Log cases where an onion failure cannot be attributed or interpreted

Create more visibility into these edge cases. The non-attributable
failure in particular can be used to disrupt sender operation and it is
therefore good to at least log these cases clearly.

Author: joostjager

lightning/src/ln/onion_utils.rs

@@ -1044,22 +1044,31 @@ where
 		let amt_to_forward = htlc_msat - route_hop.fee_msat;
 		htlc_msat = amt_to_forward;
 
-		let err_packet = match decrypt_onion_error_packet(&mut encrypted_packet, shared_secret) {
-			Ok(p) => p,
-			Err(_) => return,
-		};
+		let decrypt_result = decrypt_onion_error_packet(&mut encrypted_packet, shared_secret);
+
 		let um = gen_um_from_shared_secret(shared_secret.as_ref());
 		let mut hmac = HmacEngine::<Sha256>::new(&um);
-		hmac.input(&err_packet.encode()[32..]);
+		hmac.input(&encrypted_packet[32..]);
 
-		if !fixed_time_eq(&Hmac::from_engine(hmac).to_byte_array(), &err_packet.hmac) {
+		if !fixed_time_eq(&Hmac::from_engine(hmac).to_byte_array(), &encrypted_packet[..32]) {
 			return;
 		}
+
+		let err_packet = match decrypt_result {
+			Ok(p) => p,
+			Err(_) => {
+				log_warn!(logger, "Unreadable failure from {}", route_hop.pubkey);
+				return;
+			},
+		};
+
 		let error_code_slice = match err_packet.failuremsg.get(0..2) {
 			Some(s) => s,
 			None => {
 				// Useless packet that we can't use but it passed HMAC, so it definitely came from the peer
 				// in question
+				log_warn!(logger, "Missing error code in failure from {}", route_hop.pubkey);
+
 				let network_update = Some(NetworkUpdate::NodeFailure {
 					node_id: route_hop.pubkey,
 					is_permanent: true,
@@ -1219,6 +1228,12 @@ where
 	} else {
 		// only not set either packet unparseable or hmac does not match with any
 		// payment not retryable only when garbage is from the final node
+		log_warn!(
+			logger,
+			"Non-attributable failure encountered on route {}",
+			path.hops.iter().map(|h| h.pubkey.to_string()).collect::<Vec<_>>().join("->")
+		);
+
 		DecodedOnionFailure {
 			network_update: None,
 			short_channel_id: None,