streaming: fix snapshot cache bug

.changelog/9772.txt

@@ -0,0 +1,4 @@
+```release-note:bug
+streaming: fixes a bug caused by caching an incorrect snapshot, that would cause clients
+to error until the cache expired.
+```

agent/consul/stream/event_publisher.go

@@ -176,28 +176,27 @@ func (e *EventPublisher) Subscribe(req *SubscribeRequest) (*Subscription, error)
 	}
 
 	snapFromCache := e.getCachedSnapshotLocked(req)
-	if req.Index == 0 && snapFromCache != nil {
-		return e.subscriptions.add(req, snapFromCache.First), nil
+	if snapFromCache == nil {
+		snap := newEventSnapshot()
+		snap.appendAndSplice(*req, handler, topicHead)
+		e.setCachedSnapshotLocked(req, snap)
+		snapFromCache = snap
 	}
-	snap := newEventSnapshot()
 
-	// if the request has an Index the client view is stale and must be reset
-	// with a NewSnapshotToFollow event.
-	if req.Index > 0 {
-		snap.buffer.Append([]Event{{
-			Topic:   req.Topic,
-			Payload: newSnapshotToFollow{},
-		}})
-
-		if snapFromCache != nil {
-			snap.buffer.AppendItem(snapFromCache.First)
-			return e.subscriptions.add(req, snap.First), nil
-		}
+	// If the request.Index is 0 the client has no view, send a full snapshot.
+	if req.Index == 0 {
+		return e.subscriptions.add(req, snapFromCache.First), nil
 	}
 
-	snap.appendAndSplice(*req, handler, topicHead)
-	e.setCachedSnapshotLocked(req, snap)
-	return e.subscriptions.add(req, snap.First), nil
+	// otherwise the request has an Index, the client view is stale and must be reset
+	// with a NewSnapshotToFollow event.
+	result := newEventSnapshot()
+	result.buffer.Append([]Event{{
+		Topic:   req.Topic,
+		Payload: newSnapshotToFollow{},
+	}})
+	result.buffer.AppendItem(snapFromCache.First)
+	return e.subscriptions.add(req, result.First), nil
 }
 
 func (s *subscriptions) add(req *SubscribeRequest, head *bufferItem) *Subscription {

agent/consul/stream/event_publisher_test.go

@@ -392,6 +392,81 @@ func TestEventPublisher_SubscribeWithIndexNotZero_NewSnapshotFromCache(t *testin
 	})
 }
 
+func TestEventPublisher_SubscribeWithIndexNotZero_NewSnapshot_WithCache(t *testing.T) {
+	req := &SubscribeRequest{
+		Topic: testTopic,
+		Key:   "sub-key",
+		Index: 1,
+	}
+
+	nextEvent := Event{
+		Topic:   testTopic,
+		Index:   3,
+		Payload: simplePayload{key: "sub-key", value: "event-3"},
+	}
+
+	handlers := SnapshotHandlers{
+		testTopic: func(req SubscribeRequest, buf SnapshotAppender) (uint64, error) {
+			if req.Topic != testTopic {
+				return 0, fmt.Errorf("unexpected topic: %v", req.Topic)
+			}
+			buf.Append([]Event{testSnapshotEvent})
+			buf.Append([]Event{nextEvent})
+			return 3, nil
+		},
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	publisher := NewEventPublisher(handlers, time.Second)
+	go publisher.Run(ctx)
+	// Include the same events in the topicBuffer
+	publisher.publishEvent([]Event{testSnapshotEvent})
+	publisher.publishEvent([]Event{nextEvent})
+
+	runStep(t, "start a subscription and unsub", func(t *testing.T) {
+		sub, err := publisher.Subscribe(req)
+		require.NoError(t, err)
+		defer sub.Unsubscribe()
+
+		eventCh := runSubscription(ctx, sub)
+		next := getNextEvent(t, eventCh)
+		require.True(t, next.IsNewSnapshotToFollow(), next)
+
+		next = getNextEvent(t, eventCh)
+		require.Equal(t, testSnapshotEvent, next)
+
+		next = getNextEvent(t, eventCh)
+		require.Equal(t, nextEvent, next)
+
+		next = getNextEvent(t, eventCh)
+		require.True(t, next.IsEndOfSnapshot(), next)
+		require.Equal(t, uint64(3), next.Index)
+	})
+
+	publisher.snapshotHandlers[testTopic] = func(_ SubscribeRequest, _ SnapshotAppender) (uint64, error) {
+		return 0, fmt.Errorf("error should not be seen, cache should have been used")
+	}
+
+	runStep(t, "resume the subscription", func(t *testing.T) {
+		newReq := *req
+		newReq.Index = 0
+		sub, err := publisher.Subscribe(&newReq)
+		require.NoError(t, err)
+
+		eventCh := runSubscription(ctx, sub)
+		next := getNextEvent(t, eventCh)
+		require.Equal(t, testSnapshotEvent, next)
+
+		next = getNextEvent(t, eventCh)
+		require.Equal(t, nextEvent, next)
+
+		next = getNextEvent(t, eventCh)
+		require.True(t, next.IsEndOfSnapshot())
+	})
+}
+
 func runStep(t *testing.T, name string, fn func(t *testing.T)) {
 	t.Helper()
 	if !t.Run(name, fn) {