Move image pull throttling logic to kubelet/images

This allows runtimes in different packages (dockertools, rkt, kuberuntime) to
share the same logic. Before this change, only dockertools support this
feature. Now all three packages support image pull throttling.

pkg/kubelet/images/helpers.go

@@ -0,0 +1,50 @@
+/*
+Copyright 2016 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 images
+
+import (
+	"fmt"
+
+	"k8s.io/kubernetes/pkg/api"
+	kubecontainer "k8s.io/kubernetes/pkg/kubelet/container"
+	"k8s.io/kubernetes/pkg/util/flowcontrol"
+)
+
+// throttleImagePulling wraps kubecontainer.ImageService to throttle image
+// pulling based on the given QPS and burst limits. If QPS is zero, defaults
+// to no throttling.
+func throttleImagePulling(imageService kubecontainer.ImageService, qps float32, burst int) kubecontainer.ImageService {
+	if qps == 0.0 {
+		return imageService
+	}
+	return &throttledImageService{
+		ImageService: imageService,
+		limiter:      flowcontrol.NewTokenBucketRateLimiter(qps, burst),
+	}
+}
+
+type throttledImageService struct {
+	kubecontainer.ImageService
+	limiter flowcontrol.RateLimiter
+}
+
+func (ts throttledImageService) PullImage(image kubecontainer.ImageSpec, secrets []api.Secret) error {
+	if ts.limiter.TryAccept() {
+		return ts.ImageService.PullImage(image, secrets)
+	}
+	return fmt.Errorf("pull QPS exceeded.")
+}