Commit 5c1c24f8 authored by joeyadams's avatar joeyadams Committed by Simon Marlow
Browse files

Sealed writeChan with mask_ to prevent a theoretical bug

parent 533bcf04
......@@ -40,6 +40,7 @@ import Prelude
import System.IO.Unsafe ( unsafeInterleaveIO )
import Control.Concurrent.MVar
import Control.Exception (mask_)
import Data.Typeable
#include "Typeable.h"
......@@ -51,7 +52,7 @@ import Data.Typeable
-- |'Chan' is an abstract type representing an unbounded FIFO channel.
data Chan a
= Chan (MVar (Stream a))
(MVar (Stream a))
(MVar (Stream a)) -- Invariant: the Stream a is always an empty MVar
deriving Eq
......@@ -83,9 +84,20 @@ newChan = do
writeChan :: Chan a -> a -> IO ()
writeChan (Chan _ writeVar) val = do
new_hole <- newEmptyMVar
modifyMVar_ writeVar $ \old_hole -> do
mask_ $ do
old_hole <- takeMVar writeVar
putMVar old_hole (ChItem val new_hole)
return new_hole
putMVar writeVar new_hole
-- The reason we don't simply do this:
-- modifyMVar_ writeVar $ \old_hole -> do
-- putMVar old_hole (ChItem val new_hole)
-- return new_hole
-- is because if an asynchronous exception is received after the 'putMVar'
-- completes and before modifyMVar_ installs the new value, it will set the
-- Chan's write end to a filled hole.
-- |Read the next value from the 'Chan'.
readChan :: Chan a -> IO a
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