redis#5299 Fix blocking XREAD for streams that ran dry

The conclusion, that a xread request can be answered syncronously in case that the stream's last_id is larger than the passed last-received-id parameter, assumes, that there must be entries present, which could be returned immediately. This assumption fails for empty streams that actually contained some entries which got removed by xdel, ... . As result, the client is answered synchronously with an empty result, instead of blocking for new entries to arrive. An additional check for a non-empty stream is required.

Author: Sascha Roland

src/t_stream.c

@@ -1436,15 +1436,15 @@ void xreadCommand(client *c) {
  * synchronously in case the group top item delivered is smaller
  * than what the stream has inside. */
  streamID *last = &groups[i]->last_id;
- if (streamCompareID(&s->last_id, last) > 0) {
+ if (s->length && (streamCompareID(&s->last_id, last) > 0)) {
  serve_synchronously = 1;
  *gt = *last;
  }
  }
  } else {
  /* For consumers without a group, we serve synchronously if we can
  * actually provide at least one item from the stream. */
- if (streamCompareID(&s->last_id, gt) > 0) {
+ if (s->length && (streamCompareID(&s->last_id, gt) > 0)) {
  serve_synchronously = 1;
  }
  }