[LUCENE-1669] MMapDirectory on Windows silently fails to write to a file if also open for read

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Invalid
• Component/s: core/store
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Michael McCandless
• Created: 31/May/09 20:29
• Updated: 28/Aug/22 12:01
• Resolved: 16/Mar/13 19:22

Description

This is not a normal situation Lucene currently encounters, though we
are discussing exactly this possibility in LUCENE-1313.

I only hit it in digging down on a test failure in LUCENE-1658:

    [junit] Testcase: testIndexAndMerge(org.apache.lucene.index.TestDoc):	FAILED
    [junit] junit.framework.AssertionFailedError:
    [junit] 	at org.apache.lucene.index.FieldsWriter.addRawDocuments(FieldsWriter.java:249)
    [junit] 	at org.apache.lucene.index.SegmentMerger.mergeFields(SegmentMerger.java:350)
    [junit] 	at org.apache.lucene.index.SegmentMerger.merge(SegmentMerger.java:139)
    [junit] 	at org.apache.lucene.index.SegmentMerger.merge(SegmentMerger.java:116)
    [junit] 	at org.apache.lucene.index.TestDoc.merge(TestDoc.java:182)
    [junit] 	at org.apache.lucene.index.TestDoc.testIndexAndMerge(TestDoc.java:117)
    [junit] 	at org.apache.lucene.util.LuceneTestCase.runTest(LuceneTestCase.java:88)

That failure happens on Windows 64bit, if you use MMapDirectory, and
the index is on a remote (CIFS) mount.

This test opens a SegmentReader against doc store files that
IndexWriter still has open.

I whittled it down to this test showing the root cause:

public void testMMapWriteRead() throws Exception {
    MMapDirectory dir = new MMapDirectory(new File("readwrite"), null);
    //NIOFSDirectory dir = new NIOFSDirectory(new File("readwrite"), null);
    //SimpleFSDirectory dir = new SimpleFSDirectory(new File("readwrite"), null);
    IndexOutput out = dir.createOutput("one");
    out.writeLong(17);
    out.flush();
    // open the same file we are writing
    dir.openInput("one").close();
    // write another long
    out.writeLong(56);
    out.close();
    IndexInput in = dir.openInput("one");
    assertEquals(17, in.readLong());
    assertEquals(56, in.readLong());
    in.close();
}

NIOFSDir and SimpleFSDir pass the test fine, which is nice to know
(for LUCENE-1313). MMapDir passes fine on a local drive, but fails on
a remote CIFS mount.

I'm not sure what's going on. It seems like it could be a JRE bug –
behavior shouldn't change on local vs remote drive. I've only tested
it with java 1.6.0_11.