core/service: don't make state transitions based on cgroup state when ExitType=main

[msekletar]

Fixes #27953

[erikarvstedt]

It might make sense to add a systemd test to prevent regressions.

[msizanoen1]

Have you read redhat-plumbers/systemd-rhel8#391 (comment) or #27953 (comment)? This pull request doesn't fix all the other users of main_pid_good that got broken by ff32060. I'm pretty sure that there are other ways to achieve what you need to do on CentOS without resetting main_pid_known.

[poettering]

So this change doesn't look right to me. if we don't know the main pid and the default ExitType= is set, then we should watch the cgroup, we have nothing else.

So this doesn't look right to me.

I have merged #28000 now, which means I guess we can close this issue here. See that merge for a comment about this.