sweep: update sweeper to use AuxSweeper to add extra change addr

In this commit, we start to use the AuxSweeper (if present) to obtain a
new extra change addr we should add to the sweeping transaction. With
this, we'll take the set of inputs and our change addr, and then maybe
gain a new change addr to add to the sweep transaction.

The extra change addr will be treated as an extra required tx out,
shared across all the relevant inputs. This'll also be used in
NeedWalletInput to make sure that we add an extra input if needed to be
able to pay for the change addr.

sweep/tx_input_set.go

@@ -111,17 +111,26 @@ type BudgetInputSet struct {
 	// deadlineHeight is the height which the inputs in this set must be
 	// confirmed by.
 	deadlineHeight int32
+
+	// extraBudget is a value that should be allocated to sweep the given
+	// set of inputs. This can be used to add extra funds to the sweep
+	// transaction, for example to cover fees for additional outputs of
+	// custom channels.
+	extraBudget btcutil.Amount
 }
 
 // Compile-time constraint to ensure budgetInputSet implements InputSet.
 var _ InputSet = (*BudgetInputSet)(nil)
 
+// errEmptyInputs is returned when the input slice is empty.
+var errEmptyInputs = fmt.Errorf("inputs slice is empty")
+
 // validateInputs is used when creating new BudgetInputSet to ensure there are
 // no duplicate inputs and they all share the same deadline heights, if set.
 func validateInputs(inputs []SweeperInput, deadlineHeight int32) error {
 	// Sanity check the input slice to ensure it's non-empty.
 	if len(inputs) == 0 {
-		return fmt.Errorf("inputs slice is empty")
+		return errEmptyInputs
 	}
 
 	// inputDeadline tracks the input's deadline height. It will be updated
@@ -167,8 +176,8 @@ func validateInputs(inputs []SweeperInput, deadlineHeight int32) error {
 }
 
 // NewBudgetInputSet creates a new BudgetInputSet.
-func NewBudgetInputSet(inputs []SweeperInput,
-	deadlineHeight int32) (*BudgetInputSet, error) {
+func NewBudgetInputSet(inputs []SweeperInput, deadlineHeight int32,
+	auxSweeper fn.Option[AuxSweeper]) (*BudgetInputSet, error) {
 
 	// Validate the supplied inputs.
 	if err := validateInputs(inputs, deadlineHeight); err != nil {
@@ -186,9 +195,32 @@ func NewBudgetInputSet(inputs []SweeperInput,
 
 	log.Tracef("Created %v", bi.String())
 
+	// Attach an optional budget. This will be a no-op if the auxSweeper
+	// is not set.
+	if err := bi.attachExtraBudget(auxSweeper); err != nil {
+		return nil, err
+	}
+
 	return bi, nil
 }
 
+// attachExtraBudget attaches an extra budget to the input set, if the passed
+// aux sweeper is set.
+func (b *BudgetInputSet) attachExtraBudget(s fn.Option[AuxSweeper]) error {
+	extraBudget, err := fn.MapOptionZ(
+		s, func(aux AuxSweeper) fn.Result[btcutil.Amount] {
+			return aux.ExtraBudgetForInputs(b.Inputs())
+		},
+	).Unpack()
+	if err != nil {
+		return err
+	}
+
+	b.extraBudget = extraBudget
+
+	return nil
+}
+
 // String returns a human-readable description of the input set.
 func (b *BudgetInputSet) String() string {
 	inputsDesc := ""
@@ -212,8 +244,10 @@ func (b *BudgetInputSet) addInput(input SweeperInput) {
 func (b *BudgetInputSet) NeedWalletInput() bool {
 	var (
 		// budgetNeeded is the amount that needs to be covered from
-		// other inputs.
-		budgetNeeded btcutil.Amount
+		// other inputs. We start at the value of the extra budget,
+		// which might be needed for custom channels that add extra
+		// outputs.
+		budgetNeeded = b.extraBudget
 
 		// budgetBorrowable is the amount that can be borrowed from
 		// other inputs.