On 04/07/10 10:30, Neil Mitchell wrote:
Hi Simon,
My suspicion for the root cause of the problem is that Concurrent.Chan is incorrect. In the course of debugging this problem we found 2 bugs in Chan, and while I never tracked down any other bugs in Chan, I no longer trust it. By rewriting parts of the program, including avoiding Chan, the bugs disappeared.I don't think I'll be using Chan again until after someone has proven in correct.
Considering Chan is<150 lines of code and has been around for many years, that's amazing! Did you report the bugs? Is it anything to do with asynchronous exceptions?
Nothing to do with async exceptions. I found:
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.
Of course, there's also the async exceptions bug still around:
Yes, that's a bug (though not in Chan).
However, even after having a program with no async exceptions (I never used them), and eliminating unGetChan and isEmpyChan, I still got bugs. I have no proof they came from the Chan module, and no minimal test case was ever able to recreate them, but the same program with my own Chan implementation worked. My Chan had different properties (it queues items randomly) and a subset of the Chan functions, so it still doesn't prove any issue with Chan - but I am now sceptical.
It's surprising how difficult it is to get these MVar-based abstractions right. Some thorough testing of Chan is probably in order. Cheers, Simon