Merge bitcoindevkit/bdk#728: Add fee_amount() and fee_rate() functions to PsbtUtils trait

ab41679368be79e8645915b40be554cc07557931 Add fee_amount() and fee_rate() functions to PsbtUtils trait (Steve Myers)

Pull request description:

  ### Description

  The purpose of the PR is to provide a more convenient way to calculate the transaction fee amount and fee rate for a PSBT. This PR adds `fee_amount` and `fee_rate` functions to the existing `PsbtUtils` trait and implements them for `PartiallySignedBitcoinTransaction`. The `fee_rate` value is only valid if the PSBT it is called on is fully signed and finalized.

  See related discussion: https://github.com/bitcoindevkit/bdk-ffi/issues/179

  ### Changelog

  Added
  - PsbtUtils.fee_amount(), calculates the PSBT total transaction fee amount in Sats.
  - PsbtUtils.fee_rate(), calculates the PSBT FeeRate, the value is only accurate AFTER the PSBT is finalized.

  ### Notes to the reviewers

  Ideally I'd like `fee_rate` to return an `Option` and return `None` if the PSBT isn't finalized. But I'm not quite sure how to determine if a PSBT is finalized without having a `Wallet` and running it through the finalize code first. Or there might be a way to fill in missing signatures with properly sized fake data prior to calculating the fee rate. For now I think it's enough to do this simple approach with usage warning in the rust docs.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  afilini:
    ACK ab41679368be79e8645915b40be554cc07557931

Tree-SHA512: 5386109c9ffcf63160f18b4c51eb2c582f4121b669c7276aaba489d186cf9b97343d46f887469b1407ccd7a24448b5e7aee4c6832d86768239b21f4e68054a0f

src/psbt/mod.rs

@@ -9,11 +9,22 @@
 // You may not use this file except in accordance with one or both of these
 // licenses.
 
+use crate::FeeRate;
 use bitcoin::util::psbt::PartiallySignedTransaction as Psbt;
 use bitcoin::TxOut;
 
 pub trait PsbtUtils {
     fn get_utxo_for(&self, input_index: usize) -> Option<TxOut>;
+
+    /// The total transaction fee amount, sum of input amounts minus sum of output amounts, in Sats.
+    /// If the PSBT is missing a TxOut for an input returns None.
+    fn fee_amount(&self) -> Option<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.
+    /// If the PSBT is missing a TxOut for an input returns None.
+    fn fee_rate(&self) -> Option<FeeRate>;
 }
 
 impl PsbtUtils for Psbt {
@@ -37,6 +48,27 @@ impl PsbtUtils for Psbt {
             None
         }
     }
+
+    fn fee_amount(&self) -> Option<u64> {
+        let tx = &self.unsigned_tx;
+        let utxos: Option<Vec<TxOut>> = (0..tx.input.len()).map(|i| self.get_utxo_for(i)).collect();
+
+        utxos.map(|inputs| {
+            let input_amount: u64 = inputs.iter().map(|i| i.value).sum();
+            let output_amount: u64 = self.unsigned_tx.output.iter().map(|o| o.value).sum();
+            input_amount
+                .checked_sub(output_amount)
+                .expect("input amount must be greater than output amount")
+        })
+    }
+
+    fn fee_rate(&self) -> Option<FeeRate> {
+        let fee_amount = self.fee_amount();
+        fee_amount.map(|fee| {
+            let weight = self.clone().extract_tx().weight();
+            FeeRate::from_wu(fee, weight)
+        })
+    }
 }
 
 #[cfg(test)]
@@ -44,8 +76,9 @@ mod test {
     use crate::bitcoin::TxIn;
     use crate::psbt::Psbt;
     use crate::wallet::AddressIndex;
+    use crate::wallet::AddressIndex::New;
     use crate::wallet::{get_funded_wallet, test::get_test_wpkh};
-    use crate::SignOptions;
+    use crate::{psbt, FeeRate, SignOptions};
     use std::str::FromStr;
 
     // from bip 174
@@ -118,4 +151,83 @@ mod test {
 
         let _ = wallet.sign(&mut psbt, SignOptions::default()).unwrap();
     }
+
+    #[test]
+    fn test_psbt_fee_rate_with_witness_utxo() {
+        use psbt::PsbtUtils;
+
+        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();
+        assert!(fee_amount.is_some());
+
+        let unfinalized_fee_rate = psbt.fee_rate().unwrap();
+
+        let finalized = wallet.sign(&mut psbt, Default::default()).unwrap();
+        assert!(finalized);
+
+        let finalized_fee_rate = psbt.fee_rate().unwrap();
+        assert!(finalized_fee_rate.as_sat_per_vb() >= expected_fee_rate);
+        assert!(finalized_fee_rate.as_sat_per_vb() < unfinalized_fee_rate.as_sat_per_vb());
+    }
+
+    #[test]
+    fn test_psbt_fee_rate_with_nonwitness_utxo() {
+        use psbt::PsbtUtils;
+
+        let expected_fee_rate = 1.2345;
+
+        let (wallet, _, _) = get_funded_wallet("pkh(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();
+        assert!(fee_amount.is_some());
+        let unfinalized_fee_rate = psbt.fee_rate().unwrap();
+
+        let finalized = wallet.sign(&mut psbt, Default::default()).unwrap();
+        assert!(finalized);
+
+        let finalized_fee_rate = psbt.fee_rate().unwrap();
+        assert!(finalized_fee_rate.as_sat_per_vb() >= expected_fee_rate);
+        assert!(finalized_fee_rate.as_sat_per_vb() < unfinalized_fee_rate.as_sat_per_vb());
+    }
+
+    #[test]
+    fn test_psbt_fee_rate_with_missing_txout() {
+        use psbt::PsbtUtils;
+
+        let expected_fee_rate = 1.2345;
+
+        let (wpkh_wallet, _, _) = get_funded_wallet("wpkh(tprv8ZgxMBicQKsPd3EupYiPRhaMooHKUHJxNsTfYuScep13go8QFfHdtkG9nRkFGb7busX4isf6X9dURGCoKgitaApQ6MupRhZMcELAxTBRJgS/*)");
+        let addr = wpkh_wallet.get_address(New).unwrap();
+        let mut builder = wpkh_wallet.build_tx();
+        builder.drain_to(addr.script_pubkey()).drain_wallet();
+        builder.fee_rate(FeeRate::from_sat_per_vb(expected_fee_rate));
+        let (mut wpkh_psbt, _) = builder.finish().unwrap();
+
+        wpkh_psbt.inputs[0].witness_utxo = None;
+        wpkh_psbt.inputs[0].non_witness_utxo = None;
+        assert!(wpkh_psbt.fee_amount().is_none());
+        assert!(wpkh_psbt.fee_rate().is_none());
+
+        let (pkh_wallet, _, _) = get_funded_wallet("pkh(tprv8ZgxMBicQKsPd3EupYiPRhaMooHKUHJxNsTfYuScep13go8QFfHdtkG9nRkFGb7busX4isf6X9dURGCoKgitaApQ6MupRhZMcELAxTBRJgS/*)");
+        let addr = pkh_wallet.get_address(New).unwrap();
+        let mut builder = pkh_wallet.build_tx();
+        builder.drain_to(addr.script_pubkey()).drain_wallet();
+        builder.fee_rate(FeeRate::from_sat_per_vb(expected_fee_rate));
+        let (mut pkh_psbt, _) = builder.finish().unwrap();
+
+        pkh_psbt.inputs[0].non_witness_utxo = None;
+        assert!(pkh_psbt.fee_amount().is_none());
+        assert!(pkh_psbt.fee_rate().is_none());
+    }
 }