Implement fmt.Stringer on rest.Config to sanitize sensitive fields

It's very easy to add glog.Info(config) calls for debugging (or actual
logging). In some scenarios those configs will carry sensitive tokens
and those tokens will end up in logs or response bodies.
Leaking of those stringified configs compromises the cluster.

Also implement fmt.GoStringer.

Kubernetes-commit: c9ad1d7339b164dfba0846ec49fa4a52474d3e23

tools/clientcmd/api/types.go

@@ -17,6 +17,8 @@ limitations under the License.
 package api
 
 import (
+	"fmt"
+
 	"k8s.io/apimachinery/pkg/runtime"
 )
 
@@ -150,6 +152,25 @@ type AuthProviderConfig struct {
 	Config map[string]string `json:"config,omitempty"`
 }
 
+var _ fmt.Stringer = new(AuthProviderConfig)
+var _ fmt.GoStringer = new(AuthProviderConfig)
+
+// GoString implements fmt.GoStringer and sanitizes sensitive fields of
+// AuthProviderConfig to prevent accidental leaking via logs.
+func (c AuthProviderConfig) GoString() string {
+	return c.String()
+}
+
+// String implements fmt.Stringer and sanitizes sensitive fields of
+// AuthProviderConfig to prevent accidental leaking via logs.
+func (c AuthProviderConfig) String() string {
+	cfg := "<nil>"
+	if c.Config != nil {
+		cfg = "--- REDACTED ---"
+	}
+	return fmt.Sprintf("api.AuthProviderConfig{Name: %q, Config: map[string]string{%s}}", c.Name, cfg)
+}
+
 // ExecConfig specifies a command to provide client credentials. The command is exec'd
 // and outputs structured stdout holding credentials.
 //
@@ -172,6 +193,29 @@ type ExecConfig struct {
 	APIVersion string `json:"apiVersion,omitempty"`
 }
 
+var _ fmt.Stringer = new(ExecConfig)
+var _ fmt.GoStringer = new(ExecConfig)
+
+// GoString implements fmt.GoStringer and sanitizes sensitive fields of
+// ExecConfig to prevent accidental leaking via logs.
+func (c ExecConfig) GoString() string {
+	return c.String()
+}
+
+// String implements fmt.Stringer and sanitizes sensitive fields of ExecConfig
+// to prevent accidental leaking via logs.
+func (c ExecConfig) String() string {
+	var args []string
+	if len(c.Args) > 0 {
+		args = []string{"--- REDACTED ---"}
+	}
+	env := "[]ExecEnvVar(nil)"
+	if len(c.Env) > 0 {
+		env = "[]ExecEnvVar{--- REDACTED ---}"
+	}
+	return fmt.Sprintf("api.AuthProviderConfig{Command: %q, Args: %#v, Env: %s, APIVersion: %q}", c.Command, args, env, c.APIVersion)
+}
+
 // ExecEnvVar is used for setting environment variables when executing an exec-based
 // credential plugin.
 type ExecEnvVar struct {