invoices: adding InvoiceExpryWatcher to cancel expired invoices

This commit adds InvoiceExpryWatcher which is a separate class that
receives new invoices (and existing ones upon restart) from InvoiceRegistry
and actively watches their expiry. When an invoice is expired
InvoiceExpiryWatcher will call into InvoiceRegistry to cancel the
invoice and by that notify all subscribers about the state change.

channeldb/invoices.go

@@ -565,6 +565,83 @@ func (d *DB) LookupInvoice(paymentHash [32]byte) (Invoice, error) {
 	return invoice, nil
 }
 
+// InvoiceWithPaymentHash is used to store an invoice and its corresponding
+// payment hash. This struct is only used to store results of
+// ChannelDB.FetchAllInvoicesWithPaymentHash() call.
+type InvoiceWithPaymentHash struct {
+	// Invoice holds the invoice as selected from the invoices bucket.
+	Invoice Invoice
+
+	// PaymentHash is the payment hash for the Invoice.
+	PaymentHash lntypes.Hash
+}
+
+// FetchAllInvoicesWithPaymentHash returns all invoices and their payment hashes
+// currently stored within the database. If the pendingOnly param is true, then
+// only unsettled invoices and their payment hashes will be returned, skipping
+// all invoices that are fully settled or canceled. Note that the returned
+// array is not ordered by add index.
+func (d *DB) FetchAllInvoicesWithPaymentHash(pendingOnly bool) (
+	[]InvoiceWithPaymentHash, error) {
+
+	var result []InvoiceWithPaymentHash
+
+	err := d.View(func(tx *bbolt.Tx) error {
+		invoices := tx.Bucket(invoiceBucket)
+		if invoices == nil {
+			return ErrNoInvoicesCreated
+		}
+
+		invoiceIndex := invoices.Bucket(invoiceIndexBucket)
+		if invoiceIndex == nil {
+			// Mask the error if there's no invoice
+			// index as that simply means there are no
+			// invoices added yet to the DB. In this case
+			// we simply return an empty list.
+			return nil
+		}
+
+		return invoiceIndex.ForEach(func(k, v []byte) error {
+			// Skip the special numInvoicesKey as that does not
+			// point to a valid invoice.
+			if bytes.Equal(k, numInvoicesKey) {
+				return nil
+			}
+
+			if v == nil {
+				return nil
+			}
+
+			invoice, err := fetchInvoice(v, invoices)
+			if err != nil {
+				return err
+			}
+
+			if pendingOnly &&
+				(invoice.State == ContractSettled ||
+					invoice.State == ContractCanceled) {
+
+				return nil
+			}
+
+			invoiceWithPaymentHash := InvoiceWithPaymentHash{
+				Invoice: invoice,
+			}
+
+			copy(invoiceWithPaymentHash.PaymentHash[:], k)
+			result = append(result, invoiceWithPaymentHash)
+
+			return nil
+		})
+	})
+
+	if err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}
+
 // FetchAllInvoices returns all invoices currently stored within the database.
 // If the pendingOnly param is true, then only unsettled invoices will be
 // returned, skipping all invoices that are fully settled.