    {-# LANGUAGE Safe #-}
    {-# LANGUAGE BangPatterns #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent.QSem
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  non-portable (concurrency)
    --
    -- Simple quantity semaphores.
    --
    -----------------------------------------------------------------------------

    module Control.Concurrent.QSem
            ( -- * Simple Quantity Semaphores
              QSem,         -- abstract
              newQSem,      -- :: Int  -> IO QSem
              waitQSem,     -- :: QSem -> IO ()
              signalQSem    -- :: QSem -> IO ()
            ) where

    import Prelude
    import GHC.Internal.Control.Concurrent.MVar ( MVar, newEmptyMVar, takeMVar, tryTakeMVar
                              , putMVar, newMVar, tryPutMVar)
    import GHC.Internal.Control.Exception
    import GHC.Internal.Data.Maybe

    -- | 'QSem' is a quantity semaphore in which the resource is acquired
    -- and released in units of one. It provides guaranteed FIFO ordering
    -- for satisfying blocked `waitQSem` calls.
    --
    -- The pattern
    --
    -- > bracket_ waitQSem signalQSem (...)
    --
    -- is safe; it never loses a unit of the resource.
    --
    newtype QSem = QSem (MVar (Int, [MVar ()], [MVar ()]))

    -- The semaphore state (i, xs, ys):
    --
    --   i is the current resource value
    --
    --   (xs,ys) is the queue of blocked threads, where the queue is
    --           given by xs ++ reverse ys.  We can enqueue new blocked threads
    --           by consing onto ys, and dequeue by removing from the head of xs.
    --
    -- A blocked thread is represented by an empty (MVar ()).  To unblock
    -- the thread, we put () into the MVar.
    --
    -- A thread can dequeue itself by also putting () into the MVar, which
    -- it must do if it receives an exception while blocked in waitQSem.
    -- This means that when unblocking a thread in signalQSem we must
    -- first check whether the MVar is already full; the MVar lock on the
    -- semaphore itself resolves race conditions between signalQSem and a
    -- thread attempting to dequeue itself.

    -- |Build a new 'QSem' with a supplied initial quantity.
    --  The initial quantity must be at least 0.
    newQSem :: Int -> IO QSem
    newQSem :: Int -> IO QSem
    newQSem Int
    initial
      | Int
    initial Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    < Int
    0 = String -> IO QSem
    forall a. String -> IO a
    forall (m :: * -> *) a. MonadFail m => String -> m a
    fail String
    "newQSem: Initial quantity must be non-negative"
      | Bool
    otherwise   = do
          sem <- (Int, [MVar ()], [MVar ()])
    -> IO (MVar (Int, [MVar ()], [MVar ()]))
    forall a. a -> IO (MVar a)
    newMVar (Int
    initial, [], [])
          return (QSem sem)

    -- |Wait for a unit to become available.
    waitQSem :: QSem -> IO ()
    waitQSem :: QSem -> IO ()
    waitQSem (QSem MVar (Int, [MVar ()], [MVar ()])
    m) =
      IO () -> IO ()
    forall a. IO a -> IO a
    mask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ do
        (i,b1,b2) <- MVar (Int, [MVar ()], [MVar ()]) -> IO (Int, [MVar ()], [MVar ()])
    forall a. MVar a -> IO a
    takeMVar MVar (Int, [MVar ()], [MVar ()])
    m
        if i == 0
           then do
             b <- newEmptyMVar
             putMVar m (i, b1, b:b2)
             wait b
           else do
             let !z = Int
    iInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    1
             putMVar m (z, b1, b2)
             return ()
      where
        wait :: MVar () -> IO ()
    wait MVar ()
    b = MVar () -> IO ()
    forall a. MVar a -> IO a
    takeMVar MVar ()
    b IO () -> IO () -> IO ()
    forall a b. IO a -> IO b -> IO a
    `onException`
                    (IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ do -- Note [signal uninterruptible]
                       (i,b1,b2) <- MVar (Int, [MVar ()], [MVar ()]) -> IO (Int, [MVar ()], [MVar ()])
    forall a. MVar a -> IO a
    takeMVar MVar (Int, [MVar ()], [MVar ()])
    m
                       r <- tryTakeMVar b
                       r' <- if isJust r
                                then signal (i,b1,b2)
                                else do putMVar b (); return (i,b1,b2)
                       putMVar m r')

    -- |Signal that a unit of the 'QSem' is available.
    signalQSem :: QSem -> IO ()
    signalQSem :: QSem -> IO ()
    signalQSem (QSem MVar (Int, [MVar ()], [MVar ()])
    m) =
      IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ do -- Note [signal uninterruptible]
        r <- MVar (Int, [MVar ()], [MVar ()]) -> IO (Int, [MVar ()], [MVar ()])
    forall a. MVar a -> IO a
    takeMVar MVar (Int, [MVar ()], [MVar ()])
    m
        r' <- signal r
        putMVar m r'

    -- Note [signal uninterruptible]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --   If we have
    --
    --      bracket waitQSem signalQSem (...)
    --
    --   and an exception arrives at the signalQSem, then we must not lose
    --   the resource.  The signalQSem is masked by bracket, but taking
    --   the MVar might block, and so it would be interruptible.  Hence we
    --   need an uninterruptibleMask here.
    --
    --   This isn't ideal: during high contention, some threads won't be
    --   interruptible.  The QSemSTM implementation has better behaviour
    --   here, but it performs much worse than this one in some
    --   benchmarks.

    signal :: (Int,[MVar ()],[MVar ()]) -> IO (Int,[MVar ()],[MVar ()])
    signal :: (Int, [MVar ()], [MVar ()]) -> IO (Int, [MVar ()], [MVar ()])
    signal (Int
    i,[MVar ()]
    a1,[MVar ()]
    a2) =
     if Int
    i Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0
       then [MVar ()] -> [MVar ()] -> IO (Int, [MVar ()], [MVar ()])
    forall {a}.
    Num a =>
    [MVar ()] -> [MVar ()] -> IO (a, [MVar ()], [MVar ()])
    loop [MVar ()]
    a1 [MVar ()]
    a2
       else let !z :: Int
    z = Int
    iInt -> Int -> Int
    forall a. Num a => a -> a -> a
    +Int
    1 in (Int, [MVar ()], [MVar ()]) -> IO (Int, [MVar ()], [MVar ()])
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Int
    z, [MVar ()]
    a1, [MVar ()]
    a2)
     where
       loop :: [MVar ()] -> [MVar ()] -> IO (a, [MVar ()], [MVar ()])
    loop [] [] = (a, [MVar ()], [MVar ()]) -> IO (a, [MVar ()], [MVar ()])
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a
    1, [], [])
       loop [] [MVar ()]
    b2 = [MVar ()] -> [MVar ()] -> IO (a, [MVar ()], [MVar ()])
    loop ([MVar ()] -> [MVar ()]
    forall a. [a] -> [a]
    reverse [MVar ()]
    b2) []
       loop (MVar ()
    b:[MVar ()]
    bs) [MVar ()]
    b2 = do
         r <- MVar () -> () -> IO Bool
    forall a. MVar a -> a -> IO Bool
    tryPutMVar MVar ()
    b ()
         if r then return (0, bs, b2)
              else loop bs b2