[Request]: add "get" to "system kernel" subcommand

[banksean]

Feature or enhancement request details

There doesn't appear to be a way to check what the default kernel binary is currently set to, even though there is a way to set it with container system kernel set. I would like container to have a corresponding container system kernel get subcommand.

I've already implemented this feature (including tests) in my fork of apple/container here: https://github.com/banksean/container/tree/kernelget

If this feature request is acceptable, I can submit that as a PR.

My use case:

• my app uses a custom kernel binary with apple/container
• on its first run, the app will download the custom kernel binary and call container system kernel set ... so that container will use it
• on subsequent runs, the app has no way to check if the default kernel binary is already set to that custom binary, so the app blindly sets the default kernel again every time it runs (including a potentially lengthy download step)

Code of Conduct

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[katiewasnothere]

Hi @banksean I agree this sounds like a useful feature to have. We're in the process of designing a configuration system for container (see discussion post here) that I think would help solve this problem for you. Let me know if you think this would not be sufficient for your use case.

[banksean]

Hi @katiewasnothere thanks for the quick resonse!

apple/container#1336 looks like a good idea for managing configuration settings in general (for what my opinion is worth; I'm not very familiar with macOS-specific systems programming concepts/patterns).

In light of that, I'd like to add that my use case would be even better served by the ability to specify the kernel binary at container instance creation time (e.g. container new --kernel-binary=..., or container new --system-config=myconfig.toml where myconfig.toml specifies a kernel binary, or something else along those lines) rather than always getting it from a system-wide setting.

That would address a more fundamental potential conflict that I've been ignoring for now, but the above discussion might help solve: my app may co-exist with installations of other apps that also use apple/container, and those apps may not want to use the same kernel as my app (nor my app theirs, if they use a custom kernel too).

Having a single system-wide setting for kernel binary makes it tricky to build a robust solution to this problem: I can either be a bad neighbor and disregard other apps' kernel binary preferences, or I can try to write some kind of coordination mechanism to "over-write system kernel binary setting, start container, reset system kernel binary setting to previous value" (which might not be possible or practical at all).

Does the "Default configurations should optionally be modifiable at runtime, as appropriate" goal from the above discussion imply it would be possible to specify/override this particular system kernel setting at container creation time?