paymod: Add function to set the current step

This is necessary otherwise we would not be calling modifiers for the newly
set state which can lead to unexpected results.

plugins/libplugin-pay.c

@@ -731,7 +731,7 @@ payment_waitsendpay_finished(struct command *cmd, const char *buffer,
 		    json_tok_full_len(toks), json_tok_full(buffer, toks));
 
 	if (p->result->state == PAYMENT_COMPLETE) {
-		p->step = PAYMENT_STEP_SUCCESS;
+		payment_set_step(p, PAYMENT_STEP_SUCCESS);
 		p->end_time = time_now();
 		payment_continue(p);
 		return command_still_pending(cmd);
@@ -1230,6 +1230,12 @@ static void payment_finished(struct payment *p)
 	}
 }
 
+void payment_set_step(struct payment *p, enum payment_step newstep)
+{
+	p->current_modifier = -1;
+	p->step = newstep;
+}
+
 void payment_continue(struct payment *p)
 {
 	struct payment_modifier *mod;
@@ -1279,9 +1285,15 @@ void payment_continue(struct payment *p)
 
 void payment_fail(struct payment *p, const char *fmt, ...)
 {
+	va_list ap;
 	p->end_time = time_now();
-	p->step = PAYMENT_STEP_FAILED;
-	p->failreason = tal_steal(p, reason);
+	payment_set_step(p, PAYMENT_STEP_FAILED);
+	va_start(ap, fmt);
+	p->failreason = tal_vfmt(p, fmt, ap);
+	va_end(ap);
+
+	plugin_log(p->plugin, LOG_INFORM, "%s", p->failreason);
+
 	payment_continue(p);
 }
 
@@ -1398,7 +1410,7 @@ static inline void retry_step_cb(struct retry_mod_data *rd,
 	if (rdata->retries > 0) {
 		subpayment = payment_new(p, NULL, p, p->modifiers);
 		payment_start(subpayment);
-		p->step = PAYMENT_STEP_RETRY;
+		payment_set_step(p, PAYMENT_STEP_RETRY);
 		subpayment->why =
 		    tal_fmt(subpayment, "Still have %d attempts left",
 			    rdata->retries - 1);

plugins/libplugin-pay.h

@@ -344,6 +344,16 @@ struct payment *payment_new(tal_t *ctx, struct command *cmd,
 void payment_start(struct payment *p);
 void payment_continue(struct payment *p);
 
+/**
+ * Set the payment to the current step.
+ *
+ * This must be used by modifiers if they want to skip to a specific step. It
+ * ensures that the internal state is reset correctly and that all modifier
+ * callbacks are called once `payment_continue` is called again.
+ */
+void payment_set_step(struct payment *p, enum payment_step newstep);
+
+
 /* Fails a partial payment and continues with the core flow. */
 void payment_fail(struct payment *p, const char *fmt, ...) PRINTF_FMT(2,3);