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.shahbaz...@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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