Possible order violation in lucene library version 2.4.1 [LUCENE-5732]

Hi,

I am working on a research project on data race detection, and am using the DaCapo benchmarks for evaluation. I am using the benchmark lusearch from the 2009 suite, which uses lucene library 2.4.1.

For one test case, I am monitoring a pair of accesses
say, Lorg/apache/lucene/store/Directory;.<init> ()V:40(6) and
Lorg/apache/lucene/store/FSDirectory;.close ()V:524(1). The format is <class name>.<method name> <method desc>:line(byte code index).

During my work, I am getting AlreadyClosedExceptions on the FSDirectory from the ensureOpen() method for some threads, which I think is probably due to an order violation. I have actually introduced delays
in my instrumentation which delays threads that execute the code in
Lorg/apache/lucene/store/FSDirectory;.close ()V. This is causing the other query threads to throw an exception.

Here is the exception trace:

org.apache.lucene.store.AlreadyClosedException: this Directory is closed
at org.apache.lucene.store.Directory.ensureOpen(Directory.java:220)
at org.apache.lucene.store.FSDirectory.list(FSDirectory.java:320)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:533)
at org.apache.lucene.index.DirectoryIndexReader.open(DirectoryIndexReader.java:115)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:316)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:206)
at org.dacapo.lusearch.Search$QueryProcessor.<init>(Search.java:207)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)
org.apache.lucene.store.AlreadyClosedException: this Directory is closed
at org.apache.lucene.store.Directory.ensureOpen(Directory.java:220)
at org.apache.lucene.store.FSDirectory.list(FSDirectory.java:320)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:533)
at org.apache.lucene.index.DirectoryIndexReader.open(DirectoryIndexReader.java:115)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:316)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:206)
at org.dacapo.lusearch.Search$QueryProcessor.<init>(Search.java:207)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)
java.lang.NullPointerException
at org.dacapo.lusearch.Search$QueryProcessor.run(Search.java:226)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)
java.lang.NullPointerException
at org.dacapo.lusearch.Search$QueryProcessor.run(Search.java:226)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)
org.apache.lucene.store.AlreadyClosedException: this Directory is closed
at org.apache.lucene.store.Directory.ensureOpen(Directory.java:220)
at org.apache.lucene.store.FSDirectory.list(FSDirectory.java:320)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:533)
at org.apache.lucene.index.DirectoryIndexReader.open(DirectoryIndexReader.java:115)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:316)
at org.apache.lucene.index.IndexReader.open(IndexReader.java:206)
at org.dacapo.lusearch.Search$QueryProcessor.<init>(Search.java:207)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)
java.lang.NullPointerException
at org.dacapo.lusearch.Search$QueryProcessor.run(Search.java:226)
at org.dacapo.lusearch.Search$QueryThread.run(Search.java:179)

This exception does not happen during normal instrumentation, but can easily be reproduced by introducing just delays in the instrumentation.

---
Migrated from [LUCENE-5732](https://issues.apache.org/jira/browse/LUCENE-5732) by Swarnendu Biswas
Environment:
```
RHEL 6.5, 64 bit, Intel i5
```

Reproduce the failure with the DaCapo 2009 lusearch benchmark and the reported instrumentation delays. Start at org.apache.lucene.store.Directory.ensureOpen, FSDirectory.close/list, and the stack-trace paths through IndexReader.open; compare them with org.dacapo.lusearch.Search.QueryProcessor. The report does not define a specific fix, so completion requires confirming the ordering defect and agreeing on the expected behavior.

Written by the indexing model from the issue text.