Re: [xwiki-devs] Concurrency issue in REST server module

On Wed, May 9, 2012 at 12:12 PM, Thomas Mortagne <thomas.mortagne(a)xwiki.com> wrote: > On Wed, May 9, 2012 at 11:06 AM, Thomas Mortagne > <thomas.mortagne(a)xwiki.com> wrote: >> On Tue, May 8, 2012 at 7:12 PM, Marius Dumitru Florea >> <mariusdumitru.florea(a)xwiki.com> wrote: >>> Hi Fabio and devs, >>> >>> I found a serious concurrency issue in the REST server module while >>> debugging the instability of the Extension Manager when >>> extensions.xwiki.org repository is used (default case) . The Extension >>> Manager UI searches extensions using REST and very often it gets 500 >>> HTTP response code. See http://jira.xwiki.org/browse/XWIKI-7773 for >>> instance. The server log from xwiki.org shows that the real cause is: >>> >>> May 8, 2012 5:09:14 PM org.restlet.engine.application.StatusFilter doHandle >>> WARNING: Exception or error caught in status service >>> java.util.ConcurrentModificationException >>> at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372) >>> at java.util.AbstractList$Itr.next(AbstractList.java:343) >>> at org.xwiki.rest.XWikiSetupCleanupFilter.afterHandle(XWikiSetupCleanupFilter.java:73) >>> >>> See the full stacktrace http://pastebin.com/hnFSuwem . >>> >>> The problem is related to the way "releasable components" are managed. >>> I debugged locally both XWikiSetupCleanupFilter [1] and >>> ComponentsObjectFactory and here's what I discovered: >>> >>> * org.restlet.Context.getCurrent() is shared across HTTP requests >> >> What I don't understand is that the Context is stored in a ThreadLocal >> (see org.restlet.Context) so I don't see how it can be shared across >> HTTP requests. Or maybe it's reused by two consecutive requests ? > > Actually it could be that Restlet put the same context in all the > request using Context#setCurrent(). > > We should maybe use the XWiki Execution context which has been made > for use cases like that to be safe. Indeed. The code that manually releases REST resource components that have a per-lookup instantiation strategy looks to me like a workaround for the fact that we don't have a per-request (or per-execution) 'release' strategy (i.e. components that are automatically released at the end of the request/execution).

Thanks, Marius > >> >>> * as a consequence, restlet context attributes are shared across HTTP request >>> * RELEASABLE_COMPONENT_REFERENCES context attribute is thus also shared >>> * while a thread iterates this list in XWikiSetupCleanupFilter another >>> thread can add a component to the list in ComponentsObjectFactory >>> * the list grows indefinitely because XWikiSetupCleanupFilter only >>> releases the components; it doesn't remove them from the list >>> * older instances are re-released >>> * since the list keeps references to older instances these instances >>> can't be garbage collected >>> >>> Could someone more knowledgeable on the REST module (especially >>> Restlet) review my findings? >>> >>> Thanks, >>> Marius >>> >>> [1] https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwi… >>> [2] https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwi… >>> _______________________________________________ >>> devs mailing list >>> devs(a)xwiki.org >>> http://lists.xwiki.org/mailman/listinfo/devs >> >> >> >> -- >> Thomas Mortagne > > > > -- > Thomas Mortagne > _______________________________________________ > devs mailing list > devs(a)xwiki.org > http://lists.xwiki.org/mailman/listinfo/devs _______________________________________________ devs mailing list devs(a)xwiki.org http://lists.xwiki.org/mailman/listinfo/devs