Add global instance of *(math/rand).Rand and Reader

You can read random bytes from Reader without exhausting entropy.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

random/random.go

@@ -1,11 +1,19 @@
 package random
 
 import (
+	"io"
 	"math/rand"
 	"sync"
 	"time"
 )
 
+// Rand is a global *rand.Rand instance, which initilized with NewSource() source.
+var Rand = rand.New(NewSource())
+
+// Reader is a global, shared instance of a pseudorandom bytes generator.
+// It doesn't consume entropy.
+var Reader io.Reader = &reader{rnd: Rand}
+
 // copypaste from standard math/rand
 type lockedSource struct {
 	lk  sync.Mutex
@@ -32,3 +40,22 @@ func NewSource() rand.Source {
 		src: rand.NewSource(time.Now().UnixNano()),
 	}
 }
+
+type reader struct {
+	rnd *rand.Rand
+}
+
+func (r *reader) Read(b []byte) (int, error) {
+	i := 0
+	for {
+		val := r.rnd.Int63()
+		for val > 0 {
+			b[i] = byte(val)
+			i++
+			if i == len(b) {
+				return i, nil
+			}
+			val >>= 8
+		}
+	}
+}