kubeadm: skip LocalAPIEndpoint defaulting on worker join

FetchInitConfigurationFromCluster always called SetAPIEndpointDynamicDefaults,
which invokes ChooseAPIServerBindAddress and requires a default route on the
host. Worker nodes joining the cluster don't need a LocalAPIEndpoint and may
legitimately have no default route, causing kubeadm join to fail with:

```
unable to fetch the kubeadm-config ConfigMap: unable to select an IP from
default routes.
```

Add a skipAPIEndpoint parameter to SetInitDynamicDefaults and pass
`!getAPIEndpoint` from FetchInitConfigurationFromCluster, so the endpoint
defaulter is bypassed when the caller did not request the endpoint (worker
join, non-control-plane reset/certs/upgrade paths).

Signed-off-by: Seena Fallah <seenafallah@gmail.com>

cmd/kubeadm/app/util/config/cluster.go

@@ -68,9 +68,11 @@ func FetchInitConfigurationFromCluster(client clientset.Interface, printer outpu
 		return nil, err
 	}
 
-	// Apply dynamic defaults
-	// NB. skip CRI detection here because it won't be used at all and will be overridden later
-	if err := SetInitDynamicDefaults(cfg, true); err != nil {
+	// Apply dynamic defaults.
+	// NB. skip CRI detection here because it won't be used at all and will be overridden later.
+	// NB. skip LocalAPIEndpoint defaulting when the caller did not request the endpoint (e.g. a
+	// worker join).
+	if err := SetInitDynamicDefaults(cfg, true, !getAPIEndpoint); err != nil {
 		return nil, err
 	}
 

cmd/kubeadm/app/util/config/initconfiguration.go

@@ -57,16 +57,18 @@ var (
 	}
 )
 
-// SetInitDynamicDefaults checks and sets configuration values for the InitConfiguration object
-func SetInitDynamicDefaults(cfg *kubeadmapi.InitConfiguration, skipCRIDetect bool) error {
+// SetInitDynamicDefaults checks and sets configuration values for the InitConfiguration object.
+func SetInitDynamicDefaults(cfg *kubeadmapi.InitConfiguration, skipCRIDetect, skipAPIEndpoint bool) error {
 	if err := SetBootstrapTokensDynamicDefaults(&cfg.BootstrapTokens); err != nil {
 		return err
 	}
 	if err := SetNodeRegistrationDynamicDefaults(&cfg.NodeRegistration, true, skipCRIDetect); err != nil {
 		return err
 	}
-	if err := SetAPIEndpointDynamicDefaults(&cfg.LocalAPIEndpoint); err != nil {
-		return err
+	if !skipAPIEndpoint {
+		if err := SetAPIEndpointDynamicDefaults(&cfg.LocalAPIEndpoint); err != nil {
+			return err
+		}
 	}
 	return SetClusterDynamicDefaults(&cfg.ClusterConfiguration, &cfg.LocalAPIEndpoint, &cfg.NodeRegistration)
 }
@@ -246,7 +248,7 @@ func DefaultedInitConfiguration(versionedInitCfg *kubeadmapiv1.InitConfiguration
 	}
 
 	// Applies dynamic defaults to settings not provided with flags
-	if err := SetInitDynamicDefaults(internalcfg, opts.SkipCRIDetect); err != nil {
+	if err := SetInitDynamicDefaults(internalcfg, opts.SkipCRIDetect, false); err != nil {
 		return nil, err
 	}
 	// Validates cfg (flags/configs + defaults + dynamic defaults)
@@ -401,7 +403,7 @@ func documentMapToInitConfiguration(gvkmap kubeadmapi.DocumentMap, allowDeprecat
 	}
 
 	// Applies dynamic defaults to settings not provided with flags
-	if err := SetInitDynamicDefaults(initcfg, skipCRIDetect); err != nil {
+	if err := SetInitDynamicDefaults(initcfg, skipCRIDetect, false); err != nil {
 		return nil, err
 	}
 

cmd/kubeadm/app/util/config/initconfiguration_test.go

@@ -314,3 +314,56 @@ func TestBytesToInitConfiguration(t *testing.T) {
 		}
 	}
 }
+
+func TestSetInitDynamicDefaultsSkipAPIEndpoint(t *testing.T) {
+	// "not-an-ip" is a sentinel that would cause SetAPIEndpointDynamicDefaults to return
+	// an error if invoked. With skipAPIEndpoint=true, the value must be left untouched.
+	const sentinel = "not-an-ip"
+
+	tests := []struct {
+		name            string
+		skipAPIEndpoint bool
+		expectErr       bool
+		expectAdvertise string
+	}{
+		{
+			name:            "skip leaves AdvertiseAddress untouched",
+			skipAPIEndpoint: true,
+			expectErr:       false,
+			expectAdvertise: sentinel,
+		},
+		{
+			name:            "no skip surfaces invalid AdvertiseAddress error",
+			skipAPIEndpoint: false,
+			expectErr:       true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg := &kubeadmapi.InitConfiguration{
+				ClusterConfiguration: kubeadmapi.ClusterConfiguration{
+					KubernetesVersion: constants.CurrentKubernetesVersion.String(),
+				},
+				LocalAPIEndpoint: kubeadmapi.APIEndpoint{
+					AdvertiseAddress: sentinel,
+				},
+			}
+
+			err := SetInitDynamicDefaults(cfg, true /* skipCRIDetect */, tc.skipAPIEndpoint)
+			if tc.expectErr {
+				if err == nil {
+					t.Fatalf("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if cfg.LocalAPIEndpoint.AdvertiseAddress != tc.expectAdvertise {
+				t.Errorf("AdvertiseAddress = %q, want %q",
+					cfg.LocalAPIEndpoint.AdvertiseAddress, tc.expectAdvertise)
+			}
+		})
+	}
+}