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
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.
Instead of TChan a
, use TChan (Maybe a)
. Your normal producer (of x
) now writes Just x
. Fork an extra "ticking" process that writes Nothing
to the channel (every x seconds). Then have a reader for the channel, and abort if you get two successive Nothing
. This way, you avoid exceptions, which may cause data to get lost in your case (but I am not sure).
Use registerDelay
, an STM function, to signal a TVar
when the timeout is reached. You can then use the orElse
function or the Alternative
operator <|>
to select between the next TChan
value or the timeout.
import Control.Applicative
import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
import System.Random
-- write random values after a random delay
packetWriter :: Int -> TChan Int -> IO ()
packetWriter maxDelay chan = do
let xs = randomRs (10000 :: Int, maxDelay + 50000) (mkStdGen 24036583)
forM_ xs $ \ x -> do
threadDelay x
atomically $ writeTChan chan x
-- block (retry) until the delay TVar is set to True
fini :: TVar Bool -> STM ()
fini = check <=< readTVar
-- Read the next value from a TChan or timeout
readTChanTimeout :: Int -> TChan a -> IO (Maybe a)
readTChanTimeout timeoutAfter pktChannel = do
delay <- registerDelay timeoutAfter
atomically $
Just <$> readTChan pktChannel
<|> Nothing <$ fini delay
-- | Print packets until a timeout is reached
readLoop :: Show a => Int -> TChan a -> IO ()
readLoop timeoutAfter pktChannel = do
res <- readTChanTimeout timeoutAfter pktChannel
case res of
Nothing -> putStrLn "timeout"
Just val -> do
putStrLn $ "packet: " ++ show val
readLoop timeoutAfter pktChannel
main :: IO ()
main = do
let timeoutAfter = 1000000
-- spin up a packet writer simulation
pktChannel <- newTChanIO
tid <- forkIO $ packetWriter timeoutAfter pktChannel
readLoop timeoutAfter pktChannel
killThread tid