writeChan not properly protecting again async exceptions

[klao]

In the current code of writeChan from Control.Concurrent.Chan:

writeChan (Chan _ writeVar) val = do
  new_hole <- newEmptyMVar
  modifyMVar_ writeVar $ \old_hole -> do
    putMVar old_hole (ChItem val new_hole)
    return new_hole

if an async exception arrives "between" putMVar old_hole and return, the channel will be left in inconsistent state: the writeVar will be reverted to its original value, but old_hole would remain filled in. Thus all subsequent writers will block on this Chan indefinitely.

The proper solution is to just mask the whole operation. And it's OK, as 'putMVar old_hole' cannot ever block. I attach a patch for this.

[klao]

patch for writeChan

[klao]

Also, I'm attaching small program that demonstrates this issue. If you compile it with -O0 in a way that Control.Concurrent.Chan is also compiled with -O0 (for example by copying Control/Concurrent/Chan.hs and uncommenting import Chan instead for Control.Concurrent.Chan), then the deadlock is consistently reproduced.

[simonmar]

Thanks, this is actually already fixed, see  http://www.haskell.org/pipermail/cvs-libraries/2012-March/015116.html