Use a real rate limiter to throttle Gitiles requests.

Set the default so clients of android.googlesource.com wil not get
throttled.

Change-Id: Iace07b58bc5e45343071a99133b299eca16ea045

cmd/slothfs-expand-manifest/main.go

diff --git a/cmd/slothfs-expand-manifest/main.go b/cmd/slothfs-expand-manifest/main.go
index a9ace1d..0e5dfb6 100644
--- a/cmd/slothfs-expand-manifest/main.go
+++ b/cmd/slothfs-expand-manifest/main.go
@@ -34,7 +34,13 @@
 		log.Fatal("must set --gitiles")
 	}
 
-	service, err := gitiles.NewService(*gitilesURL)
+	// SustainedQPS is a little high, but since this is a one-shot
+	// program let's try to get away with it.
+	service, err := gitiles.NewService(*gitilesURL,
+		gitiles.Options{
+			BurstQPS:     10,
+			SustainedQPS: 5,
+		})
 	if err != nil {
 		log.Fatal(err)
 	}

cmd/slothfs-gitilesfs/main.go

diff --git a/cmd/slothfs-gitilesfs/main.go b/cmd/slothfs-gitilesfs/main.go
index 628b7f1..85ad920 100644
--- a/cmd/slothfs-gitilesfs/main.go
+++ b/cmd/slothfs-gitilesfs/main.go
@@ -48,7 +48,7 @@
 		log.Printf("NewCache: %v", err)
 	}
 
-	service, err := gitiles.NewService(*url)
+	service, err := gitiles.NewService(*url, gitiles.Options{})
 	if err != nil {
 		log.Printf("NewService: %v", err)
 	}

cmd/slothfs-manifestfs/main.go

diff --git a/cmd/slothfs-manifestfs/main.go b/cmd/slothfs-manifestfs/main.go
index 1b3199a..c3f57d5 100644
--- a/cmd/slothfs-manifestfs/main.go
+++ b/cmd/slothfs-manifestfs/main.go
@@ -57,7 +57,7 @@
 		log.Printf("NewCache: %v", err)
 	}
 
-	service, err := gitiles.NewService(*gitilesURL)
+	service, err := gitiles.NewService(*gitilesURL, gitiles.Options{})
 	if err != nil {
 		log.Printf("NewService: %v", err)
 	}

cmd/slothfs-multifs/main.go

diff --git a/cmd/slothfs-multifs/main.go b/cmd/slothfs-multifs/main.go
index 31b076b..70912b8 100644
--- a/cmd/slothfs-multifs/main.go
+++ b/cmd/slothfs-multifs/main.go
@@ -52,7 +52,7 @@
 		log.Printf("NewCache: %v", err)
 	}
 
-	service, err := gitiles.NewService(*gitilesURL)
+	service, err := gitiles.NewService(*gitilesURL, gitiles.Options{})
 	if err != nil {
 		log.Printf("NewService: %v", err)
 	}

fs/gitilesfs_test.go

diff --git a/fs/gitilesfs_test.go b/fs/gitilesfs_test.go
index bd53d25..0bbae9d 100644
--- a/fs/gitilesfs_test.go
+++ b/fs/gitilesfs_test.go
@@ -576,7 +576,8 @@
 	}
 
 	fixture.service, err = gitiles.NewService(
-		fmt.Sprintf("http://%s", fixture.testServer.listener.Addr().String()))
+		fmt.Sprintf("http://%s", fixture.testServer.listener.Addr().String()),
+		gitiles.Options{})
 	if err != nil {
 		return nil, err
 	}

gitiles/client.go

diff --git a/gitiles/client.go b/gitiles/client.go
index eb600ba..10cb86a 100644
--- a/gitiles/client.go
+++ b/gitiles/client.go
@@ -25,12 +25,15 @@
 	"net/http"
 	"net/url"
 	"path"
+
+	"golang.org/x/net/context"
+	"golang.org/x/time/rate"
 )
 
 // Service is a client for the Gitiles JSON interface.
 type Service struct {
-	throttle chan struct{}
-	addr     url.URL
+	limiter *rate.Limiter
+	addr    url.URL
 }
 
 // Addr returns the address of the gitiles service.
@@ -38,22 +41,39 @@
 	return s.addr.String()
 }
 
+// Options specifies how much load we can put on remote Gitiles servers.
+type Options struct {
+	BurstQPS     int
+	SustainedQPS float64
+}
+
 // NewService returns a new Gitiles JSON client.
-func NewService(addr string) (*Service, error) {
+func NewService(addr string, opts Options) (*Service, error) {
+	if opts.BurstQPS == 0 {
+		opts.BurstQPS = 4
+	}
+	if opts.SustainedQPS == 0.0 {
+		opts.SustainedQPS = 0.5
+	}
+
 	url, err := url.Parse(addr)
 	if err != nil {
 		return nil, err
 	}
 	return &Service{
-		throttle: make(chan struct{}, 12),
-		addr:     *url,
+		limiter: rate.NewLimiter(rate.Limit(opts.SustainedQPS), opts.BurstQPS),
+		addr:    *url,
 	}, nil
 }
 
 func (s *Service) get(u url.URL) ([]byte, error) {
-	s.throttle <- struct{}{}
+	ctx := context.Background()
+
+	if err := s.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
 	resp, err := http.Get(u.String())
-	<-s.throttle
+
 	if err != nil {
 		return nil, err
 	}