exec credential provider: ProvideClusterInfo and kubeconfig shadow

- The main idea here is that we want to 1) prevent potentially large CA
  bundles from being set in an exec plugin's environment and 2) ensure
  that the exec plugin is getting everything it needs in order to talk to
  a cluster.
- Avoid breaking existing manual declarations of rest.Config instances by
  moving exec Cluster to kubeconfig internal type.
- Use client.authentication.k8s.io/exec to qualify exec cluster extension.
- Deep copy the exec Cluster.Config when we copy a rest.Config.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

Kubernetes-commit: c4299d15d5289768808034676858e76a177eeae5

rest/exec.go

@@ -0,0 +1,85 @@
+/*
+Copyright 2020 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package rest
+
+import (
+	"fmt"
+	"net/http"
+	"net/url"
+
+	"k8s.io/client-go/pkg/apis/clientauthentication"
+	clientauthenticationapi "k8s.io/client-go/pkg/apis/clientauthentication"
+)
+
+// This file contains Config logic related to exec credential plugins.
+
+// ConfigToExecCluster creates a clientauthenticationapi.Cluster with the corresponding fields from
+// the provided Config.
+func ConfigToExecCluster(config *Config) (*clientauthenticationapi.Cluster, error) {
+	caData, err := dataFromSliceOrFile(config.CAData, config.CAFile)
+	if err != nil {
+		return nil, fmt.Errorf("failed to load CA bundle for execProvider: %v", err)
+	}
+
+	var proxyURL string
+	if config.Proxy != nil {
+		req, err := http.NewRequest("", config.Host, nil)
+		if err != nil {
+			return nil, fmt.Errorf("failed to create proxy URL request for execProvider: %w", err)
+		}
+		url, err := config.Proxy(req)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get proxy URL for execProvider: %w", err)
+		}
+		if url != nil {
+			proxyURL = url.String()
+		}
+	}
+
+	return &clientauthentication.Cluster{
+		Server:                   config.Host,
+		TLSServerName:            config.ServerName,
+		InsecureSkipTLSVerify:    config.Insecure,
+		CertificateAuthorityData: caData,
+		ProxyURL:                 proxyURL,
+		Config:                   config.ExecProvider.Config,
+	}, nil
+}
+
+// ExecClusterToConfig creates a Config with the corresponding fields from the provided
+// clientauthenticationapi.Cluster. The returned Config will be anonymous (i.e., it will not have
+// any authentication-related fields set).
+func ExecClusterToConfig(cluster *clientauthentication.Cluster) (*Config, error) {
+	var proxy func(*http.Request) (*url.URL, error)
+	if cluster.ProxyURL != "" {
+		proxyURL, err := url.Parse(cluster.ProxyURL)
+		if err != nil {
+			return nil, fmt.Errorf("cannot parse proxy URL: %w", err)
+		}
+		proxy = http.ProxyURL(proxyURL)
+	}
+
+	return &Config{
+		Host: cluster.Server,
+		TLSClientConfig: TLSClientConfig{
+			Insecure:   cluster.InsecureSkipTLSVerify,
+			ServerName: cluster.TLSServerName,
+			CAData:     cluster.CertificateAuthorityData,
+		},
+		Proxy: proxy,
+	}, nil
+}