Make test_duplicate_payment_hash_one_failure_one_success robust

`test_duplicate_payment_hash_one_failure_one_success` currently
fails if the "wrong" HTLC is picked to be claimed. Given the HTLCs
are identical, there's no way to figure out which we should claim.
The test instead relies on a magic value - the first one is the
right one....unless we change our CSPRNG implementation. When we
try to do so, the test randomly fails.

Here we change one HTLC to a lower amount so we can figure out
which transaction to broadcast to make the test robust against
CSPRNG changes.

Author: TheBlueMatt

lightning/src/ln/functional_tests.rs

@@ -4718,8 +4718,8 @@ fn test_duplicate_payment_hash_one_failure_one_success() {
 	// ACCEPTED_HTLC_SCRIPT_WEIGHT.
 	let payment_params = PaymentParameters::from_node_id(nodes[3].node.get_our_node_id())
 		.with_features(nodes[3].node.invoice_features());
-	let (route, _, _, _) = get_route_and_payment_hash!(nodes[0], nodes[3], payment_params, 900000, TEST_FINAL_CLTV - 40);
-	send_along_route_with_secret(&nodes[0], route, &[&[&nodes[1], &nodes[2], &nodes[3]]], 900000, duplicate_payment_hash, payment_secret);
+	let (route, _, _, _) = get_route_and_payment_hash!(nodes[0], nodes[3], payment_params, 800_000, TEST_FINAL_CLTV - 40);
+	send_along_route_with_secret(&nodes[0], route, &[&[&nodes[1], &nodes[2], &nodes[3]]], 800_000, duplicate_payment_hash, payment_secret);
 
 	let commitment_txn = get_local_commitment_txn!(nodes[2], chan_2.2);
 	assert_eq!(commitment_txn[0].input.len(), 1);
@@ -4739,25 +4739,34 @@ fn test_duplicate_payment_hash_one_failure_one_success() {
 
 		check_spends!(node_txn[0], commitment_txn[0]);
 		assert_eq!(node_txn[0].input.len(), 1);
+		assert_eq!(node_txn[0].output.len(), 1);
 
 		if node_txn.len() > 2 {
 			check_spends!(node_txn[1], commitment_txn[0]);
 			assert_eq!(node_txn[1].input.len(), 1);
+			assert_eq!(node_txn[2].output.len(), 1);
 			assert_eq!(node_txn[0].input[0].previous_output, node_txn[1].input[0].previous_output);
 
 			check_spends!(node_txn[2], commitment_txn[0]);
+			assert_eq!(node_txn[2].input.len(), 1);
+			assert_eq!(node_txn[2].output.len(), 1);
 			assert_ne!(node_txn[0].input[0].previous_output, node_txn[2].input[0].previous_output);
 		} else {
 			check_spends!(node_txn[1], commitment_txn[0]);
+			assert_eq!(node_txn[1].input.len(), 1);
+			assert_eq!(node_txn[1].output.len(), 1);
 			assert_ne!(node_txn[0].input[0].previous_output, node_txn[1].input[0].previous_output);
 		}
 
 		assert_eq!(node_txn[0].input[0].witness.last().unwrap().len(), ACCEPTED_HTLC_SCRIPT_WEIGHT);
 		assert_eq!(node_txn[1].input[0].witness.last().unwrap().len(), ACCEPTED_HTLC_SCRIPT_WEIGHT);
+		// Assign htlc_timeout_tx to the second HTLC (with value ~800 sats).
 		if node_txn.len() > 2 {
 			assert_eq!(node_txn[2].input[0].witness.last().unwrap().len(), ACCEPTED_HTLC_SCRIPT_WEIGHT);
+			htlc_timeout_tx = if node_txn[2].output[0].value < 900 { node_txn[2].clone() } else { node_txn[0].clone() };
+		} else {
+			htlc_timeout_tx = if node_txn[0].output[0].value < 900 { node_txn[1].clone() } else { node_txn[0].clone() };
 		}
-		htlc_timeout_tx = node_txn[0].clone();
 	}
 
 	nodes[2].node.claim_funds(our_payment_preimage);
@@ -4808,7 +4817,7 @@ fn test_duplicate_payment_hash_one_failure_one_success() {
 	// Note that the fee paid is effectively double as the HTLC value (including the nodes[1] fee
 	// and nodes[2] fee) is rounded down and then claimed in full.
 	mine_transaction(&nodes[1], &htlc_success_txn[1]);
-	expect_payment_forwarded!(nodes[1], nodes[0], nodes[2], Some(196*2), true, true);
+	expect_payment_forwarded!(nodes[1], nodes[0], nodes[2], Some(196), true, true);
 	let updates = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
 	assert!(updates.update_add_htlcs.is_empty());
 	assert!(updates.update_fail_htlcs.is_empty());