A deadlock issue occurs under the following circumstances
- IndexWriter.autoCommit == true
- IndexWriter.directory contains multiple segments
- IndexWriter.AddIndex(IndexReader) is invoked
I put together a JUnit test that recreates the deadlock, which I've attached. It is the first test method, 'testAddIndexByIndexReader()'.
In a nutshell, here is what happens:
// 1) AddIndexes(IndexReader) acquires the write lock,
// then begins optimization of destination index (this is
// prior to adding any external segments).
// 2) Main thread starts a ConcurrentMergeScheduler.MergeThread
// to merge the 2 segments.
// 3) Merging thread tries to acquire the read lock at
// IndexWriter.blockAddIndexes(boolean) in
// IndexWriter.StartCommit(), but cannot as...
// 4) Main thread still holds the write lock, and is
// waiting for the IndexWriter.runningMerges data structure
// to be devoid of merges with their optimize flag
// set (IndexWriter.optimizeMergesPending()).