invoices: fix slow startup with many expired invoices

This commit intends to fix slow first startup time when there are many
invoices that need to be canceled. The slowdown is caused by a combination
of adding invoices to the expiry watcher one-by-one and slow
cancellation. Due to slow cancellation and the unbuffered channel which
we use to pass invoices to the expiry watcher blocks the registry.
With this fix we'll instead batch add invoices to the expiry watcher and
thereby won't block the registry startup.

invoices/invoice_expiry_watcher_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/clock"
 	"github.com/lightningnetwork/lnd/lntypes"
 )
@@ -123,3 +124,31 @@ func TestInvoiceExpiryWithPendingAndExpiredInvoices(t *testing.T) {
 	test.watcher.Stop()
 	test.checkExpectations()
 }
+
+// Tests adding multiple invoices at once.
+func TestInvoiceExpiryWhenAddingMultipleInvoices(t *testing.T) {
+	t.Parallel()
+	test := newInvoiceExpiryWatcherTest(t, testTime, 5, 5)
+	var invoices []channeldb.InvoiceWithPaymentHash
+	for hash, invoice := range test.testData.expiredInvoices {
+		invoices = append(invoices,
+			channeldb.InvoiceWithPaymentHash{
+				Invoice:     *invoice,
+				PaymentHash: hash,
+			},
+		)
+	}
+	for hash, invoice := range test.testData.pendingInvoices {
+		invoices = append(invoices,
+			channeldb.InvoiceWithPaymentHash{
+				Invoice:     *invoice,
+				PaymentHash: hash,
+			},
+		)
+	}
+
+	test.watcher.AddInvoices(invoices)
+	time.Sleep(testTimeout)
+	test.watcher.Stop()
+	test.checkExpectations()
+}