[GHC] #9519: Add a way to mask thread preemption via fired timer for a computation

[GHC]

#9519: Add a way to mask thread preemption via fired timer for a computation
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       Reporter:  abbradar           |                   Owner:  simonmar
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Runtime System     |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 This follows haskell-cafe discussion [http://www.haskell.org/pipermail
 /haskell-cafe/2014-August/115656.html]. In brief: some C libraries (for
 example, SDL) use thread-local variables for error state of the last call.
 Without some way to temporarily mask timer-based lightweight thread
 preemption on one OS thread, there is no safe way to use such libraries
 and retrieve errors aside from using a global MVar (which will block all
 calls, not just ones on the same OS thread). For "errno" variable from C
 standard library, this is handled explicitly in RTS by saving its state
 for each lightweight thread.

 So the proposal is: add new prims (something in lines of
 "maskPreemption#", "unmaskPreemption#" and "getPreemptionState#") which
 will make a computation not preemptable via fired timer and temporarily
 attached to one OS thread, and corresponding functions in
 Control.Concurrent. They should be able to be used together with
 "Control.Exception.mask" to also block async exceptions, if this is
 needed.

 (I'm sorry if I've misunderstood something or did something the wrong way;
 I don't know the internal workings of RTS and this is my first GHC bug
 report).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9519>
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