On 04/07/2010 21:51, Neil Mitchell wrote:
Yup, that's a bug. Not clear if it's fixable.
That too. A very similar bug in fact, if there is a fix it will probably fix both of them. The problem is that readChan holds a lock on the read end of the Chan, so neither isEmptyChan nor unGetChan can work when a reader is blocked.
I wrote my Chan around the abstraction:
data Chan a = Chan (MVar (Either [a] [MVar a]))
The Chan either has elements in it (Left), or has readers waiting for elements (Right). To get the fairness properties on Chan you might want to make these two lists Queue's, but I think the basic principle still works. By using this abstraction my Chan was a lot simpler. With this scheme implementing isEmpyChan or unGetChan would both work nicely. My Chan was not designed for performance. (In truth I replaced the Left with IntMap a, and inserted elements with a randomly chosen key, but the basic idea is the same.)
I like the idea. But what happens if one of the blocked threads gets killed by a killThread (e.g. a timeout) while it is waiting? Won't we still give it an element of the Chan sometime in the future? Perhaps this doesn't happen in your scenario, but it seems to throw a spanner in the works for using this as a general-purpose implementation. The STM version doesn't have this bug, of course :-) But then, it doesn't have fairness either. Cheers, Simon