kubeadm: remove misleading warning on kubeadm join

If the user does not provide --config or --control-plane
but provides some other flags such as --certificate-key
kubeadm is supposed to print a warning.

The logic around printing the warning is bogus. Implement
proper checks of when to print the warning.

cmd/kubeadm/app/cmd/join.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 	"text/template"
 
 	"github.com/lithammer/dedent"
@@ -127,6 +128,7 @@ type joinOptions struct {
 	controlPlane          bool
 	ignorePreflightErrors []string
 	externalcfg           *kubeadmapiv1beta2.JoinConfiguration
+	joinControlPlane      *kubeadmapiv1beta2.JoinControlPlane
 	kustomizeDir          string
 }
 
@@ -199,7 +201,7 @@ func NewCmdJoin(out io.Writer, joinOptions *joinOptions) *cobra.Command {
 		Args: cobra.MaximumNArgs(1),
 	}
 
-	addJoinConfigFlags(cmd.Flags(), joinOptions.externalcfg)
+	addJoinConfigFlags(cmd.Flags(), joinOptions.externalcfg, joinOptions.joinControlPlane)
 	addJoinOtherFlags(cmd.Flags(), joinOptions)
 
 	joinRunner.AppendPhase(phases.NewPreflightPhase())
@@ -222,22 +224,22 @@ func NewCmdJoin(out io.Writer, joinOptions *joinOptions) *cobra.Command {
 }
 
 // addJoinConfigFlags adds join flags bound to the config to the specified flagset
-func addJoinConfigFlags(flagSet *flag.FlagSet, cfg *kubeadmapiv1beta2.JoinConfiguration) {
+func addJoinConfigFlags(flagSet *flag.FlagSet, cfg *kubeadmapiv1beta2.JoinConfiguration, jcp *kubeadmapiv1beta2.JoinControlPlane) {
 	flagSet.StringVar(
 		&cfg.NodeRegistration.Name, options.NodeName, cfg.NodeRegistration.Name,
 		`Specify the node name.`,
 	)
 	flagSet.StringVar(
-		&cfg.ControlPlane.CertificateKey, options.CertificateKey, "",
+		&jcp.CertificateKey, options.CertificateKey, jcp.CertificateKey,
 		"Use this key to decrypt the certificate secrets uploaded by init.",
 	)
 	// add control plane endpoint flags to the specified flagset
 	flagSet.StringVar(
-		&cfg.ControlPlane.LocalAPIEndpoint.AdvertiseAddress, options.APIServerAdvertiseAddress, cfg.ControlPlane.LocalAPIEndpoint.AdvertiseAddress,
+		&jcp.LocalAPIEndpoint.AdvertiseAddress, options.APIServerAdvertiseAddress, jcp.LocalAPIEndpoint.AdvertiseAddress,
 		"If the node should host a new control plane instance, the IP address the API Server will advertise it's listening on. If not set the default network interface will be used.",
 	)
 	flagSet.Int32Var(
-		&cfg.ControlPlane.LocalAPIEndpoint.BindPort, options.APIServerBindPort, cfg.ControlPlane.LocalAPIEndpoint.BindPort,
+		&jcp.LocalAPIEndpoint.BindPort, options.APIServerBindPort, jcp.LocalAPIEndpoint.BindPort,
 		"If the node should host a new control plane instance, the port for the API Server to bind to.",
 	)
 	// adds bootstrap token specific discovery flags to the specified flagset
@@ -297,11 +299,16 @@ func newJoinOptions() *joinOptions {
 	externalcfg.Discovery.BootstrapToken = &kubeadmapiv1beta2.BootstrapTokenDiscovery{}
 	externalcfg.ControlPlane = &kubeadmapiv1beta2.JoinControlPlane{}
 
+	// This object is used for storage of control-plane flags.
+	joinControlPlane := &kubeadmapiv1beta2.JoinControlPlane{}
+
 	// Apply defaults
 	kubeadmscheme.Scheme.Default(externalcfg)
+	kubeadmapiv1beta2.SetDefaults_JoinControlPlane(joinControlPlane)
 
 	return &joinOptions{
-		externalcfg: externalcfg,
+		externalcfg:      externalcfg,
+		joinControlPlane: joinControlPlane,
 	}
 }
 
@@ -309,6 +316,12 @@ func newJoinOptions() *joinOptions {
 // This func takes care of validating joinOptions passed to the command, and then it converts
 // options into the internal JoinConfiguration type that is used as input all the phases in the kubeadm join workflow
 func newJoinData(cmd *cobra.Command, args []string, opt *joinOptions, out io.Writer) (*joinData, error) {
+
+	// Validate the mixed arguments with --config and return early on errors
+	if err := validation.ValidateMixedArguments(cmd.Flags()); err != nil {
+		return nil, err
+	}
+
 	// Re-apply defaults to the public kubeadm API (this will set only values not exposed/not set as a flags)
 	kubeadmscheme.Scheme.Default(opt.externalcfg)
 
@@ -340,10 +353,26 @@ func newJoinData(cmd *cobra.Command, args []string, opt *joinOptions, out io.Wri
 		opt.externalcfg.Discovery.BootstrapToken.APIServerEndpoint = args[0]
 	}
 
-	// if not joining a control plane, unset the ControlPlane object
+	// Include the JoinControlPlane with user flags
+	opt.externalcfg.ControlPlane = opt.joinControlPlane
+
+	// If not passing --control-plane, unset the ControlPlane object
 	if !opt.controlPlane {
-		if opt.externalcfg.ControlPlane != nil {
-			klog.Warningf("[preflight] WARNING: JoinControlPane.controlPlane settings will be ignored when %s flag is not set.", options.ControlPlane)
+		// Use a defaulted JoinControlPlane object to detect if the user has passed
+		// other control-plane related flags.
+		defaultJCP := &kubeadmapiv1beta2.JoinControlPlane{}
+		kubeadmapiv1beta2.SetDefaults_JoinControlPlane(defaultJCP)
+
+		// This list must match the JCP flags in addJoinConfigFlags()
+		joinControlPlaneFlags := []string{
+			options.CertificateKey,
+			options.APIServerAdvertiseAddress,
+			options.APIServerBindPort,
+		}
+
+		if *opt.joinControlPlane != *defaultJCP {
+			klog.Warningf("[preflight] WARNING: --%s is also required when passing control-plane "+
+				"related flags such as [%s]", options.ControlPlane, strings.Join(joinControlPlaneFlags, ", "))
 		}
 		opt.externalcfg.ControlPlane = nil
 	}
@@ -361,10 +390,6 @@ func newJoinData(cmd *cobra.Command, args []string, opt *joinOptions, out io.Wri
 		}
 	}
 
-	if err := validation.ValidateMixedArguments(cmd.Flags()); err != nil {
-		return nil, err
-	}
-
 	// Either use the config file if specified, or convert public kubeadm API to the internal JoinConfiguration
 	// and validates JoinConfiguration
 	if opt.externalcfg.NodeRegistration.Name == "" {