[DI][FrameworkBundle] Allow overriding existing env vars with `EnvVarLoaderInterface`

[faizanakram99]

Description

EnvVarLoaderInterface is a nice way of providing env vars dynamically, it is particularly useful for multi-tenant apps where env vars like DATABASE_URL can be loaded from an external vault.

This comment symfony/symfony#34295 (comment) has a nice example of its usage.

The latest best practices of Symfony recommend to add default env vars to .env file, it doesn't work well with multi-tenant setup where DATABASE_URL can be different for each tenant as the EnvVarLoader won't override existing env vars which were loaded from .env file, hence the "feature request", can we add possibility of overriding existing env vars in EnvVarLoaders ?

Example

No response

[nicolas-grekas]

I think we'd better not. Doing so would mean calling env var loader backends all for the time for all vars. This might be a perf hog.

[faizanakram99]

I think we'd better not. Doing so would mean calling env var loader backends all for the time for all vars. This might be a perf hog.

What would the best way to setup default env vars then ?

Earlier we used to set them with the kernel parameters, but I don't see it documented anymore. So wondering if it might be unsupported in future.

parameters: 
    env(FOO): 'default-value'

[xabbuh]

I guess we need to add an example for that again somewhere on https://symfony.com/doc/current/configuration.html.

[nicolas-grekas]

Note that thanks to symfony/symfony#48705, Symfony 6.3 will allow unsetting an env var for prod, eg:

# .env
FOO=default_value

# .env.prod
FOO=

WIth this config, prod will fallback to env var loaders on prod.