wtwire: move message prefixing to the harness

The prefixing is done every time the harness is used, so it may as well
reside in the harness itself.

Since we already pass an empty message of the correct type, we can use
that message to get the required prefix bytes.

watchtower/wtwire/fuzz_test.go

@@ -26,17 +26,19 @@ func prefixWithMsgType(data []byte, prefix MessageType) []byte {
 func wireMsgHarness(t *testing.T, data []byte, emptyMsg Message) {
 	t.Helper()
 
-	// Create a reader with the byte array.
+	// Make sure byte array length is less than max payload size for the
-	r := bytes.NewReader(data)
+	// wire message.
-
+	payloadLen := uint32(len(data))
-	// Make sure byte array length (excluding 2 bytes for message type) is
-	// less than max payload size for the wire message.
-	payloadLen := uint32(len(data)) - 2
 	if payloadLen > emptyMsg.MaxPayloadLength(0) {
 		// Ignore this input - max payload constraint violated.
 		return
 	}
 
+	data = prefixWithMsgType(data, emptyMsg.MsgType())
+
+	// Create a reader with the byte array.
+	r := bytes.NewReader(data)
+
 	msg, err := ReadMessage(r, 0)
 	if err != nil {
 		return
@@ -57,9 +59,6 @@ func wireMsgHarness(t *testing.T, data []byte, emptyMsg Message) {
 
 func FuzzCreateSessionReply(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgCreateSessionReply.
-		data = prefixWithMsgType(data, MsgCreateSessionReply)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := CreateSessionReply{}
@@ -70,9 +69,6 @@ func FuzzCreateSessionReply(f *testing.F) {
 
 func FuzzCreateSession(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgCreateSession.
-		data = prefixWithMsgType(data, MsgCreateSession)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := CreateSession{}
@@ -83,9 +79,6 @@ func FuzzCreateSession(f *testing.F) {
 
 func FuzzDeleteSessionReply(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgDeleteSessionReply.
-		data = prefixWithMsgType(data, MsgDeleteSessionReply)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := DeleteSessionReply{}
@@ -96,9 +89,6 @@ func FuzzDeleteSessionReply(f *testing.F) {
 
 func FuzzDeleteSession(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgDeleteSession.
-		data = prefixWithMsgType(data, MsgDeleteSession)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := DeleteSession{}
@@ -109,9 +99,6 @@ func FuzzDeleteSession(f *testing.F) {
 
 func FuzzError(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgError.
-		data = prefixWithMsgType(data, MsgError)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := Error{}
@@ -122,9 +109,6 @@ func FuzzError(f *testing.F) {
 
 func FuzzInit(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgInit.
-		data = prefixWithMsgType(data, MsgInit)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := Init{}
@@ -135,9 +119,6 @@ func FuzzInit(f *testing.F) {
 
 func FuzzStateUpdateReply(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgStateUpdateReply.
-		data = prefixWithMsgType(data, MsgStateUpdateReply)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := StateUpdateReply{}
@@ -148,9 +129,6 @@ func FuzzStateUpdateReply(f *testing.F) {
 
 func FuzzStateUpdate(f *testing.F) {
 	f.Fuzz(func(t *testing.T, data []byte) {
-		// Prefix with MsgStateUpdate.
-		data = prefixWithMsgType(data, MsgStateUpdate)
-
 		// Create an empty message so that the FuzzHarness func can
 		// check if the max payload constraint is violated.
 		emptyMsg := StateUpdate{}