wallet: run coin selection by `OutputType`

Run coin selection on each OutputType separately, choosing the best
solution according to the waste metric.

This is to avoid mixing UTXOs that are of different OutputTypes,
which can hurt privacy.

If no single OutputType can fund the transaction, then coin selection
considers the entire wallet, potentially mixing (current behavior).

This is done inside AttemptSelection so that all OutputTypes are
considered at each back-off in coin selection.

1 files changed, 13 insertions, 12 deletions

diff --git a/src/wallet/spend.h b/src/wallet/spend.h
index a9216e22ee..96ecd690be 100644
--- a/src/wallet/spend.h
+++ b/src/wallet/spend.h
@@ -91,22 +91,23 @@ const CTxOut& FindNonChangeParentOutput(const CWallet& wallet, const COutPoint&
 std::map<CTxDestination, std::vector<COutput>> ListCoins(const CWallet& wallet) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
 
 std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<COutput>& outputs, const CoinSelectionParams& coin_sel_params, const CoinEligibilityFilter& filter, bool positive_only);
-
 /**
- * Attempt to find a valid input set that meets the provided eligibility filter and target.
- * Multiple coin selection algorithms will be run and the input set that produces the least waste
- * (according to the waste metric) will be chosen.
+ * Attempt to find a valid input set that preserves privacy by not mixing OutputTypes.
+ * `ChooseSelectionResult()` will be called on each OutputType individually and the best
+ * the solution (according to the waste metric) will be chosen. If a valid input cannot be found from any
+ * single OutputType, fallback to running `ChooseSelectionResult()` over all available coins.
  *
- * param@[in]  wallet                 The wallet which provides solving data for the coins
- * param@[in]  nTargetValue           The target value
- * param@[in]  eligilibity_filter     A filter containing rules for which coins are allowed to be included in this selection
- * param@[in]  available_coins        The struct of coins, organized by OutputType, available for selection prior to filtering
- * param@[in]  coin_selection_params  Parameters for the coin selection
- * returns                            If successful, a SelectionResult containing the input set
- *                                    If failed, a nullopt
+ * param@[in]  wallet                    The wallet which provides solving data for the coins
+ * param@[in]  nTargetValue              The target value
+ * param@[in]  eligilibity_filter        A filter containing rules for which coins are allowed to be included in this selection
+ * param@[in]  available_coins           The struct of coins, organized by OutputType, available for selection prior to filtering
+ * param@[in]  coin_selection_params     Parameters for the coin selection
+ * param@[in]  allow_mixed_output_types  Relax restriction that SelectionResults must be of the same OutputType
+ * returns                               If successful, a SelectionResult containing the input set
+ *                                       If failed, a nullopt
  */
 std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const CoinsResult& available_coins,
-                        const CoinSelectionParams& coin_selection_params);
+                        const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types);
 
 /**
  * Attempt to find a valid input set that meets the provided eligibility filter and target.