Display more fee information when opening a DLC channel

Cargo.toml

@@ -20,11 +20,11 @@ resolver = "2"
 # We are using our own fork of `rust-dlc` at least until we can drop all the LN-DLC features. Also,
 # `p2pderivatives/rust-dlc#master` is missing certain patches that can only be found in the LN-DLC
 # branch.
-dlc-manager = { git = "https://github.com/get10101/rust-dlc", rev = "bc31c6167e304d7886c328da91e8c17dad1afce5" }
-dlc-messages = { git = "https://github.com/get10101/rust-dlc", rev = "bc31c6167e304d7886c328da91e8c17dad1afce5" }
-dlc = { git = "https://github.com/get10101/rust-dlc", rev = "bc31c6167e304d7886c328da91e8c17dad1afce5" }
-p2pd-oracle-client = { git = "https://github.com/get10101/rust-dlc", rev = "bc31c6167e304d7886c328da91e8c17dad1afce5" }
-dlc-trie = { git = "https://github.com/get10101/rust-dlc", rev = "bc31c6167e304d7886c328da91e8c17dad1afce5" }
+dlc-manager = { git = "https://github.com/get10101/rust-dlc", rev = "1ab4bf5" }
+dlc-messages = { git = "https://github.com/get10101/rust-dlc", rev = "1ab4bf5" }
+dlc = { git = "https://github.com/get10101/rust-dlc", rev = "1ab4bf5" }
+p2pd-oracle-client = { git = "https://github.com/get10101/rust-dlc", rev = "1ab4bf5" }
+dlc-trie = { git = "https://github.com/get10101/rust-dlc", rev = "1ab4bf5" }
 
 # We should usually track the `p2pderivatives/split-tx-experiment[-10101]` branch. For now we depend
 # on a special fork which removes a panic in `rust-lightning`.

coordinator/src/collaborative_revert.rs

@@ -17,7 +17,7 @@ use diesel::r2d2::ConnectionManager;
 use diesel::r2d2::Pool;
 use diesel::r2d2::PooledConnection;
 use diesel::PgConnection;
-use dlc::util::weight_to_fee;
+use dlc::util::tx_weight_to_fee;
 use dlc_manager::channel::ClosedChannel;
 use dlc_manager::DlcChannelId;
 use dlc_manager::Signer;
@@ -86,7 +86,7 @@ pub async fn propose_collaborative_revert(
         .checked_sub(trader_amount_sats)
         .context("Could not substract trader amount from total value without overflow")?;
 
-    let fee = weight_to_fee(COLLABORATIVE_REVERT_TX_WEIGHT, fee_rate_sats_vb)
+    let fee = tx_weight_to_fee(COLLABORATIVE_REVERT_TX_WEIGHT, fee_rate_sats_vb)
         .context("Could not calculate fee")?;
 
     let fee_half = fee.checked_div(2).context("Could not divide fee")?;

crates/ln-dlc-node/src/dlc_wallet.rs

@@ -26,8 +26,6 @@ use bdk_coin_select::Target;
 use bitcoin::secp256k1::KeyPair;
 use bitcoin::Network;
 use bitcoin::TxIn;
-use bitcoin::VarInt;
-use lightning::chain::chaininterface::ConfirmationTarget;
 use ln_dlc_storage::DlcStorageProvider;
 use ln_dlc_storage::WalletStorage;
 use std::sync::Arc;
@@ -180,16 +178,12 @@ impl<D: BdkStorage, S: TenTenOneStorage, N> dlc_manager::Wallet for DlcWallet<D,
         &self,
         amount: u64,
         fee_rate: Option<u64>,
+        base_weight_wu: u64,
         lock_utxos: bool,
     ) -> Result<Vec<dlc_manager::Utxo>, dlc_manager::error::Error> {
         let network = self.on_chain_wallet.network();
 
-        let fee_rate = fee_rate.map(|fee_rate| fee_rate as f32).unwrap_or_else(|| {
-            self.on_chain_wallet
-                .fee_rate_estimator
-                .get(ConfirmationTarget::Normal)
-                .as_sat_per_vb()
-        });
+        let fee_rate = fee_rate.expect("always set by rust-dlc");
 
         // Get temporarily reserved UTXOs from in-memory storage.
         let mut reserved_outpoints = self.on_chain_wallet.locked_utxos.lock();
@@ -211,15 +205,7 @@ impl<D: BdkStorage, S: TenTenOneStorage, N> dlc_manager::Wallet for DlcWallet<D,
                     ..Default::default()
                 };
 
-                // Inspired by `rust-bitcoin:0.30.2`.
-                let segwit_weight = {
-                    let legacy_weight = {
-                        let script_sig_size = tx_in.script_sig.len();
-                        (36 + VarInt(script_sig_size as u64).len() + script_sig_size + 4) * 4
-                    };
-
-                    legacy_weight + tx_in.witness.serialized_len()
-                };
+                let segwit_weight = tx_in.segwit_weight();
 
                 // The 10101 wallet always generates SegWit addresses.
                 //
@@ -230,17 +216,13 @@ impl<D: BdkStorage, S: TenTenOneStorage, N> dlc_manager::Wallet for DlcWallet<D,
             })
             .collect::<Vec<_>>();
 
-        // This is a standard base weight (without inputs or change outputs) for on-chain DLCs. We
-        // assume that this value is still correct for DLC channels.
-        let funding_tx_base_weight = 212;
-
         let target = Target {
-            feerate: bdk_coin_select::FeeRate::from_sat_per_vb(fee_rate),
+            feerate: bdk_coin_select::FeeRate::from_sat_per_vb(fee_rate as f32),
             min_fee: 0,
             value: amount,
         };
 
-        let mut coin_selector = CoinSelector::new(&candidates, funding_tx_base_weight);
+        let mut coin_selector = CoinSelector::new(&candidates, base_weight_wu as u32);
 
         let dust_limit = 0;
         let long_term_feerate = bdk_coin_select::FeeRate::default_min_relay_fee();

crates/ln-dlc-node/src/node/dlc_channel.rs

@@ -818,3 +818,68 @@ pub fn send_dlc_message<D: BdkStorage, S: TenTenOneStorage + 'static, N: LnDlcSt
     // enqueued message ASAP.
     peer_manager.process_events();
 }
+
+/// Give an estimate for the fee reserve of a DLC channel, given a fee rate.
+///
+/// Limitations:
+///
+/// - `rust-dlc` assumes that both parties will use P2WPKH script pubkeys for their CET outputs. If
+/// they don't then the reserved fee might be slightly over or under the target fee rate.
+///
+/// - Rounding errors can cause very slight differences between what we estimate here and what
+/// `rust-dlc` will end up reserving.
+pub fn estimated_dlc_channel_fee_reserve(fee_rate_sats_per_vb: f64) -> Amount {
+    let buffer_weight_wu = dlc::channel::BUFFER_TX_WEIGHT;
+
+    let cet_or_refund_weight_wu = {
+        let cet_or_refund_base_weight_wu = dlc::CET_BASE_WEIGHT;
+        // Because the CET spends from a buffer transaction, compared to a regular DLC that spends
+        // directly from the funding transaction.
+        let cet_or_refund_extra_weight_wu = dlc::channel::CET_EXTRA_WEIGHT;
+
+        // This is the standard length of a P2WPKH script pubkey.
+        let cet_or_refund_output_spk_bytes = 22;
+
+        // Value = 8 bytes; var_int = 1 byte.
+        let cet_or_refund_output_weight_wu = (8 + 1 + cet_or_refund_output_spk_bytes) * 4;
+
+        cet_or_refund_base_weight_wu
+            + cet_or_refund_extra_weight_wu
+            // 1 output per party.
+            + (2 * cet_or_refund_output_weight_wu)
+    };
+
+    let total_weight_vb = (buffer_weight_wu + cet_or_refund_weight_wu) as f64 / 4.0;
+
+    let total_fee_reserve = total_weight_vb * fee_rate_sats_per_vb;
+    let total_fee_reserve = total_fee_reserve.ceil() as u64;
+
+    Amount::from_sat(total_fee_reserve)
+}
+
+/// Give an estimate for the fee paid to publish a DLC channel funding transaction, given a fee
+/// rate.
+///
+/// This estimate is based on a funding transaction spending _two_ P2WPKH inputs (one per party) and
+/// including _two_ P2WPKH change outputs (also one per party).
+///
+/// Values taken from
+/// https://github.com/discreetlogcontracts/dlcspecs/blob/master/Transactions.md#fees.
+pub fn estimated_funding_transaction_fee(fee_rate_sats_per_vb: f64) -> Amount {
+    let base_weight_wu = dlc::FUND_TX_BASE_WEIGHT;
+
+    let input_script_pubkey_length = 22;
+    let max_witness_length = 108;
+    let input_weight_wu = 164 + (4 * input_script_pubkey_length) + max_witness_length;
+
+    let output_script_pubkey_length = 22;
+    let output_weight_wu = 36 + (4 * output_script_pubkey_length);
+
+    let total_weight_wu = base_weight_wu + (input_weight_wu * 2) + (output_weight_wu * 2);
+    let total_weight_vb = total_weight_wu as f64 / 4.0;
+
+    let fee = total_weight_vb * fee_rate_sats_per_vb;
+    let fee = fee.ceil() as u64;
+
+    Amount::from_sat(fee)
+}

crates/ln-dlc-node/src/tests/dlc_channel.rs

@@ -1,4 +1,5 @@
 use crate::bitcoin_conversion::to_secp_pk_29;
+use crate::node::dlc_channel::estimated_dlc_channel_fee_reserve;
 use crate::node::InMemoryStore;
 use crate::node::Node;
 use crate::node::RunningNode;
@@ -260,39 +261,100 @@ async fn can_open_and_force_close_channel() {
 
 #[tokio::test(flavor = "multi_thread")]
 #[ignore]
-async fn can_open_channel_with_min_inputs() {
+async fn funding_transaction_pays_expected_fees() {
+    init_tracing();
+
+    // Arrange
+
+    let app_dlc_collateral = Amount::from_sat(10_000);
+    let coordinator_dlc_collateral = Amount::from_sat(10_000);
+
+    let fee_rate_sats_per_vb = 2;
+
+    // Give enough funds to app and coordinator so that each party can have their own change output.
+    // This is not currently enforced by `rust-dlc`, but it will be in the near future:
+    // https://github.com/p2pderivatives/rust-dlc/pull/152.
+    let (app, _running_app) = start_and_fund_app(app_dlc_collateral * 2, 1).await;
+    let (coordinator, _running_coordinator) =
+        start_and_fund_coordinator(app_dlc_collateral * 2, 1).await;
+
+    // Act
+
+    let (app_signed_channel, _) = open_channel_and_position(
+        app.clone(),
+        coordinator.clone(),
+        app_dlc_collateral,
+        coordinator_dlc_collateral,
+        Some(fee_rate_sats_per_vb),
+    )
+    .await;
+
+    // Assert
+
+    let fund_tx_outputs_amount = app_signed_channel
+        .fund_tx
+        .output
+        .iter()
+        .fold(Amount::ZERO, |acc, output| {
+            acc + Amount::from_sat(output.value)
+        });
+
+    let fund_tx_inputs_amount = Amount::from_sat(
+        app_signed_channel.own_params.input_amount + app_signed_channel.counter_params.input_amount,
+    );
+
+    let fund_tx_fee = fund_tx_inputs_amount - fund_tx_outputs_amount;
+
+    let fund_tx_weight_wu = app_signed_channel.fund_tx.weight();
+    let fund_tx_weight_vb = (fund_tx_weight_wu / 4) as u64;
+
+    let fund_tx_fee_rate_sats_per_vb = fund_tx_fee.to_sat() / fund_tx_weight_vb;
+
+    assert_eq!(fund_tx_fee_rate_sats_per_vb, fee_rate_sats_per_vb);
+}
+
+#[tokio::test(flavor = "multi_thread")]
+#[ignore]
+async fn dlc_channel_includes_expected_fee_reserve() {
     init_tracing();
 
     let app_dlc_collateral = Amount::from_sat(10_000);
     let coordinator_dlc_collateral = Amount::from_sat(10_000);
 
     // We must fix the fee rate so that we can predict how many sats `rust-dlc` will allocate
     // for transaction fees.
-    let fee_rate_sats_per_vbyte = 2;
-    let expected_fund_tx_fee = 252 * fee_rate_sats_per_vbyte;
+    let fee_rate_sats_per_vb = 2;
 
-    // This also depends on the fee rate, but the formula is a bit more involved.
-    let fee_reserve = 880;
+    let total_fee_reserve = estimated_dlc_channel_fee_reserve(fee_rate_sats_per_vb as f64);
 
-    // Fee costs are evenly split.
-    let fee_cost_per_party = (expected_fund_tx_fee + fee_reserve) / 2;
-    let fee_cost_per_party = Amount::from_sat(fee_cost_per_party);
+    let expected_fund_output_amount =
+        app_dlc_collateral + coordinator_dlc_collateral + total_fee_reserve;
 
-    let (app, _running_app) = start_and_fund_app(app_dlc_collateral + fee_cost_per_party, 1).await;
+    let (app, _running_app) = start_and_fund_app(app_dlc_collateral * 2, 1).await;
     let (coordinator, _running_coordinator) =
-        start_and_fund_coordinator(coordinator_dlc_collateral + fee_cost_per_party, 1).await;
+        start_and_fund_coordinator(coordinator_dlc_collateral * 2, 1).await;
 
     let (app_signed_channel, _) = open_channel_and_position(
         app.clone(),
         coordinator.clone(),
         app_dlc_collateral,
         coordinator_dlc_collateral,
-        Some(fee_rate_sats_per_vbyte),
+        Some(fee_rate_sats_per_vb),
     )
     .await;
 
-    // No change output means that the inputs were spent in full by the fund output.
-    assert!(app_signed_channel.fund_tx.output.len() == 1);
+    let fund_output_vout = app_signed_channel.fund_output_index;
+    let fund_output_amount = &app_signed_channel.fund_tx.output[fund_output_vout].value;
+
+    // We cannot easily assert equality because both `rust-dlc` and us have to round in several
+    // spots.
+    let epsilon = *fund_output_amount as i64 - expected_fund_output_amount.to_sat() as i64;
+
+    assert!(
+        epsilon.abs() < 5,
+        "Error out of bounds: actual {fund_output_amount} != {}",
+        expected_fund_output_amount.to_sat()
+    );
 }
 
 async fn start_and_fund_app(

mobile/lib/common/dlc_channel_service.dart

@@ -1,4 +1,5 @@
 import 'package:get_10101/common/domain/dlc_channel.dart';
+import 'package:get_10101/common/domain/model.dart';
 import 'package:get_10101/ffi.dart' as rust;
 
 class DlcChannelService {
@@ -13,4 +14,12 @@ class DlcChannelService {
   Future<void> deleteDlcChannel(String dlcChannelId) async {
     await rust.api.deleteDlcChannel(dlcChannelId: dlcChannelId);
   }
+
+  Amount getEstimatedChannelFeeReserve() {
+    return Amount(rust.api.getEstimatedChannelFeeReserve());
+  }
+
+  Amount getEstimatedFundingTxFee() {
+    return Amount(rust.api.getEstimatedFundingTxFee());
+  }
 }