kubectl: Allow namespaces with prefix

[iCiaran]

What type of PR is this?

/kind bug
/kind regression

What this PR does / why we need it:

Strip the prefix from the namespace flag if it starts with "namespace/", "namespaces/", or "ns/" (case insensitive), allowing the output of kubectl get ns -o name to be used directly in subsequent commands.

This was previously addressed in #50579, but lost in a refactor to config flag parsing (#63373).

Which issue(s) this PR fixes:

Fixes #29271

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fixed a regression in flag parsing, to allow using prefixed namespaces (kubectl -n namespace/example)

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: iCiaran / name: Ciaran Moran (680dd36)

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[iCiaran]

/sig cli

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: iCiaran
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[yankay]

/ok-to-test

[iCiaran]

Hi @pwittrock, @seans3, would be great to get some feedback on this, thanks!

[bo0ts]

Any reason why this hasn't been merged yet? That fix would be really appreciated.

[soltysh]

/hold

At this point in time, I don't consider this regression anymore.
/remove-kind regression

The <resource>/<name> pattern is good as an argument, where we need to explicitly specify resource type, for explicit flags, like -n or especially better visible with --namespace namespace/xyz it's overly verbose. Also this part can easily managed through some sort of cut invocation to skip the namespace part.

[bo0ts]

@soltysh I really consider this a worthwhile QOL change that improves many of use cases.

[iCiaran]

For me it's about having kubectl work in an intuitive way, in most (all?) other cases the get xyz -o name output can be used directly in another command. While the workaround is simple, having to remember that namespaces are an exception is a minor nuisance.

As to the long gap - it's something that's bugged me for a couple of years, I just didn't get round to trying to do something about it until now.

If not merging then a comment on #29271 noting that the original fix no longer works would be appreciated, it's where I (and I'm sure many others) ended up when searching for why you can't use -n namespace/foo.