[jira] [Created] (SLING-8051) Timing issue in SlingMainServlet

["Julian Sedding (JIRA)" <ji...@apache.org>]

Julian Sedding created SLING-8051:
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             Summary: Timing issue in SlingMainServlet
                 Key: SLING-8051
                 URL: https://issues.apache.org/jira/browse/SLING-8051
             Project: Sling
          Issue Type: Improvement
          Components: Engine
    Affects Versions: Engine 2.6.14
            Reporter: Julian Sedding
            Assignee: Julian Sedding
         Attachments: SLING-8051-stack-trace.txt

There seems to be a timing issue in the {{SlingMainServlet}}, which is rooted in the fact that it is subject to OSGi's DS life-cycle, as well as the {{Servlet}} life-cycle (as implemented in teh HTTP whiteboard.

In its {{activate}} method the {{SlingMainServlet}} registers itself as a {{Servlet}} via the HTTP whiteboard. At this point it assumes that the whiteboard immediately calls its {{init()}} method.

Directly afterwards it registers a {{SlingServletContext}} instance as a {{ServletContext}} in the service registry, which in turn contains a reference to the {{SlingMainServlet}} instance.

However, it may happen (I have observed this on Tomcat, i.e. not in standalone mode), that the HTTP whiteboard is only registered after the {{SlingMainServlet}} has been activated. This leaves a large time window, i.e. between {{SlingMainServlet.activate}} and the registration of the HTTP whiteboard, which eventually leads to {{SlingMainServlet.init(ServletConfig)}} being called. Within this time window, other DS components may pick up a reference e.g. to the {{SlingServletContext}}, which is backed by an activated but uninitialized {{SlingMainServlet}}. 

The {{SlingServletResolver}} has a static reference to the {{SlingServletContext}}, and starts initializing (Sling-)Servlets immediately, passing a {{SlingServletConfig}} backed by the {{SlingServletContext}} instance to their {{#init(ServletContext)}} method. 

Now any init method that happens to call the wrong method on {{SlingServletConfig}} is prone to {{NullPointerException}}s because {{SlingServletConfig.getServletContext().getServletConfig() == null}}.

This was observed with the {{SlingWebDavServlet}} that (via inheritance from Jackrabbit's {{SimpleWebdavServlet}}) calls {{this.getServletContext().setAttribute()}}. Attached is a stack trace of this scenario.



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