Description

While working on #50253, I realised that CacheWarmers are never executed in the kernel.build_dir (except in the case of the bug that I fixed in #50253).

For now, in a brand new Symfony project, only the compiled container is dumped in the build_dir. Looking at the documentation for kernel.build_dir (Build directory), it is described as

This directory can be used to separate read-only cache (i.e. the compiled container) from read-write cache (i.e. cache pools).

However, there's plenty of other ressources that could be considered as read-only IMHO: the router cache, Doctrine proxy classes, Symfony Config classes, translations, and probably many more.

Additionally, non-optional CacheWarmers are run during container init, in the cache_dir, which is erased if you run cache:clear command. Since the non-optional CacheWarmers are not executed in cache:clear command, because they run during container init, all the artefacts they produce are actually lost if you run cache:clear without an already existing container, which is happening on every composer install (some Doctrine CacheWarmers are concerned by this at least).

Besides, the mechanism of CacheWarmer seems a bit weird to me in some implementations of the interface. More specifically, the Symfony\Bundle\FrameworkBundle\Routing\Router that implements the WarmableInterface.

For reference, here's its implementation of the warmup function

    /**
     * @return string[] A list of classes to preload on PHP 7.4+
     */
    public function warmUp(string $cacheDir): array
    {
        $currentDir = $this->getOption('cache_dir');

        // force cache generation
        $this->setOption('cache_dir', $cacheDir);
        $this->getMatcher();
        $this->getGenerator();

        $this->setOption('cache_dir', $currentDir);

        return [
            $this->getOption('generator_class'),
            $this->getOption('matcher_class'),
        ];
    }

It seems to me that it's really error prone to have such back and forth with the cache_dir option. If someone changes the cache_dir setting of the Router, it won't be properly warmed up anymore, without any notice to the user.

So, to sum up a little bit, I can see three different issues, that may or may not be solve together.

• build_dir is under utilised and many read-only warm ups could make use of it
• Non-optional warmer artefacts are lost on initial cache:clear
• WarmableInterface does not protect from misconfiguration.

I'm willing to contribute and propose a change to solve at least the first issue but would like to get some opinions on the best way to solve it beforehand :)

Example

s new --dir symfony_build_dir_problem
cd ./symfony_build_dir_problem
echo '<?php

namespace App;

use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel
{
    use MicroKernelTrait;

    public function getBuildDir(): string
    {
        return $this->getProjectDir()."/var/build/".$this->environment;
    }
}' > ./src/Kernel.php
composer install -q

Then see the content of var/build/dev and var/cache/dev.