[Mailer] Prevent MessageLoggerListener from leaking in env=prod

[vudaltsov]

Q	A
Branch?	master
Bug fix?	no
New feature?	yes
Deprecations?	yes
License	MIT

I was trying to send a batch of emails with --env=prod when I noticed that MessageLoggerListener was still collecting all the messages and leaking the memory. I tried to do $this->getApplication()->reset(), but it didn't work because MessageLoggerListener was not tagged (now fixed in #37705).

In this PR I propose to move the declaration of MessageLoggerListener to mailer_debug.php since the only service depending on it is mailer.data_collector from mailer_debug.php. If a developer needs to collect sent emails, a custom listener could be implemented on the project side.

• deprecate the service
• add a new one to mailer_debug.php
• add a line to CHANGELOG.md
• add a line to UPGRADE-5.2.md

[fabpot]

What's the problem keeping it in prod (now that it does not leak memory anymore)?

[vudaltsov]

At first I had a hypothesis that MessageLoggerListener was declared in mailer.php(xml) by mistake.

If that's not the case, then why it is there when no other service depends on it? Obviously DI cannot automatically remove mailer.logger_message_listener, since it is a listener. But in fact it's just collecting information in prod for nothing.

And yes, it's still leaking without calling $serviceResetter->reset()! Before installing Symfony Mailer nothing was leaking in my worker in prod. Now I have to add a worker listener to do $serviceResetter->reset() just because of the MessageLoggerListener which I don't use.

I also found a comment by @nicolas-grekas :

having a non-leaking core is very important to me

This PR can help to accomplish this goal 😉

[fabpot]

Not using something in core does not mean that we should remove it.
But anyway, I don't have any problem with moving it to debug.

[nicolas-grekas]

(please mind the test failure)

[fabpot]

Thank you @vudaltsov.

[vudaltsov]

@fabpot , should I add a changelog entry?