes), size,
+			"SerializedSize = %d, actual bytes "+
+				"written = %d for message type %s (populated)",
+			size, writtenBytes, msgType)
+	})
+}