func benchWriteNestedWithSource(db *bolt.DB, options *BenchOptions, results *BenchResults, keySource func() uint32) error {
	results.WriteOps = options.Iterations

	for i := 0; i < options.Iterations; i += options.BatchSize {
		err := db.Update(func(tx *bolt.Tx) error {
			top, _ := tx.CreateBucketIfNotExists(benchBucketName)
			top.FillPercent = options.FillPercent

			var name = make([]byte, options.KeySize)
			binary.BigEndian.PutUint32(name, keySource())
			b, _ := top.CreateBucketIfNotExists(name)
			b.FillPercent = options.FillPercent

			for j := 0; j < options.BatchSize; j++ {
				var key = make([]byte, options.KeySize)
				var value = make([]byte, options.ValueSize)
				binary.BigEndian.PutUint32(key, keySource())
				if err := b.Put(key, value); err != nil {
					return err
				}
			}

			return nil
		})
		if err != nil {
			return err
		}
	}
	return nil
}
// Ensure a closed database returns an error while running a transaction block
func TestDB_Update_Closed(t *testing.T) {
	var db bolt.DB
	err := db.Update(func(tx *bolt.Tx) error {
		tx.CreateBucket([]byte("widgets"))
		return nil
	})
	equals(t, err, bolt.ErrDatabaseNotOpen)
}