pay: inject channel updates from errors ourselves.

Cut & paste from gossipd.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/libplugin-pay.c

@@ -12,6 +12,7 @@
 #include <common/type_to_string.h>
 #include <errno.h>
 #include <plugins/libplugin-pay.h>
+#include <wire/peer_wire.h>
 
 static struct gossmap *global_gossmap;
 
@@ -1340,12 +1341,105 @@ static bool assign_blame(const struct payment *p,
 	return true;
 }
 
+/* Fix up the channel_update to include the type if it doesn't currently have
+ * one. See ElementsProject/lightning#1730 and lightningnetwork/lnd#1599 for the
+ * in-depth discussion on why we break message parsing here... */
+static u8 *patch_channel_update(const tal_t *ctx, u8 *channel_update TAKES)
+{
+	u8 *fixed;
+	if (channel_update != NULL &&
+	    fromwire_peektype(channel_update) != WIRE_CHANNEL_UPDATE) {
+		/* This should be a channel_update, prefix with the
+		 * WIRE_CHANNEL_UPDATE type, but isn't. Let's prefix it. */
+		fixed = tal_arr(ctx, u8, 0);
+		towire_u16(&fixed, WIRE_CHANNEL_UPDATE);
+		towire(&fixed, channel_update, tal_bytelen(channel_update));
+		if (taken(channel_update))
+			tal_free(channel_update);
+		return fixed;
+	} else {
+		return tal_dup_talarr(ctx, u8, channel_update);
+	}
+}
+
+/* Return NULL if the wrapped onion error message has no channel_update field,
+ * or return the embedded channel_update message otherwise. */
+static u8 *channel_update_from_onion_error(const tal_t *ctx,
+					   const u8 *onion_message)
+{
+	u8 *channel_update = NULL;
+	struct amount_msat unused_msat;
+	u32 unused32;
+
+	/* Identify failcodes that have some channel_update.
+	 *
+	 * TODO > BOLT 1.0: Add new failcodes when updating to a
+	 * new BOLT version. */
+	if (!fromwire_temporary_channel_failure(ctx,
+						onion_message,
+						&channel_update) &&
+	    !fromwire_amount_below_minimum(ctx,
+					   onion_message, &unused_msat,
+					   &channel_update) &&
+	    !fromwire_fee_insufficient(ctx,
+		    		       onion_message, &unused_msat,
+				       &channel_update) &&
+	    !fromwire_incorrect_cltv_expiry(ctx,
+		    			    onion_message, &unused32,
+					    &channel_update) &&
+	    !fromwire_expiry_too_soon(ctx,
+		    		      onion_message,
+				      &channel_update))
+		/* No channel update. */
+		return NULL;
+
+	return patch_channel_update(ctx, take(channel_update));
+}
+
+
+static struct command_result *
+payment_addgossip_success(struct command *cmd, const char *buffer,
+			  const jsmntok_t *toks, struct payment *p)
+{
+	const struct node_id *errnode;
+	const struct route_hop *errchan;
+
+	if (!assign_blame(p, &errnode, &errchan)) {
+		paymod_log(p, LOG_UNUSUAL,
+			   "No erring_index set in `waitsendpay` result: %.*s",
+			   json_tok_full_len(toks),
+			   json_tok_full(buffer, toks));
+		/* FIXME: Pick a random channel to fail? */
+		payment_set_step(p, PAYMENT_STEP_FAILED);
+		payment_continue(p);
+		return command_still_pending(cmd);
+	}
+
+	if (!errchan)
+		return handle_final_failure(cmd, p, errnode,
+					    p->result->failcode);
+
+	return handle_intermediate_failure(cmd, p, errnode, errchan,
+					   p->result->failcode);
+}
+
+/* If someone gives us an invalid update, all we can do is log it */
+static struct command_result *
+payment_addgossip_failure(struct command *cmd, const char *buffer,
+			  const jsmntok_t *toks, struct payment *p)
+{
+	paymod_log(p, LOG_DBG, "Invalid channel_update: %.*s",
+		   json_tok_full_len(toks),
+		   json_tok_full(buffer, toks));
+
+	return payment_addgossip_success(cmd, NULL, NULL, p);
+}
+
 static struct command_result *
 payment_waitsendpay_finished(struct command *cmd, const char *buffer,
 			     const jsmntok_t *toks, struct payment *p)
 {
-	const struct node_id *errnode;
-	const struct route_hop *errchan;
+	u8 *update;
 
 	assert(p->route != NULL);
 
@@ -1372,23 +1466,19 @@ payment_waitsendpay_finished(struct command *cmd, const char *buffer,
 
 	payment_chanhints_apply_route(p, true);
 
-	if (!assign_blame(p, &errnode, &errchan)) {
-		paymod_log(p, LOG_UNUSUAL,
-			   "No erring_index set in `waitsendpay` result: %.*s",
-			   json_tok_full_len(toks),
-			   json_tok_full(buffer, toks));
-		/* FIXME: Pick a random channel to fail? */
-		payment_set_step(p, PAYMENT_STEP_FAILED);
-		payment_continue(p);
+	/* Tell gossipd, if we received an update */
+	update = channel_update_from_onion_error(tmpctx, p->result->raw_message);
+	if (update) {
+		struct out_req *req;
+		req = jsonrpc_request_start(p->plugin, NULL, "addgossip",
+					    payment_addgossip_success,
+					    payment_addgossip_failure, p);
+		json_add_hex_talarr(req->js, "message", update);
+		send_outreq(p->plugin, req);
 		return command_still_pending(cmd);
 	}
 
-	if (!errchan)
-		return handle_final_failure(cmd, p, errnode,
-					    p->result->failcode);
-
-	return handle_intermediate_failure(cmd, p, errnode, errchan,
-					   p->result->failcode);
+	return payment_addgossip_success(cmd, NULL, NULL, p);
 }
 
 static struct command_result *payment_sendonion_success(struct command *cmd,