Vendoring memberlist tag 0.1.0

Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>

vendor/github.com/sean-/seed/init.go

@@ -0,0 +1,84 @@
+package seed
+
+import (
+	crand "crypto/rand"
+	"fmt"
+	"math"
+	"math/big"
+	"math/rand"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+var (
+	m      sync.Mutex
+	secure int32
+	seeded int32
+)
+
+func cryptoSeed() error {
+	defer atomic.StoreInt32(&seeded, 1)
+
+	var err error
+	var n *big.Int
+	n, err = crand.Int(crand.Reader, big.NewInt(math.MaxInt64))
+	if err != nil {
+		rand.Seed(time.Now().UTC().UnixNano())
+		return err
+	}
+	rand.Seed(n.Int64())
+	atomic.StoreInt32(&secure, 1)
+	return nil
+}
+
+// Init provides best-effort seeding (which is better than running with Go's
+// default seed of 1).  If `/dev/urandom` is available, Init() will seed Go's
+// runtime with entropy from `/dev/urandom` and return true because the runtime
+// was securely seeded.  If Init() has already initialized the random number or
+// it had failed to securely initialize the random number generation, Init()
+// will return false.  See MustInit().
+func Init() (seededSecurely bool, err error) {
+	if atomic.LoadInt32(&seeded) == 1 {
+		return false, nil
+	}
+
+	// Slow-path
+	m.Lock()
+	defer m.Unlock()
+
+	if err := cryptoSeed(); err != nil {
+		return false, err
+	}
+
+	return true, nil
+}
+
+// MustInit provides guaranteed secure seeding.  If `/dev/urandom` is not
+// available, MustInit will panic() with an error indicating why reading from
+// `/dev/urandom` failed.  MustInit() will upgrade the seed if for some reason a
+// call to Init() failed in the past.
+func MustInit() {
+	if atomic.LoadInt32(&secure) == 1 {
+		return
+	}
+
+	// Slow-path
+	m.Lock()
+	defer m.Unlock()
+
+	if err := cryptoSeed(); err != nil {
+		panic(fmt.Sprintf("Unable to seed the random number generator: %v", err))
+	}
+}
+
+// Secure returns true if a cryptographically secure seed was used to
+// initialize rand.
+func Secure() bool {
+	return atomic.LoadInt32(&secure) == 1
+}
+
+// Seeded returns true if Init has seeded the random number generator.
+func Seeded() bool {
+	return atomic.LoadInt32(&seeded) == 1
+}