Implements a credentialprovider library for use by DockerPuller.

This change refactors the way Kubelet's DockerPuller handles the docker config credentials to utilize a new credentialprovider library.

The credentialprovider library is based on several of the files from the Kubelet's dockertools directory, but supports a new pluggable model for retrieving a .dockercfg-compatible JSON blob with credentials.

With this change, the Kubelet will lazily ask for the docker config from a set of DockerConfigProvider extensions each time it needs a credential.

This change provides common implementations of DockerConfigProvider for:
 - "Default": load .dockercfg from disk
 - "Caching": wraps another provider in a cache that expires after a pre-specified lifetime.

GCP-only:
 - "google-dockercfg": reads a .dockercfg from a GCE instance's metadata
 - "google-dockercfg-url": reads a .dockercfg from a URL specified in a GCE instance's metadata.
 - "google-container-registry": reads an access token from GCE metadata into a password field.

pkg/credentialprovider/provider_test.go

@@ -0,0 +1,62 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+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 credentialprovider
+
+import (
+	"testing"
+	"time"
+)
+
+func TestCachingProvider(t *testing.T) {
+	provider := &testProvider{
+		Count: 0,
+	}
+
+	cache := &CachingDockerConfigProvider{
+		Provider: provider,
+		Lifetime: 1 * time.Second,
+	}
+
+	if provider.Count != 0 {
+		t.Errorf("Unexpected number of Provide calls: %v", provider.Count)
+	}
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	if provider.Count != 1 {
+		t.Errorf("Unexpected number of Provide calls: %v", provider.Count)
+	}
+
+	time.Sleep(cache.Lifetime)
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	if provider.Count != 2 {
+		t.Errorf("Unexpected number of Provide calls: %v", provider.Count)
+	}
+
+	time.Sleep(cache.Lifetime)
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	cache.Provide()
+	if provider.Count != 3 {
+		t.Errorf("Unexpected number of Provide calls: %v", provider.Count)
+	}
+}