Cleaning up request attributes after websocket upgrade in Jetty 10

[SerCeMan]

Jetty version
10.0.0

Description

Hi, Jetty maintainers!

I'm currently in the progress of migrating from 9.4.x to 10.0.0, and I stumbled across a difference in the behaviour of the JettyWebSocketCreator. In Jetty 9.4.x, the upgrade request allows for removing servlet attributes:
https://github.com/eclipse/jetty.project/blob/403d5ec318ffaa20f1f2c0b62df217cfc99ebbe0/jetty-websocket/websocket-servlet/src/main/java/org/eclipse/jetty/websocket/servlet/UpgradeHttpServletRequest.java#L407-L414

However, 10.0.0 throws an exception when an attempt to modify the request attributes is made:
https://github.com/eclipse/jetty.project/blob/7dfb44f7ad7c70d5dda509587629b825bacafce4/jetty-websocket/websocket-core-server/src/main/java/org/eclipse/jetty/websocket/core/server/internal/UpgradeHttpServletRequest.java#L406-L410

The issue with not being able to remove the attributes is that the WebSocket applications often maintain a large number of connections and additional attributes on the request lead to a significant increase in memory usage. In 9.4.x, it would be possible to read the attributes within JettyWebSocketCreator, act upon them, and then establish a socket and completely remove the attributes freeing up a significant amount of memory. Is there a way to achieve similar behaviour in Jetty 10 where attributes are removed after establishing the WebSocket connection?

[SerCeMan]

Hey, @lachlan-roberts! I'm just wondering if you could suggest a workaround for this issue. I'm thinking of adding another filter that re-packs all attributes into a holder attribute which can later nullify all of the references, however, the workaround feels incredibly hackish.

[gregw]

@SerCeMan that approach would work, but is a bit hackish. Best is that we allow you to remove those attributes. I've flagged this to be fixed in the next release.

[SerCeMan]

Thanks, @gregw! Let me know if you'd prefer me to submit a pull request, I could try to give it a go.

[lachlan-roberts]

@SerCeMan I have opened PR #5935 to fix this, feel free to add your review. With the PR you should now be able to set request attributes in the JettyWebSocketCreator.