Expose a Balance for inbound HTLCs even without a preimage

If we don't currently have the preimage for an inbound HTLC, that
does not guarantee we can never claim it, but instead only that we
cannot claim it unless we receive the preimage from the channel we
forwarded the channel out on.

Thus, we cannot consider a channel to have no claimable balances if
the only remaining output on the commitment ransaction is an
inbound HTLC for which we do not have the preimage, as we may be
able to claim it in the future.

This commit addresses this issue by adding a new `Balance` variant
- `MaybePreimageClaimableHTLCAwaitingTimeout`, which is generated
until the HTLC output is spent.

Fixes #1620

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -579,13 +579,24 @@ pub enum Balance {
 	/// fees) if the counterparty does not know the preimage for the HTLCs. These are somewhat
 	/// likely to be claimed by our counterparty before we do.
 	MaybeClaimableHTLCAwaitingTimeout {
-		/// The amount available to claim, in satoshis, excluding the on-chain fees which will be
-		/// required to do so.
+		/// The amount potentially available to claim, in satoshis, excluding the on-chain fees
+		/// which will be required to do so.
 		claimable_amount_satoshis: u64,
 		/// The height at which we will be able to claim the balance if our counterparty has not
 		/// done so.
 		claimable_height: u32,
 	},
+	/// HTLCs which we received from our counterparty which are claimable with a preimage which we
+	/// do not currently have. This will only be claimable if we receive the preimage from the node
+	/// to which we forwarded this HTLC before the timeout.
+	MaybePreimageClaimableHTLC {
+		/// The amount potentially available to claim, in satoshis, excluding the on-chain fees
+		/// which will be required to do so.
+		claimable_amount_satoshis: u64,
+		/// The height at which our counterparty will be able to claim the balance if we have not
+		/// yet received the preimage and claimed it ourselves.
+		expiry_height: u32,
+	},
 	/// The channel has been closed, and our counterparty broadcasted a revoked commitment
 	/// transaction.
 	///
@@ -1565,6 +1576,11 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 					timeout_height: htlc.cltv_expiry,
 				});
 			}
+		} else if htlc_resolved.is_none() {
+			return Some(Balance::MaybePreimageClaimableHTLC {
+				claimable_amount_satoshis: htlc.amount_msat / 1000,
+				expiry_height: htlc.cltv_expiry,
+			});
 		}
 		None
 	}
@@ -1728,6 +1744,13 @@ impl<Signer: Sign> ChannelMonitor<Signer> {
 					});
 				} else if us.payment_preimages.get(&htlc.payment_hash).is_some() {
 					claimable_inbound_htlc_value_sat += htlc.amount_msat / 1000;
+				} else {
+					// As long as the HTLC is still in our latest commitment state, treat
+					// it as potentially claimable, even if it has long-since expired.
+					res.push(Balance::MaybePreimageClaimableHTLC {
+						claimable_amount_satoshis: htlc.amount_msat / 1000,
+						expiry_height: htlc.cltv_expiry,
+					});
 				}
 			}
 			res.push(Balance::ClaimableOnChannelClose {

lightning/src/ln/monitor_tests.rs

@@ -288,10 +288,16 @@ fn do_test_claim_value_force_close(prev_commitment_tx: bool) {
 			claimable_height: htlc_cltv_timeout,
 		}]),
 		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
-	assert_eq!(vec![Balance::ClaimableOnChannelClose {
+	assert_eq!(sorted_vec(vec![Balance::ClaimableOnChannelClose {
 			claimable_amount_satoshis: 1_000,
-		}],
-		nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances());
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 3_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 4_000,
+			expiry_height: htlc_cltv_timeout,
+		}]),
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
 
 	nodes[1].node.claim_funds(payment_preimage);
 	check_added_monitors!(nodes[1], 1);
@@ -747,6 +753,238 @@ fn test_balances_on_local_commitment_htlcs() {
 	test_spendable_output(&nodes[0], &as_txn[1]);
 }
 
+#[test]
+fn test_no_preimage_inbound_htlc_balances() {
+	// Tests that MaybePreimageClaimableHTLC are generated for inbound HTLCs for which we do not
+	// have a preimage.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let (_, _, chan_id, funding_tx) = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 500_000_000, InitFeatures::known(), InitFeatures::known());
+	let funding_outpoint = OutPoint { txid: funding_tx.txid(), index: 0 };
+
+	// Send two HTLCs, one from A to B, and one from B to A.
+	let to_b_failed_payment_hash = route_payment(&nodes[0], &[&nodes[1]], 10_000_000).1;
+	let to_a_failed_payment_hash = route_payment(&nodes[1], &[&nodes[0]], 20_000_000).1;
+	let htlc_cltv_timeout = nodes[0].best_block_info().1 + TEST_FINAL_CLTV + 1; // Note ChannelManager adds one to CLTV timeouts for safety
+
+	let chan_feerate = get_feerate!(nodes[0], chan_id) as u64;
+	let opt_anchors = get_opt_anchors!(nodes[0], chan_id);
+
+	// Both A and B will have an HTLC that's claimable on timeout and one that's claimable if they
+	// receive the preimage. These will remain the same through the channel closure and until the
+	// HTLC output is spent.
+
+	assert_eq!(sorted_vec(vec![Balance::ClaimableOnChannelClose {
+			claimable_amount_satoshis: 1_000_000 - 500_000 - 10_000 - chan_feerate *
+				(channel::commitment_tx_base_weight(opt_anchors) + 2 * channel::COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 20_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::MaybeClaimableHTLCAwaitingTimeout {
+			claimable_amount_satoshis: 10_000,
+			claimable_height: htlc_cltv_timeout,
+		}]),
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	assert_eq!(sorted_vec(vec![Balance::ClaimableOnChannelClose {
+			claimable_amount_satoshis: 500_000 - 20_000,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 10_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::MaybeClaimableHTLCAwaitingTimeout {
+			claimable_amount_satoshis: 20_000,
+			claimable_height: htlc_cltv_timeout,
+		}]),
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	// Get nodes[0]'s commitment transaction and HTLC-Timeout transaction
+	let as_txn = get_local_commitment_txn!(nodes[0], chan_id);
+	assert_eq!(as_txn.len(), 2);
+	check_spends!(as_txn[1], as_txn[0]);
+	check_spends!(as_txn[0], funding_tx);
+
+	// Now close the channel by confirming A's commitment transaction on both nodes, checking the
+	// claimable balances remain the same except for the non-HTLC balance changing variant.
+	let node_a_commitment_claimable = nodes[0].best_block_info().1 + BREAKDOWN_TIMEOUT as u32;
+	let as_pre_spend_claims = sorted_vec(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 1_000_000 - 500_000 - 10_000 - chan_feerate *
+				(channel::commitment_tx_base_weight(opt_anchors) + 2 * channel::COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000,
+			confirmation_height: node_a_commitment_claimable,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 20_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::MaybeClaimableHTLCAwaitingTimeout {
+			claimable_amount_satoshis: 10_000,
+			claimable_height: htlc_cltv_timeout,
+		}]);
+
+	mine_transaction(&nodes[0], &as_txn[0]);
+	nodes[0].tx_broadcaster.txn_broadcasted.lock().unwrap().clear();
+	check_added_monitors!(nodes[0], 1);
+	check_closed_broadcast!(nodes[0], true);
+	check_closed_event!(nodes[0], 1, ClosureReason::CommitmentTxConfirmed);
+
+	assert_eq!(as_pre_spend_claims,
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	mine_transaction(&nodes[1], &as_txn[0]);
+	check_added_monitors!(nodes[1], 1);
+	check_closed_broadcast!(nodes[1], true);
+	check_closed_event!(nodes[1], 1, ClosureReason::CommitmentTxConfirmed);
+
+	let node_b_commitment_claimable = nodes[1].best_block_info().1 + ANTI_REORG_DELAY - 1;
+	let mut bs_pre_spend_claims = sorted_vec(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 500_000 - 20_000,
+			confirmation_height: node_b_commitment_claimable,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 10_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::MaybeClaimableHTLCAwaitingTimeout {
+			claimable_amount_satoshis: 20_000,
+			claimable_height: htlc_cltv_timeout,
+		}]);
+	assert_eq!(bs_pre_spend_claims,
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	// We'll broadcast the HTLC-Timeout transaction one block prior to the htlc's expiration (as it
+	// is confirmable in the next block), but will still include the same claimable balances as no
+	// HTLC has been spent, even after the HTLC expires. We'll also fail the inbound HTLC, but it
+	// won't do anything as the channel is already closed.
+
+	connect_blocks(&nodes[0], TEST_FINAL_CLTV - 1);
+	let as_htlc_timeout_claim = nodes[0].tx_broadcaster.txn_broadcasted.lock().unwrap().split_off(0);
+	assert_eq!(as_htlc_timeout_claim.len(), 1);
+	check_spends!(as_htlc_timeout_claim[0], as_txn[0]);
+	expect_pending_htlcs_forwardable_conditions!(nodes[0],
+		[HTLCDestination::FailedPayment { payment_hash: to_a_failed_payment_hash }]);
+
+	assert_eq!(as_pre_spend_claims,
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	connect_blocks(&nodes[0], 1);
+	assert_eq!(as_pre_spend_claims,
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	// For node B, we'll get the non-HTLC funds claimable after ANTI_REORG_DELAY confirmations
+	connect_blocks(&nodes[1], ANTI_REORG_DELAY - 1);
+	test_spendable_output(&nodes[1], &as_txn[0]);
+	bs_pre_spend_claims.retain(|e| if let Balance::ClaimableAwaitingConfirmations { .. } = e { false } else { true });
+
+	// The next few blocks for B look the same as for A, though for the opposite HTLC
+	nodes[1].tx_broadcaster.txn_broadcasted.lock().unwrap().clear();
+	connect_blocks(&nodes[1], TEST_FINAL_CLTV - (ANTI_REORG_DELAY - 1) - 1);
+	expect_pending_htlcs_forwardable_conditions!(nodes[1],
+		[HTLCDestination::FailedPayment { payment_hash: to_b_failed_payment_hash }]);
+	let bs_htlc_timeout_claim = nodes[1].tx_broadcaster.txn_broadcasted.lock().unwrap().split_off(0);
+	assert_eq!(bs_htlc_timeout_claim.len(), 1);
+	check_spends!(bs_htlc_timeout_claim[0], as_txn[0]);
+
+	assert_eq!(bs_pre_spend_claims,
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	connect_blocks(&nodes[1], 1);
+	assert_eq!(bs_pre_spend_claims,
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	// Now confirm the two HTLC timeout transactions for A, checking that the inbound HTLC resolves
+	// after ANTI_REORG_DELAY confirmations and the other takes BREAKDOWN_TIMEOUT confirmations.
+	mine_transaction(&nodes[0], &as_htlc_timeout_claim[0]);
+	let as_timeout_claimable_height = nodes[0].best_block_info().1 + (BREAKDOWN_TIMEOUT as u32) - 1;
+	assert_eq!(sorted_vec(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 1_000_000 - 500_000 - 10_000 - chan_feerate *
+				(channel::commitment_tx_base_weight(opt_anchors) + 2 * channel::COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000,
+			confirmation_height: node_a_commitment_claimable,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 20_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 10_000,
+			confirmation_height: as_timeout_claimable_height,
+		}]),
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	mine_transaction(&nodes[0], &bs_htlc_timeout_claim[0]);
+	assert_eq!(sorted_vec(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 1_000_000 - 500_000 - 10_000 - chan_feerate *
+				(channel::commitment_tx_base_weight(opt_anchors) + 2 * channel::COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000,
+			confirmation_height: node_a_commitment_claimable,
+		}, Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 20_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 10_000,
+			confirmation_height: as_timeout_claimable_height,
+		}]),
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	// Once as_htlc_timeout_claim[0] reaches ANTI_REORG_DELAY confirmations, we should get a
+	// payment failure event.
+	connect_blocks(&nodes[0], ANTI_REORG_DELAY - 2);
+	expect_payment_failed!(nodes[0], to_b_failed_payment_hash, true);
+
+	connect_blocks(&nodes[0], 1);
+	assert_eq!(sorted_vec(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 1_000_000 - 500_000 - 10_000 - chan_feerate *
+				(channel::commitment_tx_base_weight(opt_anchors) + 2 * channel::COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000,
+			confirmation_height: node_a_commitment_claimable,
+		}, Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 10_000,
+			confirmation_height: core::cmp::max(as_timeout_claimable_height, htlc_cltv_timeout),
+		}]),
+		sorted_vec(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	connect_blocks(&nodes[0], node_a_commitment_claimable - nodes[0].best_block_info().1);
+	assert_eq!(vec![Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 10_000,
+			confirmation_height: core::cmp::max(as_timeout_claimable_height, htlc_cltv_timeout),
+		}],
+		nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances());
+	test_spendable_output(&nodes[0], &as_txn[0]);
+
+	connect_blocks(&nodes[0], as_timeout_claimable_height - nodes[0].best_block_info().1);
+	assert!(nodes[0].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances().is_empty());
+	test_spendable_output(&nodes[0], &as_htlc_timeout_claim[0]);
+
+	// The process for B should be completely identical as well, noting that the non-HTLC-balance
+	// was already claimed.
+	mine_transaction(&nodes[1], &bs_htlc_timeout_claim[0]);
+	let bs_timeout_claimable_height = nodes[1].best_block_info().1 + ANTI_REORG_DELAY - 1;
+	assert_eq!(sorted_vec(vec![Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 10_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 20_000,
+			confirmation_height: bs_timeout_claimable_height,
+		}]),
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	mine_transaction(&nodes[1], &as_htlc_timeout_claim[0]);
+	assert_eq!(sorted_vec(vec![Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 10_000,
+			expiry_height: htlc_cltv_timeout,
+		}, Balance::ClaimableAwaitingConfirmations {
+			claimable_amount_satoshis: 20_000,
+			confirmation_height: bs_timeout_claimable_height,
+		}]),
+		sorted_vec(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances()));
+
+	connect_blocks(&nodes[1], ANTI_REORG_DELAY - 2);
+	expect_payment_failed!(nodes[1], to_a_failed_payment_hash, true);
+
+	assert_eq!(vec![Balance::MaybePreimageClaimableHTLC {
+			claimable_amount_satoshis: 10_000,
+			expiry_height: htlc_cltv_timeout,
+		}],
+		nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances());
+	test_spendable_output(&nodes[1], &bs_htlc_timeout_claim[0]);
+
+	connect_blocks(&nodes[1], 1);
+	assert!(nodes[1].chain_monitor.chain_monitor.get_monitor(funding_outpoint).unwrap().get_claimable_balances().is_empty());
+}
+
 fn sorted_vec_with_additions<T: Ord + Clone>(v_orig: &Vec<T>, extra_ts: &[&T]) -> Vec<T> {
 	let mut v = v_orig.clone();
 	for t in extra_ts {