fix(ckdoge): use correct transaction fees (#7660)

Follow-up on #7360 to use the correct parameters to compute the fees
involved in a transaction sent by the minter to the Dogecoin network in
case of a withdrawal.

The implementation introduces a new trait `FeeEstimator`, of which there
are two implementations (one for Bitcoin and another one for Dogecoin),
and parametrize the existing methods to build transactions by an
instance of that trait. A follow-up PR will add the
`estimate_withdrawal_fee` endpoint.

Author: gregorydemay

rs/bitcoin/ckbtc/agent/src/lib.rs

@@ -1,6 +1,6 @@
 use candid::{CandidType, Deserialize, Principal};
 use ic_agent::Agent;
-use ic_ckbtc_minter::queries::RetrieveBtcStatusRequest;
+use ic_ckbtc_minter::queries::{EstimateFeeArg, RetrieveBtcStatusRequest, WithdrawalFee};
 use ic_ckbtc_minter::state::RetrieveBtcStatus;
 use ic_ckbtc_minter::state::eventlog::{Event, GetEventsArg};
 use ic_ckbtc_minter::updates::{
@@ -114,6 +114,19 @@ impl CkBtcMinterAgent {
         .await
     }
 
+    pub async fn estimate_withdrawal_fee(
+        &self,
+        amount: u64,
+    ) -> Result<WithdrawalFee, CkBtcMinterAgentError> {
+        self.query(
+            "estimate_withdrawal_fee",
+            EstimateFeeArg {
+                amount: Some(amount),
+            },
+        )
+        .await
+    }
+
     pub async fn distribute_kyt_fee(&self) -> Result<(), CkBtcMinterAgentError> {
         self.update("distribute_kyt_fee", ()).await
     }

rs/bitcoin/ckbtc/minter/src/fees/mod.rs

@@ -0,0 +1,137 @@
+use crate::state::CkBtcMinterState;
+use crate::tx::UnsignedTransaction;
+use crate::{Network, fake_sign};
+use ic_btc_interface::{MillisatoshiPerByte, Satoshi};
+use std::cmp::max;
+
+pub trait FeeEstimator {
+    const DUST_LIMIT: u64;
+
+    /// Estimate the median fees based on the given fee percentiles (slice of fee rates in milli base unit per vbyte/byte).
+    fn estimate_median_fee(
+        &self,
+        fee_percentiles: &[MillisatoshiPerByte],
+    ) -> Option<MillisatoshiPerByte>;
+
+    /// Evaluate the fee necessary to cover the minter's cycles consumption.
+    fn evaluate_minter_fee(&self, num_inputs: u64, num_outputs: u64) -> Satoshi;
+
+    /// Evaluate transaction fee with the given fee rate (in milli base unit per vbyte/byte)
+    fn evaluate_transaction_fee(&self, tx: &UnsignedTransaction, fee_rate: u64) -> u64;
+
+    /// Compute a new minimum withdrawal amount based on the current fee rate
+    fn fee_based_minimum_withdrawal_amount(&self, median_fee: MillisatoshiPerByte) -> Satoshi;
+}
+
+pub struct BitcoinFeeEstimator {
+    /// The Bitcoin network that the minter will connect to
+    network: Network,
+    /// Minimum amount of bitcoin that can be retrieved
+    retrieve_btc_min_amount: u64,
+    /// The fee for a single Bitcoin check request.
+    check_fee: u64,
+}
+
+impl BitcoinFeeEstimator {
+    /// The minter's address is of type P2WPKH which means it has a dust limit of 294 sats.
+    /// For additional safety, we round that value up.
+    pub const MINTER_ADDRESS_P2PWPKH_DUST_LIMIT: Satoshi = 300;
+
+    pub fn new(network: Network, retrieve_btc_min_amount: u64, check_fee: u64) -> Self {
+        Self {
+            network,
+            retrieve_btc_min_amount,
+            check_fee,
+        }
+    }
+
+    pub fn from_state(state: &CkBtcMinterState) -> Self {
+        Self::new(
+            state.btc_network,
+            state.retrieve_btc_min_amount,
+            state.check_fee,
+        )
+    }
+
+    /// An estimated fee per vbyte of 142 millisatoshis per vbyte was selected around 2025.06.21 01:09:50 UTC
+    /// for Bitcoin Mainnet, whereas the median fee around that time should have been 2_000.
+    /// Until we know the root cause, we ensure that the estimated fee has a meaningful minimum value.
+    const fn minimum_fee_per_vbyte(&self) -> MillisatoshiPerByte {
+        match &self.network {
+            Network::Mainnet => 1_500,
+            Network::Testnet => 1_000,
+            Network::Regtest => 0,
+        }
+    }
+}
+
+impl FeeEstimator for BitcoinFeeEstimator {
+    // The default dustRelayFee is 3 sat/vB,
+    // which translates to a dust threshold of 546 satoshi for P2PKH outputs.
+    // The threshold for other types is lower,
+    // so we simply use 546 satoshi as the minimum amount per output.
+    const DUST_LIMIT: u64 = 546;
+
+    fn estimate_median_fee(
+        &self,
+        fee_percentiles: &[MillisatoshiPerByte],
+    ) -> Option<MillisatoshiPerByte> {
+        /// The default fee we use on regtest networks.
+        const DEFAULT_REGTEST_FEE: MillisatoshiPerByte = 5_000;
+
+        let median_fee = match &self.network {
+            Network::Mainnet | Network::Testnet => {
+                if fee_percentiles.len() < 100 {
+                    return None;
+                }
+                Some(fee_percentiles[50])
+            }
+            Network::Regtest => Some(DEFAULT_REGTEST_FEE),
+        };
+        median_fee.map(|f| f.max(self.minimum_fee_per_vbyte()))
+    }
+
+    fn evaluate_minter_fee(&self, num_inputs: u64, num_outputs: u64) -> u64 {
+        const MINTER_FEE_PER_INPUT: u64 = 146;
+        const MINTER_FEE_PER_OUTPUT: u64 = 4;
+        const MINTER_FEE_CONSTANT: u64 = 26;
+
+        max(
+            MINTER_FEE_PER_INPUT * num_inputs
+                + MINTER_FEE_PER_OUTPUT * num_outputs
+                + MINTER_FEE_CONSTANT,
+            Self::MINTER_ADDRESS_P2PWPKH_DUST_LIMIT,
+        )
+    }
+
+    /// Returns the minimum withdrawal amount based on the current median fee rate (in millisatoshi per byte).
+    /// The returned amount is in satoshi.
+    fn fee_based_minimum_withdrawal_amount(&self, median_fee: MillisatoshiPerByte) -> Satoshi {
+        match self.network {
+            Network::Mainnet | Network::Testnet => {
+                const PER_REQUEST_RBF_BOUND: u64 = 22_100;
+                const PER_REQUEST_VSIZE_BOUND: u64 = 221;
+                const PER_REQUEST_MINTER_FEE_BOUND: u64 = 305;
+
+                let median_fee_rate = median_fee / 1_000;
+                ((PER_REQUEST_RBF_BOUND
+                    + PER_REQUEST_VSIZE_BOUND * median_fee_rate
+                    + PER_REQUEST_MINTER_FEE_BOUND
+                    + self.check_fee)
+                    / 50_000) //TODO DEFI-2187: adjust increment of minimum withdrawal amount to be a multiple of retrieve_btc_min_amount/2
+                    * 50_000
+                    + self.retrieve_btc_min_amount
+            }
+            Network::Regtest => self.retrieve_btc_min_amount,
+        }
+    }
+
+    fn evaluate_transaction_fee(
+        &self,
+        unsigned_tx: &UnsignedTransaction,
+        fee_per_vbyte: u64,
+    ) -> u64 {
+        let tx_vsize = fake_sign(unsigned_tx).vsize();
+        (tx_vsize as u64 * fee_per_vbyte) / 1000
+    }
+}