paymod Collect the final failure when aggregating the tree results

We're lucky that we can distinguish the severity of the failure based on the
failcode, so we bubble up the one with the maximum failcode, and let callers
inspect details if they need more information.

plugins/libplugin-pay.c

@@ -91,6 +91,9 @@ struct payment_tree_result payment_collect_result(struct payment *p)
 	res.treestates = p->step;
 	res.leafstates = 0;
 	res.preimage = NULL;
+	res.failure = NULL;
+	if (p->step == PAYMENT_STEP_FAILED && p->result != NULL)
+		res.failure = p->result;
 
 	if (numchildren == 0) {
 		res.leafstates |= p->step;
@@ -122,6 +125,14 @@ struct payment_tree_result payment_collect_result(struct payment *p)
 		res.leafstates |= cres.leafstates;
 		res.treestates |= cres.treestates;
 		res.attempts += cres.attempts;
+
+		/* We bubble the failure result with the highest failcode up
+		 * to the root. */
+		if (res.failure == NULL ||
+		    (cres.failure != NULL &&
+		     cres.failure->failcode > res.failure->failcode)) {
+			res.failure = cres.failure;
+		}
 	}
 	return res;
 }

plugins/libplugin-pay.h

@@ -137,6 +137,11 @@ struct payment_tree_result {
 	struct preimage *preimage;
 
 	u32 attempts;
+
+	/* Pointer to the failure that caused the payment to fail. We just
+	 * take the one with the highest failcode, since that happen to match
+	 * the severity of the error. */
+	struct payment_result *failure;
 };
 
 struct payment {