# LANGUAGE CPP , NamedFieldPuns , RecordWildCards , ScopedTypeVariables , RankNTypes , DeriveDataTypeable #

#if MIN_VERSION_monad_control(0,3,0)
# LANGUAGE FlexibleContexts #
#endif

#if !MIN_VERSION_base(4,3,0)
{-# LANGUAGE RankNTypes #-}
#endif

-- |
-- Module:      Data.Pool
  Copyright :    ( c ) 2011 MailRank , Inc.
  License :      BSD3
  Maintainer :     <  > ,
                  <  >
-- Stability:   experimental
-- Portability: portable
--
-- A high-performance striped pooling abstraction for managing
-- flexibly-sized collections of resources such as database
-- connections.
--
-- \"Striped\" means that a single 'Pool' consists of several
-- sub-pools, each managed independently.  A single stripe is fine for
-- many applications, and probably what you should choose by default.
-- More stripes will lead to reduced contention in high-performance
-- multicore applications, at a trade-off of causing the maximum
-- number of simultaneous resources in use to grow.
module Data.Pool
    (
      Pool(idleTime, maxResources, numStripes)
    , LocalPool
    , createPool
    , withResource
    , takeResource
    , tryWithResource
    , tryTakeResource
    , destroyResource
    , putResource
    , destroyAllResources
    ) where

import Control.Applicative ((<$>))
import Control.Concurrent (ThreadId, forkIOWithUnmask, killThread, myThreadId, threadDelay)
import Control.Concurrent.STM
import Control.Exception (SomeException, onException, mask_)
import Control.Monad (forM_, forever, join, liftM3, unless, when)
import Data.Hashable (hash)
import Data.IORef (IORef, newIORef, mkWeakIORef)
import Data.List (partition)
import Data.Time.Clock (NominalDiffTime, UTCTime, diffUTCTime, getCurrentTime)
import Data.Typeable (Typeable)
import GHC.Conc.Sync (labelThread)
import qualified Control.Exception as E
import qualified Data.Vector as V

#if MIN_VERSION_monad_control(0,3,0)
import Control.Monad.Trans.Control (MonadBaseControl, control)
import Control.Monad.Base (liftBase)
#else
import Control.Monad.IO.Control (MonadControlIO, controlIO)
import Control.Monad.IO.Class (liftIO)
#define control controlIO
#define liftBase liftIO
#endif

#if MIN_VERSION_base(4,3,0)
import Control.Exception (mask)
#else
-- Don't do any async exception protection for older GHCs.
mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
mask f = f id
#endif

-- | A single resource pool entry.
data Entry a = Entry {
      entry :: a
    , lastUse :: UTCTime
    -- ^ Time of last return.
    }

-- | A single striped pool.
data LocalPool a = LocalPool {
      inUse :: TVar Int
    -- ^ Count of open entries (both idle and in use).
    , entries :: TVar [Entry a]
    -- ^ Idle entries.
    , lfin :: IORef ()
    -- ^ empty value used to attach a finalizer to (internal)
    } deriving (Typeable)

data Pool a = Pool {
      create :: IO a
    -- ^ Action for creating a new entry to add to the pool.
    , destroy :: a -> IO ()
    -- ^ Action for destroying an entry that is now done with.
    , numStripes :: Int
    -- ^ The number of stripes (distinct sub-pools) to maintain.
  The smallest acceptable value is 1 .
    , idleTime :: NominalDiffTime
    -- ^ Amount of time for which an unused resource is kept alive.
  The smallest acceptable value is 0.5 seconds .
    --
    -- The elapsed time before closing may be a little longer than
  requested , as the reaper thread wakes at 1 - second intervals .
    , maxResources :: Int
    -- ^ Maximum number of resources to maintain per stripe.  The
  smallest acceptable value is 1 .
    --
    -- Requests for resources will block if this limit is reached on a
    -- single stripe, even if other stripes have idle resources
    -- available.
    , localPools :: V.Vector (LocalPool a)
    -- ^ Per-capability resource pools.
    , fin :: IORef ()
    -- ^ empty value used to attach a finalizer to (internal)
    } deriving (Typeable)

instance Show (Pool a) where
    show Pool{..} = "Pool {numStripes = " ++ show numStripes ++ ", " ++
                    "idleTime = " ++ show idleTime ++ ", " ++
                    "maxResources = " ++ show maxResources ++ "}"

-- | Create a striped resource pool.
--
-- Although the garbage collector will destroy all idle resources when
-- the pool is garbage collected it's recommended to manually
-- 'destroyAllResources' when you're done with the pool so that the
-- resources are freed up as soon as possible.
createPool
    :: IO a
    -- ^ Action that creates a new resource.
    -> (a -> IO ())
    -- ^ Action that destroys an existing resource.
    -> Int
    -- ^ The number of stripes (distinct sub-pools) to maintain.
  The smallest acceptable value is 1 .
    -> NominalDiffTime
    -- ^ Amount of time for which an unused resource is kept open.
  The smallest acceptable value is 0.5 seconds .
    --
    -- The elapsed time before destroying a resource may be a little
  longer than requested , as the reaper thread wakes at 1 - second
    -- intervals.
    -> Int
    -- ^ Maximum number of resources to keep open per stripe.  The
  smallest acceptable value is 1 .
    --
    -- Requests for resources will block if this limit is reached on a
    -- single stripe, even if other stripes have idle resources
    -- available.
     -> IO (Pool a)
createPool create destroy numStripes idleTime maxResources = do
  when (numStripes < 1) $
    modError "pool " $ "invalid stripe count " ++ show numStripes
  when (idleTime < 0.5) $
    modError "pool " $ "invalid idle time " ++ show idleTime
  when (maxResources < 1) $
    modError "pool " $ "invalid maximum resource count " ++ show maxResources
  localPools <- V.replicateM numStripes $
                liftM3 LocalPool (newTVarIO 0) (newTVarIO []) (newIORef ())
  reaperId <- forkIOLabeledWithUnmask "resource-pool: reaper" $ \unmask ->
                unmask $ reaper destroy idleTime localPools
  fin <- newIORef ()
  let p = Pool {
            create
          , destroy
          , numStripes
          , idleTime
          , maxResources
          , localPools
          , fin
          }
  mkWeakIORef fin (killThread reaperId) >>
    V.mapM_ (\lp -> mkWeakIORef (lfin lp) (purgeLocalPool destroy lp)) localPools
  return p

-- TODO: Propose 'forkIOLabeledWithUnmask' for the base library.

-- | Sparks off a new thread using 'forkIOWithUnmask' to run the given
  IO computation , but first labels the thread with the given label
  ( using '  ' ) .
--
-- The implementation makes sure that asynchronous exceptions are
-- masked until the given computation is executed. This ensures the
-- thread will always be labeled which guarantees you can always
  easily find it in the GHC event log .
--
-- Like 'forkIOWithUnmask', the given computation is given a function
-- to unmask asynchronous exceptions. See the documentation of that
-- function for the motivation of this.
--
-- Returns the 'ThreadId' of the newly created thread.
forkIOLabeledWithUnmask :: String
                        -> ((forall a. IO a -> IO a) -> IO ())
                        -> IO ThreadId
forkIOLabeledWithUnmask label m = mask_ $ forkIOWithUnmask $ \unmask -> do
                                    tid <- myThreadId
                                    labelThread tid label
                                    m unmask

-- | Periodically go through all pools, closing any resources that
-- have been left idle for too long.
reaper :: (a -> IO ()) -> NominalDiffTime -> V.Vector (LocalPool a) -> IO ()
reaper destroy idleTime pools = forever $ do
  threadDelay (1 * 1000000)
  now <- getCurrentTime
  let isStale Entry{..} = now `diffUTCTime` lastUse > idleTime
  V.forM_ pools $ \LocalPool{..} -> do
    resources <- atomically $ do
      (stale,fresh) <- partition isStale <$> readTVar entries
      unless (null stale) $ do
        writeTVar entries fresh
        modifyTVar_ inUse (subtract (length stale))
      return (map entry stale)
    forM_ resources $ \resource -> do
      destroy resource `E.catch` \(_::SomeException) -> return ()

-- | Destroy all idle resources of the given 'LocalPool' and remove them from
-- the pool.
purgeLocalPool :: (a -> IO ()) -> LocalPool a -> IO ()
purgeLocalPool destroy LocalPool{..} = do
  resources <- atomically $ do
    idle <- swapTVar entries []
    modifyTVar_ inUse (subtract (length idle))
    return (map entry idle)
  forM_ resources $ \resource ->
    destroy resource `E.catch` \(_::SomeException) -> return ()

-- | Temporarily take a resource from a 'Pool', perform an action with
-- it, and return it to the pool afterwards.
--
-- * If the pool has an idle resource available, it is used
--   immediately.
--
-- * Otherwise, if the maximum number of resources has not yet been
--   reached, a new resource is created and used.
--
-- * If the maximum number of resources has been reached, this
--   function blocks until a resource becomes available.
--
-- If the action throws an exception of any type, the resource is
-- destroyed, and not returned to the pool.
--
-- It probably goes without saying that you should never manually
-- destroy a pooled resource, as doing so will almost certainly cause
-- a subsequent user (who expects the resource to be valid) to throw
-- an exception.
withResource ::
#if MIN_VERSION_monad_control(0,3,0)
    (MonadBaseControl IO m)
#else
    (MonadControlIO m)
#endif
  => Pool a -> (a -> m b) -> m b
{-# SPECIALIZE withResource :: Pool a -> (a -> IO b) -> IO b #-}
withResource pool act = control $ \runInIO -> mask $ \restore -> do
  (resource, local) <- takeResource pool
  ret <- restore (runInIO (act resource)) `onException`
            destroyResource pool local resource
  putResource local resource
  return ret
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE withResource #
#endif

-- | Take a resource from the pool, following the same results as
-- 'withResource'. Note that this function should be used with caution, as
-- improper exception handling can lead to leaked resources.
--
  This function returns both a resource and the @LocalPool@ it came from so
  that it may either be destroyed ( via '  ' ) or returned to the
  pool ( via '  ' ) .
takeResource :: Pool a -> IO (a, LocalPool a)
takeResource pool@Pool{..} = do
  local@LocalPool{..} <- getLocalPool pool
  resource <- liftBase . join . atomically $ do
    ents <- readTVar entries
    case ents of
      (Entry{..}:es) -> writeTVar entries es >> return (return entry)
      [] -> do
        used <- readTVar inUse
        when (used == maxResources) retry
        writeTVar inUse $! used + 1
        return $
          create `onException` atomically (modifyTVar_ inUse (subtract 1))
  return (resource, local)
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE takeResource #
#endif

-- | Similar to 'withResource', but only performs the action if a resource could
-- be taken from the pool /without blocking/. Otherwise, 'tryWithResource'
  returns immediately with ' Nothing ' ( ie . the action function is /not/ called ) .
-- Conversely, if a resource can be borrowed from the pool without blocking, the
-- action is performed and it's result is returned, wrapped in a 'Just'.
tryWithResource :: forall m a b.
#if MIN_VERSION_monad_control(0,3,0)
    (MonadBaseControl IO m)
#else
    (MonadControlIO m)
#endif
  => Pool a -> (a -> m b) -> m (Maybe b)
tryWithResource pool act = control $ \runInIO -> mask $ \restore -> do
  res <- tryTakeResource pool
  case res of
    Just (resource, local) -> do
      ret <- restore (runInIO (Just <$> act resource)) `onException`
                destroyResource pool local resource
      putResource local resource
      return ret
    Nothing -> restore . runInIO $ return (Nothing :: Maybe b)
#if __GLASGOW_HASKELL__ >= 700
{-# INLINABLE tryWithResource #-}
#endif

-- | A non-blocking version of 'takeResource'. The 'tryTakeResource' function
-- returns immediately, with 'Nothing' if the pool is exhausted, or @'Just' (a,
-- 'LocalPool' a)@ if a resource could be borrowed from the pool successfully.
tryTakeResource :: Pool a -> IO (Maybe (a, LocalPool a))
tryTakeResource pool@Pool{..} = do
  local@LocalPool{..} <- getLocalPool pool
  resource <- liftBase . join . atomically $ do
    ents <- readTVar entries
    case ents of
      (Entry{..}:es) -> writeTVar entries es >> return (return . Just $ entry)
      [] -> do
        used <- readTVar inUse
        if used == maxResources
          then return (return Nothing)
          else do
            writeTVar inUse $! used + 1
            return $ Just <$>
              create `onException` atomically (modifyTVar_ inUse (subtract 1))
  return $ (flip (,) local) <$> resource
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE tryTakeResource #
#endif

  | Get a ( Thread-)'LocalPool '
--
  Internal , just to not repeat code for ' takeResource ' and ' tryTakeResource '
getLocalPool :: Pool a -> IO (LocalPool a)
getLocalPool Pool{..} = do
  i <- liftBase $ ((`mod` numStripes) . hash) <$> myThreadId
  return $ localPools V.! i
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE getLocalPool #
#endif

-- | Destroy a resource. Note that this will ignore any exceptions in the
-- destroy function.
destroyResource :: Pool a -> LocalPool a -> a -> IO ()
destroyResource Pool{..} LocalPool{..} resource = do
   destroy resource `E.catch` \(_::SomeException) -> return ()
   atomically (modifyTVar_ inUse (subtract 1))
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE destroyResource #
#endif

-- | Return a resource to the given 'LocalPool'.
putResource :: LocalPool a -> a -> IO ()
putResource LocalPool{..} resource = do
    now <- getCurrentTime
    atomically $ modifyTVar_ entries (Entry resource now:)
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE putResource #
#endif

-- | Destroy all resources in all stripes in the pool. Note that this
-- will ignore any exceptions in the destroy function.
--
-- This function is useful when you detect that all resources in the
-- pool are broken. For example after a database has been restarted
-- all connections opened before the restart will be broken. In that
-- case it's better to close those connections so that 'takeResource'
-- won't take a broken connection from the pool but will open a new
-- connection instead.
--
-- Another use-case for this function is that when you know you are
-- done with the pool you can destroy all idle resources immediately
-- instead of waiting on the garbage collector to destroy them, thus
-- freeing up those resources sooner.
destroyAllResources :: Pool a -> IO ()
destroyAllResources Pool{..} = V.forM_ localPools $ purgeLocalPool destroy

modifyTVar_ :: TVar a -> (a -> a) -> STM ()
modifyTVar_ v f = readTVar v >>= \a -> writeTVar v $! f a

modError :: String -> String -> a
modError func msg =
    error $ "Data.Pool." ++ func ++ ": " ++ msg

# LANGUAGE RankNTypes #
 |
 Module:      Data.Pool
 Stability:   experimental
 Portability: portable

 A high-performance striped pooling abstraction for managing
 flexibly-sized collections of resources such as database
 connections.

 \"Striped\" means that a single 'Pool' consists of several
 sub-pools, each managed independently.  A single stripe is fine for
 many applications, and probably what you should choose by default.
 More stripes will lead to reduced contention in high-performance
 multicore applications, at a trade-off of causing the maximum
 number of simultaneous resources in use to grow.
 Don't do any async exception protection for older GHCs.
 | A single resource pool entry.
 ^ Time of last return.
 | A single striped pool.
 ^ Count of open entries (both idle and in use).
 ^ Idle entries.
 ^ empty value used to attach a finalizer to (internal)
 ^ Action for creating a new entry to add to the pool.
 ^ Action for destroying an entry that is now done with.
 ^ The number of stripes (distinct sub-pools) to maintain.
 ^ Amount of time for which an unused resource is kept alive.

 The elapsed time before closing may be a little longer than
 ^ Maximum number of resources to maintain per stripe.  The

 Requests for resources will block if this limit is reached on a
 single stripe, even if other stripes have idle resources
 available.
 ^ Per-capability resource pools.
 ^ empty value used to attach a finalizer to (internal)
 | Create a striped resource pool.

 Although the garbage collector will destroy all idle resources when
 the pool is garbage collected it's recommended to manually
 'destroyAllResources' when you're done with the pool so that the
 resources are freed up as soon as possible.
 ^ Action that creates a new resource.
 ^ Action that destroys an existing resource.
 ^ The number of stripes (distinct sub-pools) to maintain.
 ^ Amount of time for which an unused resource is kept open.

 The elapsed time before destroying a resource may be a little
 intervals.
 ^ Maximum number of resources to keep open per stripe.  The

 Requests for resources will block if this limit is reached on a
 single stripe, even if other stripes have idle resources
 available.
 TODO: Propose 'forkIOLabeledWithUnmask' for the base library.
 | Sparks off a new thread using 'forkIOWithUnmask' to run the given

 The implementation makes sure that asynchronous exceptions are
 masked until the given computation is executed. This ensures the
 thread will always be labeled which guarantees you can always

 Like 'forkIOWithUnmask', the given computation is given a function
 to unmask asynchronous exceptions. See the documentation of that
 function for the motivation of this.

 Returns the 'ThreadId' of the newly created thread.
 | Periodically go through all pools, closing any resources that
 have been left idle for too long.
 | Destroy all idle resources of the given 'LocalPool' and remove them from
 the pool.
 | Temporarily take a resource from a 'Pool', perform an action with
 it, and return it to the pool afterwards.

 * If the pool has an idle resource available, it is used
   immediately.

 * Otherwise, if the maximum number of resources has not yet been
   reached, a new resource is created and used.

 * If the maximum number of resources has been reached, this
   function blocks until a resource becomes available.

 If the action throws an exception of any type, the resource is
 destroyed, and not returned to the pool.

 It probably goes without saying that you should never manually
 destroy a pooled resource, as doing so will almost certainly cause
 a subsequent user (who expects the resource to be valid) to throw
 an exception.
# SPECIALIZE withResource :: Pool a -> (a -> IO b) -> IO b #
 | Take a resource from the pool, following the same results as
 'withResource'. Note that this function should be used with caution, as
 improper exception handling can lead to leaked resources.

 | Similar to 'withResource', but only performs the action if a resource could
 be taken from the pool /without blocking/. Otherwise, 'tryWithResource'
 Conversely, if a resource can be borrowed from the pool without blocking, the
 action is performed and it's result is returned, wrapped in a 'Just'.
# INLINABLE tryWithResource #
 | A non-blocking version of 'takeResource'. The 'tryTakeResource' function
 returns immediately, with 'Nothing' if the pool is exhausted, or @'Just' (a,
 'LocalPool' a)@ if a resource could be borrowed from the pool successfully.

 | Destroy a resource. Note that this will ignore any exceptions in the
 destroy function.
 | Return a resource to the given 'LocalPool'.
 | Destroy all resources in all stripes in the pool. Note that this
 will ignore any exceptions in the destroy function.

 This function is useful when you detect that all resources in the
 pool are broken. For example after a database has been restarted
 all connections opened before the restart will be broken. In that
 case it's better to close those connections so that 'takeResource'
 won't take a broken connection from the pool but will open a new
 connection instead.

 Another use-case for this function is that when you know you are
 done with the pool you can destroy all idle resources immediately
 instead of waiting on the garbage collector to destroy them, thus
 freeing up those resources sooner.
# LANGUAGE CPP , NamedFieldPuns , RecordWildCards , ScopedTypeVariables , RankNTypes , DeriveDataTypeable #

#if MIN_VERSION_monad_control(0,3,0)
# LANGUAGE FlexibleContexts #
#endif

#if !MIN_VERSION_base(4,3,0)
#endif

  Copyright :    ( c ) 2011 MailRank , Inc.
  License :      BSD3
  Maintainer :     <  > ,
                  <  >
module Data.Pool
    (
      Pool(idleTime, maxResources, numStripes)
    , LocalPool
    , createPool
    , withResource
    , takeResource
    , tryWithResource
    , tryTakeResource
    , destroyResource
    , putResource
    , destroyAllResources
    ) where

import Control.Applicative ((<$>))
import Control.Concurrent (ThreadId, forkIOWithUnmask, killThread, myThreadId, threadDelay)
import Control.Concurrent.STM
import Control.Exception (SomeException, onException, mask_)
import Control.Monad (forM_, forever, join, liftM3, unless, when)
import Data.Hashable (hash)
import Data.IORef (IORef, newIORef, mkWeakIORef)
import Data.List (partition)
import Data.Time.Clock (NominalDiffTime, UTCTime, diffUTCTime, getCurrentTime)
import Data.Typeable (Typeable)
import GHC.Conc.Sync (labelThread)
import qualified Control.Exception as E
import qualified Data.Vector as V

#if MIN_VERSION_monad_control(0,3,0)
import Control.Monad.Trans.Control (MonadBaseControl, control)
import Control.Monad.Base (liftBase)
#else
import Control.Monad.IO.Control (MonadControlIO, controlIO)
import Control.Monad.IO.Class (liftIO)
#define control controlIO
#define liftBase liftIO
#endif

#if MIN_VERSION_base(4,3,0)
import Control.Exception (mask)
#else
mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
mask f = f id
#endif

data Entry a = Entry {
      entry :: a
    , lastUse :: UTCTime
    }

data LocalPool a = LocalPool {
      inUse :: TVar Int
    , entries :: TVar [Entry a]
    , lfin :: IORef ()
    } deriving (Typeable)

data Pool a = Pool {
      create :: IO a
    , destroy :: a -> IO ()
    , numStripes :: Int
  The smallest acceptable value is 1 .
    , idleTime :: NominalDiffTime
  The smallest acceptable value is 0.5 seconds .
  requested , as the reaper thread wakes at 1 - second intervals .
    , maxResources :: Int
  smallest acceptable value is 1 .
    , localPools :: V.Vector (LocalPool a)
    , fin :: IORef ()
    } deriving (Typeable)

instance Show (Pool a) where
    show Pool{..} = "Pool {numStripes = " ++ show numStripes ++ ", " ++
                    "idleTime = " ++ show idleTime ++ ", " ++
                    "maxResources = " ++ show maxResources ++ "}"

createPool
    :: IO a
    -> (a -> IO ())
    -> Int
  The smallest acceptable value is 1 .
    -> NominalDiffTime
  The smallest acceptable value is 0.5 seconds .
  longer than requested , as the reaper thread wakes at 1 - second
    -> Int
  smallest acceptable value is 1 .
     -> IO (Pool a)
createPool create destroy numStripes idleTime maxResources = do
  when (numStripes < 1) $
    modError "pool " $ "invalid stripe count " ++ show numStripes
  when (idleTime < 0.5) $
    modError "pool " $ "invalid idle time " ++ show idleTime
  when (maxResources < 1) $
    modError "pool " $ "invalid maximum resource count " ++ show maxResources
  localPools <- V.replicateM numStripes $
                liftM3 LocalPool (newTVarIO 0) (newTVarIO []) (newIORef ())
  reaperId <- forkIOLabeledWithUnmask "resource-pool: reaper" $ \unmask ->
                unmask $ reaper destroy idleTime localPools
  fin <- newIORef ()
  let p = Pool {
            create
          , destroy
          , numStripes
          , idleTime
          , maxResources
          , localPools
          , fin
          }
  mkWeakIORef fin (killThread reaperId) >>
    V.mapM_ (\lp -> mkWeakIORef (lfin lp) (purgeLocalPool destroy lp)) localPools
  return p


  IO computation , but first labels the thread with the given label
  ( using '  ' ) .
  easily find it in the GHC event log .
forkIOLabeledWithUnmask :: String
                        -> ((forall a. IO a -> IO a) -> IO ())
                        -> IO ThreadId
forkIOLabeledWithUnmask label m = mask_ $ forkIOWithUnmask $ \unmask -> do
                                    tid <- myThreadId
                                    labelThread tid label
                                    m unmask

reaper :: (a -> IO ()) -> NominalDiffTime -> V.Vector (LocalPool a) -> IO ()
reaper destroy idleTime pools = forever $ do
  threadDelay (1 * 1000000)
  now <- getCurrentTime
  let isStale Entry{..} = now `diffUTCTime` lastUse > idleTime
  V.forM_ pools $ \LocalPool{..} -> do
    resources <- atomically $ do
      (stale,fresh) <- partition isStale <$> readTVar entries
      unless (null stale) $ do
        writeTVar entries fresh
        modifyTVar_ inUse (subtract (length stale))
      return (map entry stale)
    forM_ resources $ \resource -> do
      destroy resource `E.catch` \(_::SomeException) -> return ()

purgeLocalPool :: (a -> IO ()) -> LocalPool a -> IO ()
purgeLocalPool destroy LocalPool{..} = do
  resources <- atomically $ do
    idle <- swapTVar entries []
    modifyTVar_ inUse (subtract (length idle))
    return (map entry idle)
  forM_ resources $ \resource ->
    destroy resource `E.catch` \(_::SomeException) -> return ()

withResource ::
#if MIN_VERSION_monad_control(0,3,0)
    (MonadBaseControl IO m)
#else
    (MonadControlIO m)
#endif
  => Pool a -> (a -> m b) -> m b
withResource pool act = control $ \runInIO -> mask $ \restore -> do
  (resource, local) <- takeResource pool
  ret <- restore (runInIO (act resource)) `onException`
            destroyResource pool local resource
  putResource local resource
  return ret
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE withResource #
#endif

  This function returns both a resource and the @LocalPool@ it came from so
  that it may either be destroyed ( via '  ' ) or returned to the
  pool ( via '  ' ) .
takeResource :: Pool a -> IO (a, LocalPool a)
takeResource pool@Pool{..} = do
  local@LocalPool{..} <- getLocalPool pool
  resource <- liftBase . join . atomically $ do
    ents <- readTVar entries
    case ents of
      (Entry{..}:es) -> writeTVar entries es >> return (return entry)
      [] -> do
        used <- readTVar inUse
        when (used == maxResources) retry
        writeTVar inUse $! used + 1
        return $
          create `onException` atomically (modifyTVar_ inUse (subtract 1))
  return (resource, local)
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE takeResource #
#endif

  returns immediately with ' Nothing ' ( ie . the action function is /not/ called ) .
tryWithResource :: forall m a b.
#if MIN_VERSION_monad_control(0,3,0)
    (MonadBaseControl IO m)
#else
    (MonadControlIO m)
#endif
  => Pool a -> (a -> m b) -> m (Maybe b)
tryWithResource pool act = control $ \runInIO -> mask $ \restore -> do
  res <- tryTakeResource pool
  case res of
    Just (resource, local) -> do
      ret <- restore (runInIO (Just <$> act resource)) `onException`
                destroyResource pool local resource
      putResource local resource
      return ret
    Nothing -> restore . runInIO $ return (Nothing :: Maybe b)
#if __GLASGOW_HASKELL__ >= 700
#endif

tryTakeResource :: Pool a -> IO (Maybe (a, LocalPool a))
tryTakeResource pool@Pool{..} = do
  local@LocalPool{..} <- getLocalPool pool
  resource <- liftBase . join . atomically $ do
    ents <- readTVar entries
    case ents of
      (Entry{..}:es) -> writeTVar entries es >> return (return . Just $ entry)
      [] -> do
        used <- readTVar inUse
        if used == maxResources
          then return (return Nothing)
          else do
            writeTVar inUse $! used + 1
            return $ Just <$>
              create `onException` atomically (modifyTVar_ inUse (subtract 1))
  return $ (flip (,) local) <$> resource
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE tryTakeResource #
#endif

  | Get a ( Thread-)'LocalPool '
  Internal , just to not repeat code for ' takeResource ' and ' tryTakeResource '
getLocalPool :: Pool a -> IO (LocalPool a)
getLocalPool Pool{..} = do
  i <- liftBase $ ((`mod` numStripes) . hash) <$> myThreadId
  return $ localPools V.! i
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE getLocalPool #
#endif

destroyResource :: Pool a -> LocalPool a -> a -> IO ()
destroyResource Pool{..} LocalPool{..} resource = do
   destroy resource `E.catch` \(_::SomeException) -> return ()
   atomically (modifyTVar_ inUse (subtract 1))
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE destroyResource #
#endif

putResource :: LocalPool a -> a -> IO ()
putResource LocalPool{..} resource = do
    now <- getCurrentTime
    atomically $ modifyTVar_ entries (Entry resource now:)
#if __GLASGOW_HASKELL__ >= 700
# INLINABLE putResource #
#endif

destroyAllResources :: Pool a -> IO ()
destroyAllResources Pool{..} = V.forM_ localPools $ purgeLocalPool destroy

modifyTVar_ :: TVar a -> (a -> a) -> STM ()
modifyTVar_ v f = readTVar v >>= \a -> writeTVar v $! f a

modError :: String -> String -> a
modError func msg =
    error $ "Data.Pool." ++ func ++ ": " ++ msg
