[GHC] #1048: Add forkIO that starts in 'block' mode

[GHC]

#1048: Add forkIO that starts in 'block' mode
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    Reporter:  ChrisKuklewicz   |       Owner:             
        Type:  feature request  |      Status:  new        
    Priority:  normal           |   Milestone:             
   Component:  libraries/base   |     Version:  6.6        
    Severity:  normal           |    Keywords:  concurrency
  Difficulty:  Easy (1 hr)      |    Testcase:             
Architecture:  Multiple         |          Os:  Multiple   
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The current forkIO starts in 'unblock' mode and the spawned thread can
 receive asynchronous exceptions (and die) before the passed (IO ()) action
 can start to run.

 Even if the user tried (forkIO (block io)) the new thread can be killed
 before entering the scope of block.

 Proposal: Add forkBlockedIO that spawns the new thread in a 'block' state.
 Alternative: Add forkInheritIO that spawns the new thread in the same
 block or unblock state as the parent thread is at the forkInheritIO call.
 Thus (forkBlockedIO = block . forkInheritIO) and (forkIO = unblock .
 forkInheritIO).

 Work around: At the moment a fresh MVar can immediately be used by the
 parent to wait until the child has signaled that it is executing withing
 the scope of the block (or exception handler, etc).

 This ought to also be extended to forkBlockedOS or forkInheritOS.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1048>
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