fix NewULID for systems with low time resolution (#270)

NewULID uses a pool of rand.Source using time.Now() as seed. In some systems with high concurrency and lower time resolution, this leads to multiple sources with the same sead, making NewULID return equal values on subsequent calls.

This commit changes the seed to add a random number from the global shared rand.Source (which is thread-safe).

Signed-off-by: Santiago M. Mola <santi@mola.io>

model.go

@@ -229,7 +229,8 @@ type Record interface {
 
 var randPool = &sync.Pool{
 	New: func() interface{} {
-		return rand.NewSource(time.Now().UnixNano())
+		seed := time.Now().UnixNano() + rand.Int63()
+		return rand.NewSource(seed)
 	},
 }
 

model_test.go

@@ -1,6 +1,7 @@
 package kallax
 
 import (
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -23,16 +24,34 @@ func TestULID_Value(t *testing.T) {
 
 func TestUULID_ThreeNewIDsAreDifferent(t *testing.T) {
 	r := require.New(t)
-	id1 := NewULID()
-	id2 := NewULID()
-	id3 := NewULID()
 
-	r.NotEqual(id1, id2)
-	r.NotEqual(id1, id3)
-	r.NotEqual(id2, id3)
-
-	r.True(id1 == id1)
-	r.False(id1 == id2)
+	goroutines := 100
+	ids_per_goroutine := 1000
+
+	ids := make(map[ULID]bool, ids_per_goroutine*goroutines)
+	m := &sync.Mutex{}
+
+	wg := &sync.WaitGroup{}
+	wg.Add(goroutines)
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			var oids []ULID
+			for j := 0; j < ids_per_goroutine; j++ {
+				oids = append(oids, NewULID())
+			}
+
+			m.Lock()
+			for _, id := range oids {
+				ids[id] = true
+			}
+			m.Unlock()
+			wg.Done()
+		}()
+	}
+
+	wg.Wait()
+
+	r.Equal(goroutines*ids_per_goroutine, len(ids))
 }
 
 func TestULID_ScanValue(t *testing.T) {