Symfony version(s) affected: 4.4.x and 5.x

The issue was detected on Platform.sh using PHP 7.4 trying to enable the Preloading support.

Description

On Platform.sh the file var/cache/prod/srcApp_KernelProdContainer.preload.php is NOT generated when it is working locally.

How to reproduce

Just deploy an empty app on Platform.sh.
OR (and the following explanation will explain why)

• Install Symfony locally in your root directory /
• run bin/console
• check the cache directory,
• you will see that the preload file is NOT generated

Reasons

This is happening for 3 combined reasons:

• first on Platform.sh (and I guess Symfony Cloud) application is installed in /app
• https://github.com/symfony/symfony/pull/33529/files/2eae30053766181acd502a11bcf8689a335d26bf#diff-f7b23d463cba27ac5e4cb677f2be7623
• https://github.com/symfony/symfony/blob/master/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php#L203

The return of getAutoloadFile is always null is this context.
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php#L306
And that is even explained in the code:

// Build a regexp where the first root dirs are mandatory,
// but every other sub-dir is optional up to the full path in $dir
// Mandate at least 2 root dirs and not more that 5 optional dirs.

Traces that explain the bug better
preg_match: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php#L2055

When it works:

string(54) "ComposerAutoloaderInit7a88e581ff3256a1c9451ad0a87173b8"
string(65) "/Users/plopix/DOCKFILES/NOVACTIVE/INTERNAL/XXX/application/vendor"
string(78) "/Users/plopix/DOCKFILES/NOVACTIVE/INTERNAL/XXX/application/vendor/autoload.php"
string(90) "#/Users/plopix/DOCKFILES/NOVACTIVE/INTERNAL(/XXX(/application(/var(/cache(/dev)?)?)?)?)?#A"

here the preg_match works

When it DOES NOT work (on Platform.sh):

string(54) "ComposerAutoloaderInitb7e173ef0544858adc8ec39ee3ef34c1"
string(11) "/app/vendor"
string(24) "/app/vendor/autoload.php"
string(28) "#/app/var(/cache(/pro_)?)?#A"

here the preg_match DOES NOT work

Solutions
That is a bit deep in the stack and I don't know enough to propose something. But if you tell me I could do it ;)

ping @nicolas-grekas