[GHC] #7216: Compositional blocking on file descriptors

[GHC]

#7216: Compositional blocking on file descriptors
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    Reporter:  AndreasVoellmy    |       Owner:                  
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:  7.8.1           
   Component:  libraries/base    |     Version:  7.4.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by AndreasVoellmy):

 The last patch addresses the previous comment by igloo.  I made the
 threadWaitSTM return an IO action that unregisters interest in the file
 descriptor.  By returning an `IO ()` value we avoid having to export the
 `FdKey` type and `unregsterFd` functions.

 I also added some haddock comments to `threadWaitReadSTM` and
 `threadWaitWriteSTM`.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7216#comment:6>
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