Re: [boost] Probably bug in condition::timed_wait()

From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2006-10-08 09:20:51

Peter Dimov wrote:
> It does matter. Consider the case where two threads are awaiting an element 
> to be pushed into a queue, one uses timed_wait, the other uses wait. The 
> "element pushed" signal arrives concurrently with the timeout, the 
> timed_wait thread is awakened, sees timeout, returns false to its caller, 
> who consequently doesn't pop the element. The other thread waits forever, 
> and the element remains in the queue. 

I can see the problem, but you simply would get the wrong result from
timed_wait, making the caller believe, the operation hasn't timed out.
I think the caller should do something like:

if (!timed_wait(lk, xt, pred))
{ // handle timeout case
   if (pred()) {
     // we got the signal after timeout
      ...
   }
   else {
     // we received a pure timeout
   }
}
else {
   // signal received in time

}

or the following which is equivalent to your suggestion:
if (!timed_wait(lk, xt, pred) && !pred()) {
   // handle timeout
}

The current definition makes it possible for the user code
to disambiguate for both cases, while your suggestion would
drop the timeout information.

Does this make sense?

However the issue should be cleary documented.

Roland