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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