Merge #483: Fix BTC/kB conversion logic

2bb341b Add test for BTC/kb conversion (Tobin C. Harding)
eea11c3 Fix BTC/kb conversion (Tobin C. Harding)

Pull request description:

  Old RPC calls use BTC/kB, we need a `FeeRate` but `v0.32` cannot elegantly handle the conversion so we do it manually - and surprise surprise its buggy.

  This bug is why we spent so much time on the `FeeRate` API in `rust-bitcoin`.

  The original code was written by me, was untested, and was buggy (even after I worked on the API in `rust-bitcoin`) - bad Tobin no biscuit.

  Patch 2 adds a unit test and can be put before patch 1 to see the bug.

  Fix: #482

ACKs for top commit:
  jamillambert:
    ACK 2bb341b

Tree-SHA512: f40b7d7201d723e0b747c7af6601b6e2609792476d9f5a57706c5848f74aa415bcf2c3bdd74264e74c0d1aacb530da18e740320e1f8a2e0c02ac0bff1597a510

Author: tcharding

types/src/lib.rs

@@ -90,13 +90,9 @@ impl std::error::Error for NumericError {}
 
 /// Converts `fee_rate` in BTC/kB to `FeeRate`.
 fn btc_per_kb(btc_per_kb: f64) -> Result<Option<FeeRate>, ParseAmountError> {
-    let sats_per_kb = Amount::from_btc(btc_per_kb)?;
-    let sats_per_byte = sats_per_kb.to_sat() / 1000;
-
-    // Virtual bytes equal bytes before segwit.
-    let rate = FeeRate::from_sat_per_vb(sats_per_byte);
-
-    Ok(rate)
+    // TODO: After upgrade to bitcoin `v0.33` use `FeeRate::from_sat_per_kvb()`.
+    let per_kb = Amount::from_btc(btc_per_kb)?;
+    Ok(FeeRate::from_sat_per_vb(per_kb.to_sat()).and_then(|fee_rate| fee_rate.checked_div(1000)))
 }
 
 // TODO: Remove this function if a new `Witness` constructor gets added.
@@ -292,3 +288,16 @@ impl ScriptSig {
         ScriptBuf::from_hex(&self.hex)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn convert_btc_per_kb() {
+        // per kB = per kvB because this is a conversion of legacy transaction weights.
+        let f: f64 = 0.000001;
+        let got = btc_per_kb(f).unwrap();
+        assert_eq!(got, Some(FeeRate::from_sat_per_kwu(25)))
+    }
+}