logging: Buffer the logs before config is loaded

[francislavoie]

Fixes #7209 and probably multiple other issues we've had in the past.

My idea to solve this was:

• At the start of caddy run, set up a temporary in-memory log buffer and capture all logs into that
• If caddy run fails at any point before config is loaded, flush the buffer directly to the default logger (stderr, effectively)
• Once the config is loaded successfully, flush the buffer through the configured logger (so they're formatted and written out as the user wants)

To test this, I used this config:

{
	admin localhost:2020
	log {
		output file ./test.log
		level info
		format console {
			level_format upper
		}
	}
}

:2021 {
	respond "Hello, Caddy!"
}

Then simply running caddy run on that Caddyfile, seeing that all logs end up in test.log.

If I make a change like removing a { to produce a config error, I see this in my terminal, as expected:

2025/09/07 02:43:59.760 INFO    maxprocs: Leaving GOMAXPROCS=12: CPU quota undefined
2025/09/07 02:43:59.760 INFO    GOMEMLIMIT is updated   {"GOMEMLIMIT": 30849878016, "previous": 9223372036854775807}
2025/09/07 02:43:59.760 INFO    using config from file  {"file": "Caddyfile.test2"}
Error: adapting config using caddyfile: unexpected EOF, at Caddyfile:12

Assistance Disclosure

I used RooCode + GPT-4.1 to ideate, and it produced the logbuffer.go implementation, I wrote the remainder.

[mholt]

Oh neat, this is a clever solution. Thanks for working on this. You're spoiling our user base with niceties. 😉