multi: replace htlcResolution with an interface

This commit repalces the htlcResolution struct with an interface.
This interface is implemeted by failure, settle and accept resolution
structs. Only settles and fails are exported because the existing
code that handles htlc resolutions uses a nil resolution to indicate
that a htlc was accepted. The accept resolution is used internally
to report on the resolution result of the accepted htlc, but a nil
resolution is surfaced. Further refactoring of all the functions
that call NotifyExitHopHtlc to handle a htlc accept case (rather than
having a nil check) is required.

invoices/resolution.go

@@ -0,0 +1,124 @@
+package invoices
+
+import (
+	"time"
+
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/lntypes"
+)
+
+// HtlcResolution describes how an htlc should be resolved.
+type HtlcResolution interface {
+	// CircuitKey returns the circuit key for the htlc that we have a
+	// resolution for.
+	CircuitKey() channeldb.CircuitKey
+}
+
+// HtlcFailResolution is an implementation of the HtlcResolution interface
+// which is returned when a htlc is failed.
+type HtlcFailResolution struct {
+	// circuitKey is the key of the htlc for which we have a resolution.
+	circuitKey channeldb.CircuitKey
+
+	// AcceptHeight is the original height at which the htlc was accepted.
+	AcceptHeight int32
+
+	// outcome indicates the outcome of the invoice registry update.
+	Outcome ResolutionResult
+}
+
+// NewFailResolution returns a htlc failure resolution.
+func NewFailResolution(key channeldb.CircuitKey,
+	acceptHeight int32, outcome ResolutionResult) *HtlcFailResolution {
+
+	return &HtlcFailResolution{
+		circuitKey:   key,
+		AcceptHeight: acceptHeight,
+		Outcome:      outcome,
+	}
+}
+
+// CircuitKey returns the circuit key for the htlc that we have a
+// resolution for.
+//
+// Note: it is part of the HtlcResolution interface.
+func (f *HtlcFailResolution) CircuitKey() channeldb.CircuitKey {
+	return f.circuitKey
+}
+
+// HtlcSettleResolution is an implementation of the HtlcResolution interface
+// which is returned when a htlc is settled.
+type HtlcSettleResolution struct {
+	// Preimage is the htlc preimage. Its value is nil in case of a cancel.
+	Preimage lntypes.Preimage
+
+	// circuitKey is the key of the htlc for which we have a resolution.
+	circuitKey channeldb.CircuitKey
+
+	// acceptHeight is the original height at which the htlc was accepted.
+	AcceptHeight int32
+
+	// Outcome indicates the outcome of the invoice registry update.
+	Outcome ResolutionResult
+}
+
+// NewSettleResolution returns a htlc resolution which is associated with a
+// settle.
+func NewSettleResolution(preimage lntypes.Preimage, key channeldb.CircuitKey,
+	acceptHeight int32, outcome ResolutionResult) *HtlcSettleResolution {
+
+	return &HtlcSettleResolution{
+		Preimage:     preimage,
+		circuitKey:   key,
+		AcceptHeight: acceptHeight,
+		Outcome:      outcome,
+	}
+}
+
+// CircuitKey returns the circuit key for the htlc that we have a
+// resolution for.
+//
+// Note: it is part of the HtlcResolution interface.
+func (s *HtlcSettleResolution) CircuitKey() channeldb.CircuitKey {
+	return s.circuitKey
+}
+
+// htlcAcceptResolution is an implementation of the HtlcResolution interface
+// which is returned when a htlc is accepted. This struct is not exported
+// because the codebase uses a nil resolution to indicate that a htlc was
+// accepted. This struct is used internally in the invoice registry to
+// surface accept resolution results. When an invoice update returns an
+// acceptResolution, a nil resolution should be surfaced.
+type htlcAcceptResolution struct {
+	// circuitKey is the key of the htlc for which we have a resolution.
+	circuitKey channeldb.CircuitKey
+
+	// autoRelease signals that the htlc should be automatically released
+	// after a timeout.
+	autoRelease bool
+
+	// acceptTime is the time at which this htlc was accepted.
+	acceptTime time.Time
+
+	// outcome indicates the outcome of the invoice registry update.
+	outcome ResolutionResult
+}
+
+// newAcceptResolution returns a htlc resolution which is associated with a
+// htlc accept.
+func newAcceptResolution(key channeldb.CircuitKey,
+	outcome ResolutionResult) *htlcAcceptResolution {
+
+	return &htlcAcceptResolution{
+		circuitKey: key,
+		outcome:    outcome,
+	}
+}
+
+// CircuitKey returns the circuit key for the htlc that we have a
+// resolution for.
+//
+// Note: it is part of the HtlcResolution interface.
+func (a *htlcAcceptResolution) CircuitKey() channeldb.CircuitKey {
+	return a.circuitKey
+}