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From "Robert Munteanu (JIRA)" <>
Subject [jira] [Commented] (SLING-8051) Timing issue in SlingMainServlet
Date Fri, 26 Oct 2018 14:24:00 GMT


Robert Munteanu commented on SLING-8051:

[~jsedding] - thanks for the detailed explanation. The fix seems reasonable, but I'm not very
familiar with the way the HTTP service and the SCR interact, so my opinion is not "authoritative"

> Timing issue in SlingMainServlet
> --------------------------------
>                 Key: SLING-8051
>                 URL:
>             Project: Sling
>          Issue Type: Improvement
>          Components: Engine
>    Affects Versions: Engine 2.6.14
>            Reporter: Julian Sedding
>            Assignee: Julian Sedding
>            Priority: Major
>         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 [^SLING-8051-stack-trace.txt]
> cc [~cziegeler], [~joerghoh]

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