System/Timeout.hs

    {-# LANGUAGE Safe #-}
    {-# LANGUAGE CPP #-}
    {-# LANGUAGE StandaloneDeriving #-}

    -------------------------------------------------------------------------------
    -- |
    -- Module      :  System.Timeout
    -- Copyright   :  (c) The University of Glasgow 2007
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable
    --
    -- Attach a timeout event to arbitrary 'IO' computations.
    --
    -------------------------------------------------------------------------------

    module System.Timeout ( timeout ) where

    #ifndef mingw32_HOST_OS
    import Control.Monad
    import GHC.Event           (getSystemTimerManager,
                                registerTimeout, unregisterTimeout)
    #endif

    import Control.Concurrent
    import Control.Exception   (Exception(..), handleJust, bracket,
                                uninterruptibleMask_,
                                asyncExceptionToException,
                                asyncExceptionFromException)
    import Data.Unique         (Unique, newUnique)

    -- An internal type that is thrown as a dynamic exception to
    -- interrupt the running IO computation when the timeout has
    -- expired.

    newtype Timeout = Timeout Unique deriving (Eq)

    instance Show Timeout where
        show _ = "<<timeout>>"

    -- Timeout is a child of SomeAsyncException
    instance Exception Timeout where
      toException = asyncExceptionToException
      fromException = asyncExceptionFromException

    -- |Wrap an 'IO' computation to time out and return @Nothing@ in case no result
    -- is available within @n@ microseconds (@1\/10^6@ seconds). In case a result
    -- is available before the timeout expires, @Just a@ is returned. A negative
    -- timeout interval means \"wait indefinitely\". When specifying long timeouts,
    -- be careful not to exceed @maxBound :: Int@.
    --
    -- The design of this combinator was guided by the objective that @timeout n f@
    -- should behave exactly the same as @f@ as long as @f@ doesn't time out. This
    -- means that @f@ has the same 'myThreadId' it would have without the timeout
    -- wrapper. Any exceptions @f@ might throw cancel the timeout and propagate
    -- further up. It also possible for @f@ to receive exceptions thrown to it by
    -- another thread.
    --
    -- A tricky implementation detail is the question of how to abort an @IO@
    -- computation. This combinator relies on asynchronous exceptions internally.
    -- The technique works very well for computations executing inside of the
    -- Haskell runtime system, but it doesn't work at all for non-Haskell code.
    -- Foreign function calls, for example, cannot be timed out with this
    -- combinator simply because an arbitrary C function cannot receive
    -- asynchronous exceptions. When @timeout@ is used to wrap an FFI call that
    -- blocks, no timeout event can be delivered until the FFI call returns, which
    -- pretty much negates the purpose of the combinator. In practice, however,
    -- this limitation is less severe than it may sound. Standard I\/O functions
    -- like 'System.IO.hGetBuf', 'System.IO.hPutBuf', Network.Socket.accept, or
    -- 'System.IO.hWaitForInput' appear to be blocking, but they really don't
    -- because the runtime system uses scheduling mechanisms like @select(2)@ to
    -- perform asynchronous I\/O, so it is possible to interrupt standard socket
    -- I\/O or file I\/O using this combinator.

    timeout :: Int -> IO a -> IO (Maybe a)
    timeout n f
        | n <  0    = fmap Just f
        | n == 0    = return Nothing
    #ifndef mingw32_HOST_OS
        | rtsSupportsBoundThreads = do
            -- In the threaded RTS, we use the Timer Manager to delay the
            -- (fairly expensive) 'forkIO' call until the timeout has expired.
            --
            -- An additional thread is required for the actual delivery of
            -- the Timeout exception because killThread (or another throwTo)
            -- is the only way to reliably interrupt a throwTo in flight.
            pid <- myThreadId
            ex  <- fmap Timeout newUnique
            tm  <- getSystemTimerManager
            -- 'lock' synchronizes the timeout handler and the main thread:
            --  * the main thread can disable the handler by writing to 'lock';
            --  * the handler communicates the spawned thread's id through 'lock'.
            -- These two cases are mutually exclusive.
            lock <- newEmptyMVar
            let handleTimeout = do
                    v <- isEmptyMVar lock
                    when v $ void $ forkIOWithUnmask $ \unmask -> unmask $ do
                        v2 <- tryPutMVar lock =<< myThreadId
                        when v2 $ throwTo pid ex
                cleanupTimeout key = uninterruptibleMask_ $ do
                    v <- tryPutMVar lock undefined
                    if v then unregisterTimeout tm key
                         else takeMVar lock >>= killThread
            handleJust (\e -> if e == ex then Just () else Nothing)
                       (\_ -> return Nothing)
                       (bracket (registerTimeout tm n handleTimeout)
                                cleanupTimeout
                                (\_ -> fmap Just f))
    #endif
        | otherwise = do
            pid <- myThreadId
            ex  <- fmap Timeout newUnique
            handleJust (\e -> if e == ex then Just () else Nothing)
                       (\_ -> return Nothing)
                       (bracket (forkIOWithUnmask $ \unmask ->
                                     unmask $ threadDelay n >> throwTo pid ex)
                                (uninterruptibleMask_ . killThread)
                                (\_ -> fmap Just f))
            -- #7719 explains why we need uninterruptibleMask_ above.