[jira] [Created] (LUCENE-7570) Tragic events during merges can lead to deadlock

["Joey Echeverria (JIRA)" <ji...@apache.org>]

Joey Echeverria created LUCENE-7570:
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             Summary: Tragic events during merges can lead to deadlock
                 Key: LUCENE-7570
                 URL: https://issues.apache.org/jira/browse/LUCENE-7570
             Project: Lucene - Core
          Issue Type: Bug
          Components: core/index
    Affects Versions: 5.5, master (7.0)
            Reporter: Joey Echeverria


When an {{IndexWriter#commit()}} is stalled due to too many pending merges, you can get a deadlock if the currently active merge thread hits a tragic event.

# The thread performing the commit synchronizes on the the {{commitLock}} in {{commitInternal}}.
# The thread goes on to to call {{ConcurrentMergeScheduler#doStall()}} which {{waits()}} on the {{ConcurrentMergeScheduler}} object. This release the merge scheduler's monitor lock, but not the {{commitLock}} in {{IndexWriter}}.
# Sometime after this wait begins, the merge thread gets a tragic exception can calls {{IndexWriter#tragicEvent()}} which in turn calls {{IndexWriter#rollbackInternal()}}.
# The {{IndexWriter#rollbackInternal()}} synchronizes on the {{commitLock}} which is still held by the committing thread from (1) above which is waiting on the merge(s) to complete. Hence, deadlock.

We hit this bug with Lucene 5.5, but I looked at the code in the master branch and it looks like the deadlock still exists there as well.



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