Calculate Trampoline onion size dynamically

For outbound Trampoline payments, we will need to generate onion
packets whose size matches their content. This allows them to be
included in individual hops at arbitrary positions.

A future spec change may involve defining a static size to improve
privacy from Trampoline nodes.

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -427,12 +427,30 @@ pub(super) fn construct_onion_packet(
 #[allow(unused)]
 pub(super) fn construct_trampoline_onion_packet(
 	payloads: Vec<msgs::OutboundTrampolinePayload>, onion_keys: Vec<OnionKeys>,
-	prng_seed: [u8; 32], associated_data: &PaymentHash, length: u16,
+	prng_seed: [u8; 32], associated_data: &PaymentHash, length: Option<u16>,
 ) -> Result<msgs::TrampolineOnionPacket, ()> {
-	let mut packet_data = vec![0u8; length as usize];
+	let minimum_packet_length = payloads.iter().map(|p| p.serialized_length() + 32).sum();
 
+	debug_assert!(
+		minimum_packet_length < ONION_DATA_LEN,
+		"Trampoline onion packet must be smaller than outer onion"
+	);
+	if minimum_packet_length >= ONION_DATA_LEN {
+		return Err(());
+	}
+
+	let packet_length = length.map(|l| usize::from(l)).unwrap_or(minimum_packet_length);
+	debug_assert!(
+		packet_length >= minimum_packet_length,
+		"Packet length cannot be smaller than the payloads require."
+	);
+	if packet_length < minimum_packet_length {
+		return Err(());
+	}
+
+	let mut packet_data = vec![0u8; packet_length];
 	let mut chacha = ChaCha20::new(&prng_seed, &[0; 8]);
-	chacha.process(&vec![0u8; length as usize], &mut packet_data);
+	chacha.process_in_place(&mut packet_data);
 
 	construct_onion_packet_with_init_noise::<_, _>(
 		payloads,