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Add command "kubectl config get-contexts"

```release-note
* A new command "kubectl config get-contexts" has been added.
```

fixes #25383

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Migrate some integration tests to run in dedicated namespace.

Note: Context is not a runtime object (doesn't have Kind and Version) so
      we can't use the resource_printer

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Fix logic of consecutive DELETE calls when gracefull deletion is enabled

Previously it was possible to extend delete timestamp to infinity by issuing repeated DELETE calls. This PR prevents that. Ref. discussion in #27422

cc @wojtek-t @smarterclayton @lavalamp @piosz @dchen1107 

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cacher: replace usable lock with conditional variable

Perviously we use a rwlock to indicate the ready information of the cacher. I feel it is not straightforward. Also it requires a few comments to explain. The pull request tries to replace the lock with a conditional variable for readability reason.

/cc @lavalamp @wojtek-t   

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Handle more cases in diff.ObjectReflectDiff

Interfaces and nested map values were not being recursed.

@karkagis since @wojtek-t is out

Interfaces and nested map values were not being recursed.

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e2e: increase timeout when waiting for deployment pods to be deleted

Use the same timeout as the one used for waiting for the deployment
reaper to complete.

Takes a stab at https://github.com/kubernetes/kubernetes/issues/28067

@kubernetes/deployment  PTAL

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Simplify registries initialization

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Increase timeouts in multiple node pool e2e tests for cluster autoscaler

cc: @piosz @fgrzadkowski @jszczepkowski 

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Unify DeleteAllEtcdKeys in integration tests & start using namespaces in some tests

Getting rid of DeleteAllEtcdKeys() and running all tests in separate namespace is necessary to run integration tests in parallel.

Aside from that, this is first step of reducing amount of annoying logs in integration tests...

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WIP - Handle map[]struct{} in DeepCopy

Deep copy was not properly handling the empty struct case we use for Sets.

@lavalamp I need your expertise when you have some time - the go2idl parser is turning sets.String into the following tree:

    type:         sets.String           kind: Alias
      underlying: map[string]sets.Empty kind: Map
        key:      string                kind: Builtin
        elem:     set.Empty             kind: Struct
                                              ^
                                              should be Alias

Looking at tc.Named, I'm not sure what the expected outcome would be and why you flatten there.

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Make generated build tag parameterizable for go2idl

Downstream generators that want to reuse the upstream generated types
need to be able to define a different ignore tag (so that they can see
the already generated types).

@wojtek-t allows third parties to reuse conversions in api/v1 (for instance) because otherwise they are hidden when go2idl parses the dependency packages.

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Run builds as the calling user

This means no 'sudo' is needed to copy files out.

@wojtek-t @smarterclayton 

This means no 'sudo' is needed to copy files out.

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Cleanup a TODO from godeps -> vendor change

This logs a false "error" message, so it's time to go.  It was needed to ensure
nobody has stale build images laying around, but that was quite a while ago, so
it's probably safe now.

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kubectl: ignore only update conflicts in the scaler

@kubernetes/kubectl is there any reason to retry any other errors? 

While testing this fix in OpenShift it was discovered that the
PackageConstraint was overly aggressive - types that declare a public
copy function should always return true.  PackageConstraint is intended
to limit packages where we might generate deep copy function, rather
than to prevent external packages from being consumed.

Downstream generators that want to reuse the upstream generated types
need to be able to define a different ignore tag (so that they can see
the already generated types).

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Make validators return error strings

Part of the larger validation PR, broken out for easier review and merge.  Builds on previous PRs in the series.  This is a group of smaller commits, so I don't waster precious PR numbers.

Also treat 0.0.0.0 as special, like loopback and multicast.

This logs a false "error" message, so it's time to go.  It was needed to ensure
nobody has stale build images laying around, but that was quite a while ago, so
it's probably safe now.