go routine deadlock with single channel

Question

I started learning go recently and I am stuck on a problem. I have a simple go routine which either returns or pushes value to a channel. And my main fn delegates work to this routine till it meets condition or data is exhausted. This code seem to deadlock on "found" channel. What am I doing wrong?

There are multiple workers
Item can be found in more than one worker at the same time
Once item is found, all workers should be stopped.

.

func workerRoutine(data Data, found chan bool, wg *sync.WaitGroup){

   defer (*wg).Done()
   // data processing
   // return on false 

   // multiple routines can set this at the same time
   found <-true
}

func main {

   // ....
   found:=make(chan bool)
   var wg sync.WaitGroup
   itemFound:=false
       Loop:
          for i:=0; i<limit; i++ {
              select {
                 case <-found:
                    itemFound = true
                    break Loop
                 default:
                    if(some_check) {
                       wg.Add(1)
                       go workerRoutine(mdata,found,&wg)
                    }
              }
       }

   wg.Wait()

   // use itemFound
}

Can it happen that an item is found multiple times, i.e. `found <- true` is executed multiple times? — mrd0ll4r, Oct 09 '15 at 10:28
Are you aware that break just breaks select, not whole loop? — Kokos, Oct 09 '15 at 10:31
`<-found` case of select may also not manage to trigger before loop ends (default kicks in as soon as possible) — Kokos, Oct 09 '15 at 10:42
@Kokos Yes. I forgot to add the label for break above. Question edited! — Rijin, Oct 09 '15 at 20:06

score 1 · Accepted Answer · answered Oct 09 '15 at 10:47

One possible solution is to avoid select statement and use separate goroutine for receiver (or sender, or both). Example:

package main    
import "sync"

func worker(res chan bool, wg *sync.WaitGroup) {
    res <- true
    wg.Done()
}

func receiver(res chan bool, wg *sync.WaitGroup) {
    for range res {
    }
    wg.Done()
}

func main() {
    var wg, wg2 sync.WaitGroup
    wg.Add(1)
    wg2.Add(10)
    found := make(chan bool)
    go receiver(found, &wg)
    for i := 0; i < 10; i++ {
        go worker(found, &wg2)
    }
    wg2.Wait()
    close(found)
    wg.Done()
}

You are right. Looks like the loop is finishing before <-found is fired. The solution you gave is excellent. But I would still like to find out an elegant way to modify my existing code to make it work. — Rijin, Oct 09 '15 at 20:32

go routine deadlock with single channel

1 Answers1

Linked