stream: fix a snapshot cache bug

Previously a snapshot created as part of a resumse-stream request could have incorrectly
cached the newSnapshotToFollow event. This would cause clients to error because they
received an unexpected framing event.

.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 {