connectd: gate uniform message padding behind --dev-uniform-padding

Uniform padding (sending all messages as 1460-byte chunks) breaks
peers running LND-based nodes: LND disconnects on receiving a
ping(num_pong_bytes=65535) with "pong bytes exceeded" instead of
ignoring it as required by BOLT #1.
Gate the feature behind --dev-uniform-padding so it is opt-in rather
than forced on all connections. Nodes that only peer with CLN can
enable it for the traffic analysis defence.
Changelog-Changed: uniform message padding is now opt-in via the --dev-uniform-padding flag.

Author: sangbida

connectd/multiplex.c

@@ -484,10 +484,16 @@ static bool have_empty_encrypted_queue(const struct peer *peer)
 /* (Continue) writing the encrypted_peer_out array */
 static struct io_plan *write_encrypted_to_peer(struct peer *peer)
 {
-	assert(membuf_num_elems(&peer->encrypted_peer_out) >= UNIFORM_MESSAGE_SIZE);
+	size_t avail = membuf_num_elems(&peer->encrypted_peer_out);
+	/* With padding: always a full uniform-size chunk.
+	 * Without: flush whatever we have (caller ensures non-zero). */
+	size_t write_size = peer->daemon->dev_uniform_padding
+		? UNIFORM_MESSAGE_SIZE : avail;
+
+	assert(avail >= write_size && write_size > 0);
 	return io_write_partial(peer->to_peer,
 				membuf_elems(&peer->encrypted_peer_out),
-				UNIFORM_MESSAGE_SIZE,
+				write_size,
 				&peer->encrypted_peer_out_sent,
 				write_to_peer, peer);
 }
@@ -1244,8 +1250,11 @@ static struct io_plan *write_to_peer(struct io_conn *peer_conn,
 				/* Wait for them to wake us */
 				return msg_queue_wait(peer_conn, peer->peer_outq, write_to_peer, peer);
 			}
-			/* OK, add padding. */
-			pad_encrypted_queue(peer);
+			/* OK, add padding (only if --dev-uniform-padding enabled). */
+			if (peer->daemon->dev_uniform_padding)
+				pad_encrypted_queue(peer);
+			else
+				break;
 		} else {
 			if (peer->draining_state == WRITING_TO_PEER)
 				status_peer_debug(&peer->id, "draining, but sending %s.",
@@ -1263,6 +1272,14 @@ static struct io_plan *write_to_peer(struct io_conn *peer_conn,
 	}
 
 	peer->nonurgent_flush_timer = tal_free(peer->nonurgent_flush_timer);
+
+	/* With uniform padding the buffer is always a full UNIFORM_MESSAGE_SIZE.
+	 * Without it, write whatever we have; if nothing, go back to waiting. */
+	if (have_empty_encrypted_queue(peer)) {
+		io_wake(&peer->subds);
+		io_wake(&peer->peer_in);
+		return msg_queue_wait(peer_conn, peer->peer_outq, write_to_peer, peer);
+	}
 	return write_encrypted_to_peer(peer);
 }