Deprecate --mode option kubelet-registration-probe

[mauriciopoppe]

In #143 we added a workaround for a race condition happening in Windows only in the kubelet registration mechanism, as part of the workaround I added the flag mode in #152 which through a livenessProbe would crash the Pod if the kubelet registration failed, fortunately @luckerby found the race in kubernetes/kubernetes#104584 (comment) and fixed it upstream.

With the above fix the mode flag is no longer needed and can be deprecated, its introduction created issues with the probe file it created in readOnly filesystems as seen in #244.

I still need to define the deprecation timeline and the steps to remove the flag.

Tasks

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1. Remove code that creates the probe
2. Update the docs removing references of this flag

As an alternative we can keep the feature but under the hood we would do nothing when --mode=kubelet-registration-probe, if so then I can safely remove the code that tries to create a file.

[mauriciopoppe]

Another approach:

• Make --mode=kubelet-registration-probe a no-op. If it's a no-op then we can remove all the code I wrote in Provide different running modes for node-driver-registrar, add a run mode to detect if the kubelet plugin registration failed #152 safely too, if there're people that still rely on --mode=kubelet-registration-probe it'd always succeed e.g. it'd be a command that always returns exit code 0. Then we can deprecate the other mode safely and remove the value in a future release.
  • This option isn't as drastic as deprecating the mode flag, instead we're just deprecating one value so that's a plus.

[jsafrane]

We can remove a cmdline option only by bumping the major version.

I would deprecate the --mode option in the next v2.x release (with a big release note) and remove it afterwards and release v3.0. IMO it is not very helpful to remove implementation of --mode, but keep the option there.

[mauriciopoppe]

Sounds good, yes that's comment #309 (comment). Changing the title to reflect the deprecation of the --mode option kubelet-registration-probe only.

/retitle Deprecate --mode option kubelet-registration-probe