stream: fix a bug with creating a snapshot

The head of the topic buffer was being ignored when creating a snapshot. This commit fixes
the bug by ensuring that the head of the topic buffer is included in the snapshot
before handing it off to the subscription.

agent/consul/stream/event_snapshot.go

@@ -49,34 +49,33 @@ func (s *eventSnapshot) appendAndSplice(req SubscribeRequest, fn SnapshotFunc, t
 func (s *eventSnapshot) spliceFromTopicBuffer(topicBufferHead *bufferItem, idx uint64) {
 	item := topicBufferHead
 	for {
-		next := item.NextNoBlock()
 		switch {
-		case next == nil:
+		case item.Err != nil:
-			// This is the head of the topic buffer (or was just now which is after
-			// the snapshot completed). We don't want any of the events (if any) in
-			// the snapshot buffer as they came before the snapshot but we do need to
-			// wait for the next update.
-			s.buffer.AppendItem(item.NextLink())
-			return
-
-		case next.Err != nil:
 			// This case is not currently possible because errors can only come
 			// from a snapshot func, and this is consuming events from a topic
 			// buffer which does not contain a snapshot.
 			// Handle this case anyway in case errors can come from other places
 			// in the future.
-			s.buffer.AppendItem(next)
+			s.buffer.AppendItem(item)
 			return
 
-		case len(next.Events) > 0 && next.Events[0].Index > idx:
+		case len(item.Events) > 0 && item.Events[0].Index > idx:
 			// We've found an update in the topic buffer that happened after our
 			// snapshot was taken, splice it into the snapshot buffer so subscribers
 			// can continue to read this and others after it.
-			s.buffer.AppendItem(next)
+			s.buffer.AppendItem(item)
 			return
 		}
 
-		// We don't need this item, continue to next
+		next := item.NextNoBlock()
+		if next == nil {
+			// We reached the head of the topic buffer. We don't want any of the
+			// events in the topic buffer as they came before the snapshot.
+			// Append a link to any future items.
+			s.buffer.AppendItem(item.NextLink())
+			return
+		}
+		// Proceed to the next item in the topic buffer
 		item = next
 	}
 }