Move BootLoaderConfig to context manager

[schaefi]

Change the BootLoaderConfig class to be a context manager. All code using BootLoaderConfig was updated to the following with statement:

with BootLoaderConfig.new(...) as bootloader_config:
    bootloader_config.some_member()

This is related to Issue #2412

[schaefi]

I created a Staging release with this patch... waiting for the integration tests to complete

[schaefi]

All tests passing, open for review :)

[dcermak]

I think it would be cleaner if the BootLoaderConfigBase would use a contextlib.ExitStack, as the volume_mounts are already context managers

[schaefi]

I think it would be cleaner if the BootLoaderConfigBase would use a contextlib.ExitStack, as the volume_mounts are already context managers

The reason why I did not push it into an ExitStack was because I think the elements of the ExitStack will disappear one after the other when we go forward moving the other classes into context managers. With each follup pull request my plan is to come more and more into that nested structure of context managers which will reduce the existing ExitStack until we reach the point where it is empty.

The current implementation of some builders keeps resources active for way too long. BootLoaderConfig is one example. It was created super early and kept until the end of the complete build process. But within that process there is only one time where it actually operates and depending on the bootloader mounts stuff, does its bootloader config job and should now free the resources but it holds it too long. One of the nice effects of moving into the context managers is to find out how to release as early as possible.

That's why I did not use the ExitStack because for the BootLoaderConfig I knew when I need it and when I can release.

Hope this makes sense to you too ?

[dcermak]

With each follup pull request my plan is to come more and more into that nested structure of context managers which will reduce the existing ExitStack until we reach the point where it is empty.

Well, the idea of the ExitStack is that you do not have to nest contextmanagers and just have one exitstack that cleans them all up. But I think we can add these later on if they turn out to be useful.

[dcermak]

LGTM, I've added two small nit-picky comments