NPE from WebInfConfiguration.deconfigure during WebAppContext shutdown

[joakime]

Jetty version
9.4.x

Description
When shutting down WebAppContext, there occasionally is seen a NullPointerException.

java.lang.NullPointerException
	at org.eclipse.jetty.util.IO.delete(IO.java:368)
	at org.eclipse.jetty.webapp.WebInfConfiguration.deconfigure(WebInfConfiguration.java:360)
	at org.eclipse.jetty.webapp.WebAppContext.stopContext(WebAppContext.java:1433)
	at org.eclipse.jetty.server.handler.ContextHandler.doStop(ContextHandler.java:1115)
	at org.eclipse.jetty.servlet.ServletContextHandler.doStop(ServletContextHandler.java:297)
	at org.eclipse.jetty.webapp.WebAppContext.doStop(WebAppContext.java:547)
	at org.eclipse.jetty.util.component.AbstractLifeCycle.stop(AbstractLifeCycle.java:93)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.stop(ContainerLifeCycle.java:180)

[joakime]

It's easy to add the safety checks for this, but why was WebAppContext.getTempDirectory() null during WebInfConfiguration.deconfigure(WebAppContext context) ?

[joakime]

The only ways that context.getTempDirectory() could return a null during WebInfConfiguration.deconfigure() is ...

1. There was never a Temp Directory setup for the context.
2. A process outside of Jetty intentionally called context.setTempDirectory(null)
3. The WebInfConfiguration.deconfigure(WebAppContext) was run twice against the same WebAppContext.

For scenario 1, that's not really possible if WebInfConfiguration is in play.
As the WebInfConfiguration.preConfigure(WebAppContext) ensures a Temp Directory.

For scenario 2, the safety checks will help, nothing we can do about that.

For scenario 3, the safety checks will help again, but that's a bad configuration on the installation side.

[joakime]

Opened PR #5481