Async search allows users to retrieve partial results for a running search. For partial results, the number of successful shards does not include the skipped shards, while the response returned to users should.

When a fatal failure happened after some partial results were processed, for instance because all of the shards failed to fetch their hits, the number of successful shards should be reset to align it with what ordinary search would return.

Also, we recently had a bug where async search would miss tracking shard failures, which would have been caught if we had assertions in place that verified that whenever we get the last response, the number of failures included in it is the same as the failures that were tracked through the listener notifications.