Add randomness pool mode for V4 UUID

uuid.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"strings"
+	"sync"
 )
 
 // A UUID is a 128 bit (16 byte) Universal Unique IDentifier as defined in RFC
@@ -33,7 +34,15 @@ const (
 	Future                    // Reserved for future definition.
 )
 
-var rander = rand.Reader // random function
+const randPoolSize = 16 * 16
+
+var (
+	rander      = rand.Reader // random function
+	poolEnabled = false
+	poolMu      sync.Mutex
+	poolPos     = randPoolSize     // protected with poolMu
+	pool        [randPoolSize]byte // protected with poolMu
+)
 
 type invalidLengthError struct{ len int }
 
@@ -255,3 +264,31 @@ func SetRand(r io.Reader) {
 	}
 	rander = r
 }
+
+// EnableRandPool enables internal randomness pool used for Random
+// (Version 4) UUID generation. The pool contains random bytes read from
+// the random number generator on demand in batches. Enabling the pool
+// may improve the UUID generation throughput significantly.
+//
+// Since the pool is stored on the Go heap, this feature may be a bad fit
+// for security sensitive applications.
+//
+// Both EnableRandPool and DisableRandPool are not thread-safe and should
+// only be called when there is no possibility that New or any other
+// UUID Version 4 generation function will be called concurrently.
+func EnableRandPool() {
+	poolEnabled = true
+}
+
+// DisableRandPool disables the randomness pool if it was previously
+// enabled with EnableRandPool.
+//
+// Both EnableRandPool and DisableRandPool are not thread-safe and should
+// only be called when there is no possibility that New or any other
+// UUID Version 4 generation function will be called concurrently.
+func DisableRandPool() {
+	poolEnabled = false
+	defer poolMu.Unlock()
+	poolMu.Lock()
+	poolPos = randPoolSize
+}

uuid_test.go

@@ -179,6 +179,26 @@ func TestRandomUUID(t *testing.T) {
 	}
 }
 
+func TestRandomUUID_Pooled(t *testing.T) {
+	defer DisableRandPool()
+	EnableRandPool()
+	m := make(map[string]bool)
+	for x := 1; x < 128; x++ {
+		uuid := New()
+		s := uuid.String()
+		if m[s] {
+			t.Errorf("NewRandom returned duplicated UUID %s", s)
+		}
+		m[s] = true
+		if v := uuid.Version(); v != 4 {
+			t.Errorf("Random UUID of version %s", v)
+		}
+		if uuid.Variant() != RFC4122 {
+			t.Errorf("Random UUID is variant %d", uuid.Variant())
+		}
+	}
+}
+
 func TestNew(t *testing.T) {
 	m := make(map[UUID]bool)
 	for x := 1; x < 32; x++ {
@@ -517,6 +537,22 @@ func TestRandomFromReader(t *testing.T) {
 	}
 }
 
+func TestRandPool(t *testing.T) {
+	myString := "8059ddhdle77cb52"
+	EnableRandPool()
+	SetRand(strings.NewReader(myString))
+	_, err := NewRandom()
+	if err == nil {
+		t.Errorf("expecting an error as reader has no more bytes")
+	}
+	DisableRandPool()
+	SetRand(strings.NewReader(myString))
+	_, err = NewRandom()
+	if err != nil {
+		t.Errorf("failed generating UUID from a reader")
+	}
+}
+
 func TestWrongLength(t *testing.T) {
 	_, err := Parse("12345")
 	if err == nil {
@@ -641,3 +677,26 @@ func BenchmarkParseLen36Corrupted(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkUUID_New(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			_, err := NewRandom()
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}
+
+func BenchmarkUUID_NewPooled(b *testing.B) {
+	EnableRandPool()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			_, err := NewRandom()
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}

version4.go

@@ -27,6 +27,8 @@ func NewString() string {
 // The strength of the UUIDs is based on the strength of the crypto/rand
 // package.
 //
+// Uses the randomness pool if it was enabled with EnableRandPool.
+//
 // A note about uniqueness derived from the UUID Wikipedia entry:
 //
 //  Randomly generated UUIDs have 122 random bits.  One's annual risk of being
@@ -35,7 +37,10 @@ func NewString() string {
 //  equivalent to the odds of creating a few tens of trillions of UUIDs in a
 //  year and having one duplicate.
 func NewRandom() (UUID, error) {
-	return NewRandomFromReader(rander)
+	if !poolEnabled {
+		return NewRandomFromReader(rander)
+	}
+	return newRandomFromPool()
 }
 
 // NewRandomFromReader returns a UUID based on bytes read from a given io.Reader.
@@ -49,3 +54,23 @@ func NewRandomFromReader(r io.Reader) (UUID, error) {
 	uuid[8] = (uuid[8] & 0x3f) | 0x80 // Variant is 10
 	return uuid, nil
 }
+
+func newRandomFromPool() (UUID, error) {
+	var uuid UUID
+	poolMu.Lock()
+	if poolPos == randPoolSize {
+		_, err := io.ReadFull(rander, pool[:])
+		if err != nil {
+			poolMu.Unlock()
+			return Nil, err
+		}
+		poolPos = 0
+	}
+	copy(uuid[:], pool[poolPos:(poolPos+16)])
+	poolPos += 16
+	poolMu.Unlock()
+
+	uuid[6] = (uuid[6] & 0x0f) | 0x40 // Version 4
+	uuid[8] = (uuid[8] & 0x3f) | 0x80 // Variant is 10
+	return uuid, nil
+}