spend: a temporary partial fix for LowestFee

src/spend.rs

@@ -201,7 +201,27 @@ where
         if drain.is_none() && self.must_have_change {
             None
         } else {
-            self.lowest_fee.score(cs)
+            // This is a temporary partial fix for https://github.com/bitcoindevkit/coin-select/issues/6
+            // until it has been fixed upstream.
+            // TODO: Revert this change once upstream fix has been made.
+            // When calculating the score, the excess should be added to changeless solutions instead of
+            // those with change.
+            // Given a solution has been found, this fix adds or removes the excess to its incorrectly
+            // calculated score as required so that two changeless solutions can be differentiated
+            // if one has higher excess (and therefore pays a higher fee).
+            // Note that the `bound` function is also affected by this bug, which could mean some branches
+            // are not considered when running BnB, but at least this fix will mean the score for those
+            // solutions that are found is correct.
+            self.lowest_fee.score(cs).map(|score| {
+                // See https://github.com/bitcoindevkit/coin-select/blob/29b187f5509a01ba125a0354f6711e317bb5522a/src/metrics/lowest_fee.rs#L35-L45
+                assert!(cs.selected_value() >= self.lowest_fee.target.value);
+                let excess = (cs.selected_value() - self.lowest_fee.target.value) as f32;
+                bdk_coin_select::float::Ordf32(if drain.is_none() {
+                    score.0 + excess
+                } else {
+                    score.0 - excess
+                })
+            })
         }
     }
 

tests/test_spend.py

@@ -1,5 +1,5 @@
 from fixtures import *
-from test_framework.serializations import PSBT
+from test_framework.serializations import PSBT, uint256_from_str
 from test_framework.utils import wait_for, COIN, RpcError
 
 
@@ -332,6 +332,23 @@ def test_coin_selection(lianad, bitcoind):
     assert auto_psbt.tx.vout[1].nValue == manual_psbt.tx.vout[1].nValue
 
 
+def test_coin_selection_changeless(lianad, bitcoind):
+    """We choose the changeless solution with lowest fee."""
+    # Get two coins with similar amounts.
+    txid_a = bitcoind.rpc.sendtoaddress(lianad.rpc.getnewaddress()["address"], 0.00031)
+    txid_b = bitcoind.rpc.sendtoaddress(lianad.rpc.getnewaddress()["address"], 0.00032)
+    bitcoind.generate_block(1, wait_for_mempool=[txid_a, txid_b])
+    wait_for(lambda: len(lianad.rpc.listcoins(["confirmed"])["coins"]) == 2)
+    # Send an amount that can be paid by just one of our coins.
+    res = lianad.rpc.createspend({bitcoind.rpc.getnewaddress(): 30800}, [], 1)
+    psbt = PSBT.from_base64(res["psbt"])
+    # Only one input needed.
+    assert len(psbt.i) == 1
+    # Coin A is used as input.
+    txid_a = uint256_from_str(bytes.fromhex(txid_a)[::-1])
+    assert psbt.tx.vin[0].prevout.hash == txid_a
+
+
 def test_sweep(lianad, bitcoind):
     """
     Test we can leverage the change_address parameter to partially or completely sweep