REQ plugin

[alexgleason]

I haven't fully thought this through yet, but I sometimes wish I could hook into client requests. Like, to be able to get ["REQ", ...] and potentially change the filter or response, or trigger side-effects. Maybe AUTH could even be implemented in a req plugin.

I still don't know if this is even a good idea yet, but I figured I'd drop it in case it strikes anyone else who might be having similar ideas.

[hoytech]

Hey, yeah this is a cool idea and would open up a lot of use-cases. You might even be able to do AUTH with it, as you suggest.

However, I'm considering the performance implications of this. Handling the REQ messages is on the critical path for all read requests, so the user-specifiable code would need a very low overhead to not impact relay performance. I think the way we do the write policy plugins would not work here, except for low-traffic relays.

I'm not totally sure what the answer is, but I think eventually we'll want to figure something out about this.