{-# LANGUAGE DeriveDataTypeable #-}
{-# OPTIONS_GHC -funbox-strict-fields #-}
module Distribution.Client.Compat.Semaphore
    ( QSem
    , newQSem
    , waitQSem
    , signalQSem
    ) where

import Prelude (IO, return, Eq (..), Int, Bool (..), ($), ($!), Num (..), flip)

import Control.Concurrent.STM (TVar, atomically, newTVar, readTVar, retry,
                               writeTVar)
import Control.Exception (mask_, onException)
import Control.Monad (join, unless)
import Data.Typeable (Typeable)
import Data.List.NonEmpty (NonEmpty (..))
import qualified Data.List.NonEmpty as NE

-- | '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.
--
data QSem = QSem !(TVar Int) !(TVar [TVar Bool]) !(TVar [TVar Bool])
  deriving (QSem -> QSem -> Bool
forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
/= :: QSem -> QSem -> Bool
$c/= :: QSem -> QSem -> Bool
== :: QSem -> QSem -> Bool
$c== :: QSem -> QSem -> Bool
Eq, Typeable)

newQSem :: Int -> IO QSem
newQSem :: Int -> IO QSem
newQSem Int
i = forall a. STM a -> IO a
atomically forall a b. (a -> b) -> a -> b
$ do
  TVar Int
q <- forall a. a -> STM (TVar a)
newTVar Int
i
  TVar [TVar Bool]
b1 <- forall a. a -> STM (TVar a)
newTVar []
  TVar [TVar Bool]
b2 <- forall a. a -> STM (TVar a)
newTVar []
  forall (m :: * -> *) a. Monad m => a -> m a
return (TVar Int -> TVar [TVar Bool] -> TVar [TVar Bool] -> QSem
QSem TVar Int
q TVar [TVar Bool]
b1 TVar [TVar Bool]
b2)

waitQSem :: QSem -> IO ()
waitQSem :: QSem -> IO ()
waitQSem s :: QSem
s@(QSem TVar Int
q TVar [TVar Bool]
_b1 TVar [TVar Bool]
b2) =
  forall a. IO a -> IO a
mask_ forall a b. (a -> b) -> a -> b
$ forall (m :: * -> *) a. Monad m => m (m a) -> m a
join forall a b. (a -> b) -> a -> b
$ forall a. STM a -> IO a
atomically forall a b. (a -> b) -> a -> b
$ do
        -- join, because if we need to block, we have to add a TVar to
        -- the block queue.
        -- mask_, because we need a chance to set up an exception handler
        -- after the join returns.
     Int
v <- forall a. TVar a -> STM a
readTVar TVar Int
q
     if Int
v forall a. Eq a => a -> a -> Bool
== Int
0
        then do TVar Bool
b <- forall a. a -> STM (TVar a)
newTVar Bool
False
                [TVar Bool]
ys <- forall a. TVar a -> STM a
readTVar TVar [TVar Bool]
b2
                forall a. TVar a -> a -> STM ()
writeTVar TVar [TVar Bool]
b2 (TVar Bool
bforall a. a -> [a] -> [a]
:[TVar Bool]
ys)
                forall (m :: * -> *) a. Monad m => a -> m a
return (TVar Bool -> IO ()
wait TVar Bool
b)
        else do forall a. TVar a -> a -> STM ()
writeTVar TVar Int
q forall a b. (a -> b) -> a -> b
$! Int
v forall a. Num a => a -> a -> a
- Int
1
                forall (m :: * -> *) a. Monad m => a -> m a
return (forall (m :: * -> *) a. Monad m => a -> m a
return ())
  where
    --
    -- very careful here: if we receive an exception, then we need to
    --  (a) write True into the TVar, so that another signalQSem doesn't
    --      try to wake up this thread, and
    --  (b) if the TVar is *already* True, then we need to do another
    --      signalQSem to avoid losing a unit of the resource.
    --
    -- The 'wake' function does both (a) and (b), so we can just call
    -- it here.
    --
    wait :: TVar Bool -> IO ()
wait TVar Bool
t =
      forall a b c. (a -> b -> c) -> b -> a -> c
flip forall a b. IO a -> IO b -> IO a
onException (QSem -> TVar Bool -> IO ()
wake QSem
s TVar Bool
t) forall a b. (a -> b) -> a -> b
$
      forall a. STM a -> IO a
atomically forall a b. (a -> b) -> a -> b
$ do
        Bool
b <- forall a. TVar a -> STM a
readTVar TVar Bool
t
        forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
unless Bool
b forall a. STM a
retry


wake :: QSem -> TVar Bool -> IO ()
wake :: QSem -> TVar Bool -> IO ()
wake QSem
s TVar Bool
x = forall (m :: * -> *) a. Monad m => m (m a) -> m a
join forall a b. (a -> b) -> a -> b
$ forall a. STM a -> IO a
atomically forall a b. (a -> b) -> a -> b
$ do
      Bool
b <- forall a. TVar a -> STM a
readTVar TVar Bool
x
      if Bool
b then forall (m :: * -> *) a. Monad m => a -> m a
return (QSem -> IO ()
signalQSem QSem
s)
           else do forall a. TVar a -> a -> STM ()
writeTVar TVar Bool
x Bool
True
                   forall (m :: * -> *) a. Monad m => a -> m a
return (forall (m :: * -> *) a. Monad m => a -> m a
return ())

{-
 property we want:

   bracket waitQSem (\_ -> signalQSem) (\_ -> ...)

 never loses a unit of the resource.
-}

signalQSem :: QSem -> IO ()
signalQSem :: QSem -> IO ()
signalQSem s :: QSem
s@(QSem TVar Int
q TVar [TVar Bool]
b1 TVar [TVar Bool]
b2) =
  forall a. IO a -> IO a
mask_ forall a b. (a -> b) -> a -> b
$ forall (m :: * -> *) a. Monad m => m (m a) -> m a
join forall a b. (a -> b) -> a -> b
$ forall a. STM a -> IO a
atomically forall a b. (a -> b) -> a -> b
$ do
      -- join, so we don't force the reverse inside the txn
      -- mask_ is needed so we don't lose a wakeup
    Int
v <- forall a. TVar a -> STM a
readTVar TVar Int
q
    if Int
v forall a. Eq a => a -> a -> Bool
/= Int
0
       then do forall a. TVar a -> a -> STM ()
writeTVar TVar Int
q forall a b. (a -> b) -> a -> b
$! Int
v forall a. Num a => a -> a -> a
+ Int
1
               forall (m :: * -> *) a. Monad m => a -> m a
return (forall (m :: * -> *) a. Monad m => a -> m a
return ())
       else do [TVar Bool]
xs <- forall a. TVar a -> STM a
readTVar TVar [TVar Bool]
b1
               [TVar Bool] -> STM (IO ())
checkwake1 [TVar Bool]
xs
  where
    checkwake1 :: [TVar Bool] -> STM (IO ())
checkwake1 [] = do
      [TVar Bool]
ys <- forall a. TVar a -> STM a
readTVar TVar [TVar Bool]
b2
      [TVar Bool] -> STM (IO ())
checkwake2 [TVar Bool]
ys
    checkwake1 (TVar Bool
x:[TVar Bool]
xs) = do
      forall a. TVar a -> a -> STM ()
writeTVar TVar [TVar Bool]
b1 [TVar Bool]
xs
      forall (m :: * -> *) a. Monad m => a -> m a
return (QSem -> TVar Bool -> IO ()
wake QSem
s TVar Bool
x)

    checkwake2 :: [TVar Bool] -> STM (IO ())
checkwake2 [] = do
      forall a. TVar a -> a -> STM ()
writeTVar TVar Int
q Int
1
      forall (m :: * -> *) a. Monad m => a -> m a
return (forall (m :: * -> *) a. Monad m => a -> m a
return ())
    checkwake2 (TVar Bool
y:[TVar Bool]
ys) = do
      let (TVar Bool
z:|[TVar Bool]
zs) = forall a. NonEmpty a -> NonEmpty a
NE.reverse (TVar Bool
yforall a. a -> [a] -> NonEmpty a
:|[TVar Bool]
ys)
      forall a. TVar a -> a -> STM ()
writeTVar TVar [TVar Bool]
b1 [TVar Bool]
zs
      forall a. TVar a -> a -> STM ()
writeTVar TVar [TVar Bool]
b2 []
      forall (m :: * -> *) a. Monad m => a -> m a
return (QSem -> TVar Bool -> IO ()
wake QSem
s TVar Bool
z)