Discourage fee sniping with nLockTime
By default bdk sets the transaction's nLockTime to current_height to discourage fee sniping. current_height can be provided by the user through TxParams; if the user didn't provide it, we use the last sync height, or 0 if we never synced. Fixes #533
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@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
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## [Unreleased]
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- New MSRV set to `1.56.1`
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- Fee sniping discouraging through nLockTime - if the user specifies a `current_height`, we use that as a nlocktime, otherwise we use the last sync height (or 0 if we never synced)
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## [v0.19.0] - [v0.18.0]
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@ -619,9 +619,27 @@ where
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_ => 1,
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};
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// We use a match here instead of a map_or_else as it's way more readable :)
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let current_height = match params.current_height {
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// If they didn't tell us the current height, we assume it's the latest sync height.
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None => self
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.database()
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.get_sync_time()?
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.map(|sync_time| sync_time.block_time.height),
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h => h,
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};
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let lock_time = match params.locktime {
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// No nLockTime, default to 0
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None => requirements.timelock.unwrap_or(0),
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// When no nLockTime is specified, we try to prevent fee sniping, if possible
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None => {
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// Fee sniping can be partially prevented by setting the timelock
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// to current_height. If we don't know the current_height,
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// we default to 0.
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let fee_sniping_height = current_height.unwrap_or(0);
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// We choose the biggest between the required nlocktime and the fee sniping
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// height
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std::cmp::max(requirements.timelock.unwrap_or(0), fee_sniping_height)
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}
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// Specific nLockTime required and we have no constraints, so just set to that value
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Some(x) if requirements.timelock.is_none() => x,
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// Specific nLockTime required and it's compatible with the constraints
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@ -1980,9 +1998,59 @@ pub(crate) mod test {
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builder.add_recipient(addr.script_pubkey(), 25_000);
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let (psbt, _) = builder.finish().unwrap();
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// Since we never synced the wallet we don't have a last_sync_height
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// we could use to try to prevent fee sniping. We default to 0.
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assert_eq!(psbt.unsigned_tx.lock_time, 0);
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}
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#[test]
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fn test_create_tx_fee_sniping_locktime_provided_height() {
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let (wallet, _, _) = get_funded_wallet(get_test_wpkh());
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let addr = wallet.get_address(New).unwrap();
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let mut builder = wallet.build_tx();
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builder.add_recipient(addr.script_pubkey(), 25_000);
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let sync_time = SyncTime {
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block_time: BlockTime {
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height: 24,
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timestamp: 0,
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},
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};
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wallet
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.database
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.borrow_mut()
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.set_sync_time(sync_time)
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.unwrap();
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let current_height = 25;
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builder.set_current_height(current_height);
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let (psbt, _) = builder.finish().unwrap();
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// current_height will override the last sync height
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assert_eq!(psbt.unsigned_tx.lock_time, current_height);
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}
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#[test]
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fn test_create_tx_fee_sniping_locktime_last_sync() {
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let (wallet, _, _) = get_funded_wallet(get_test_wpkh());
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let addr = wallet.get_address(New).unwrap();
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let mut builder = wallet.build_tx();
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builder.add_recipient(addr.script_pubkey(), 25_000);
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let sync_time = SyncTime {
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block_time: BlockTime {
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height: 25,
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timestamp: 0,
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},
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};
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wallet
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.database
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.borrow_mut()
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.set_sync_time(sync_time.clone())
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.unwrap();
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let (psbt, _) = builder.finish().unwrap();
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// If there's no current_height we're left with using the last sync height
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assert_eq!(psbt.unsigned_tx.lock_time, sync_time.block_time.height);
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}
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#[test]
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fn test_create_tx_default_locktime_cltv() {
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let (wallet, _, _) = get_funded_wallet(get_test_single_sig_cltv());
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@ -2001,9 +2069,13 @@ pub(crate) mod test {
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let mut builder = wallet.build_tx();
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builder
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.add_recipient(addr.script_pubkey(), 25_000)
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.set_current_height(630_001)
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.nlocktime(630_000);
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let (psbt, _) = builder.finish().unwrap();
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// When we explicitly specify a nlocktime
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// we don't try any fee sniping prevention trick
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// (we ignore the current_height)
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assert_eq!(psbt.unsigned_tx.lock_time, 630_000);
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}
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@ -147,6 +147,7 @@ pub(crate) struct TxParams {
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pub(crate) add_global_xpubs: bool,
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pub(crate) include_output_redeem_witness_script: bool,
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pub(crate) bumping_fee: Option<PreviousFee>,
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pub(crate) current_height: Option<u32>,
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}
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#[derive(Clone, Copy, Debug)]
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@ -543,6 +544,17 @@ impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>, Ctx: TxBuilderContext>
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self.params.rbf = Some(RbfValue::Value(nsequence));
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self
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}
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/// Set the current blockchain height.
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///
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/// This will be used to set the nLockTime for preventing fee sniping. If the current height is
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/// not provided, the last sync height will be used instead.
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///
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/// **Note**: This will be ignored if you manually specify a nlocktime using [`TxBuilder::nlocktime`].
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pub fn set_current_height(&mut self, height: u32) -> &mut Self {
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self.params.current_height = Some(height);
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self
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}
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}
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impl<'a, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>> TxBuilder<'a, D, Cs, CreateTx> {
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