perf: cache computations in WithRequestDocumentRoot

[dunglas]

Alternative to #1152.

Improve performance by ~10% on my Mac with the default config.

[AlliBalliBaba]

In the meantime, here are some quick benchmarks to put you at ease 😄

Docker Linux amd64 20 CPU cores i9 wrk -t4 -c180 -d30 http://localhost/worker-with-watcher.php

In dunglas/frankenphp:1.25
php_server 40 threads: ~52000 RPS
php 40 threads: ~70000 RPS
php 8 threads: ~69000 RPS

In dunglas/frankenphp:1.3
php_server 40 threads: ~66000 RPS
php 40 threads: ~93000 RPS
php 8 threads (ideal): ~122000 RPS

The benchmark script is the one in testdata/worker-with-watcher.php. This doesn't capture all performance improvements though. Improvements to the $_SERVER import scale linearly with the size of the environment. And dispatching to workers in order should improve IO with external services (hard to quantify unless you're benchmarking a prod environment - which I plan to do with k6 and AWS ECS)

[mholt]

Yeah that's smart. Not sure why Go std lib doesn't do this already. (Can the wd change?)

[dunglas]

@AlliBalliBaba thank you very much!!

@mholt the working directory can change if the chdir syscall is used (or os.Chdir directly, in Go), that's why os.Getwd() cannot cache the value: https://www.man7.org/linux/man-pages/man2/chdir.2.html

[mholt]

Ah... I forgot about that. Thanks