[jira] [Created] (LUCENE-9405) IndexWriter incorrectly calls closeMergeReaders twice when the merged segment is 100% deleted

["Michael McCandless (Jira)" <ji...@apache.org>]

Michael McCandless created LUCENE-9405:
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             Summary: IndexWriter incorrectly calls closeMergeReaders twice when the merged segment is 100% deleted
                 Key: LUCENE-9405
                 URL: https://issues.apache.org/jira/browse/LUCENE-9405
             Project: Lucene - Core
          Issue Type: Bug
          Components: core/index
            Reporter: Michael McCandless


This is the first spinoff from a [controversial PR to add a new index-time feature to Lucene to merge small segments during commit|https://github.com/apache/lucene-solr/pull/1552].  This can substantially reduce the number of small index segments to search.

See specifically [this discussion there|https://github.com/apache/lucene-solr/pull/1552#discussion_r440298695].

{{IndexWriter}} seems to be missing a {{success = true}} inside {{mergeMiddle}} in the case where all segments being merged have 100% deletions and the segments will simply be dropped.

In this case, in master today, I think we are incorrectly calling {{closeMergedReaders}} twice, first with {{suppressExceptions = false}} and second time with {{true}}.

There is a [dedicated test case here showing the issue|https://github.com/apache/lucene-solr/commit/cab5ef5e6f2bdcda59fd669a298ec1377777af9d], but that test case relies on changes in the controversial feature (added {{MergePolicy.findFullFlushMerges}}). I think it should be possible to make another test case show the bug without that controversial feature, and I am unsure why our existing randomized tests have not uncovered this yet ...



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