Drop return value from fail_htlc_backwards, clarify docs

`ChannelManager::fail_htlc_backwards`' bool return value is quite
confusing - just because it returns false doesn't mean the payment
wasn't (already) failed. Worse, in some race cases around shutdown
where a payment was claimed before an unclean shutdown and then
retried on startup, `fail_htlc_backwards` could return true even
though (a duplicate copy of the same payment) was claimed, but the
claim event has not been seen by the user yet.

While its possible to use it correctly, its somewhat confusing to
have a return value at all, and definitely lends itself to misuse.

Instead, we should push users towards a model where they don't care
if `fail_htlc_backwards` succeeds - either they've locally marked
the payment as failed (prior to seeing any `PaymentReceived`
events) and will fail any attempts to pay it, or they have not and
the payment is still receivable until its timeout time is reached.

We can revisit this decision based on user feedback, but will need
to very carefully document the potential failure modes here if we
do.

Author: TheBlueMatt

fuzz/src/chanmon_consistency.rs

@@ -840,7 +840,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out) {
 						events::Event::PaymentReceived { payment_hash, .. } => {
 							if claim_set.insert(payment_hash.0) {
 								if $fail {
-									assert!(nodes[$node].fail_htlc_backwards(&payment_hash));
+									nodes[$node].fail_htlc_backwards(&payment_hash);
 								} else {
 									nodes[$node].claim_funds(PaymentPreimage(payment_hash.0));
 								}

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -831,7 +831,7 @@ fn do_test_monitor_update_fail_raa(test_ignore_second_cs: bool) {
 	let (_, payment_hash_1, _) = route_payment(&nodes[0], &[&nodes[1], &nodes[2]], 1000000);
 
 	// Fail the payment backwards, failing the monitor update on nodes[1]'s receipt of the RAA
-	assert!(nodes[2].node.fail_htlc_backwards(&payment_hash_1));
+	nodes[2].node.fail_htlc_backwards(&payment_hash_1);
 	expect_pending_htlcs_forwardable!(nodes[2]);
 	check_added_monitors!(nodes[2], 1);
 
@@ -1696,7 +1696,7 @@ fn test_monitor_update_on_pending_forwards() {
 	send_payment(&nodes[0], &[&nodes[1], &nodes[2]], 5000000);
 
 	let (_, payment_hash_1, _) = route_payment(&nodes[0], &[&nodes[1], &nodes[2]], 1000000);
-	assert!(nodes[2].node.fail_htlc_backwards(&payment_hash_1));
+	nodes[2].node.fail_htlc_backwards(&payment_hash_1);
 	expect_pending_htlcs_forwardable!(nodes[2]);
 	check_added_monitors!(nodes[2], 1);
 
@@ -2468,7 +2468,7 @@ fn do_test_reconnect_dup_htlc_claims(htlc_status: HTLCStatusAtDupClaim, second_f
 		payment_preimage,
 	};
 	if second_fails {
-		assert!(nodes[2].node.fail_htlc_backwards(&payment_hash));
+		nodes[2].node.fail_htlc_backwards(&payment_hash);
 		expect_pending_htlcs_forwardable!(nodes[2]);
 		check_added_monitors!(nodes[2], 1);
 		get_htlc_update_msgs!(nodes[2], nodes[1].node.get_our_node_id());

lightning/src/ln/channelmanager.rs

@@ -3479,9 +3479,17 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// Indicates that the preimage for payment_hash is unknown or the received amount is incorrect
 	/// after a PaymentReceived event, failing the HTLC back to its origin and freeing resources
 	/// along the path (including in our own channel on which we received it).
-	/// Returns false if no payment was found to fail backwards, true if the process of failing the
-	/// HTLC backwards has been started.
-	pub fn fail_htlc_backwards(&self, payment_hash: &PaymentHash) -> bool {
+	///
+	/// Note that in some cases around unclean shutdown, it is possible the payment may have
+	/// already been claimed by you via [`ChannelManager::claim_funds`] prior to you seeing (a
+	/// second copy of) the [`events::Event::PaymentReceived`] event. Alternatively, the payment
+	/// may have already been failed automatically by LDK if it was nearing its expiration time.
+	///
+	/// While LDK will never claim a payment automatically on your behalf (i.e. without you calling
+	/// [`ChannelManager::claim_funds`]), you should still monitor for
+	/// [`events::Event::PaymentClaimed`] events even for payments you intend to fail, especially on
+	/// startup during which time claims which were in-progress at shutdown may be replayed.
+	pub fn fail_htlc_backwards(&self, payment_hash: &PaymentHash) {
 		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(&self.total_consistency_lock, &self.persistence_notifier);
 
 		let mut channel_state = Some(self.channel_state.lock().unwrap());
@@ -3496,8 +3504,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 						HTLCSource::PreviousHopData(htlc.prev_hop), payment_hash,
 						HTLCFailReason::Reason { failure_code: 0x4000 | 15, data: htlc_msat_height_data });
 			}
-			true
-		} else { false }
+		}
 	}
 
 	/// Gets an HTLC onion failure code and error data for an `UPDATE` error, given the error code

lightning/src/ln/functional_test_utils.rs

@@ -1717,7 +1717,7 @@ pub fn fail_payment_along_route<'a, 'b, 'c>(origin_node: &Node<'a, 'b, 'c>, expe
 	for path in expected_paths.iter() {
 		assert_eq!(path.last().unwrap().node.get_our_node_id(), expected_paths[0].last().unwrap().node.get_our_node_id());
 	}
-	assert!(expected_paths[0].last().unwrap().node.fail_htlc_backwards(&our_payment_hash));
+	expected_paths[0].last().unwrap().node.fail_htlc_backwards(&our_payment_hash);
 	expect_pending_htlcs_forwardable!(expected_paths[0].last().unwrap());
 
 	pass_failed_payment_back(origin_node, expected_paths, skip_last, our_payment_hash);
@@ -1823,7 +1823,7 @@ pub fn pass_failed_payment_back<'a, 'b, 'c>(origin_node: &Node<'a, 'b, 'c>, expe
 	}
 
 	// Ensure that fail_htlc_backwards is idempotent.
-	assert!(!expected_paths[0].last().unwrap().node.fail_htlc_backwards(&our_payment_hash));
+	expected_paths[0].last().unwrap().node.fail_htlc_backwards(&our_payment_hash);
 	assert!(expected_paths[0].last().unwrap().node.get_and_clear_pending_events().is_empty());
 	assert!(expected_paths[0].last().unwrap().node.get_and_clear_pending_msg_events().is_empty());
 	check_added_monitors!(expected_paths[0].last().unwrap(), 0);

lightning/src/ln/functional_tests.rs

@@ -3052,7 +3052,7 @@ fn do_test_commitment_revoked_fail_backward_exhaustive(deliver_bs_raa: bool, use
 	let (_, second_payment_hash, _) = route_payment(&nodes[0], &[&nodes[1], &nodes[2]], value);
 	let (_, third_payment_hash, _) = route_payment(&nodes[0], &[&nodes[1], &nodes[2]], value);
 
-	assert!(nodes[2].node.fail_htlc_backwards(&first_payment_hash));
+	nodes[2].node.fail_htlc_backwards(&first_payment_hash);
 	expect_pending_htlcs_forwardable!(nodes[2]);
 	check_added_monitors!(nodes[2], 1);
 	let updates = get_htlc_update_msgs!(nodes[2], nodes[1].node.get_our_node_id());
@@ -3065,7 +3065,7 @@ fn do_test_commitment_revoked_fail_backward_exhaustive(deliver_bs_raa: bool, use
 	let bs_raa = commitment_signed_dance!(nodes[1], nodes[2], updates.commitment_signed, false, true, false, true);
 	// Drop the last RAA from 3 -> 2
 
-	assert!(nodes[2].node.fail_htlc_backwards(&second_payment_hash));
+	nodes[2].node.fail_htlc_backwards(&second_payment_hash);
 	expect_pending_htlcs_forwardable!(nodes[2]);
 	check_added_monitors!(nodes[2], 1);
 	let updates = get_htlc_update_msgs!(nodes[2], nodes[1].node.get_our_node_id());
@@ -3082,7 +3082,7 @@ fn do_test_commitment_revoked_fail_backward_exhaustive(deliver_bs_raa: bool, use
 	nodes[2].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &as_raa);
 	check_added_monitors!(nodes[2], 1);
 
-	assert!(nodes[2].node.fail_htlc_backwards(&third_payment_hash));
+	nodes[2].node.fail_htlc_backwards(&third_payment_hash);
 	expect_pending_htlcs_forwardable!(nodes[2]);
 	check_added_monitors!(nodes[2], 1);
 	let updates = get_htlc_update_msgs!(nodes[2], nodes[1].node.get_our_node_id());
@@ -5438,10 +5438,10 @@ fn do_test_fail_backwards_unrevoked_remote_announce(deliver_last_raa: bool, anno
 
 	// Now fail back three of the over-dust-limit and three of the under-dust-limit payments in one go.
 	// Fail 0th below-dust, 4th above-dust, 8th above-dust, 10th below-dust HTLCs
-	assert!(nodes[4].node.fail_htlc_backwards(&payment_hash_1));
-	assert!(nodes[4].node.fail_htlc_backwards(&payment_hash_3));
-	assert!(nodes[4].node.fail_htlc_backwards(&payment_hash_5));
-	assert!(nodes[4].node.fail_htlc_backwards(&payment_hash_6));
+	nodes[4].node.fail_htlc_backwards(&payment_hash_1);
+	nodes[4].node.fail_htlc_backwards(&payment_hash_3);
+	nodes[4].node.fail_htlc_backwards(&payment_hash_5);
+	nodes[4].node.fail_htlc_backwards(&payment_hash_6);
 	check_added_monitors!(nodes[4], 0);
 	expect_pending_htlcs_forwardable!(nodes[4]);
 	check_added_monitors!(nodes[4], 1);
@@ -5454,8 +5454,8 @@ fn do_test_fail_backwards_unrevoked_remote_announce(deliver_last_raa: bool, anno
 	commitment_signed_dance!(nodes[3], nodes[4], four_removes.commitment_signed, false);
 
 	// Fail 3rd below-dust and 7th above-dust HTLCs
-	assert!(nodes[5].node.fail_htlc_backwards(&payment_hash_2));
-	assert!(nodes[5].node.fail_htlc_backwards(&payment_hash_4));
+	nodes[5].node.fail_htlc_backwards(&payment_hash_2);
+	nodes[5].node.fail_htlc_backwards(&payment_hash_4);
 	check_added_monitors!(nodes[5], 0);
 	expect_pending_htlcs_forwardable!(nodes[5]);
 	check_added_monitors!(nodes[5], 1);
@@ -5880,7 +5880,7 @@ fn do_htlc_claim_previous_remote_commitment_only(use_dust: bool, check_revoke_no
 	// actually revoked.
 	let htlc_value = if use_dust { 50000 } else { 3000000 };
 	let (_, our_payment_hash, _) = route_payment(&nodes[0], &[&nodes[1]], htlc_value);
-	assert!(nodes[1].node.fail_htlc_backwards(&our_payment_hash));
+	nodes[1].node.fail_htlc_backwards(&our_payment_hash);
 	expect_pending_htlcs_forwardable!(nodes[1]);
 	check_added_monitors!(nodes[1], 1);
 
@@ -7056,7 +7056,7 @@ fn do_test_failure_delay_dust_htlc_local_commitment(announce_latest: bool) {
 	let as_prev_commitment_tx = get_local_commitment_txn!(nodes[0], chan.2);
 
 	// Fail one HTLC to prune it in the will-be-latest-local commitment tx
-	assert!(nodes[1].node.fail_htlc_backwards(&payment_hash_2));
+	nodes[1].node.fail_htlc_backwards(&payment_hash_2);
 	check_added_monitors!(nodes[1], 0);
 	expect_pending_htlcs_forwardable!(nodes[1]);
 	check_added_monitors!(nodes[1], 1);

lightning/src/ln/onion_route_tests.rs

@@ -1134,7 +1134,7 @@ fn test_phantom_failure_reject_payment() {
 	expect_pending_htlcs_forwardable_ignore!(nodes[1]);
 	nodes[1].node.process_pending_htlc_forwards();
 	expect_payment_received!(nodes[1], payment_hash, payment_secret, recv_amt_msat);
-	assert!(nodes[1].node.fail_htlc_backwards(&payment_hash));
+	nodes[1].node.fail_htlc_backwards(&payment_hash);
 	expect_pending_htlcs_forwardable_ignore!(nodes[1]);
 	nodes[1].node.process_pending_htlc_forwards();