sync: add WaitTimeout method to Cond

[mei-rune]

can add func (c *Cond) WaitTimeout(timeout time.Duration) bool for sync.Cond,

example:
...
if !this.c.WaitTimeout(10 * time.Second) {
return errors.New("time out")
}
...

[adg]

Here's how you can do it:

done := make(chan struct{})
go func() {
  cond.Wait()
  close(done)
}()
select {
case <-time.After(timeout):
  // timed out
case <-done:
  // Wait returned
}

[bradfitz]

That's how you do it at a lot of cost (goroutine + 2 channels). The OP might be asking for scheduler integration.

@runner-mei, why do you want this?

[minux]

The problem of timed wait is that for normal use cases, you also
want kill the task to avoid leaking/wasting resources. but that's not
going to happen for goroutine, you will need to do your own framework
to have killable tasks, but in that case, you will definitely need to write
your own timed wait.

However, if the requirement is not waiting and killing the task if it
executes too long, I think using sync.Cond is the wrong solution.
You should use channel to signal readiness and then it's trivial
to use select and time.After for timeout.

It all depend on the use case.

[mei-rune]

my case is:

next_id := 0

// goroutine (net read):
for {
   id := recv() // read from network
   mu.Lock()
   next_id = id
   mu.Unlock()
   cond.Broadcast()
}

// goroutine n:  0 <= n < 100 in any time
func wait(id int, t time.Duration) bool {
   mu.Lock()
   defer mu.Unlock()
   start_at := time.Now()
   for next_id < id {
      if  t >= 0 {
         return false
      }

      if !cond.WaitTimeout(t) {
        return false
      }
      t = time.Now().Sub(start_at)
   }
   return true
}

Create cost of channel is too high, because calling too often

[ianlancetaylor]

Condition variables are generally not the right thing to use in Go, it's hard for me to see why we should add features to them.

[rsc]

I agree with Ian.

[dtromb]

"Condition variables are generally not the right thing to use in Go, it's hard for me to see why we should add features to them."

@ianlancetaylor - Please provide a reasonable alternative or a "Go-like" way to do this, here, then?

You haven't neglected to add a feature, you've taken /away/ a feature from the more-or-less standard set of stuff any systems programmer expects to find where a Mutex/Cond pattern is provided. All thre threading libraries I know directly support this, and I cannot find any good alternatives. (Yes, I've read https://blog.golang.org/pipelines The question(s) are about accomplishing this standard pattern w/o the cost of creating lightweight tasks or additional object allocations - it often happens in extremely tight loops)

[ianlancetaylor]

If you carefully design an abstract problem so that you must have condition variables and can not use channels, then, yes, there are cases where a timeout on a condition variable becomes useful. In Go, however, we prefer channels. Mutexes have their uses in Go. Condition variables are almost always better implemented using channels.

To say more requires discussing a specific problem.

[ianlancetaylor]

Sorry, I should also have said: this is a discussion to have on the mailing list [email protected], not on a closed issue.