invoices: fix synchronization issue with single invoice subscribers

This commit fixes a synchronization issue where a single invoice
subscriber could receive duplicate and/or out of order invoice updates.

invoices/invoiceregistry.go

@@ -67,9 +67,11 @@ type InvoiceRegistry struct {
 	singleNotificationClients map[uint32]*SingleInvoiceSubscription
 
 	newSubscriptions    chan *InvoiceSubscription
-	newSingleSubscriptions chan *SingleInvoiceSubscription
 	subscriptionCancels chan uint32
-	invoiceEvents          chan *invoiceEvent
+
+	// invoiceEvents is a single channel over which both invoice updates and
+	// new single invoice subscriptions are carried.
+	invoiceEvents chan interface{}
 
 	// debugInvoices is a map which stores special "debug" invoices which
 	// should be only created/used when manual tests require an invoice
@@ -112,9 +114,8 @@ func NewRegistry(cdb *channeldb.DB, decodeFinalCltvExpiry func(invoice string) (
 		notificationClients:       make(map[uint32]*InvoiceSubscription),
 		singleNotificationClients: make(map[uint32]*SingleInvoiceSubscription),
 		newSubscriptions:          make(chan *InvoiceSubscription),
-		newSingleSubscriptions:    make(chan *SingleInvoiceSubscription),
 		subscriptionCancels:       make(chan uint32),
-		invoiceEvents:             make(chan *invoiceEvent, 100),
+		invoiceEvents:             make(chan interface{}, 100),
 		hodlSubscriptions:         make(map[lntypes.Hash]map[chan<- interface{}]struct{}),
 		hodlReverseSubscriptions:  make(map[chan<- interface{}]map[lntypes.Hash]struct{}),
 		decodeFinalCltvExpiry:     decodeFinalCltvExpiry,
@@ -176,23 +177,6 @@ func (i *InvoiceRegistry) invoiceEventNotifier() {
 			// continue.
 			i.notificationClients[newClient.id] = newClient
 
-		// A new single invoice subscription has arrived. We'll query
-		// for any backlog notifications, then add it to the set of
-		// clients.
-		case newClient := <-i.newSingleSubscriptions:
-			err := i.deliverSingleBacklogEvents(newClient)
-			if err != nil {
-				log.Errorf("Unable to deliver backlog invoice "+
-					"notifications: %v", err)
-			}
-
-			log.Infof("New single invoice subscription "+
-				"client: id=%v, hash=%v",
-				newClient.id, newClient.hash,
-			)
-
-			i.singleNotificationClients[newClient.id] = newClient
-
 		// A client no longer wishes to receive invoice notifications.
 		// So we'll remove them from the set of active clients.
 		case clientID := <-i.subscriptionCancels:
@@ -202,18 +186,39 @@ func (i *InvoiceRegistry) invoiceEventNotifier() {
 			delete(i.notificationClients, clientID)
 			delete(i.singleNotificationClients, clientID)
 
-		// A sub-systems has just modified the invoice state, so we'll
-		// dispatch notifications to all registered clients.
+		// An invoice event has come in. This can either be an update to
+		// an invoice or a new single invoice subscriber. Both type of
+		// events are passed in via the same channel, to make sure that
+		// subscribers get a consistent view of the event sequence.
 		case event := <-i.invoiceEvents:
-			// For backwards compatibility, do not notify all
-			// invoice subscribers of cancel and accept events.
-			state := event.invoice.Terms.State
+			switch e := event.(type) {
+
+			// A sub-systems has just modified the invoice state, so
+			// we'll dispatch notifications to all registered
+			// clients.
+			case *invoiceEvent:
+				// For backwards compatibility, do not notify
+				// all invoice subscribers of cancel and accept
+				// events.
+				state := e.invoice.Terms.State
 				if state != channeldb.ContractCanceled &&
 					state != channeldb.ContractAccepted {
 
-				i.dispatchToClients(event)
+					i.dispatchToClients(e)
+				}
+				i.dispatchToSingleClients(e)
+
+			// A new single invoice subscription has arrived. Add it
+			// to the set of clients. It is important to do this in
+			// sequence with any other invoice events, because an
+			// initial invoice update has already been sent out to
+			// the subscriber.
+			case *SingleInvoiceSubscription:
+				log.Infof("New single invoice subscription "+
+					"client: id=%v, hash=%v", e.id, e.hash)
+
+				i.singleNotificationClients[e.id] = e
 			}
-			i.dispatchToSingleClients(event)
 
 		case <-i.quit:
 			return
@@ -367,7 +372,9 @@ func (i *InvoiceRegistry) deliverSingleBacklogEvents(
 
 	// It is possible that the invoice does not exist yet, but the client is
 	// already watching it in anticipation.
-	if err == channeldb.ErrInvoiceNotFound {
+	if err == channeldb.ErrInvoiceNotFound ||
+		err == channeldb.ErrNoInvoicesCreated {
+
 		return nil
 	}
 	if err != nil {
@@ -896,7 +903,7 @@ func (i *InvoiceRegistry) SubscribeNotifications(addIndex, settleIndex uint64) *
 // SubscribeSingleInvoice returns an SingleInvoiceSubscription which allows the
 // caller to receive async notifications for a specific invoice.
 func (i *InvoiceRegistry) SubscribeSingleInvoice(
-	hash lntypes.Hash) *SingleInvoiceSubscription {
+	hash lntypes.Hash) (*SingleInvoiceSubscription, error) {
 
 	client := &SingleInvoiceSubscription{
 		Updates: make(chan *channeldb.Invoice),
@@ -949,12 +956,24 @@ func (i *InvoiceRegistry) SubscribeSingleInvoice(
 		}
 	}()
 
-	select {
-	case i.newSingleSubscriptions <- client:
-	case <-i.quit:
+	// Within the lock, we both query the invoice state and pass the client
+	// subscription to the invoiceEvents channel. This is to make sure that
+	// the client receives a consistent stream of events.
+	i.Lock()
+	defer i.Unlock()
+
+	err := i.deliverSingleBacklogEvents(client)
+	if err != nil {
+		return nil, err
 	}
 
-	return client
+	select {
+	case i.invoiceEvents <- client:
+	case <-i.quit:
+		return nil, ErrShuttingDown
+	}
+
+	return client, nil
 }
 
 // notifyHodlSubscribers sends out the hodl event to all current subscribers.

invoices/invoiceregistry_test.go

@@ -71,7 +71,10 @@ func TestSettleInvoice(t *testing.T) {
 	defer allSubscriptions.Cancel()
 
 	// Subscribe to the not yet existing invoice.
-	subscription := registry.SubscribeSingleInvoice(hash)
+	subscription, err := registry.SubscribeSingleInvoice(hash)
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer subscription.Cancel()
 
 	if subscription.hash != hash {
@@ -225,7 +228,10 @@ func TestCancelInvoice(t *testing.T) {
 	}
 
 	// Subscribe to the not yet existing invoice.
-	subscription := registry.SubscribeSingleInvoice(hash)
+	subscription, err := registry.SubscribeSingleInvoice(hash)
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer subscription.Cancel()
 
 	if subscription.hash != hash {
@@ -329,7 +335,10 @@ func TestHoldInvoice(t *testing.T) {
 	defer allSubscriptions.Cancel()
 
 	// Subscribe to the not yet existing invoice.
-	subscription := registry.SubscribeSingleInvoice(hash)
+	subscription, err := registry.SubscribeSingleInvoice(hash)
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer subscription.Cancel()
 
 	if subscription.hash != hash {

lnrpc/invoicesrpc/invoices_server.go

@@ -178,7 +178,10 @@ func (s *Server) SubscribeSingleInvoice(req *SubscribeSingleInvoiceRequest,
 		return err
 	}
 
-	invoiceClient := s.cfg.InvoiceRegistry.SubscribeSingleInvoice(hash)
+	invoiceClient, err := s.cfg.InvoiceRegistry.SubscribeSingleInvoice(hash)
+	if err != nil {
+		return err
+	}
 	defer invoiceClient.Cancel()
 
 	for {