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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