onion: Print the extracted channel_update and onionreply to debug

This adds one line with the onion and the channel_update we extract from
it. This in turn allows us to check that the channel_update in the onion is not
type prefixed, and that we patch it correctly before passing it to gossipd.

CHANGELOG.md

@@ -53,6 +53,8 @@ changes.
 - Fixed crash when shutting down during opening a channel (#1737)
 - Don't lose track of our own output when applying penalty transaction (#1738)
 - Protocol: `channel_update` inside error messages now refers to correct channel.
+- Stripping type prefix from `channel_update`s that are nested in an onion reply
+  to be compatible with eclair and lnd (#1730).
 
 ### Security
 

lightningd/pay.c

@@ -297,7 +297,8 @@ remote_routing_failure(const tal_t *ctx,
 		       bool *p_retry_plausible,
 		       bool *p_report_to_gossipd,
 		       const struct wallet_payment *payment,
-		       const struct onionreply *failure)
+		       const struct onionreply *failure,
+		       struct log *log)
 {
 	enum onion_type failcode = fromwire_peektype(failure->msg);
 	u8 *channel_update;
@@ -318,6 +319,11 @@ remote_routing_failure(const tal_t *ctx,
 	channel_update
 		= channel_update_from_onion_error(routing_failure,
 						  failure->msg);
+	if (channel_update)
+		log_debug(log, "Extracted channel_update %s from onionreply %s",
+			  tal_hex(tmpctx, channel_update),
+			  tal_hex(tmpctx, failure->msg));
+
 	retry_plausible = true;
 	report_to_gossipd = true;
 
@@ -521,7 +527,8 @@ void payment_failed(struct lightningd *ld, const struct htlc_out *hout,
 			fail = remote_routing_failure(tmpctx,
 						      &retry_plausible,
 						      &report_to_gossipd,
-						      payment, reply);
+						      payment, reply,
+						      hout->key.channel->log);
 		}
 	}
 

tests/test_lightningd.py

@@ -4725,6 +4725,13 @@ class LightningDTests(BaseLightningDTests):
         self.assertRaisesRegex(RpcError, 'WIRE_TEMPORARY_CHANNEL_FAILURE',
                                l1.rpc.waitsendpay, inv['payment_hash'])
 
+        # Make sure we get an onionreply, without the type prefix of the nested
+        # channel_update, and it should patch it to include a type prefix. The
+        # prefix 0x0102 should be in the channel_update, but not in the
+        # onionreply (negation of 0x0102 in the RE)
+        l1.daemon.wait_for_log(r'Extracted channel_update 0102.*from onionreply 10070080(?!.*0102)')
+
+        # And now monitor for l1 to apply the channel_update we just extracted
         l1.daemon.wait_for_log('Received channel_update for channel {}\(.\) now DISABLED was ACTIVE \(from error\)'.format(chanid2))
 
     def test_address(self):