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From "Ellis Pritchard (JIRA)" <>
Subject [jira] Commented: (COCOON-1985) AbstractCachingProcessingPipeline locking with IncludeTransformer may hang pipeline
Date Sun, 17 Feb 2008 17:06:34 GMT


Ellis Pritchard commented on COCOON-1985:

I'm still not happy with this.

1/ The original patch for 2.1.x had a problem (as I explained) where parallel includes can
dead-lock; this is neatly by-passed in 2.2.x, in a way that is not possible in 2.1.x (see However...

2/ I'm convinced that this code could still dead-lock; the waiter relies on being woken by
lock.notifyAll(), however, this notification could conceivably happen between getting the
lock object from the transient store, and actually doing a lock.wait() to wait to be notified,
thus the waiter would not be woken by the thread that holds the lock:

    protected boolean waitForLock(Object key) {
        if(transientStore != null) {
            Object lock = null;
            synchronized(transientStore) {
                String lockKey = PIPELOCK_PREFIX+key;
                if(transientStore.containsKey(lockKey)) {
                    // cache content is currently being generated, wait for other thread
                    lock = transientStore.get(lockKey);
             *** lock.notifyAll() could be called here, and we'd not hear about it.
            // Avoid deadlock with self (see JIRA COCOON-1985).
            if(lock != null && lock != Thread.currentThread()) {
                try {
                    // become owner of monitor
                    synchronized(lock) {
                        /*** WAIT UNTIL OTHER THREAD CALLS notifyAll() again  ***/
                } catch (InterruptedException e) {
                    if(getLogger().isDebugEnabled()) {
                        getLogger().debug("Got interrupted waiting for other pipeline to finish
processing, retrying...",e);
                    return false;
                if(getLogger().isDebugEnabled()) {
                    getLogger().debug("Other pipeline finished processing, retrying to get
cached response.");
                return false;
        return true;

Fortunately, because I used the Thread object as the lock object in my patch for 2.1.x, the
unfortunate waiting thread should only (!) have to wait until the processor thread that set
the lock is re-used for generating cacheable content, (and runs notifyAll() in releaseLock()),
but that could still be a significant period. However, with the new 2.2 patch, the RequestAttributes
object is never going to be re-used, so will never be notified, so we are in a complete dead-lock

A work-around (i.e. hack) for this is, instead of using Object#wait(), use Object#wait(long
timeout), i.e. replace

above, with

i.e. wait for notification, or 1000ms (pick a number); the calling code loops calling waitForLock()
so exiting without being notified is safe, however, it's still a hack.

Basically, the whole idea behind pipe-line locking is a great one, but this implementation
is full of danger, and, worse, the sort of danger you only encounter on a very busy system.
My original recommendation was to remove this code; I still stand by that.

> AbstractCachingProcessingPipeline locking with IncludeTransformer may hang pipeline
> -----------------------------------------------------------------------------------
>                 Key: COCOON-1985
>                 URL:
>             Project: Cocoon
>          Issue Type: Bug
>          Components: * Cocoon Core
>    Affects Versions: 2.1.9, 2.1.10, 2.1.11, 2.2-dev (Current SVN)
>            Reporter: Ellis Pritchard
>            Priority: Critical
>             Fix For: 2.2-dev (Current SVN)
>         Attachments: caching-trials.patch, 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
> 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

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