sweep: introduce Bumper interface to handle RBF

This commit adds a new interface, `Bumper`, to handle RBF for a given
input set. It's responsible for creating the sweeping tx using the input
set, and monitors its confirmation status to decide whether a RBF should
be attempted or not.

We leave implementation details to future commits, and focus on mounting
this `Bumper` interface to our sweeper in this commit.

sweep/fee_bumper.go

@@ -0,0 +1,142 @@
+package sweep
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/lightningnetwork/lnd/input"
+	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
+)
+
+var (
+	// ErrInvalidBumpResult is returned when the bump result is invalid.
+	ErrInvalidBumpResult = errors.New("invalid bump result")
+)
+
+// Bumper defines an interface that can be used by other subsystems for fee
+// bumping.
+type Bumper interface {
+	// Broadcast is used to publish the tx created from the given inputs
+	// specified in the request. It handles the tx creation, broadcasts it,
+	// and monitors its confirmation status for potential fee bumping. It
+	// returns a chan that the caller can use to receive updates about the
+	// broadcast result and potential RBF attempts.
+	Broadcast(req *BumpRequest) (<-chan *BumpResult, error)
+}
+
+// BumpEvent represents the event of a fee bumping attempt.
+type BumpEvent uint8
+
+const (
+	// TxPublished is sent when the broadcast attempt is finished.
+	TxPublished BumpEvent = iota
+
+	// TxFailed is sent when the broadcast attempt fails.
+	TxFailed
+
+	// TxReplaced is sent when the original tx is replaced by a new one.
+	TxReplaced
+
+	// TxConfirmed is sent when the tx is confirmed.
+	TxConfirmed
+
+	// sentinalEvent is used to check if an event is unknown.
+	sentinalEvent
+)
+
+// String returns a human-readable string for the event.
+func (e BumpEvent) String() string {
+	switch e {
+	case TxPublished:
+		return "Published"
+	case TxFailed:
+		return "Failed"
+	case TxReplaced:
+		return "Replaced"
+	case TxConfirmed:
+		return "Confirmed"
+	default:
+		return "Unknown"
+	}
+}
+
+// Unknown returns true if the event is unknown.
+func (e BumpEvent) Unknown() bool {
+	return e >= sentinalEvent
+}
+
+// BumpRequest is used by the caller to give the Bumper the necessary info to
+// create and manage potential fee bumps for a set of inputs.
+type BumpRequest struct {
+	// Budget givens the total amount that can be used as fees by these
+	// inputs.
+	Budget btcutil.Amount
+
+	// Inputs is the set of inputs to sweep.
+	Inputs []input.Input
+
+	// DeadlineHeight is the block height at which the tx should be
+	// confirmed.
+	DeadlineHeight int32
+
+	// DeliveryAddress is the script to send the change output to.
+	DeliveryAddress []byte
+
+	// MaxFeeRate is the maximum fee rate that can be used for fee bumping.
+	MaxFeeRate chainfee.SatPerKWeight
+}
+
+// BumpResult is used by the Bumper to send updates about the tx being
+// broadcast.
+type BumpResult struct {
+	// Event is the type of event that the result is for.
+	Event BumpEvent
+
+	// Tx is the tx being broadcast.
+	Tx *wire.MsgTx
+
+	// ReplacedTx is the old, replaced tx if a fee bump is attempted.
+	ReplacedTx *wire.MsgTx
+
+	// FeeRate is the fee rate used for the new tx.
+	FeeRate chainfee.SatPerKWeight
+
+	// Fee is the fee paid by the new tx.
+	Fee btcutil.Amount
+
+	// Err is the error that occurred during the broadcast.
+	Err error
+}
+
+// Validate validates the BumpResult so it's safe to use.
+func (b *BumpResult) Validate() error {
+	// Every result must have a tx.
+	if b.Tx == nil {
+		return fmt.Errorf("%w: nil tx", ErrInvalidBumpResult)
+	}
+
+	// Every result must have a known event.
+	if b.Event.Unknown() {
+		return fmt.Errorf("%w: unknown event", ErrInvalidBumpResult)
+	}
+
+	// If it's a replacing event, it must have a replaced tx.
+	if b.Event == TxReplaced && b.ReplacedTx == nil {
+		return fmt.Errorf("%w: nil replacing tx", ErrInvalidBumpResult)
+	}
+
+	// If it's a failed event, it must have an error.
+	if b.Event == TxFailed && b.Err == nil {
+		return fmt.Errorf("%w: nil error", ErrInvalidBumpResult)
+	}
+
+	// If it's a confirmed event, it must have a fee rate and fee.
+	if b.Event == TxConfirmed && (b.FeeRate == 0 || b.Fee == 0) {
+		return fmt.Errorf("%w: missing fee rate or fee",
+			ErrInvalidBumpResult)
+	}
+
+	return nil
+}