SOLR-18125 Make LukeRequestHandler::getFirstLiveDoc get the first live doc

[kotman12]

Description

LukeRequestHandler::getFirstLiveDoc is completely broken.

Funny little expedition as I was testing a patch I had to support distrib=true for LukeRequestHandler. It turns out the logic to fetch the first live doc from the index for a particular field (used to populate the index flags for that field) is broken in two surprising ways. As far as I can tell it's been broken since 2012. The net effect is you'll just randomly not get index flags for fields as I show in the test.

Timeline:

• LUCENE-4355 Before this change docsEnum = reader.termDocsEnum(liveDocs, field, termText, offset) apparently would return null, perhaps when there were no live docs for this particular term. There used to be a guard in the loop condition which ensured that it would exit on the first non-null docsEnum it encountered across all terms. Not entirely sure why exactly this extra condition was necessary; it would only matter if the first non-null docsEnum also didn't have any docs. Either away, reaching such a state apparently warranted ending the search altogether. LUCENE-4355 changed the semantics to termsEnum.docs(liveDocs, docsEnum, offset) which could not return null, as much is obvious from the fact that the inner null check disappeared. However, the earlier docsEnum == null remained in the loop condition, almost certainly by mistake.
• LUCENE-6553 this massive change seemed to introduce the exact opposite liveDocs check you'd expect from this method (somehow exiting the loop with no result when a live doc was encountered). I suspect it is, again, an oversight given how large the change was and how little discussion there was around it.

Solution

Fix the bug and git rid of a vestigial check. Diffs of how we got here mentioned in JIRA.

Tests

Quite easily show that you get no index flags when you should get index flags.

Checklist

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