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From "Uwe Schindler (Updated) (JIRA)" <>
Subject [jira] [Updated] (LUCENE-3800) Readers wrapping other readers don't prevent usage if any of their subreaders was closed
Date Mon, 20 Feb 2012 16:51:35 GMT


Uwe Schindler updated LUCENE-3800:

    Attachment: LUCENE-3800.patch

Patch for trunk that enforces a "soft-close" (make it unuseable) once a child-reader (subreader,
delegate, parallel reader) is closed.

The implementation works like that:
- A parent reader that delegates to one or more child readers (its currently BaseCompositeReader,
SlowCompositeReaderWrapper, FilterAtomicReader, ParallelAtomicReader), register itsself on
every child/delegate reader. The registration is done using a weak reference, so there is
no circular ref problems. To be able to use WeakHashMap, this patch explicitely enforces identity
equals/hashcode on IndexReader (which we always assumedin the past for FieldCache and so on,
now its enforced like in MMapIndexInput)
- When a reader closes, it simply iterates over all registered parents and "marks" them as
"unuseable" (they are not really closed).
- When somebody calls any method of a reader of which any child was closed, ensureOpen() will
throw an AlreadyClosedEx with corresponding message.
- You are still able to decRef/close readers which are marked as invalid (otherwise tests
would fail). The child reader closed -> disable parent reader is simply done for safety.
- To make the ensureOpen() checks not more expensive than before, the extra boolean was not
made volatile, instead happens before and volatile behaviour as explained in Michael Busch's
talks was used to guard the "closedByChild" boolean.
- The patch does not use readerClosedListener for 2 reasons: (1) RCL has no weak refs, so
we don't produce circular references between parents and childs, (2) We have to recursively
go up the parent chain, which may confuse conventional readerClosedListeners. Also the parent
readers are not closed, so its not applicable to call the listeners.
> Readers wrapping other readers don't prevent usage if any of their subreaders was closed
> ----------------------------------------------------------------------------------------
>                 Key: LUCENE-3800
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: core/index
>    Affects Versions: 4.0
>            Reporter: Uwe Schindler
>            Assignee: Uwe Schindler
>             Fix For: 4.0
>         Attachments: LUCENE-3800.patch
> On recent trunk test we got this problem:
> org.apache.lucene.index.TestReaderClosed.test
> fails because the inner reader is closed but the wrapped outer ones are still open.
> I fixed the issue partially for SlowCompositeReaderWrapper and ParallelAtomicReader but
it failed again. The cool thing with this test is the following:
> The test opens an DirectoryReader and then creates a searcher, closes the reader and
executes a search. This is not an issue, if the reader is closed that the search is running
on. This test uses LTC.newSearcher(wrap=true), which randomly wraps the passed Reader with
SlowComposite or ParallelReader - or with both!!! If you then close the original inner reader,
the close is not detected when excuting search. This can cause SIGSEGV when MMAP is used.
> The problem in (in Slow* and Parallel*) is, that both have their own Fields instances
thats are kept alive until the reader itsself is closed. If the child reader is closed, the
wrapping reader does not know and still uses its own Fields instance that delegates to the
inner readers. On this step no more ensureOpen checks are done, causing the failures.
> The first fix done in Slow and Parallel was to call ensureOpen() on the subReader, too
when requesting fields(). This works fine until you wrap two times: ParallelAtomicReader(SlowCompositeReaderWrapper(StandardDirectoryReader(segments_1:3:nrt
> One solution would be to make ensureOpen also check all subreaders, but that would do
the volatile checks way too often (with n is the total number of subreaders and m is the number
of hierarchical levels this is n^m) - we cannot do this. Currently we only have n*m which
is fine.
> The proposal how to solve this (closing subreaders under the hood of parent readers is
to use the readerClosedListeners. Whenever a composite or slow reader wraps another readers,
it registers itself as interested in readerClosed events. When a subreader is then forcefully
closed (e.g by a programming error or this crazy test), we automatically close the parents,
> We should also fix this in 3.x, if we have similar problems there (needs investigation).

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