feat: CompareAndSwapInt32 may be better than AddInt32 (#2077)

core/mr/mapreduce.go

@@ -376,9 +376,7 @@ type onceChan struct {
 }
 
 func (oc *onceChan) write(val interface{}) {
-	if atomic.AddInt32(&oc.wrote, 1) > 1 {
+	if atomic.CompareAndSwapInt32(&oc.wrote, 0, 1) {
-		return
-	}
-
 		oc.channel <- val
+	}
 }