[test] Ignore BIP324 decoy messages

Also allow P2PConnection::send_message() to send decoy messages for
writing tests.

- v2 P2P msg = 3 byte len + 1-13 byte message_type + payload + 16 byte MAC
- In the 3 bytes len, 23 bits are used to represent length and 1 bit is
used to indicate if it's a decoy message.
(hence max msg len allowed is 2^23-1)

test/functional/test_framework/p2p.py

@@ -303,6 +303,9 @@ def _on_data(self):
                     msg_len, msg = self.v2_conn.v2_dec_msg(self.recvbuf)
                     if msg is None:
                         return
+                    self.recvbuf = self.recvbuf[3+msg_len+16:]
+                    if msg == b'': # reject decoy messages and messages longer than permitted payload length
+                        return
                     size_or_shortid = msg[0]
                     if 0 < size_or_shortid <= 12: # TODO: check if this works
                         # a string command
@@ -316,7 +319,6 @@ def _on_data(self):
                         else:
                             msgtype = "unknown-" + str(msg[1]) # TODO: check if this works
                     f = BytesIO(payload)
-                    self.recvbuf = self.recvbuf[3+msg_len+16:]
 
                 # TODO: Remove later, depending on disconnection logic.
                 # not using an "else" here so that if v2 doesn't work out, normal v1 handling can be done.
@@ -356,12 +358,12 @@ def on_message(self, message):
 
     # Socket write methods
 
-    def send_message(self, message):
+    def send_message(self, message, is_decoy=False):
         """Send a P2P message over the socket.
 
         This method takes a P2P payload, builds the P2P header and adds
         the message to the send buffer to be sent over the socket."""
-        tmsg = self.build_message(message)
+        tmsg = self.build_message(message, is_decoy)
         self._log_message("send", message)
         return self.send_raw_message(tmsg)
 
@@ -379,7 +381,7 @@ def maybe_write():
 
     # Class utility methods
 
-    def build_message(self, message):
+    def build_message(self, message, is_decoy=False):
         """Build a serialized P2P message"""
         if self.support_v2_p2p:
             msgtype = message.msgtype
@@ -391,7 +393,7 @@ def build_message(self, message):
                 tmsg += bytes(msgtype, 'ascii')
             tmsg += data
             # Need to return (3 bytes length + encrypted payload + MAC tag)
-            return self.v2_conn.v2_enc_msg(tmsg)
+            return self.v2_conn.v2_enc_msg(tmsg, is_decoy)
         else:
             msgtype = message.msgtype
             data = message.serialize()