    {-# LANGUAGE NoImplicitPrelude #-}

    module GHC.Conc.Signal
            ( Signal
            , HandlerFun
            , setHandler
            , runHandlers
            ) where

    import Control.Concurrent.MVar (MVar, newMVar, withMVar)
    import Data.Dynamic (Dynamic)
    import Data.Maybe (Maybe(..))
    import Foreign.C.Types (CInt)
    import Foreign.ForeignPtr (ForeignPtr)
    import Foreign.StablePtr (castPtrToStablePtr, castStablePtrToPtr,
                              deRefStablePtr, freeStablePtr, newStablePtr)
    import Foreign.Ptr (Ptr, castPtr)
    import GHC.Arr (inRange)
    import GHC.Base
    import GHC.Conc.Sync (forkIO)
    import GHC.IO (mask_, unsafePerformIO)
    import GHC.IOArray (IOArray, boundsIOArray, newIOArray, unsafeReadIOArray,
                        unsafeWriteIOArray)
    import GHC.Real (fromIntegral)
    import GHC.Word (Word8)

    ------------------------------------------------------------------------
    -- Signal handling

    type Signal = CInt

    maxSig :: Int
    maxSig = 64

    type HandlerFun = ForeignPtr Word8 -> IO ()

    -- Lock used to protect concurrent access to signal_handlers.  Symptom
    -- of this race condition is GHC bug #1922, although that bug was on
    -- Windows a similar bug also exists on Unix.
    signal_handlers :: MVar (IOArray Int (Maybe (HandlerFun,Dynamic)))
    signal_handlers = unsafePerformIO $ do
      arr <- newIOArray (0, maxSig) Nothing
      m <- newMVar arr
      sharedCAF m getOrSetGHCConcSignalSignalHandlerStore
    {-# NOINLINE signal_handlers #-}

    foreign import ccall unsafe "getOrSetGHCConcSignalSignalHandlerStore"
      getOrSetGHCConcSignalSignalHandlerStore :: Ptr a -> IO (Ptr a)

    setHandler :: Signal -> Maybe (HandlerFun, Dynamic)
               -> IO (Maybe (HandlerFun, Dynamic))
    setHandler sig handler = do
      let int = fromIntegral sig
      withMVar signal_handlers $ \arr ->
        if not (inRange (boundsIOArray arr) int)
          then error "GHC.Conc.setHandler: signal out of range"
          else do old <- unsafeReadIOArray arr int
                  unsafeWriteIOArray arr int handler
                  return old

    runHandlers :: ForeignPtr Word8 -> Signal -> IO ()
    runHandlers p_info sig = do
      let int = fromIntegral sig
      withMVar signal_handlers $ \arr ->
        if not (inRange (boundsIOArray arr) int)
          then return ()
          else do handler <- unsafeReadIOArray arr int
                  case handler of
                    Nothing -> return ()
                    Just (f,_)  -> do _ <- forkIO (f p_info)
                                      return ()

    -- Machinery needed to ensure that we only have one copy of certain
    -- CAFs in this module even when the base package is present twice, as
    -- it is when base is dynamically loaded into GHCi.  The RTS keeps
    -- track of the single true value of the CAF, so even when the CAFs in
    -- the dynamically-loaded base package are reverted, nothing bad
    -- happens.
    --
    sharedCAF :: a -> (Ptr a -> IO (Ptr a)) -> IO a
    sharedCAF a get_or_set =
      mask_ $ do
        stable_ref <- newStablePtr a
        let ref = castPtr (castStablePtrToPtr stable_ref)
        ref2 <- get_or_set ref
        if ref == ref2
          then return a
          else do freeStablePtr stable_ref
                  deRefStablePtr (castPtrToStablePtr (castPtr ref2))