Herbert Valerio Riedel
(bracket (E.registerTimeout em to (throwTo tid ex)) (E.unregisterTimeout em) (\_ -> fmap Just f))
...after some discussion on #ghc, I've realized, that 'registerTimeout' is dangerous if used improperly; it should be avoided to call any blocking operation (or throw exceptions) in the timeout-handler, as otherwise the I/O manager loop stops processing new events (at least with GHC-7.6.2) until the timeout-action completes; the following code demonstrates this issue by triggering a neverending timeout-action which effectively makes the Haskell process non-responsive. --8<---------------cut here---------------start------------->8--- import Control.Concurrent import qualified GHC.Event as E messupEventManager :: IO () messupEventManager = do mv <- newMVar () Just em <- E.getSystemEventManager E.registerTimeout em 5000000 (putStrLn "...blocking NOW!" >> putMVar mv ()) putStrLn "...in about 5 seconds the I/O manager will get stuck..." --8<---------------cut here---------------end--------------->8--- So maybe a warning in the documentation of registerTimeout may be appropriate telling users of registerTimeout that care should be taken to avoid operations blocking for non-negligible time (or throwing exceptions) in the timeout-handler, as otherwise in the best case the I/O processing latency suffers and in the worst case the I/O manager may come to a halt altogether. cheers, hvr