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/interfaces.go

@@ -21,13 +21,20 @@ type InvoiceDatabase interface {
 	// NotifyExitHopHtlc attempts to mark an invoice as settled. If the
 	// invoice is a debug invoice, then this method is a noop as debug
 	// invoices are never fully settled. The return value describes how the
-	// htlc should be resolved.
-	NotifyExitHopHtlc(rhash lntypes.Hash, amt lnwire.MilliSatoshi) (
-		*invoices.HodlEvent, error)
+	// htlc should be resolved. If the htlc cannot be resolved immediately,
+	// the resolution is sent on the passed in hodlChan later.
+	NotifyExitHopHtlc(payHash lntypes.Hash, paidAmount lnwire.MilliSatoshi,
+		hodlChan chan<- interface{}) (*invoices.HodlEvent, error)
 
 	// CancelInvoice attempts to cancel the invoice corresponding to the
 	// passed payment hash.
 	CancelInvoice(payHash lntypes.Hash) error
+
+	// SettleHodlInvoice settles a hold invoice.
+	SettleHodlInvoice(preimage lntypes.Preimage) error
+
+	// HodlUnsubscribeAll unsubscribes from all hodl events.
+	HodlUnsubscribeAll(subscriber chan<- interface{})
 }
 
 // ChannelLink is an interface which represents the subsystem for managing the