messageID: fix atomic operation suggested by brainexe

name                  old time/op    new time/op    delta
NewSafeID-20            7.60ns ± 1%    5.93ns ± 1%  -21.97%  (p=0.008 n=5+5)
NewSafeIDParallel-20    21.0ns ± 1%    21.0ns ± 2%     ~     (p=0.952 n=5+5)

name                  old alloc/op   new alloc/op   delta
NewSafeID-20             0.00B          0.00B          ~     (all equal)
NewSafeIDParallel-20     8.00B ± 0%     8.00B ± 0%     ~     (all equal)

name                  old allocs/op  new allocs/op  delta
NewSafeID-20              0.00           0.00          ~     (all equal)
NewSafeIDParallel-20      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

See also:
- #1035 (review)

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

messageID.go

@@ -19,9 +19,8 @@ type safeID struct {
 	nextID int64
 }
 
-func (s *safeID) Next() (id int) {
-	id = int(atomic.LoadInt64(&s.nextID))
-	atomic.AddInt64(&s.nextID, 1)
+func (s *safeID) Next() int {
+	id := atomic.AddInt64(&s.nextID, 1)
 
-	return id
+	return int(id)
 }