renepay: add precondition check + error msg

Signed-off-by: Lagrang3 <[email protected]>

plugins/renepay/mods.c

@@ -654,7 +654,8 @@ static struct command_result *compute_routes_cb(struct payment *payment)
 
 	/* How much are we still trying to send? */
 	if (!amount_msat_sub(&remaining, payment->payment_info.amount,
-			     payment->total_delivering)) {
+			     payment->total_delivering) ||
+	    amount_msat_zero(remaining)) {
 		plugin_log(pay_plugin->plugin, LOG_UNUSUAL,
 			   "%s: Payment is pending with full amount already "
 			   "committed. We skip the computation of new routes.",

plugins/renepay/routebuilder.c

@@ -165,6 +165,11 @@ struct route **get_routes(const tal_t *ctx,
 				 "Failed to build disabled_bitmap.");
 		goto function_fail;
 	}
+	if (amount_msat_zero(amount_to_deliver)) {
+		tal_report_error(ctx, ecode, fail, RENEPAY_PRECONDITION_ERROR,
+				 "amount to deliver is zero");
+		goto function_fail;
+	}
 
 	/* Also disable every channel that we don't have in the chan_extra_map.
 	 * We might have channels in the gossmap that are not usable for