fix http3 outdated certificates after config reload

[WeidiDeng]

Caddy http3 will serve outdated certificates is config is reloaded because caddy tries to reuse existing quic listeners. However, because after config reload, old tls.Config won't update its certificates anymore. We could disable quic listener reuse, but by changing GetConfigForClient field, quic listener will use latest certificates anyway.

Fix https://github.com/caddyserver/caddy/issues/4849.

[mholt]

I'm intrigued -- thanks for the patch!! Just one question; we need to make sure this is thread-safe.

[francislavoie]

I think Matt is better equipped to review the logic, but this is just some style nits

[mholt]

This is looking pretty good -- does it work alright @WeidiDeng ?

Thanks for working on this!

[WeidiDeng]

This is looking pretty good -- does it work alright @WeidiDeng ?

Thanks for working on this!

It works fine when new config is ok. I haven't been able to test how it will react when new config load failed (i.e. when http3 is starting ok but other parts fail), but theoretically it will use the older tls.Config in this case.

[mholt]

Thanks for the updates. I'm willing to give this a try in the 2.7 beta 👍