Adding InheritedListeners to already-started components can cause IllegalStateException

[janbartel]

Jetty 10.0.x

When a webapp is hot-deployed to an already-started jetty via the deploy module, the o.e.j.deploy.bindings.StandardStarter does:

        if (contexts.isStarted() && handler.isStopped())
        {
            // start the handler manually
            handler.start();

            // After starting let the context manage state
            contexts.manage(handler);
        }

So the webapp is first started, and then the ContextHandlerCollection is told to manage it. During the call to manage, if an o.e.j.u.component.Container.InheritedListener is registered with jetty, then it will be added to every component of the newly started webapp.

If one of those components happens to be a o.e.j.io.ClientConnector, when the InheritedListener is registered with the connector's o.e.j.io.ClientConnector.ClientSelectorManager, then the call to addEventListener(EventListener) will fail because the ClientConnector.ClientSelectorManager is already in the started state, see:
https://github.com/eclipse/jetty.project/blob/jetty-10.0.x/jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java#L399.

This does not happen if the webapp is deployed during startup of jetty, because none of the component hierarchy is already in the started state, it is just starting.

This can be reproduced in the standard jetty distro by:

1. enabling the jmx module (which has an InheritedListener)
2. enabling the deploy module, along with http and websocket
3. starting jetty the usual way
4. deploying the cometd-6.0.5 demo webapp (which will contain a ClientConnector and ClientSelectorManager)

[janbartel]

This problem does not affect jetty-9.4.

This problem is present in the jetty-10 series from 10.0.2 onwards.

[sbordet]

This issue has been introduced with 25f8c65 in #5840.

What happen is that the JavaxWebSocketClientContainer is added to the WebAppContext lazily when it's first used.
In CometD, this happens when the WebAppContext is started, from a Servlet.init() method, which is earlier than usual (typically at the first WebSocket upgrade).
This adds an HttpClient instance as a child bean of JavaxWebSocketClientContainer, which is then already started when the Servlet.init() mechanism completes.

After the WebAppContext is started, like explained above, contexts.manage(handler) happens, which triggers the "inheritance" of the MBeanContainer listener on the component subtree.

I don't see a reason for SelectorManager.[add|remove]EventListener(...) to throw IllegalStateException if running, also because they are the only overridden methods that add the extra check that throws (other overrides do not).