Funding_tx: add anti-fee sniping recommendation and check if final

lightning/src/ln/channelmanager.rs

@@ -2783,6 +2783,9 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// Returns an [`APIError::APIMisuseError`] if the funding_transaction spent non-SegWit outputs
 	/// or if no output was found which matches the parameters in [`Event::FundingGenerationReady`].
 	///
+	/// Returns [`APIError::APIMisuseError`] if the funding transaction is not final for propagation
+	/// across the p2p network.
+	///
 	/// Returns [`APIError::ChannelUnavailable`] if a funding transaction has already been provided
 	/// for the channel or if the channel has been closed as indicated by [`Event::ChannelClosed`].
 	///
@@ -2798,6 +2801,11 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// not currently support replacing a funding transaction on an existing channel. Instead,
 	/// create a new channel with a conflicting funding transaction.
 	///
+	/// Note to keep the miner incentives aligned in moving the blockchain forward, we recommend
+	/// the wallet software generating the funding transaction to apply anti-fee sniping as
+	/// implemented by Bitcoin Core wallet. See <https://bitcoinops.org/en/topics/fee-sniping/>
+	/// for more details.
+	///
 	/// [`Event::FundingGenerationReady`]: crate::util::events::Event::FundingGenerationReady
 	/// [`Event::ChannelClosed`]: crate::util::events::Event::ChannelClosed
 	pub fn funding_transaction_generated(&self, temporary_channel_id: &[u8; 32], counterparty_node_id: &PublicKey, funding_transaction: Transaction) -> Result<(), APIError> {
@@ -2810,6 +2818,18 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				});
 			}
 		}
+		{
+			let height = self.best_block.read().unwrap().height();
+			// Transactions are evaluated as final by network mempools at the next block. However, the modules
+			// constituting our Lightning node might not have perfect sync about their blockchain views. Thus, if
+			// the wallet module is in advance on the LDK view, allow one more block of headroom.
+			// TODO: updated if/when https://github.com/rust-bitcoin/rust-bitcoin/pull/994 landed and rust-bitcoin bumped.
+			if !funding_transaction.input.iter().all(|input| input.sequence == 0xffffffff) && funding_transaction.lock_time < 500_000_000 && funding_transaction.lock_time > height + 2 {
+				return Err(APIError::APIMisuseError {
+					err: "Funding transaction absolute timelock is non-final".to_owned()
+				});
+			}
+		}
 		self.funding_transaction_generated_intern(temporary_channel_id, counterparty_node_id, funding_transaction, |chan, tx| {
 			let mut output_index = None;
 			let expected_spk = chan.get_funding_redeemscript().to_v0_p2wsh();

lightning/src/ln/functional_tests.rs

@@ -41,6 +41,8 @@ use bitcoin::blockdata::script::Builder;
 use bitcoin::blockdata::opcodes;
 use bitcoin::blockdata::constants::genesis_block;
 use bitcoin::network::constants::Network;
+use bitcoin::{Transaction, TxIn, TxOut, Witness};
+use bitcoin::OutPoint as BitcoinOutPoint;
 
 use bitcoin::secp256k1::Secp256k1;
 use bitcoin::secp256k1::{PublicKey,SecretKey};
@@ -10329,3 +10331,45 @@ fn test_max_dust_htlc_exposure() {
 	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, false);
 	do_test_max_dust_htlc_exposure(false, ExposureEvent::AtUpdateFeeOutbound, true);
 }
+
+#[test]
+fn test_non_final_funding_tx() {
+	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 nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let temp_channel_id = nodes[0].node.create_channel(nodes[1].node.get_our_node_id(), 100_000, 0, 42, None).unwrap();
+	let open_channel_message = get_event_msg!(nodes[0], MessageSendEvent::SendOpenChannel, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_open_channel(&nodes[0].node.get_our_node_id(), InitFeatures::known(), &open_channel_message);
+	let accept_channel_message = get_event_msg!(nodes[1], MessageSendEvent::SendAcceptChannel, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_accept_channel(&nodes[1].node.get_our_node_id(), InitFeatures::known(), &accept_channel_message);
+
+	let best_height = nodes[0].node.best_block.read().unwrap().height();
+
+	let chan_id = *nodes[0].network_chan_count.borrow();
+	let events = nodes[0].node.get_and_clear_pending_events();
+	let input = TxIn { previous_output: BitcoinOutPoint::null(), script_sig: bitcoin::Script::new(), sequence: 0x1, witness: Witness::from_vec(vec!(vec!(1))) };
+	assert_eq!(events.len(), 1);
+	let mut tx = match events[0] {
+		Event::FundingGenerationReady { ref channel_value_satoshis, ref output_script, .. } => {
+			// Timelock the transaction _beyond_ the best client height + 2.
+			Transaction { version: chan_id as i32, lock_time: best_height + 3, input: vec![input], output: vec![TxOut {
+				value: *channel_value_satoshis, script_pubkey: output_script.clone(),
+			}]}
+		},
+		_ => panic!("Unexpected event"),
+	};
+	// Transaction should fail as it's evaluated as non-final for propagation.
+	match nodes[0].node.funding_transaction_generated(&temp_channel_id, &nodes[1].node.get_our_node_id(), tx.clone()) {
+		Err(APIError::APIMisuseError { err }) => {
+			assert_eq!(format!("Funding transaction absolute timelock is non-final"), err);
+		},
+		_ => panic!()
+	}
+
+	// However, transaction should be accepted if it's in a +2 headroom from best block.
+	tx.lock_time -= 1;
+	assert!(nodes[0].node.funding_transaction_generated(&temp_channel_id, &nodes[1].node.get_our_node_id(), tx.clone()).is_ok());
+	get_event_msg!(nodes[0], MessageSendEvent::SendFundingCreated, nodes[1].node.get_our_node_id());
+}