The race is between the len call and the use of the channel. Entries can
have been added in between.
On Sun, Aug 28, 2016, 17:40 dc0d <kaveh.shahbaz...@gmail.com> wrote:
> Would you please elaborate on that?
>
> As I understand it there is no concurrent use of len happening here. It's
> a for loop and all calling to len is happening sequentially. Unless the
> channels make the code inside cases of one select statement concurrent -
> which will be super confusing for me.
>
>
> On Sunday, August 28, 2016 at 8:03:10 PM UTC+4:30, Jan Mercl wrote:
>
>> Using len(ch) like this in a concurrency scenario is a big no because
>> then the value you get carries 0 bits of useful information. It's not a
>> data race, it's worse, the race is semantic and not fixable without
>> removing the use of len(ch).
>>
>> On Sun, Aug 28, 2016, 17:26 dc0d <kaveh.sh...@gmail.com> wrote:
>>
> TL;DR
>>>
>>> Does assigning a (buffered) channel, already in a variable, to a second
>>> variable, affects the result of len function?
>>>
>>> Long version:
>>> What is happening here? - Code at the end; Go 1.7.
>>>
>>> *Output 1*:
>>>
>>> Nine times:
>>> [ info ] 2016/08/28 19:51:28 LEN_BEFORE=0
>>>
>>> [ info ] 2016/08/28 19:51:28 LEN=7
>>>
>>> [ info ] 2016/08/28 19:51:28 S00=7
>>>
>>> But if the second case gets commented like this:
>>>
>>> case <-limiter.C:
>>> // if len(actualBuffer) > 0 {
>>> // buffer = actualBuffer
>>> // } else {
>>> // buffer = nil
>>> // }
>>>
>>> It works as expected; *Output2*:
>>>
>>> [ info ] 2016/08/28 19:54:28 LEN_BEFORE=0
>>>
>>> [ info ] 2016/08/28 19:54:28 LEN=7
>>>
>>> [ info ] 2016/08/28 19:54:28 S00=7
>>>
>>> [ info ] 2016/08/28 19:54:29 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:30 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:31 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:32 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:33 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:34 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:35 LEN_BEFORE=7
>>>
>>> [ info ] 2016/08/28 19:54:36 LEN_BEFORE=7
>>> ...
>>>
>>> Code:
>>>
>>> package main
>>>
>>>
>>> import (
>>> "log"
>>> "time"
>>>
>>>
>>> "github.com/comail/colog"
>>> )
>>>
>>>
>>> func status00Channeler() {
>>> <-start
>>>
>>>
>>> limiter := time.NewTicker(time.Second / maxMsgPerSec)
>>> fetchLimiter := time.NewTicker(time.Second)
>>>
>>>
>>> var buffer chan *Data
>>> actualBuffer := make(chan *Data, maxMsgPerSec)
>>> db, err := newDB()
>>> if err != nil {
>>> log.Panic(err)
>>> }
>>>
>>>
>>> FIRST:
>>> for {
>>> select {
>>> case <-interrupted:
>>> break FIRST
>>> case <-limiter.C:
>>> // if len(actualBuffer) > 0 {
>>> // buffer = actualBuffer
>>> // } else {
>>> // buffer = nil
>>> // }
>>> case i := <-buffer:
>>> select {
>>> case status00 <- i: // will block here
>>> case <-interrupted:
>>> break FIRST
>>> }
>>> case <-fetchLimiter.C:
>>> log.Printf("LEN_BEFORE=%d", len(actualBuffer))
>>> if len(actualBuffer) > 0 {
>>> continue
>>> }
>>>
>>>
>>> s00, err := db.GetIncomings()
>>> if err != nil {
>>> log.Println(`error:`, err)
>>> time.Sleep(time.Second)
>>> continue
>>> }
>>> if s00 == nil || len(s00) == 0 {
>>> continue
>>> }
>>>
>>>
>>> FILL_BUFFER:
>>> for _, v := range s00 {
>>> select {
>>> case actualBuffer <- v:
>>> default:
>>> break FILL_BUFFER
>>> }
>>> }
>>>
>>>
>>> log.Printf("LEN=%d", len(actualBuffer))
>>> log.Printf("S00=%d", len(s00))
>>> }
>>> }
>>> }
>>>
>>>
>>> func main() {
>>> go status00Channeler()
>>> close(start)
>>>
>>>
>>> <-time.After(time.Second * 30)
>>> }
>>>
>>>
>>> var (
>>> status00 = make(chan *Data, maxMsgPerSec)
>>> )
>>>
>>>
>>> type Data struct{}
>>>
>>>
>>> const (
>>> maxMsgPerSec = 60
>>> )
>>>
>>>
>>> func newDB() (*DB, error) {
>>> res := new(DB)
>>> return res, nil
>>> }
>>>
>>>
>>> func (db *DB) GetIncomings() ([]*Data, error) {
>>> var res []*Data
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>> res = append(res, &Data{})
>>>
>>>
>>> return res, nil
>>> }
>>>
>>>
>>> type DB struct{}
>>>
>>>
>>> func init() {
>>> colog.Register()
>>> }
>>>
>>>
>>> var (
>>> start = make(chan struct{})
>>> interrupted = make(chan struct{}) // comes from sys interrupts SIGINT,
>>> SIGTERM, etc
>>> )
>>>
>>>
>>> --
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>> --
>>
>> -j
>>
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