haskell - TimeoutManager 使用 tryPutMVar 什么都不放

Question

在 Snap 源Snap.Internal.Http.Server.TimeoutManager

------------------------------------------------------------------------------
-- | Register a new connection with the TimeoutManager.
register :: IO ()               -- ^ action to run when the timeout deadline is
                                -- exceeded.
         -> TimeoutManager      -- ^ manager to register with.
         -> IO TimeoutHandle
register killAction tm = do
    now <- getTime
    let !state = Deadline $ now + toEnum defaultTimeout
    stateRef <- newIORef state

    let !h = TimeoutHandle killAction stateRef getTime
    atomicModifyIORef connections $ \x -> (h:x, ())

    inact <- readIORef inactivity
    when inact $ do
        -- wake up manager thread
        writeIORef inactivity False
        _ <- tryPutMVar morePlease ()
        return ()
    return h

  where
    getTime        = _getTime tm
    inactivity     = _inactivity tm
    morePlease     = _morePlease tm
    connections    = _connections tm
    defaultTimeout = _defaultTimeout tm

为什么会有 _morePlease 字段？
做什么_ <- tryPutMVar morePlease ()？

Answer 1

managerThread :: TimeoutManager -> IO ()
managerThread tm = loop `finally` (readIORef connections >>= destroyAll)
  where
    --------------------------------------------------------------------------
    connections = _connections tm
    getTime     = _getTime tm
    inactivity  = _inactivity tm
    morePlease  = _morePlease tm
    waitABit    = threadDelay 5000000

    --------------------------------------------------------------------------
    loop = do
        waitABit
        handles <- atomicModifyIORef connections (\x -> ([],x))

        if null handles
          then do
            -- we're inactive, go to sleep until we get new threads
            writeIORef inactivity True
            takeMVar morePlease
          else do
            now   <- getTime
            dlist <- processHandles now handles id
            atomicModifyIORef connections (\x -> (dlist x, ()))

        loop

    --------------------------------------------------------------------------
    processHandles !now handles initDlist = go handles initDlist
      where
        go [] !dlist = return dlist

        go (x:xs) !dlist = do
            state   <- readIORef $ _state x
            !dlist' <- case state of
                         Canceled   -> return dlist
                         Deadline t -> if t <= now
                                         then do
                                           _killAction x
                                           return dlist
                                         else return (dlist . (x:))
            go xs dlist'

    --------------------------------------------------------------------------
    destroyAll = mapM_ diediedie

    --------------------------------------------------------------------------
    diediedie x = do
        state <- readIORef $ _state x
        case state of
          Canceled -> return ()
          _        -> _killAction x

如果没有要处理的句柄，managerThread将被takeMVar morePlease. _ <- tryPutMVar morePlease ()叫醒他。