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Date: Sun, 19 Feb 2006 00:06:26 +0100 (CET)
From: "Sven Boden (JIRA)" <ibatis-dev@incubator.apache.org>
To: dev@ibatis.apache.org
Subject: [jira] Closed: (IBATIS-249) Race conditions in Throttle lead to
 thread blockage popping items from ThrottledPools under stress
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     [ http://issues.apache.org/jira/browse/IBATIS-249?page=all ]
     
Sven Boden closed IBATIS-249:
-----------------------------

    Fix Version: 2.2.0
     Resolution: Fixed
      Assign To: Sven Boden

Thanks for the IBATIS-249 patch Jonathan, changes checked in:

- Verified by at least 2 other people
- I played a little bit with it
- Logically the changes look correct
- No unit test case as it's difficult to simulate


> Race conditions in Throttle lead to thread blockage popping items from ThrottledPools under stress
> --------------------------------------------------------------------------------------------------
>
>          Key: IBATIS-249
>          URL: http://issues.apache.org/jira/browse/IBATIS-249
>      Project: iBatis for Java
>         Type: Bug
>   Components: SQL Maps
>     Versions: 2.1.7
>     Reporter: Jonathan Burstein
>     Assignee: Sven Boden
>      Fix For: 2.2.0
>  Attachments: IBATIS-249.diff
>
> com.ibatis.common.util.Throttle.increment contains a synchronization error. Currently, when a waiting thread returns from LOCK.wait it will increment count and return without checking that count is below limit. There are a number of situations where LOCK.wait can complete but the count will not be less than the limit:
> 1) The wait was interrupted
> 2) There was a spurious thread wakeup
> 3) Another thread obtained the lock between the time LOCK.notify was called (by a thread calling decrement) and the wait returned.
> The fix here is to re-check the value of count after exiting the wait (using a while loop). A small amount of extra logic is necessary to satisfy maxWait properly.
> ThrottledPool.pop attempts to work around these race conditions by catching swallowing all exceptions from throttle.increment and pool.remove(0) and looping until an item is obtained. Since the increment call is within the loop this logic can lead to multiple increments with no corresponding decrements. Note that an IndexOutOfBound exception from pool.remove(0) is an artifact of the bug in Throttle.increment -- this could never occur if Throttle behaved correctly.
> This routine should simple call throttle.increment and pool.remove(0). It should most certainly not swallow exceptions.
> Finally, ThrottledPool.push incorrectly calls throttle.decrement if the parameter is invalid (null or of an incorrect type).

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