Using TChan with Timeout

Question

I have a TChan as input for a thread which should behave like this:

If sombody writes to the TChan within a specific time, the content should be retrieved. If there is n

Answer 1

The thumb rule of concurrency is: if adding a sleep in some point inside an IO action matters, your program is not safe.

To understand why the code timeout 1000000 $ atomically $ readTChan pktChannel does not work, consider the following alternative implementation of atomically:

atomically' :: STM a -> IO a
atomically' action = do
  result <- atomically action
  threadDelay someTimeAmount
  return result

The above is equal to atomically, but for an extra innocent delay. Now it is easy to see that if timeout kills the thread during the threadDelay, the atomic action has completed (consuming a message from the channel), yet timeout will return Nothing.

A simple fix to timeout n $ atomically ... could be the following

smartTimeout :: Int -> STM a -> IO (Maybe a)
smartTimeout n action = do
   v <- atomically $ newEmptyTMvar
   _ <- timeout n $ atomically $ do
          result <- action
          putTMvar v result
   atomically $ tryTakeTMvar v

The above uses an extra transactional variable v to do the trick. The result value of the action is stored into v inside the same atomic block in which the action is run. The return value of timeout is not trusted, since it does not tell us if action was run or not. After that, we check the TMVar v, which will be full if and only if action was run.