Add SendError enum for onion messages and error on too-big packets

XXX test

Author: valentinewallace

lightning/src/onion_message.rs

@@ -403,6 +403,18 @@ impl Destination {
 	}
 }
 
+/// Errors that may occur when [sending an onion message].
+///
+/// [sending an onion message]: OnionMessenger::send_onion_message
+#[derive(Debug)]
+pub enum SendError {
+	/// Errored computing onion message packet keys.
+	Secp256k1(secp256k1::Error),
+	/// Because implementations such as Eclair will drop onion messages where the message packet
+	/// exceeds 32834 bytes, we refuse to send messages where the packet exceeds this size.
+	TooBigPacket,
+}
+
 /// A sender, receiver and forwarder of onion messages. In upcoming releases, this object will be
 /// used to retrieve invoices and fulfill invoice requests from [offers].
 ///
@@ -438,7 +450,7 @@ impl<Signer: Sign, K: Deref, L: Deref> OnionMessenger<Signer, K, L>
 	}
 
 	/// Send an empty onion message to `destination`, routing it through `intermediate_nodes`.
-	pub fn send_onion_message(&self, intermediate_nodes: Vec<PublicKey>, destination: Destination) -> Result<(), secp256k1::Error> {
+	pub fn send_onion_message(&self, intermediate_nodes: Vec<PublicKey>, destination: Destination) -> Result<(), SendError> {
 		let blinding_secret_bytes = self.keys_manager.get_secure_random_bytes();
 		let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
 		let (introduction_node_id, blinding_point) = if intermediate_nodes.len() != 0 {
@@ -451,9 +463,17 @@ impl<Signer: Sign, K: Deref, L: Deref> OnionMessenger<Signer, K, L>
 			}
 		};
 		let (encrypted_data_keys, onion_packet_keys) = construct_sending_keys(
-			&self.secp_ctx, &intermediate_nodes, &destination, &blinding_secret)?;
+			&self.secp_ctx, &intermediate_nodes, &destination, &blinding_secret)
+			.map_err(|e| SendError::Secp256k1(e))?;
 		let payloads = build_payloads(intermediate_nodes, destination, encrypted_data_keys);
 
+		// Check whether the onion message is too big to send.
+		let payloads_serialized_len = payloads.iter()
+			.fold(0, |total, next_payload| total + next_payload.serialized_length() + 32 /* HMAC */ );
+		if payloads_serialized_len > BIG_PACKET_HOP_DATA_LEN {
+			return Err(SendError::TooBigPacket)
+		}
+
 		let prng_seed = self.keys_manager.get_secure_random_bytes();
 		let onion_packet = onion_utils::construct_onion_message_packet(payloads, onion_packet_keys, prng_seed);