ReRanking/LTR does not work in combination with custom sort and SolrCloud

TomGilke, tboeghk and I are currently working a lot on the Solr LTR feature. In our setup we're using a custom sort function for a solid base scoring of the documents.

We found out that a plain SolrCloud will return random sorted docs if you define a custom sort field or function. This problem is also mentioned here and is not really fixed.

You can do the following steps to reproduce this issue: here.

In the meantime we're also investing on this issue and try to debug the QueryComponent. But if there is any hint or thought available, we're very happy to hear from you. Maybe cpoerschke can help here a little bit?

What we've found out so far:

• It only happens in SolrCloud mode (shards > 1)
• Maybe is has something to do with the MergeStrategy in the QueryComponent.

You can also use the pull request to reproduce the problem.