htlcswitch: hodl invoice

This commit modifies the invoice registry to handle invoices for which
the preimage is not known yet (hodl invoices). In that case, the
resolution channel passed in from links and resolvers is stored until we
either learn the preimage or want to cancel the htlc.

htlcswitch/mock.go

@@ -759,18 +759,23 @@ func (i *mockInvoiceRegistry) LookupInvoice(rHash lntypes.Hash) (channeldb.Invoi
 	return i.registry.LookupInvoice(rHash)
 }
 
-func (i *mockInvoiceRegistry) NotifyExitHopHtlc(rhash lntypes.Hash,
-	amt lnwire.MilliSatoshi) (*invoices.HodlEvent, error) {
+func (i *mockInvoiceRegistry) SettleHodlInvoice(preimage lntypes.Preimage) error {
+	return i.registry.SettleHodlInvoice(preimage)
+}
 
-	event, err := i.registry.NotifyExitHopHtlc(rhash, amt)
+func (i *mockInvoiceRegistry) NotifyExitHopHtlc(rhash lntypes.Hash,
+	amt lnwire.MilliSatoshi, hodlChan chan<- interface{}) (
+	*invoices.HodlEvent, error) {
+
+	event, err := i.registry.NotifyExitHopHtlc(rhash, amt, hodlChan)
 	if err != nil {
-		return event, err
+		return nil, err
 	}
 	if i.settleChan != nil {
 		i.settleChan <- rhash
 	}
 
-	return event, err
+	return event, nil
 }
 
 func (i *mockInvoiceRegistry) CancelInvoice(payHash lntypes.Hash) error {
@@ -784,6 +789,10 @@ func (i *mockInvoiceRegistry) AddInvoice(invoice channeldb.Invoice,
 	return err
 }
 
+func (i *mockInvoiceRegistry) HodlUnsubscribeAll(subscriber chan<- interface{}) {
+	i.registry.HodlUnsubscribeAll(subscriber)
+}
+
 var _ InvoiceDatabase = (*mockInvoiceRegistry)(nil)
 
 type mockSigner struct {