core: Implement FastAbs to avoid repeated os.Getwd calls

[francislavoie]

Needs benchmarking

[mholt]

I'm not sure about the build tag (would need more time to investigate myself, but it sounds right, the _unix implies not Windows or any other non-Unix OS) -- but overall this LGTM. I wonder if we should add to our docs that, for performance reasons, we don't support chdir on the process while it's running.

I would definitely want to see some benchmark though to get a sense for the improvement here to justify this. 👍

[dunglas]

@mholt For chdir, if there is a use case, we could even provide our own wrapper that will just refresh the wd variable after the change (it will not cover some use cases like an external library using directly os.Chdir, the C function or the syscall, but it should cover most use cases).

[mholt]

Gotcha. Good idea @dunglas !

@francislavoie Is there anything left to settle about the build tag/filename?

[dunglas]

@mholt the similar change in FrankenPHP improves our benchmark performance by about 7%, which is quite significant. As it's not exactly the same case, it may not apply there, but it will likely improve performance for PHP FPM and static files users significantly.

Creating a Go benchmark should be enough to measure that.

[francislavoie]

Apparently _unix.go is not an actual build constraint. See golang/go#51572

This is ready to go now.

[francislavoie]

For ref, this was a collab with dunglas/frankenphp#1154, this change was mirrored in FrankenPHP. This was something like a 7% performance increase by avoiding the syscalls to os.Getwd, calling it just once for the whole server.

This does mean plugins should never call os.Chdir (could cause bad things to happen with caddy.FastAbs) but I think it's pretty unlikely.