Add a rate limiter, use it to rate limit docker pulls.

pkg/util/throttle.go

@@ -0,0 +1,104 @@
+/*
+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 util
+
+import (
+	"sync"
+	"time"
+)
+
+type RateLimiter interface {
+	// CanAccept returns true if the rate is below the limit, false otherwise
+	CanAccept() bool
+	// Stop stops the rate limiter, subsequent calls to CanAccept will return false
+	Stop()
+}
+
+type tickRateLimiter struct {
+	lock   sync.Mutex
+	tokens chan bool
+	ticker <-chan time.Time
+	stop   chan bool
+}
+
+// NewTokenBucketRateLimiter creates a rate limiter which implements a token bucket approach.
+// The rate limiter allows bursts of up to 'burst' to exceed the QPS, while still maintaining a
+// smoothed qps rate of 'qps'.
+// The bucket is initially filled with 'burst' tokens, the rate limiter spawns a go routine
+// which refills the bucket with one token at a rate of 'qps'.  The maximum number of tokens in
+// the bucket is capped at 'burst'.
+// When done with the limiter, Stop() must be called to halt the associated goroutine.
+func NewTokenBucketRateLimiter(qps float32, burst int) RateLimiter {
+	ticker := time.Tick(time.Duration(float32(time.Second) / qps))
+	rate := newTokenBucketRateLimiterFromTicker(ticker, burst)
+	go rate.run()
+	return rate
+}
+
+func newTokenBucketRateLimiterFromTicker(ticker <-chan time.Time, burst int) *tickRateLimiter {
+	if burst < 1 {
+		panic("burst must be a positive integer")
+	}
+	rate := &tickRateLimiter{
+		tokens: make(chan bool, burst),
+		ticker: ticker,
+		stop:   make(chan bool),
+	}
+	for i := 0; i < burst; i++ {
+		rate.tokens <- true
+	}
+	return rate
+}
+
+func (t *tickRateLimiter) CanAccept() bool {
+	select {
+	case <-t.tokens:
+		return true
+	default:
+		return false
+	}
+}
+
+func (t *tickRateLimiter) Stop() {
+	close(t.stop)
+}
+
+func (r *tickRateLimiter) run() {
+	for {
+		if !r.step() {
+			break
+		}
+	}
+}
+
+func (r *tickRateLimiter) step() bool {
+	select {
+	case <-r.ticker:
+		r.increment()
+		return true
+	case <-r.stop:
+		return false
+	}
+}
+
+func (t *tickRateLimiter) increment() {
+	// non-blocking send
+	select {
+	case t.tokens <- true:
+	default:
+	}
+}