Adds HTTP Get rate limitation on cache

We don't want to have a large bandwidth on bedetheque.com, so the pace
at which we perform GET request is limited by a maximal number of
request over a window (per example no more than 10 request over 10
second)

If more request are required, the request is simply paused until we go
back within the limit ;). Go rulez !

rate_limited_getter.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"net/http"
+	"time"
+)
+
+// An HTTPGetter can GET over HTTP
+type HTTPGetter interface {
+	Get(URL string) (*http.Response, error)
+}
+
+type simpleHTTPGetter struct{}
+
+// NewHTTPGetter returns the simplest object that can GET over HTTP
+func NewHTTPGetter() HTTPGetter {
+	return &simpleHTTPGetter{}
+}
+
+func (g *simpleHTTPGetter) Get(URL string) (*http.Response, error) {
+	return http.Get(URL)
+}
+
+type rateLimitedHTTPGetter struct {
+	tokens chan bool
+	window time.Duration
+}
+
+// NewRateLimitedGetter returns an HTTPGetter that is limited by a
+// maximal amount of request over a time window
+func NewRateLimitedGetter(maxRequest uint, window time.Duration) HTTPGetter {
+	return &rateLimitedHTTPGetter{
+		tokens: make(chan bool, maxRequest),
+		window: window,
+	}
+}
+
+func (g *rateLimitedHTTPGetter) Get(URL string) (*http.Response, error) {
+	g.tokens <- true
+	defer func() {
+		go func() {
+			time.Sleep(g.window)
+			<-g.tokens
+		}()
+	}()
+	return http.Get(URL)
+}