[xwiki-devs] Concurrency issue in REST server module

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 * 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…