Merge bitcoin/bitcoin#28366: Fix waste calculation in SelectionResult

bd34dd85e7b8b4cc26d2173d84bbeda2e9c27624 Use `exact_target` shorthand in coinselector_tests (Murch)
7aa7e30441fe77bf8e8092916e36b004bbbfe2a7 Fold GetSelectionWaste() into ComputeAndSetWaste() (Murch)

Pull request description:

  PR #26152 moved waste calculation into SelectionResult to be able to correct the waste score on basis of the bump_fee_group_discount for overlapping ancestries. This left two functions with largely overlapping purpose, where one was simply a wrapper of the other. This PR cleans up the overlap, and fixes the double-meaning of `change_cost` where the `GetChange()` function assumed that no change was created when `change_cost` was set to 0. This behavior was exploited in a bunch of tests, but is problematic, because a `change_cost` of 0 is permitted with custom settings for feerate and discard_feerate (i.e. when they’re both 0).

ACKs for top commit:
  achow101:
    ACK bd34dd85e7b8b4cc26d2173d84bbeda2e9c27624
  furszy:
    Code ACK bd34dd85e7b8b4cc26d2173d84bbeda2e9c27624
  ismaelsadeeq:
    Code Review ACK bd34dd85e7b8b4cc26d2173d84bbeda2e9c27624

Tree-SHA512: 83a2688d45d719dc61a64b5180fe136107faccf401a59df65245c05d701748a03e85ed56fde8c9b7ef39a3ab54374dd3718c559bda5b3f55dafedfd7fed25161

1 files changed, 23 insertions, 36 deletions

diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
index 42615b5d42..f1706b6800 100644
--- a/src/wallet/coinselection.cpp
+++ b/src/wallet/coinselection.cpp
@@ -194,7 +194,7 @@ util::Result<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool
     for (const size_t& i : best_selection) {
         result.AddInput(utxo_pool.at(i));
     }
-    result.ComputeAndSetWaste(cost_of_change, cost_of_change, CAmount{0});
+    result.RecalculateWaste(cost_of_change, cost_of_change, CAmount{0});
     assert(best_waste == result.GetWaste());
 
     return result;
@@ -792,35 +792,6 @@ void OutputGroupTypeMap::Push(const OutputGroup& group, OutputType type, bool in
     }
 }
 
-CAmount SelectionResult::GetSelectionWaste(CAmount change_cost, CAmount target, bool use_effective_value)
-{
-    // This function should not be called with empty inputs as that would mean the selection failed
-    assert(!m_selected_inputs.empty());
-
-    // Always consider the cost of spending an input now vs in the future.
-    CAmount waste = 0;
-    for (const auto& coin_ptr : m_selected_inputs) {
-        const COutput& coin = *coin_ptr;
-        waste += coin.GetFee() - coin.long_term_fee;
-    }
-    // Bump fee of whole selection may diverge from sum of individual bump fees
-    waste -= bump_fee_group_discount;
-
-    if (change_cost) {
-        // Consider the cost of making change and spending it in the future
-        // If we aren't making change, the caller should've set change_cost to 0
-        assert(change_cost > 0);
-        waste += change_cost;
-    } else {
-        // When we are not making change (change_cost == 0), consider the excess we are throwing away to fees
-        CAmount selected_effective_value = use_effective_value ? GetSelectedEffectiveValue() : GetSelectedValue();
-        assert(selected_effective_value >= target);
-        waste += selected_effective_value - target;
-    }
-
-    return waste;
-}
-
 CAmount GenerateChangeTarget(const CAmount payment_value, const CAmount change_fee, FastRandomContext& rng)
 {
     if (payment_value <= CHANGE_LOWER / 2) {
@@ -839,16 +810,32 @@ void SelectionResult::SetBumpFeeDiscount(const CAmount discount)
     bump_fee_group_discount = discount;
 }
 
-
-void SelectionResult::ComputeAndSetWaste(const CAmount min_viable_change, const CAmount change_cost, const CAmount change_fee)
+void SelectionResult::RecalculateWaste(const CAmount min_viable_change, const CAmount change_cost, const CAmount change_fee)
 {
-    const CAmount change = GetChange(min_viable_change, change_fee);
+    // This function should not be called with empty inputs as that would mean the selection failed
+    assert(!m_selected_inputs.empty());
+
+    // Always consider the cost of spending an input now vs in the future.
+    CAmount waste = 0;
+    for (const auto& coin_ptr : m_selected_inputs) {
+        const COutput& coin = *coin_ptr;
+        waste += coin.GetFee() - coin.long_term_fee;
+    }
+    // Bump fee of whole selection may diverge from sum of individual bump fees
+    waste -= bump_fee_group_discount;
 
-    if (change > 0) {
-        m_waste = GetSelectionWaste(change_cost, m_target, m_use_effective);
+    if (GetChange(min_viable_change, change_fee)) {
+        // if we have a minimum viable amount after deducting fees, account for
+        // cost of creating and spending change
+        waste += change_cost;
     } else {
-        m_waste = GetSelectionWaste(0, m_target, m_use_effective);
+        // When we are not making change (GetChange(…) == 0), consider the excess we are throwing away to fees
+        CAmount selected_effective_value = m_use_effective ? GetSelectedEffectiveValue() : GetSelectedValue();
+        assert(selected_effective_value >= m_target);
+        waste += selected_effective_value - m_target;
     }
+
+    m_waste = waste;
 }
 
 void SelectionResult::SetAlgoCompleted(bool algo_completed)