[GHC] #6153: writeChan not properly protecting again async exceptions

[GHC]

#6153: writeChan not properly protecting again async exceptions
------------------------------+---------------------------------------------
 Reporter:  klao              |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  libraries/base  
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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 In the current code of writeChan from Control.Concurrent.Chan:

 {{{
 writeChan (Chan _ writeVar) val = do
   new_hole <- newEmptyMVar
   modifyMVar_ writeVar $ \old_hole -> do
     putMVar old_hole (ChItem val new_hole)
     return new_hole
 }}}

 if an async exception arrives "between" putMVar old_hole and return, the
 channel will be left in inconsistent state: the writeVar will be reverted
 to its original value, but old_hole would remain filled in. Thus all
 subsequent writers will block on this Chan indefinitely.

 The proper solution is to just mask the whole operation. And it's OK, as
 'putMVar old_hole' cannot ever block. I attach a patch for this.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6153>
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