Claim HTLC output on-chain if preimage is recv'd after force-close

If we receive a preimage for an outgoing HTLC on a force-closed channel,
we need to claim the HTLC output on-chain.

Co-authored-by: Antoine Riard <[email protected]>
Co-authored-by: Valentine Wallace <[email protected]>

Author: valentinewallace

lightning/src/chain/channelmonitor.rs

@@ -64,11 +64,18 @@ pub struct ChannelMonitorUpdate {
 	pub(crate) updates: Vec<ChannelMonitorUpdateStep>,
 	/// The sequence number of this update. Updates *must* be replayed in-order according to this
 	/// sequence number (and updates may panic if they are not). The update_id values are strictly
-	/// increasing and increase by one for each new update.
+	/// increasing and increase by one for each new update, with one exception specified below.
 	///
 	/// This sequence number is also used to track up to which points updates which returned
 	/// ChannelMonitorUpdateErr::TemporaryFailure have been applied to all copies of a given
 	/// ChannelMonitor when ChannelManager::channel_monitor_updated is called.
+	///
+	/// The only instance where update_id values are not strictly increasing is the case where: (1)
+	/// the channel has been force closed and (2) we receive a preimage from a forward link that
+	/// allows us to spend an HTLC output on this channel's (the backward link's) broadcasted
+	/// commitment transaction. In this case, we allow the `ChannelManager` to send a
+	/// `ChannelMonitorUpdate` with an update_id of `std::u64::MAX`, with the update providing said
+	/// payment preimage. No other update types are allowed after force-close.
 	pub update_id: u64,
 }
 
@@ -1150,6 +1157,46 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 		    L::Target: Logger,
 	{
 		self.payment_preimages.insert(payment_hash.clone(), payment_preimage.clone());
+
+		// If the channel is force closed, try to claim the output from this preimage.
+		// First check if a counterparty commitment transaction has been broadcasted:
+		macro_rules! claim_htlcs {
+			($commitment_number: expr, $txid: expr) => {
+				let (htlc_claim_reqs, set_script) = self.get_counterparty_htlc_output_claim_reqs($commitment_number, $txid, None);
+				if set_script {
+					self.counterparty_payment_script = {
+						// Note that the Network here is ignored as we immediately drop the address for the
+						// script_pubkey version
+						let payment_hash160 = WPubkeyHash::hash(&self.keys.pubkeys().payment_point.serialize());
+						Builder::new().push_opcode(opcodes::all::OP_PUSHBYTES_0).push_slice(&payment_hash160[..]).into_script()
+					};
+				}
+				self.onchain_tx_handler.update_claims_view(&Vec::new(), htlc_claim_reqs, None, broadcaster, fee_estimator, logger);
+			}
+		}
+		if let Some(txid) = self.current_counterparty_commitment_txid {
+			if let Some(commitment_number) = self.counterparty_commitment_txn_on_chain.get(&txid) {
+				claim_htlcs!(*commitment_number, txid);
+				return;
+			}
+		}
+		if let Some(txid) = self.prev_counterparty_commitment_txid {
+			if let Some(commitment_number) = self.counterparty_commitment_txn_on_chain.get(&txid) {
+				claim_htlcs!(*commitment_number, txid);
+				return;
+			}
+		}
+
+		// Then if a holder commitment transaction has been signed, broadcast transactions claiming
+		// the HTLC output from each of the holder commitment transactions.
+		if self.holder_tx_signed {
+			let (claim_reqs, _, _) = self.broadcast_by_holder_state(None, &self.current_holder_commitment_tx);
+			self.onchain_tx_handler.update_claims_view(&Vec::new(), claim_reqs, None, broadcaster, fee_estimator, logger);
+			if let Some(ref tx) = self.prev_holder_signed_commitment_tx {
+				let (claim_reqs, _, _) = self.broadcast_by_holder_state(None, &tx);
+				self.onchain_tx_handler.update_claims_view(&Vec::new(), claim_reqs, None, broadcaster, fee_estimator, logger);
+			}
+		}
 	}
 
 	pub(crate) fn broadcast_latest_holder_commitment_txn<B: Deref, L: Deref>(&mut self, broadcaster: &B, logger: &L)
@@ -1171,7 +1218,18 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 		    F::Target: FeeEstimator,
 		    L::Target: Logger,
 	{
-		if self.latest_update_id + 1 != updates.update_id {
+		// ChannelMonitor updates may be applied after force close if we receive a
+		// preimage for a broadcasted commitment transaction HTLC output that we'd
+		// like to claim on-chain. If this is the case, we no longer have guaranteed
+		// access to the monitor's update ID, so we use a sentinel value instead.
+		if updates.update_id == std::u64::MAX {
+			match updates.updates[0] {
+				ChannelMonitorUpdateStep::PaymentPreimage { .. } => {},
+				_ => panic!("Attempted to apply post-force-close ChannelMonitorUpdate that wasn't providing a payment preimage"),
+			}
+			assert_eq!(updates.updates.len(), 1);
+		}
+		if updates.update_id != std::u64::MAX && self.latest_update_id + 1 != updates.update_id {
 			panic!("Attempted to apply ChannelMonitorUpdates out of order, check the update_id before passing an update to update_monitor!");
 		}
 		for update in updates.updates.iter() {
@@ -1439,39 +1497,61 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 				check_htlc_fails!(txid, "previous", 'prev_loop);
 			}
 
+			let (htlc_claim_reqs, set_script) = self.get_counterparty_htlc_output_claim_reqs(commitment_number, commitment_txid, Some(tx));
+			if set_script {
+				self.counterparty_payment_script = {
+					// Note that the Network here is ignored as we immediately drop the address for the
+					// script_pubkey version
+					let payment_hash160 = WPubkeyHash::hash(&self.keys.pubkeys().payment_point.serialize());
+					Builder::new().push_opcode(opcodes::all::OP_PUSHBYTES_0).push_slice(&payment_hash160[..]).into_script()
+				};
+			}
+			for req in htlc_claim_reqs {
+				claimable_outpoints.push(req);
+			}
+
+		}
+		(claimable_outpoints, (commitment_txid, watch_outputs))
+	}
+
+	fn get_counterparty_htlc_output_claim_reqs(&self, commitment_number: u64, commitment_txid: Txid, tx: Option<&Transaction>) -> (Vec<ClaimRequest>, bool) {
+		let mut claims = Vec::new();
+		let mut set_counterparty_payment_script = false;
+		if let Some(htlc_outputs) = self.counterparty_claimable_outpoints.get(&commitment_txid) {
 			if let Some(revocation_points) = self.their_cur_revocation_points {
 				let revocation_point_option =
 					if revocation_points.0 == commitment_number { Some(&revocation_points.1) }
-					else if let Some(point) = revocation_points.2.as_ref() {
-						if revocation_points.0 == commitment_number + 1 { Some(point) } else { None }
-					} else { None };
+				else if let Some(point) = revocation_points.2.as_ref() {
+					if revocation_points.0 == commitment_number + 1 { Some(point) } else { None }
+				} else { None };
 				if let Some(revocation_point) = revocation_point_option {
-					self.counterparty_payment_script = {
-						// Note that the Network here is ignored as we immediately drop the address for the
-						// script_pubkey version
-						let payment_hash160 = WPubkeyHash::hash(&self.keys.pubkeys().payment_point.serialize());
-						Builder::new().push_opcode(opcodes::all::OP_PUSHBYTES_0).push_slice(&payment_hash160[..]).into_script()
-					};
+					set_counterparty_payment_script = true;
 
 					// Then, try to find htlc outputs
-					for (_, &(ref htlc, _)) in per_commitment_data.iter().enumerate() {
+					for (_, &(ref htlc, _)) in htlc_outputs.iter().enumerate() {
 						if let Some(transaction_output_index) = htlc.transaction_output_index {
-							if transaction_output_index as usize >= tx.output.len() ||
-									tx.output[transaction_output_index as usize].value != htlc.amount_msat / 1000 {
-								return (claimable_outpoints, (commitment_txid, watch_outputs)); // Corrupted per_commitment_data, fuck this user
+							if let Some(transaction) = tx {
+								if transaction_output_index as usize >= transaction.output.len() ||
+									transaction.output[transaction_output_index as usize].value != htlc.amount_msat / 1000 {
+										return (claims, set_counterparty_payment_script) // Corrupted per_commitment_data, fuck this user
+									}
 							}
-							let preimage = if htlc.offered { if let Some(p) = self.payment_preimages.get(&htlc.payment_hash) { Some(*p) } else { None } } else { None };
+							let preimage = if htlc.offered {
+								if let Some(p) = self.payment_preimages.get(&htlc.payment_hash) {
+									Some(*p)
+								} else { None }
+							} else { None };
 							let aggregable = if !htlc.offered { false } else { true };
 							if preimage.is_some() || !htlc.offered {
 								let witness_data = InputMaterial::CounterpartyHTLC { per_commitment_point: *revocation_point, counterparty_delayed_payment_base_key: self.counterparty_tx_cache.counterparty_delayed_payment_base_key, counterparty_htlc_base_key: self.counterparty_tx_cache.counterparty_htlc_base_key, preimage, htlc: htlc.clone() };
-								claimable_outpoints.push(ClaimRequest { absolute_timelock: htlc.cltv_expiry, aggregable, outpoint: BitcoinOutPoint { txid: commitment_txid, vout: transaction_output_index }, witness_data });
+								claims.push(ClaimRequest { absolute_timelock: htlc.cltv_expiry, aggregable, outpoint: BitcoinOutPoint { txid: commitment_txid, vout: transaction_output_index }, witness_data });
 							}
 						}
 					}
 				}
 			}
 		}
-		(claimable_outpoints, (commitment_txid, watch_outputs))
+		(claims, set_counterparty_payment_script)
 	}
 
 	/// Attempts to claim a counterparty HTLC-Success/HTLC-Timeout's outputs using the revocation key
@@ -1501,7 +1581,7 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 		(claimable_outpoints, Some((htlc_txid, outputs)))
 	}
 
-	fn broadcast_by_holder_state(&self, commitment_tx: &Transaction, holder_tx: &HolderSignedTx) -> (Vec<ClaimRequest>, Vec<(u32, TxOut)>, Option<(Script, PublicKey, PublicKey)>) {
+	fn broadcast_by_holder_state(&self, commitment_tx: Option<&Transaction>, holder_tx: &HolderSignedTx) -> (Vec<ClaimRequest>, Vec<(u32, TxOut)>, Option<(Script, PublicKey, PublicKey)>) {
 		let mut claim_requests = Vec::with_capacity(holder_tx.htlc_outputs.len());
 		let mut watch_outputs = Vec::with_capacity(holder_tx.htlc_outputs.len());
 
@@ -1522,7 +1602,9 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 							} else { None },
 						amount: htlc.amount_msat,
 				}});
-				watch_outputs.push((transaction_output_index, commitment_tx.output[transaction_output_index as usize].clone()));
+				if let Some(commitment_tx) = commitment_tx {
+					watch_outputs.push((transaction_output_index, commitment_tx.output[transaction_output_index as usize].clone()));
+				}
 			}
 		}
 
@@ -1574,13 +1656,13 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 		if self.current_holder_commitment_tx.txid == commitment_txid {
 			is_holder_tx = true;
 			log_trace!(logger, "Got latest holder commitment tx broadcast, searching for available HTLCs to claim");
-			let mut res = self.broadcast_by_holder_state(tx, &self.current_holder_commitment_tx);
+			let mut res = self.broadcast_by_holder_state(Some(tx), &self.current_holder_commitment_tx);
 			append_onchain_update!(res);
 		} else if let &Some(ref holder_tx) = &self.prev_holder_signed_commitment_tx {
 			if holder_tx.txid == commitment_txid {
 				is_holder_tx = true;
 				log_trace!(logger, "Got previous holder commitment tx broadcast, searching for available HTLCs to claim");
-				let mut res = self.broadcast_by_holder_state(tx, holder_tx);
+				let mut res = self.broadcast_by_holder_state(Some(tx), holder_tx);
 				append_onchain_update!(res);
 			}
 		}
@@ -1749,7 +1831,7 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 			self.pending_monitor_events.push(MonitorEvent::CommitmentTxBroadcasted(self.funding_info.0));
 			if let Some(commitment_tx) = self.onchain_tx_handler.get_fully_signed_holder_tx(&self.funding_redeemscript) {
 				self.holder_tx_signed = true;
-				let (mut new_outpoints, new_outputs, _) = self.broadcast_by_holder_state(&commitment_tx, &self.current_holder_commitment_tx);
+				let (mut new_outpoints, new_outputs, _) = self.broadcast_by_holder_state(Some(&commitment_tx), &self.current_holder_commitment_tx);
 				if !new_outputs.is_empty() {
 					watch_outputs.push((self.current_holder_commitment_tx.txid.clone(), new_outputs));
 				}
@@ -1777,7 +1859,7 @@ impl<ChanSigner: ChannelKeys> ChannelMonitor<ChanSigner> {
 			}
 		}
 
-		self.onchain_tx_handler.block_connected(&txn_matched, claimable_outpoints, height, &*broadcaster, &*fee_estimator, &*logger);
+		self.onchain_tx_handler.update_claims_view(&txn_matched, claimable_outpoints, Some(height), &&*broadcaster, &&*fee_estimator, &&*logger);
 		self.last_block_hash = block_hash;
 
 		// Determine new outputs to watch by comparing against previously known outputs to watch,

lightning/src/ln/functional_tests.rs

@@ -8503,3 +8503,126 @@ fn test_htlc_no_detection() {
         connect_blocks(&nodes[0], ANTI_REORG_DELAY - 1, 201, true, header_201.block_hash());
         expect_payment_failed!(nodes[0], our_payment_hash, true);
 }
+
+fn do_test_onchain_htlc_settlement_after_close(broadcast_alice: bool) {
+	// If we route an HTLC, then learn the HTLC's preimage after the upstream
+	// channel has been force-closed by our counterparty, we must claim that HTLC
+	// on-chain. (Given an HTLC forwarded from Alice --> Bob --> Carol, Alice
+	// would be the upstream node, and Carol the downstream.)
+	//
+	// Steps of the test:
+	// 1) Alice sends a HTLC to Carol through Bob.
+	// 2) Carol doesn't settle the HTLC.
+	// 3) If broadcast_alice is true, Alice force-closes her channel with Bob. Else Bob force closes.
+	// 4) Bob sees the Alice's commitment on his chain or vice versa. An offered output is present
+	//    but can't be claimed as Bob doesn't have yet knowledge of the preimage.
+	// 5) Carol release the preimage to Bob off-chain.
+	// 6) Bob claims the offered output on the broadcasted commitment.
+	let chanmon_cfgs = create_chanmon_cfgs(3);
+	let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, None, None]);
+	let nodes = create_network(3, &node_cfgs, &node_chanmgrs);
+
+	// Create some initial channels
+	let chan_ab = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100000, 10001, InitFeatures::known(), InitFeatures::known());
+	create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 100000, 10001, InitFeatures::known(), InitFeatures::known());
+
+	// Steps (1) and (2):
+	// Send an HTLC Alice --> Bob --> Carol, but Carol doesn't settle the HTLC back.
+	let (payment_preimage, _payment_hash) = route_payment(&nodes[0], &vec!(&nodes[1], &nodes[2]), 3_000_000);
+
+	// Check that Alice's commitment transaction now contains an output for this HTLC.
+	let alice_txn = get_local_commitment_txn!(nodes[0], chan_ab.2);
+	check_spends!(alice_txn[0], chan_ab.3);
+	assert_eq!(alice_txn[0].output.len(), 2);
+	check_spends!(alice_txn[1], alice_txn[0]); // 2nd transaction is a non-final HTLC-timeout
+	assert_eq!(alice_txn[1].input[0].witness.last().unwrap().len(), OFFERED_HTLC_SCRIPT_WEIGHT);
+	assert_eq!(alice_txn.len(), 2);
+
+	// Steps (3) and (4):
+	// Broadcast the relevant commitment transaction and check that Bob responds by (1) broadcasting
+	// a channel update and (2) adding a new ChannelMonitor.
+	let mut txn_to_broadcast = alice_txn;
+	if !broadcast_alice { txn_to_broadcast = get_local_commitment_txn!(nodes[1], chan_ab.2); }
+	let header = BlockHeader { version: 0x20000000, prev_blockhash: Default::default(), merkle_root: Default::default(), time: 42, bits: 42, nonce: 42};
+	connect_block(&nodes[1], &Block { header, txdata: vec![txn_to_broadcast[0].clone()]}, 1);
+	check_closed_broadcast!(nodes[1], false);
+	check_added_monitors!(nodes[1], 1);
+	{
+		let mut bob_txn = nodes[1].tx_broadcaster.txn_broadcasted.lock().unwrap(); // ChannelManager : 1 (commitment tx)
+		assert_eq!(bob_txn.len(), 1);
+		check_spends!(bob_txn[0], chan_ab.3);
+		bob_txn.clear();
+	}
+
+	// Step (5):
+	// Carol then claims the funds and sends an update_fulfill message to Bob, and they go
+	// through the process of removing the HTLC from their commitment transactions.
+	assert!(nodes[2].node.claim_funds(payment_preimage, &None, 3_000_000));
+	check_added_monitors!(nodes[2], 1);
+	let carol_updates = get_htlc_update_msgs!(nodes[2], nodes[1].node.get_our_node_id());
+	assert!(carol_updates.update_add_htlcs.is_empty());
+	assert!(carol_updates.update_fail_htlcs.is_empty());
+	assert!(carol_updates.update_fail_malformed_htlcs.is_empty());
+	assert!(carol_updates.update_fee.is_none());
+	assert_eq!(carol_updates.update_fulfill_htlcs.len(), 1);
+
+	nodes[1].node.handle_update_fulfill_htlc(&nodes[2].node.get_our_node_id(), &carol_updates.update_fulfill_htlcs[0]);
+	nodes[1].node.handle_commitment_signed(&nodes[2].node.get_our_node_id(), &carol_updates.commitment_signed);
+	// One monitor update for the preimage, one monitor update for the new commitment tx info
+	check_added_monitors!(nodes[1], 2);
+
+	let events = nodes[1].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 2);
+	let bob_revocation = match events[0] {
+		MessageSendEvent::SendRevokeAndACK { ref node_id, ref msg } => {
+			assert_eq!(*node_id, nodes[2].node.get_our_node_id());
+			(*msg).clone()
+		},
+		_ => panic!("Unexpected event"),
+	};
+	let bob_updates = match events[1] {
+		MessageSendEvent::UpdateHTLCs { ref node_id, ref updates } => {
+			assert_eq!(*node_id, nodes[2].node.get_our_node_id());
+			(*updates).clone()
+		},
+		_ => panic!("Unexpected event"),
+	};
+
+	nodes[2].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &bob_revocation);
+	check_added_monitors!(nodes[2], 1);
+	nodes[2].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &bob_updates.commitment_signed);
+	check_added_monitors!(nodes[2], 1);
+
+	let events = nodes[2].node.get_and_clear_pending_msg_events();
+	assert_eq!(events.len(), 1);
+	let carol_revocation = match events[0] {
+		MessageSendEvent::SendRevokeAndACK { ref node_id, ref msg } => {
+			assert_eq!(*node_id, nodes[1].node.get_our_node_id());
+			(*msg).clone()
+		},
+		_ => panic!("Unexpected event"),
+	};
+	nodes[1].node.handle_revoke_and_ack(&nodes[2].node.get_our_node_id(), &carol_revocation);
+	check_added_monitors!(nodes[1], 1);
+
+	// Step (6):
+	// Finally, check that Bob broadcasted a preimage-claiming transaction for the HTLC
+	// output on the broadcasted commitment transaction.
+	{
+		let bob_txn = nodes[1].tx_broadcaster.txn_broadcasted.lock().unwrap().clone();
+		assert_eq!(bob_txn.len(), 1);
+		check_spends!(bob_txn[0], txn_to_broadcast[0]);
+		let script_weight = match broadcast_alice {
+			true => OFFERED_HTLC_SCRIPT_WEIGHT,
+			false => ACCEPTED_HTLC_SCRIPT_WEIGHT
+		};
+		assert_eq!(bob_txn[0].input[0].witness.last().unwrap().len(), script_weight);
+	}
+}
+
+#[test]
+fn test_onchain_htlc_settlement_after_close() {
+	do_test_onchain_htlc_settlement_after_close(true);
+	do_test_onchain_htlc_settlement_after_close(false);
+}