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116 lines
3.7 KiB
Haskell
Executable File
116 lines
3.7 KiB
Haskell
Executable File
{-# LANGUAGE ScopedTypeVariables #-}
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{-
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The auto-update package:
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https://hackage.haskell.org/package/auto-update
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Users found a possible deadlock and livelock:
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https://www.reddit.com/r/haskell/comments/2i5d7m/updating_autoupdate/
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This is the code from Control.AutoUpdate modified to use the
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@MonadConc@ abstraction, with tests added to verify that the issues
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identified are caught.. The original code is available under the MIT
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license, which is reproduced below.
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- - - - -
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Copyright (c) 2014 Michael Snoyman
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Permission is hereby granted, free of charge, to any person obtaining
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a copy of this software and associated documentation files (the
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"Software"), to deal in the Software without restriction, including
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without limitation the rights to use, copy, modify, merge, publish,
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distribute, sublicense, and/or sell copies of the Software, and to
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permit persons to whom the Software is furnished to do so, subject to
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the following conditions:
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The above copyright notice and this permission notice shall be included
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in all copies or substantial portions of the Software.
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
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CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
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TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
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SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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-}
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module Examples.AutoUpdate where
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import Control.Exception (SomeException)
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import Control.Monad
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import Control.Monad.Conc.Class
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-- test imports
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import Common
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import Test.DejaFu (Failure(..), gives)
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tests :: [TestTree]
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tests = toTestList
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[ T "deadlocks" deadlocks (gives [Left Deadlock, Right ()])
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, T "nondeterministic" nondeterministic (gives [Left Deadlock, Right 0, Right 1])
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]
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-- This exhibits a deadlock with no preemptions.
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deadlocks :: MonadConc m => m ()
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deadlocks = join (mkAutoUpdate defaultUpdateSettings)
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-- This exhibits nondeterminism with three preemptions. However, as
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-- the program explicitly yields, the bounds don't need changing.
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nondeterministic :: forall m. MonadConc m => m Int
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nondeterministic = do
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var <- newIORef 0
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let settings = (defaultUpdateSettings :: UpdateSettings m ())
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{ updateAction = atomicModifyIORef var (\x -> (x+1, x)) }
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auto <- mkAutoUpdate settings
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void auto
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auto
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-------------------------------------------------------------------------------
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data UpdateSettings m a = UpdateSettings
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{ updateFreq :: Int
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, updateSpawnThreshold :: Int
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, updateAction :: m a
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}
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defaultUpdateSettings :: MonadConc m => UpdateSettings m ()
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defaultUpdateSettings = UpdateSettings
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{ updateFreq = 1000000
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, updateSpawnThreshold = 3
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, updateAction = pure ()
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}
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mkAutoUpdate :: MonadConc m => UpdateSettings m a -> m (m a)
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mkAutoUpdate us = do
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currRef <- newIORef Nothing
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needsRunning <- newEmptyMVar
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lastValue <- newEmptyMVar
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void $ fork $ forever $ do
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takeMVar needsRunning
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a <- catchSome $ updateAction us
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writeIORef currRef $ Just a
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void $ tryTakeMVar lastValue
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putMVar lastValue a
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threadDelay $ updateFreq us
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writeIORef currRef Nothing
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void $ takeMVar lastValue
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pure $ do
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mval <- readIORef currRef
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case mval of
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Just val -> pure val
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Nothing -> do
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void $ tryPutMVar needsRunning ()
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readMVar lastValue
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catchSome :: MonadConc m => m a -> m a
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catchSome act = catch act $
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\e -> throw (e :: SomeException)
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