[GHC] #13995: provide hook for 'setNumCapabilities'

[GHC]

#13995: provide hook for 'setNumCapabilities'
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           Reporter:  winter         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This request if straight forward: i want to run some code when user call
 `setNumCapabilities`.

 I'm making a
 [https://github.com/winterland1989/stdio/blob/newFD/System/LowResTimer.hs
 stripped timing wheel implementation], and the problem is obvious: we keep
 an array of timer managers just like I/O managers, but we have to ask user
 to call something like `lowResTimerManagerCapabilitiesChanged` so that we
 can re-allocate new array, which is unreasonable.

 The same problem exists in some stripped resource pool implementation, so
 i proposed

 {{{
 -- this is not exported
 capabilityChangedHook :: IORef (IO ())
 capabilityChangedHook = unsafePerformIO $ newIORef (return ())

 -- this is exported to user
 addCapabilityChangedHook :: IO () -> IO ()
 addCapabilityChangedHook f = do
     atomicModifyIORef' capabilityChangedHook $ \ g -> (g >> f, ())

 setNumCapabilities :: ...
 setNumCapabilities = ...
 ...
    -- somewhere inside setNumCapabilities
    hook <- readIORef capabilityChangedHook
    hook
 ...
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13995>
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