[firestore] Set the cursor to empty when the end is reached (gravitat…

…ional#7448)

* nil firestore cursor on end

* added explainer

lib/events/firestoreevents/firestoreevents.go

@@ -470,6 +470,7 @@ func (l *Log) SearchEvents(fromUTC, toUTC time.Time, namespace string, eventType
 	var values []events.EventFields
 	var parsedStartKey int64
 	var err error
+	reachedEnd := false
 	totalSize := 0
 
 	if startKey != "" {
@@ -501,6 +502,15 @@ func (l *Log) SearchEvents(fromUTC, toUTC time.Time, namespace string, eventType
 		return nil, "", firestorebk.ConvertGRPCError(err)
 	}
 
+	// Correctly detecting if you've reached the end of a query in firestore is
+	// tricky since it doesn't set any flag when it finds that there are no further events.
+	// This solution here seems to be the the most common but I haven't been able to find
+	// any documented hard guarantees on firestore not early returning for some reason like response
+	// size like DynamoDB does. In short, this should work in all cases for lack of a better solution.
+	if len(docSnaps) < limit {
+		reachedEnd = true
+	}
+
 	g.WithFields(log.Fields{"duration": time.Since(start)}).Debugf("Query completed.")
 	for _, docSnap := range docSnaps {
 		var e event
@@ -546,7 +556,7 @@ func (l *Log) SearchEvents(fromUTC, toUTC time.Time, namespace string, eventType
 	}
 
 	var lastKeyString string
-	if lastKey != 0 {
+	if lastKey != 0 && !reachedEnd {
 		lastKeyString = fmt.Sprintf("%d", lastKey)
 	}
 

lib/events/firestoreevents/firestoreevents_test.go

@@ -105,3 +105,7 @@ func (s *FirestoreeventsSuite) TearDownTest(c *check.C) {
 func (s *FirestoreeventsSuite) TestSessionEventsCRUD(c *check.C) {
 	s.SessionEventsCRUD(c)
 }
+
+func (s *FirestoreeventsSuite) TestPagination(c *check.C) {
+	s.EventPagination(c)
+}