Add wallet::utils::FeeInfo trait and implement for PartiallySignedBitcoinTransaction

Author: notmandatory

src/wallet/mod.rs

@@ -5414,4 +5414,33 @@ pub(crate) mod test {
         // ...and checking that everything is fine
         assert_fee_rate!(psbt, details.fee.unwrap_or(0), fee_rate);
     }
+
+    #[test]
+    fn test_finalized_single_xprv_fee_rate() {
+        use utils::FeeInfo;
+
+        let expected_fee_rate = 1.2345;
+
+        let (wallet, _, _) = get_funded_wallet("wpkh(tprv8ZgxMBicQKsPd3EupYiPRhaMooHKUHJxNsTfYuScep13go8QFfHdtkG9nRkFGb7busX4isf6X9dURGCoKgitaApQ6MupRhZMcELAxTBRJgS/*)");
+        let addr = wallet.get_address(New).unwrap();
+        let mut builder = wallet.build_tx();
+        builder.drain_to(addr.script_pubkey()).drain_wallet();
+        builder.fee_rate(FeeRate::from_sat_per_vb(expected_fee_rate));
+        let (mut psbt, _) = builder.finish().unwrap();
+
+        let fee_amount = psbt.fee_amount();
+        dbg!(fee_amount);
+
+        let unfinalized_fee_rate = psbt.fee_rate();
+        dbg!(unfinalized_fee_rate.as_sat_vb());
+
+        let finalized = wallet.sign(&mut psbt, Default::default()).unwrap();
+        assert!(finalized);
+
+        let finalized_fee_rate = psbt.fee_rate();
+        dbg!(finalized_fee_rate.as_sat_vb());
+
+        assert!(finalized_fee_rate.as_sat_vb() >= expected_fee_rate);
+        assert!(finalized_fee_rate.as_sat_vb() < unfinalized_fee_rate.as_sat_vb());
+    }
 }

src/wallet/utils.rs

@@ -10,8 +10,10 @@
 // licenses.
 
 use bitcoin::blockdata::script::Script;
+use bitcoin::psbt::PartiallySignedTransaction;
 use bitcoin::secp256k1::{All, Secp256k1};
 
+use crate::FeeRate;
 use miniscript::{MiniscriptKey, Satisfier, ToPublicKey};
 
 // MSB of the nSequence. If set there's no consensus-constraint, so it must be disabled when
@@ -137,6 +139,38 @@ impl<Pk: MiniscriptKey + ToPublicKey> Satisfier<Pk> for Older {
 
 pub(crate) type SecpCtx = Secp256k1<All>;
 
+/// Trait for extracting fee amount and rate information from a `PartiallySignedTransaction`.
+pub trait FeeInfo {
+    /// The total transaction fee amount, sum of input amounts minus sum of output amounts, in Sats.
+    fn fee_amount(&self) -> u64;
+
+    /// The transaction's fee rate. This value will only be accurate if calculated AFTER the
+    /// `PartiallySignedTransaction` is finalized and all witness/signature data is added to the
+    /// transaction.
+    fn fee_rate(&self) -> FeeRate;
+}
+
+impl FeeInfo for PartiallySignedTransaction {
+    fn fee_amount(&self) -> u64 {
+        let input_amount = &self
+            .inputs
+            .iter()
+            .fold(0, |acc, i| acc + i.witness_utxo.as_ref().unwrap().value);
+        let output_amount = &self
+            .unsigned_tx
+            .output
+            .iter()
+            .fold(0, |acc, o| acc + o.value);
+        input_amount - output_amount
+    }
+
+    fn fee_rate(&self) -> FeeRate {
+        let fee_amount = self.fee_amount();
+        let weight = self.clone().extract_tx().weight();
+        FeeRate::from_wu(fee_amount, weight)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::{