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https://issues.apache.org/jira/browse/COCOON-1985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12466844
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Ellis Pritchard commented on COCOON-1985:
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Cheers ;)
However, as I alluded to in the original report, my patch is *not* a full
solution to this problem.
I believe deadlock (this time with the main thread and sub-threads) will happen
if the same test is run with the IncludeTransformer in parallel-processing
mode. The reason is that the main thread cannot complete until the include
threads have completed, and they can't complete because the main thread holds
the pipeline lock.
This should be easy to test by adding:
<map:parameter name="parallel" value="true"/>
to the sitemap inside the <map:transformer type="include"/>.
It seems like my semi-patch fall-through logic should be modified to compare
the thread's main Request against the Request of the pipeline holder.
Maybe the original author of the code can help?
> AbstractCachingProcessingPipeline locking with IncludeTransformer may hang
> pipeline
> -----------------------------------------------------------------------------------
>
> Key: COCOON-1985
> URL: https://issues.apache.org/jira/browse/COCOON-1985
> Project: Cocoon
> Issue Type: Bug
> Components: * Cocoon Core
> Affects Versions: 2.1.9, 2.1.10, 2.1.11-dev (Current SVN)
> Reporter: Ellis Pritchard
> Priority: Critical
> Fix For: 2.1.11-dev (Current SVN)
>
> Attachments: includer.xsl, patch.txt, sitemap.xmap
>
>
> Cocoon 2.1.9 introduced the concept of a lock in
> AbstractCachingProcessingPipeline, an optimization to prevent two concurrent
> requests from generating the same cached content. The first request adds the
> pipeline key to the transient cache to 'lock' the cache entry for that
> pipeline, subsequent concurrent requests wait for the first request to cache
> the content (by Object.lock()ing the pipeline key entry) before proceeding,
> and can then use the newly cached content.
> However, this has introduced an incompatibility with the IncludeTransformer:
> if the inclusions access the same yet-to-be-cached content as the root
> pipeline, the whole assembly hangs, since a lock will be made on a lock
> already held by the same thread, and which cannot be satisfied.
> e.g.
> i) Root pipeline generates using sub-pipeline cocoon:/foo.xml
> ii) the cocoon:/foo.xml sub-pipeline adds it's pipeline key to the transient
> store as a lock.
> iii) subsequently in the root pipeline, the IncludeTransformer is run.
> iv) one of the inclusions also generates with cocoon:/foo.xml, this
> sub-pipeline locks in AbstractProcessingPipeline.waitForLock() because the
> sub-pipeline key is already present.
> v) deadlock.
> I've found a (partial, see below) solution for this: instead of a plain
> Object being added to the transient store as the lock object, the
> Thread.currentThread() is added; when waitForLock() is called, if the lock
> object exists, it checks that it is not the same thread before attempting to
> lock it; if it is the same thread, then waitForLock() returns success, which
> allows generation to proceed. You loose the efficiency of generating the
> cache only once in this case, but at least it doesn't hang! With JDK1.5 this
> can be made neater by using Thread#holdsLock() instead of adding the thread
> object itself to the transient store.
> See patch file.
> However, even with this fix, parallel includes (when enabled) may still hang,
> because they pass the not-the-same-thread test, but fail because the root
> pipeline, which holds the initial lock, cannot complete (and therefore
> statisfy the lock condition for the parallel threads), before the threads
> themselves have completed, which then results in a deadlock again.
> The complete solution is probably to avoid locking if the lock is held by the
> same top-level Request, but that requires more knowledge of Cocoon's
> processing than I (currently) have!
> IMHO unless a complete solution is found to this, then this optimization
> should be removed completely, or else made optional by configuration, since
> it renders the IncludeTransformer dangerous.
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