Scope payment preimage in do_test_keysend_payments

b0d4ab8 fixed a nasty bug where
we'd failed to include the payment preimage in keysend onions at
all. Ultimately, this was a test failure - the existing test suite
should which did keysend payments were not structured in a way that
would fail in this case, instead using the same preimage variable
both for sending and receiving.

Here we improve the main keysend test tweaked by b0d4ab8
to make absolutely sure it cannot work if the preimage doesn't come
from the onion. We make the payment preimage on the sending side a
variable inside a scope which only exists for the send call. Once
that scope completes the payment preimage only exists in the
sending `ChannelManager`, which must have put it in the onion in
order for the receiving node to have it.

Author: TheBlueMatt

lightning/src/ln/payment_tests.rs

@@ -15,7 +15,7 @@ use crate::chain::{ChannelMonitorUpdateStatus, Confirm, Listen, Watch};
 use crate::chain::channelmonitor::{ANTI_REORG_DELAY, HTLC_FAIL_BACK_BUFFER, LATENCY_GRACE_PERIOD_BLOCKS};
 use crate::sign::EntropySource;
 use crate::chain::transaction::OutPoint;
-use crate::events::{ClosureReason, Event, HTLCDestination, MessageSendEvent, MessageSendEventsProvider, PathFailure, PaymentFailureReason};
+use crate::events::{ClosureReason, Event, HTLCDestination, MessageSendEvent, MessageSendEventsProvider, PathFailure, PaymentFailureReason, PaymentPurpose};
 use crate::ln::channel::EXPIRE_PREV_CONFIG_TICKS;
 use crate::ln::channelmanager::{BREAKDOWN_TIMEOUT, ChannelManager, MPP_TIMEOUT_TICKS, MIN_CLTV_EXPIRY_DELTA, PaymentId, PaymentSendFailure, IDEMPOTENCY_TIMEOUT_TICKS, RecentPaymentDetails, RecipientOnionFields, HTLCForwardInfo, PendingHTLCRouting, PendingAddHTLCInfo};
 use crate::ln::features::Bolt11InvoiceFeatures;
@@ -274,22 +274,31 @@ fn do_test_keysend_payments(public_node: bool, with_retry: bool) {
 		nodes[0].logger, &scorer, &(), &random_seed_bytes
 	).unwrap();
 
-	let test_preimage = PaymentPreimage([42; 32]);
-	let payment_hash = if with_retry {
-		nodes[0].node.send_spontaneous_payment_with_retry(Some(test_preimage),
-			RecipientOnionFields::spontaneous_empty(), PaymentId(test_preimage.0),
-			route_params, Retry::Attempts(1)).unwrap()
-	} else {
-		nodes[0].node.send_spontaneous_payment(&route, Some(test_preimage),
-			RecipientOnionFields::spontaneous_empty(), PaymentId(test_preimage.0)).unwrap()
-	};
+	{
+		let test_preimage = PaymentPreimage([42; 32]);
+		if with_retry {
+			nodes[0].node.send_spontaneous_payment_with_retry(Some(test_preimage),
+				RecipientOnionFields::spontaneous_empty(), PaymentId(test_preimage.0),
+				route_params, Retry::Attempts(1)).unwrap()
+		} else {
+			nodes[0].node.send_spontaneous_payment(&route, Some(test_preimage),
+				RecipientOnionFields::spontaneous_empty(), PaymentId(test_preimage.0)).unwrap()
+		};
+	}
 	check_added_monitors!(nodes[0], 1);
-	let mut events = nodes[0].node.get_and_clear_pending_msg_events();
-	assert_eq!(events.len(), 1);
-	let event = events.pop().unwrap();
-	let path = vec![&nodes[1]];
-	pass_along_path(&nodes[0], &path, 10000, payment_hash, None, event, true, Some(test_preimage));
-	claim_payment(&nodes[0], &path, test_preimage);
+	let send_event = SendEvent::from_node(&nodes[0]);
+	nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &send_event.msgs[0]);
+	do_commitment_signed_dance(&nodes[1], &nodes[0], &send_event.commitment_msg, false, false);
+	expect_pending_htlcs_forwardable!(nodes[1]);
+	// Previously, a refactor caused us to stop including the payment preimage in the onion which
+	// is sent as a part of keysend payments. Thus, to be extra careful here, we scope the preimage
+	// above to demonstrate that we have no way to get the preimage at this point except by
+	// extracting it from the onion nodes[1] received.
+	let event = nodes[1].node.get_and_clear_pending_events();
+	assert_eq!(event.len(), 1);
+	if let Event::PaymentClaimable { purpose: PaymentPurpose::SpontaneousPayment(preimage), .. } = event[0] {
+		claim_payment(&nodes[0], &[&nodes[1]], preimage);
+	} else { panic!(); }
 }
 
 #[test]