Basic rate limiting implementation.

Added `--limit-upload` and `--limit-download` flags to rate limit
backups and restores.

internal/limiter/static_limiter.go

@@ -0,0 +1,53 @@
+package limiter
+
+import (
+	"io"
+
+	"github.com/juju/ratelimit"
+)
+
+type staticLimiter struct {
+	upstream   *ratelimit.Bucket
+	downstream *ratelimit.Bucket
+}
+
+// NewStaticLimiter constructs a Limiter with a fixed (static) upload and
+// download rate cap
+func NewStaticLimiter(uploadKb, downloadKb int) Limiter {
+	var (
+		upstreamBucket   *ratelimit.Bucket
+		downstreamBucket *ratelimit.Bucket
+	)
+
+	if uploadKb > 0 {
+		upstreamBucket = ratelimit.NewBucketWithRate(toByteRate(uploadKb), int64(toByteRate(uploadKb)))
+	}
+
+	if downloadKb > 0 {
+		downstreamBucket = ratelimit.NewBucketWithRate(toByteRate(downloadKb), int64(toByteRate(downloadKb)))
+	}
+
+	return staticLimiter{
+		upstream:   upstreamBucket,
+		downstream: downstreamBucket,
+	}
+}
+
+func (l staticLimiter) Upstream(r io.Reader) io.Reader {
+	return l.limit(r, l.upstream)
+}
+
+func (l staticLimiter) Downstream(r io.Reader) io.Reader {
+	return l.limit(r, l.downstream)
+}
+
+func (l staticLimiter) limit(r io.Reader, b *ratelimit.Bucket) io.Reader {
+	if b == nil {
+		return r
+	}
+	return ratelimit.Reader(r, b)
+}
+
+func toByteRate(val int) float64 {
+	return float64(val) * 1024.
+}