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{-# LANGUAGE NamedFieldPuns, RecordWildCards, ScopedTypeVariables #-}

-- |
-- Module: Data.Pool
-- Copyright: (c) 2011 MailRank, Inc.
-- License: BSD3
-- Maintainer: Bryan O'Sullivan <bos@mailrank.com>
-- 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 stripe size of 1 is fine
-- for many applications, and probably what you should choose by
-- default. Larger stripe sizes 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)
    , createPool
    , withResource
    ) where

import Control.Applicative ((<$>))
import Control.Concurrent (forkIO, killThread, myThreadId, threadDelay)
import Control.Concurrent.STM
import Control.Exception (SomeException, catch, onException)
import Control.Monad (forM_, forever, join, liftM2, unless, when)
import Data.Hashable (hash)
import Data.List (partition)
import Data.Time.Clock (NominalDiffTime, UTCTime, diffUTCTime, getCurrentTime)
import Prelude hiding (catch)
import System.Mem.Weak (addFinalizer)
import qualified Data.Vector as V

-- | 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.
    }

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
    -- ^ Stripe count. The number of 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 2-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.
    }

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

createPool
    :: IO a
    -- ^ Action that creates a new resource.
    -> (a -> IO ())
    -- ^ Action that destroys an existing resource.
    -> Int
    -- ^ Stripe count. The number of 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 <- atomically . V.replicateM numStripes $
                liftM2 LocalPool (newTVar 0) (newTVar [])
  reaperId <- forkIO $ reaper destroy idleTime localPools
  let p = Pool {
            create
          , destroy
          , numStripes
          , idleTime
          , maxResources
          , localPools
          }
  addFinalizer p $ killThread reaperId
  return p

-- | 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
      -- debug "reaper" "destroying idle resource"
      destroy resource `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 :: Pool a -> (a -> IO b) -> IO b
withResource Pool{..} act = do
  i <- ((`mod` numStripes) . hash) <$> myThreadId
  let LocalPool{..} = localPools V.! i
  resource <- join . atomically $ do
    ents <- readTVar entries
    case ents of
      (Entry{..}:es) -> writeTVar entries es >> return create
      [] -> do
        used <- readTVar inUse
        when (used == maxResources) retry
        writeTVar inUse $! used + 1
        return $ do
          create `onException` atomically (modifyTVar_ inUse (subtract 1))
  ret <- act resource `onException` do
           destroy resource `catch` \(_::SomeException) -> return ()
           atomically (modifyTVar_ inUse (subtract 1))
  now <- getCurrentTime
  atomically $ modifyTVar_ entries (Entry resource now:)
  return ret

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
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