{-# LANGUAGE DeriveDataTypeable #-} -- | -- Module : Control.Concurrent.MSemN -- Copyright : (c) Chris Kuklewicz 2011 -- License : 3 clause BSD-style (see the file LICENSE) -- -- Maintainer : haskell@list.mightyreason.com -- Stability : experimental -- Portability : non-portable (concurrency) -- -- Quantity semaphores in which each thread may wait for an arbitrary amount. This modules is -- intended to improve on "Control.Concurrent.QSemN". -- -- This semaphore gracefully handles threads which die while blocked waiting for quantity. The -- fairness guarantee is that blocked threads are FIFO. An early thread waiting for a large -- quantity will prevent a later thread waiting for a small quantity from jumping the queue. -- -- If 'with' is used to guard a critical section then no quantity of the semaphore will be lost -- if the activity throws an exception. -- module Control.Concurrent.MSemN (MSemN ,new ,with ,wait ,signal ,withF ,waitF ,signalF ,peekAvail ) where import Control.Concurrent.MVar(MVar,withMVar,modifyMVar,modifyMVar_,newMVar,newEmptyMVar,putMVar,takeMVar,tryTakeMVar) import Control.Exception(bracket,uninterruptibleMask_,onException,evaluate,mask_) import Data.Typeable(Typeable) {- The only MVars allocated are the three created be 'new'. Their three roles are 1) to have a FIFO queue of waiters 2) for the head waiter to block on 3) to protect the quantity state of the semaphore and the head waiter subtle design notes: with, wait, and signal pattern match the quantity against 0 which has two effect: it avoids locking in the easy case and it ensures strict evaluation of the quantity before any locks are taken. Originally withF, waitF, and signal did not strictly evalaute the function they are passed before locks are taken because there is no real point since the function may throw an error when computing the size. But then I realized forcing 'f' might run forever with the locks held and I could move this particular hang outside the locks by first evaluating 'f'. -} -- MS has an invariant that "maybe True (> avail) headWants" is always True. data MS = MS { avail :: !Integer -- ^ This is the quantity available to be taken from the semaphore. Often updated. , headWants :: !(Maybe Integer) -- ^ If there is waiter then this is Just the amount being waited for. Often updated. , headWait :: MVar () -- ^ The head of the waiter queue blocks on headWait. Never updated. } deriving (Eq,Typeable) -- | A 'MSemN' is a quantity semaphore, in which the available quantity may be signalled or -- waited for in arbitrary amounts. data MSemN = MSemN { mSem :: !(MVar MS) -- ^ Used to lock access to state of semaphore quantity. Never updated. , queueWait :: !(MVar ()) -- ^ Used as FIFO queue for waiter, held by head of queue. Never updated. } deriving (Eq,Typeable) -- |'new' allows positive, zero, and negative initial values. The initial value is forced here to -- better localize errors. new :: Integer -> IO MSemN new initial = do newHeadWait <- newEmptyMVar newQueueWait <- newMVar () newMS <- newMVar $! (MS { avail = initial , headWants = Nothing , headWait = newHeadWait }) return (MSemN { mSem = newMS , queueWait = newQueueWait }) -- | 'with' takes a quantity of the semaphore to take and hold while performing the provided -- operation. 'with' ensures the quantity of the sempahore cannot be lost if there are exceptions. -- This uses 'bracket' to ensure 'wait' and 'signal' get called correctly. with :: MSemN -> Integer -> IO a -> IO a with _ 0 = id with m wanted = bracket (wait m wanted) (\() -> signal m wanted) . const -- | 'withF' takes a pure function and an operation. The pure function converts the available -- quantity to a pair of the wanted quantity and a returned value. The operation takes the result -- of the pure function. 'withF' ensures the quantity of the sempahore cannot be lost if there -- are exceptions. This uses 'bracket' to ensure 'waitF' and 'signal' get called correctly. -- -- Note: A long running pure function will block all other access to the 'MSemN' while it is -- evaluated. withF :: MSemN -> (Integer -> (Integer,b)) -> ((Integer,b) -> IO a) -> IO a withF m f = seq f $ bracket (waitF m f) (\(wanted,_) -> signal m wanted) -- |'wait' allow positive, zero, and negative wanted values. Waiters may block, and will be handled -- fairly in FIFO order. -- -- If 'wait' returns without interruption then it left the 'MSemN' with a remaining quantity that was -- greater than or equal to zero. If 'wait' is interrupted then no quantity is lost. If 'wait' -- returns without interruption then it is known that each earlier waiter has definitely either been -- interrupted or has retured without interruption. wait :: MSemN -> Integer -> IO () wait _ 0 = return () wait m wanted = fmap snd $ waitF m (const (wanted,())) -- | 'waitWith' takes the 'MSemN' and a pure function that takes the available quantity and computes the -- amount wanted and a second value. The value wanted is stricly evaluated but the second value is -- returned lazily. -- -- 'waitF' allow positive, zero, and negative wanted values. Waiters may block, and will be handled -- fairly in FIFO order. -- -- If 'waitF' returns without interruption then it left the 'MSemN' with a remaining quantity that was -- greater than or equal to zero. If 'waitF' or the provided function are interrupted then no -- quantity is lost. If 'waitF' returns without interruption then it is known that each previous -- waiter has each definitely either been interrupted or has retured without interruption. -- -- Note: A long running pure function will block all other access to the 'MSemN' while it is -- evaluated. waitF :: MSemN -> (Integer -> (Integer,b)) -> IO (Integer,b) waitF (MSemN sem advance) f = seq f $ mask_ $ withMVar advance $ \ () -> do (out@(wanted,_),todo) <- modifyMVar sem $ \ m -> do let outVal@(wantedVal,_) = f (avail m) -- assert that headDown is Nothing via new or signal or cleanup -- wantedVal gets forced by the (<=) condition here: if wantedVal <= avail m then do let avail'down = avail m - wantedVal m' <- evaluate $ m { avail = avail'down } return (m', (outVal,Nothing)) else do m' <- evaluate $ m { headWants = Just wantedVal } return (m', (outVal,Just (headWait m))) -- mask_ is needed above because either (Just wantedVal) may be set here and this means we need to -- get the `onException` setup without being interrupted, or avail'down was set and we must finish -- 'waitF' without being interrupted so that a 'bracket' can ensure a matching 'signal' can -- protect the returned quantity. case todo of Nothing -> return () Just hw -> do let cleanup = uninterruptibleMask_ $ modifyMVar_ sem $ \m -> do mStale <- tryTakeMVar (headWait m) let avail' = avail m + maybe 0 (const wanted) mStale evaluate $ m {avail = avail', headWants = Nothing} takeMVar hw `onException` cleanup -- may not block if a 'signal' has already arrived. return out -- |'signal' allows positive, zero, and negative values, thus this is also way to remove quantity -- that skips any threads in the 'wait'/'waitF' queue. If the new total is greater than the next -- value being waited for (if present) then the first waiter is woken. If there are queued waiters -- then the next one will wake after a waiter has proceeded and notice the remaining value; thus a -- single 'signal' may result in several waiters obtaining values. Waking waiting threads is -- asynchronous. -- -- 'signal' may block, but it cannot be interrupted, which allows it to dependably restore value to -- the 'MSemN'. All 'signal', 'signalF', 'peekAvail', and the head waiter may momentarily block in a -- fair FIFO manner. signal :: MSemN -> Integer -> IO () signal _ 0 = return () signal m size = uninterruptibleMask_ $ fmap snd $ signalF m (const (size,())) -- | Instead of providing a fixed change to the available quantity, 'signalF' applies a provided -- pure function to the available quantity to compute the change and a second value. The -- requested change is stricly evaluated but the second value is returned lazily. If the new total is -- greater than the next value being waited for then the first waiter is woken. If there are queued -- waiters then the next one will wake after a waiter has proceeded and notice the remaining value; -- thus a single 'signalF' may result in several waiters obtaining values. Waking waiting threads -- is asynchronous. -- -- 'signalF' may block, and it can be safely interrupted. If the provided function throws an error -- or is interrupted then it leaves the 'MSemN' unchanged. All 'signal', 'signalF', 'peekAvail', and -- the head waiter may momentarily block in a fair FIFO manner. -- -- Note: A long running pure function will block all other access to the 'MSemN' while it is -- evaluated. signalF :: MSemN -> (Integer -> (Integer,b)) -> IO (Integer,b) signalF (MSemN sem _) f = seq f $ modifyMVar sem $ \ m -> do let out@(size,_) = f (avail m) avail' <- evaluate $ avail m + size -- this forces 'size' case headWants m of Just wanted | wanted <= avail' -> do let avail'down = avail' - wanted m' <- evaluate $ m { avail = avail'down, headWants = Nothing } putMVar (headWait m') () -- will always succeed without blocking return (m',out) _ -> do m' <- evaluate $ m { avail = avail' } return (m',out) -- | 'peekAvail' skips the queue of any blocked 'wait' and 'waitF' threads, but may momentarily -- block on 'signal', 'signalF', other 'peekAvail', and the head waiter. This returns the amount of -- value available to be taken. Using this value without producing unwanted race conditions is left -- up to the programmer. -- -- 'peekAvail' is an optimized form of \"signalF m (\x -> (0,x))\". -- -- A version of 'peekAvail' that joins the FIFO queue of 'wait' and 'waitF' can be acheived by -- \"waitF m (\x -> (0,x))\" peekAvail :: MSemN -> IO Integer peekAvail (MSemN sem _) = withMVar sem (return . avail)