[commit: base] ghc-7.4: Sealed writeChan with mask_ to prevent a theoretical bug (b24a359)

[Paolo Capriotti]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/base

On branch  : ghc-7.4

http://hackage.haskell.org/trac/ghc/changeset/b24a3598764592ae341f3e90d7223ae04bce931b

>---------------------------------------------------------------

commit b24a3598764592ae341f3e90d7223ae04bce931b
Author: Joey Adams <joeyadams3.14159 at gmail.com>
Date:   Sun Feb 12 16:08:07 2012 -0500

    Sealed writeChan with mask_ to prevent a theoretical bug
    
    MERGED from commit 48df85c45e50d18a1e34f2db82a58e74dea6bbef

>---------------------------------------------------------------

 Control/Concurrent/Chan.hs |   18 +++++++++++++++---
 1 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/Control/Concurrent/Chan.hs b/Control/Concurrent/Chan.hs
index 23b21bb..2b84503 100644
--- a/Control/Concurrent/Chan.hs
+++ b/Control/Concurrent/Chan.hs
@@ -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
 
 INSTANCE_TYPEABLE1(Chan,chanTc,"Chan")
@@ -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