Last mile work for issue #6: content blocking and new user moderation

[jsco2t]

This is a wrap-up PR for the work that NH started.

The original issue is here:

https://github.com/rocky-linux/mattermost-plugin-community-toolkit/issues/6

The original PR from NH is here:

https://github.com/rocky-linux/mattermost-plugin-community-toolkit/pull/7

To summarize all of the new changes in this branch:

• Added the ability to deploy a local-dev mattermost instance to test the plugin.
• Resolved some bugs related to domain and user name blocking
• Added in more test automation + tests in depth for new features
• Documentation cleanup
• Fixed issues reported by golang linting tools
• Tuned LRUCache for slight performance improvemen

[NeilHanlon]

Thanks very much for your work on this and your patience in awaiting a review. I began reviewing it but am having trouble getting a good grasp on all the changes because there's a lot of unrelated work bundled together. With a 10k line delta across 28 files it's difficult to give each part the attention it deserves.

Looking at the bullet points in the PR, could you break this up into separate PRs? Something like:

1. Local dev environment (podman/makefile/scripts)
2. Test infrastructure improvements
3. Domain/username validation features
4. Content blocking features (links/images/dms)
5. User cleanup/enforcement logic
6. Documentation updates

This would make it much easier to review each piece properly and understand the interactions between changes. It would also make rollback & bisection simpler if any issues come up later.

I did spot one issue (I think) in the LRU cache implementation: in Get(), there's a small gap between releasing the read lock and acquiring the write lock where another goroutine can evict or remove the entry. Because the previously-fetched pointer is still used after reacquiring the write lock, Get() can return a "hit" for a key that is no longer present in the map. It’s not a data race, but it does break expected LRU semantics.

A straightforward fix would be to re-check the entry once the write lock is held (or just hold the write lock for the entire Get() path). Either approach removes the TOCTOU window.

Happy to dig into this (and everything else) once the PR is split up.

[jsco2t]

Thanks for the feedback @NeilHanlon - I'll work on breaking this up.