Check transaction fees in transaction builder (#669)

src/subcommand/wallet/transaction_builder.rs

@@ -62,9 +62,9 @@ pub(crate) struct TransactionBuilder {
 type Result<T> = std::result::Result<T, Error>;
 
 impl TransactionBuilder {
-  const TARGET_POSTAGE: Amount = Amount::from_sat(10_000);
   const MAX_POSTAGE: Amount = Amount::from_sat(2 * 10_000);
-  const FEE_RATE: usize = 1;
+  const TARGET_FEE_RATE: Amount = Amount::from_sat(1);
+  const TARGET_POSTAGE: Amount = Amount::from_sat(10_000);
 
   pub(crate) fn build_transaction(
     ranges: BTreeMap<OutPoint, Vec<(u64, u64)>>,
@@ -171,8 +171,30 @@ impl TransactionBuilder {
   fn deduct_fee(mut self) -> Result<Self> {
     let ordinal_offset = self.calculate_ordinal_offset();
 
-    let tx = self.build()?;
-    let fee = Amount::from_sat((Self::FEE_RATE * tx.vsize()).try_into().unwrap());
+    let dummy_transaction = Transaction {
+      version: 1,
+      lock_time: PackedLockTime::ZERO,
+      input: self
+        .inputs
+        .iter()
+        .map(|_| TxIn {
+          previous_output: OutPoint::null(),
+          script_sig: Script::new(),
+          sequence: Sequence::ENABLE_RBF_NO_LOCKTIME,
+          witness: Witness::new(),
+        })
+        .collect(),
+      output: self
+        .outputs
+        .iter()
+        .map(|(address, amount)| TxOut {
+          value: amount.to_sat(),
+          script_pubkey: address.script_pubkey(),
+        })
+        .collect(),
+    };
+
+    let fee = Self::TARGET_FEE_RATE * dummy_transaction.vsize().try_into().unwrap();
 
     let total_output_amount = self
       .outputs
@@ -194,7 +216,7 @@ impl TransactionBuilder {
     Ok(self)
   }
 
-  fn build(&self) -> Result<Transaction> {
+  fn build(self) -> Result<Transaction> {
     let ordinal = self.ordinal.n();
     let recipient = self.recipient.script_pubkey();
     let transaction = Transaction {
@@ -287,6 +309,28 @@ impl TransactionBuilder {
       offset += output.value;
     }
 
+    let mut fee = Amount::ZERO;
+    for input in &transaction.input {
+      fee += Amount::from_sat(
+        self.ranges[&input.previous_output]
+          .iter()
+          .map(|(start, end)| end - start)
+          .sum::<u64>(),
+      );
+    }
+    for output in &transaction.output {
+      fee -= Amount::from_sat(output.value);
+    }
+
+    let fee_rate = fee.to_sat() as f64 / transaction.vsize() as f64;
+    let target_fee_rate = Self::TARGET_FEE_RATE.to_sat() as f64;
+    assert!(
+      fee_rate == target_fee_rate,
+      "invariant: fee rate is equal to target fee rate: actual fee rate: {} target_fee rate: {}",
+      fee_rate,
+      target_fee_rate,
+    );
+
     Ok(transaction)
   }
 
@@ -438,7 +482,7 @@ mod tests {
           "tb1qakxxzv9n7706kc3xdcycrtfv8cqv62hnwexc0l"
             .parse()
             .unwrap(),
-          Amount::from_sat(2000),
+          Amount::from_sat(1774),
         ),
       ],
     };
@@ -490,7 +534,7 @@ mod tests {
               .script_pubkey(),
           },
           TxOut {
-            value: 2000,
+            value: 1774,
             script_pubkey: "tb1qakxxzv9n7706kc3xdcycrtfv8cqv62hnwexc0l"
               .parse::<Address>()
               .unwrap()
@@ -880,4 +924,37 @@ mod tests {
     .build()
     .unwrap();
   }
+
+  #[test]
+  #[should_panic(expected = "invariant: fee rate is equal to target fee rate")]
+  fn fee_is_at_least_target_fee_rate() {
+    let utxos = vec![(
+      "1111111111111111111111111111111111111111111111111111111111111111:1"
+        .parse()
+        .unwrap(),
+      vec![(0, 10_000)],
+    )];
+
+    TransactionBuilder::new(
+      utxos.into_iter().collect(),
+      Ordinal(0),
+      "tb1q6en7qjxgw4ev8xwx94pzdry6a6ky7wlfeqzunz"
+        .parse()
+        .unwrap(),
+      vec![
+        "tb1qjsv26lap3ffssj6hfy8mzn0lg5vte6a42j75ww"
+          .parse()
+          .unwrap(),
+        "tb1qakxxzv9n7706kc3xdcycrtfv8cqv62hnwexc0l"
+          .parse()
+          .unwrap(),
+      ],
+    )
+    .select_ordinal()
+    .unwrap()
+    .strip_excess_postage()
+    .unwrap()
+    .build()
+    .unwrap();
+  }
 }