kubeadm: Fix upgrade plan for air-gapped setups

A bug was discovered in the `enforceRequirements` func for `upgrade plan`.
If a command line argument that specifies the target Kubernetes version is
supplied, the returned `ClusterConfiguration` by `enforceRequirements` will
have its `KubernetesVersion` field set to the new version.
If no version was specified, the returned `KubernetesVersion` points to the
currently installed one.

This remained undetected for a couple of reasons
- It's only `upgrade plan` that allows for the version command line argument to
  be optional (in `upgrade plan` it's mandatory)
- Prior to 1.19, the implementation of `upgrade plan` did not make use of the
  `KubernetesVersion` returned by `enforceRequirements`.

`upgrade plan` supports this optional command line argument to enable
air-gapped setups (as not specifying a version on the command line will end up
looking for the latest version over the Interned).

Hence, the only option is to make `enforceRequirements` consistent in the
`upgrade plan` case and always return the currently installed version in the
`KubernetesVersion` field.

Signed-off-by: Rostislav M. Georgiev <[email protected]>

Author: rosti

cmd/kubeadm/app/cmd/upgrade/common.go

@@ -185,7 +185,15 @@ func enforceRequirements(flags *applyPlanFlags, args []string, dryRun bool, upgr
 	// If option was specified in both args and config file, args will overwrite the config file.
 	if len(args) == 1 {
 		newK8sVersion = args[0]
-		cfg.KubernetesVersion = newK8sVersion
+		if upgradeApply {
+			// The `upgrade apply` version always overwrites the KubernetesVersion in the returned cfg with the target
+			// version. While this is not the same for `upgrade plan` where the KubernetesVersion should be the old
+			// one (because the call to getComponentConfigVersionStates requires the currently installed version).
+			// This also makes the KubernetesVersion value returned for `upgrade plan` consistent as that command
+			// allows to not specify a target version in which case KubernetesVersion will always hold the currently
+			// installed one.
+			cfg.KubernetesVersion = newK8sVersion
+		}
 	}
 
 	// If features gates are passed to the command line, use it (otherwise use featureGates from configuration)