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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-4018) ArrayIndexOutOfBoundsException in TopService.inService under heavy multithreaded use of EmbeddedDriver
Date Fri, 26 Jun 2009 15:13:07 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4018?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-4018:
--------------------------------------

    Attachment: d4018-1a.diff

d4018-1a is a partial fix for the problem, and it should be sufficient to fix the ArrayIndexOutOfBoundsException
in TopService.inService(). I ran the full repro on two machines on which I could reproduce
the problem reliably without the fix, and now I don't see the exception. I don't see the deadlock
that I saw with the previous patch either.

This fix factors out two for loops which iterate over moduleInstances into a helper method
which synchronizes on the moduleInstances object over the entire loop. This prevents other
threads from removing elements from the vector, and therefore the value returned by Vector.size()
should still be valid when we call Vector.get(), and no AIOOBE should be thrown. I believe
that it is safe, since the calls to size() and get() are already synchronized on the Vector,
and the loop doesn't do anything except unsynchronized accessing a field in the object fetched
from the Vector. So there should be no new ordering of how the synchronization locks are obtained,
which was the problem with the previous patch.

There are still a couple of places where there's an unsynchronized window between the checking
of the size and the actual retrieval from the Vector. I'll see if I can address those in a
follow-up patch.

> ArrayIndexOutOfBoundsException in TopService.inService under heavy multithreaded use
of EmbeddedDriver
> ------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-4018
>                 URL: https://issues.apache.org/jira/browse/DERBY-4018
>             Project: Derby
>          Issue Type: Bug
>          Components: Services
>    Affects Versions: 10.4.2.0
>         Environment: Windows XP SP2
> Intel Core 2 Duo 3GHz, 3GB RAM
> Java JDK 1.6.0_07
>            Reporter: Jeff Clary
>            Assignee: Knut Anders Hatlen
>         Attachments: BeatDerbyToPieces.java, d4018-1a.diff, program_output.txt, sync.diff,
trunk_stacktrace.txt
>
>
> I have created a test program that reproduces this issue.  The program pounds on Derby
by running jobs on a pool of 10 threads.  Each job creates a new Derby database, a schema,
a table, and an index.  Then it shuts down the database and finishes.  The program runs until
an error occurs or until 1000 jobs have finished.
> The problem is very intermittent, and related I think to multithreaded access of a vector
"moduleInstances" in TopService.java.  I am seeing the error on my machine about half the
time I run the test program.  Sometimes it happens after 100 or 200 jobs...but sometimes not
until 500 or more, or not at all.
> I am using the 10.4.2 derby.jar downloaded from http://db.apache.org/derby/releases/release-10.4.2.0.cgi,
without any modifications on my part.
> I will attach the test program plus output from a run that failed with the ArrayIndexOutOfBoundsException.
 (The actual exception thrown to the application from Derby is SQLException, but the underlying
cause is an ArrayIndexOutOfBoundsException.)  BE AWARE that if you run this test program it
can use up a GByte or more of disk space in your Java tmp directory.

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