Feature: Multi-threading in PgBouncer

[JelteF]

We currently have support for multi-process PgBouncer using so_reuseport, but actual multi-threading is preferable since it is not super easy to set up so_reuseport together with peering. Also, pool limits are not shared when using multi-process, as well as stats queries only showing stats of one process.

Things that at least would need to happen:

1. Make the lists that contain clients/servers thread safe to access
2. Stats would need to be gathered in a thread safe way

[vitabaks]

+1

[Catwoolfii]

+1

[jonahharris]

Mason and I are in the process of porting the multithreaded PgBouncer I wrote for NEXTGRES to the latest version. Will push an Alpha next week, which will require a bit of clean-up and decisions as to how multithreading is handled and whether it gets ifdef'd or mainlined. Preliminary code will be in this branch

[JelteF]

That sounds amazing! Please open a PR once you have that Alpha ready then I can take a look at it.

I think we wouldn't want it ifdef-ed, since then it will be hard to test for people using pre-built binaries. But we'd probably want it opt-in with a config flag for a few versions after merging. To allow the community to help find some bugs/perf issues, while having an easy way to roll back to the single threaded mode.