paymod: Allow callers to opt out of shadow routing amount fuzzing

With MPP we require that the sum of parts is equal to the `total_msat` amount
declared in the onion. Since that can't be changed once the first part arrives
we need a way to disable amount fuzzing for MPP.

plugins/libplugin-pay.c

@@ -1700,10 +1700,13 @@ REGISTER_PAYMENT_MODIFIER(exemptfee, struct exemptfee_data *,
 
 static struct shadow_route_data *shadow_route_init(struct payment *p)
 {
-	if (p->parent != NULL)
+	if (p->parent != NULL) {
 		return payment_mod_shadowroute_get_data(p->parent);
-	else
+	} else {
-		return tal(p, struct shadow_route_data);
+		struct shadow_route_data *d = tal(p, struct shadow_route_data);
+		d->fuzz_amount = true;
+		return d;
+	}
 }
 
 /* Mutual recursion */
@@ -1810,8 +1813,12 @@ static struct command_result *shadow_route_listchannels(struct command *cmd,
 
 		/* And now the thing that caused all of this: adjust the call
 		 * to getroute. */
-		ok &= amount_msat_add(&p->getroute->amount, p->getroute->amount,
+		if (d->fuzz_amount) {
-				      best_fee);
+			/* Only fuzz the amount to route to the destination if
+			 * we didn't opt-out earlier. */
+			ok &= amount_msat_add(&p->getroute->amount,
+					      p->getroute->amount, best_fee);
+		}
 		p->getroute->cltv += best->cltv_expiry_delta;
 		assert(ok);
 	}

plugins/libplugin-pay.h

@@ -307,6 +307,11 @@ struct shadow_route_data {
 	struct payment_constraints constraints;
 	struct node_id destination;
 	struct route_hop *route;
+
+	/* multi-part payments require the sum of parts to be the exact
+	 * amount, so we allow the payment flow to opt out of fuzzing the
+	 * amount. */
+	bool fuzz_amount;
 };
 
 struct direct_pay_data {