NullPointerException in QueryComponent.unmarshalSortValues

[Hendrik Haddorp <he...@gmx.net>]

Hi,

I'm doing a simple *:* search on an empty multi sharded collection using
Solr 7.6 and am getting this exception:

NullPointerException
     at
org.apache.solr.handler.component.QueryComponent.unmarshalSortValues(QueryComponent.java:1034)
     at
org.apache.solr.handler.component.QueryComponent.mergeIds(QueryComponent.java:885)
     at
org.apache.solr.handler.component.QueryComponent.handleRegularResponses(QueryComponent.java:585)
     at
org.apache.solr.handler.component.QueryComponent.handleResponses(QueryComponent.java:564)
     at
org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:426)
     at
org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:199)
     at org.apache.solr.core.SolrCore.execute(SolrCore.java:2541)

This is the same exception as reported in
https://issues.apache.org/jira/browse/SOLR-12060 and likely also
https://issues.apache.org/jira/browse/SOLR-11643. Sometimes I can do
multiple requests and some work and some pass. And this test is done on
a single node that for testing uses a collection with four shards.

I looked a bit into the code:
https://github.com/apache/lucene-solr/blob/branch_7_6/solr/core/src/java/org/apache/solr/handler/component/QueryComponent.java
I seems like
https://github.com/apache/lucene-solr/blob/branch_7_6/solr/core/src/java/org/apache/solr/handler/component/QueryComponent.java#L884
return null.
This should mean that
https://github.com/apache/lucene-solr/blob/branch_7_6/solr/core/src/java/org/apache/solr/handler/component/QueryComponent.java#L482
did not get invoked, which would happen if FIELD_SORT_VALUES is not set
to true. And indeed if I add fsv=true to my query the NPE does not show
up. So there are a few questions:
1) what is fsv=true about?
2) why do I need to set it?
3) why don't I get the NPE all the time?

Earlier it looked as if the problem only showed up if I enabled the
suggester or spellcheck component. But after having done tons of tests
things are not that consistent.

thanks,
Hendrik