Only configure smtp_settings if provided on configuration

[andrehjr]

Summary

There's a bug when ActionMailer::Base.smtp_settings= is set before ActionMailer gets initialized and config.action_mailer.smtp_settings is not set. This ends up overriding ActionMailer::Base.smtp_settings.

Some users may configure ActionMailer::Base.smtp_directly instead of using it via config.action_mailer.smtp_settings.

In order to fix that this PR does not try to configure smtp_settings when it's not present on the config hash.

Fixes #44161 and Follow up from #44061

[Edouard-chin]

Thank you for the Pull Request @andrehjr . The issue you are describing is a common mistake where users would prematurely reference a constant (such as ActionMailer::Base, ActionController::Base) which messes up the boot process and ultimately result in the problem you were seeing.

Application shouldn't be doing that and should instead use a Lazy Load hook. In your example it would look something like

ActiveSupport.on_load(:action_mailer) do
  self.smtp_settings = { ... }
end
# This code will be executed once Rails itself uses the ActionMailer::Base constant. 

[andrehjr]

You're totally right @Edouard-chin. I also advise on using Lazy Load Hooks and avoiding calling those constants prematurely. Thanks for taking a look into this PR.

The issue is that on #44061 there is still another behavior missing. Something that used to work on previous Rails versions is not working on Rails 7+ properly. This PR attempts to keep that between versions.

That means fewer surprises when upgrading if someone by mistake was using ActionMailer::Base directly. But, as it's not a good practice, rails may also decide not to fix it.

[Edouard-chin]

Ah yeah I understand the nice intention to keep everyone's application working the same way despite the new changes, but IMO this kind of problem will always arise at some and it's better for users to fix the root cause.

Just my 2cents, I'll let the core team decides.