Re: contextVersion NullPointerException due to race condition

[Mark Thomas <ma...@apache.org>]

On 29/02/2024 13:32, FRANTS Patrick wrote:
> Not sure this is the right mailing list or that it should go to dev.

users@ is fine. Generally, if you aren't sure use users@.

> One of our unit tests will occasionally have a null pointer exception durin=
> 
> g shutdown. Unfortunately I have not been able to create a small reproducti=
> 
> on yet.
> 
> The scenario is as follows.
> 
>    *   SpringApplicationShutdownHook calls Tomcat.stop() and a contextVersio=
> 
> n used by a SSE request is removed through MapperListener.unregisterContext
> 
>    *   A tomcat thread processes an IOException ("Connection reset by peer")=
> 
>   exception for that SSE request and ends up in Mapper.map() and gets a null=
> 
>   result
> 
> from contextObjectToContextVersionMap.get(context). In the end this causes =
> 
> a NullPointerException in internalMapWrapper when
> 
> calling contextVersion.path.length()
> 
> 
> This seems to be only in Tomcat 10.1.12+. Tomcat 10.1.11 does not have the =
> 
> problem.

I think I can see what the problem is.

When stopping a Context (which can happen during shutdown or during 
normal usage) Tomcat first removes the Context from the mapper to 
prevent any further requests from reaching the Context, then it stops 
the Context. That in turn stops the individual Wrappers (servlets). Note 
that that process waits for in progress requests to complete before the 
Wrapper is stopped.

The problem here is that the application won't be notified that it is 
being stopped until the Context is stopped. But RequestDistaptchers will 
start failing from the point the Context is removed from the mapper.

Without some major refactoring (and I'm not even sure that would be 
successful) I don't think we can address this. The best I think we could 
do is ensure that getRequestDispatcher(String) returns null in this 
case. Although that might just move the source of the NPE to your code.

Mark

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