Proposal: Add Control.Concurrent.forkIOWithUnmask, deprecate forkIOUnmasked

[Simon Marlow]

Ticket:

   http://hackage.haskell.org/trac/ghc/ticket/4858

We added forkIOUnmasked as part of the new asynchronous exceptions API 
in base 4.3.0.0 (GHC 7.0.1). Unfortunately, forkIOUnmasked isn't very 
useful: the computation in the child thread is executed in the unmasked 
state, so the child doesn't have a way to reliably set up an exception 
handler before an asynchronous exception is raised.

This proposal is to replace forkIOUnmasked with forkIOWithUnmask:

-- | Like 'forkIO', but the child thread is passed a function that can
-- be used to unmask asynchronous exceptions.  This function is
-- typically used in the following way
--
-- >  ... mask_ $ forkIOWithUnmask $ \unmask ->
-- >                 catch (unmask ...) handler
--
-- so that the exception handler in the child thread is established
-- with asynchronous exceptions masked, meanwhile the main body of
-- the child thread is executed in the unmasked state.
--
-- Note that the unmask function passed to the child thread should
-- only be used in that thread; the behaviour is undefined if it is
-- invoked in a different thread.
--
forkIOWithUnmask :: ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
forkIOWithUnmask io = forkIO (io unsafeUnmask)

forkIOUnmasked will be deprecated.

Above is almost the entire implementation, hence I didn't bother 
attaching a patch to the ticket.

Some discussion leading up to this can be found in 
http://hackage.haskell.org/trac/ghc/ticket/3837.

Discussion period: 3 weeks (until 12 Jan 2011)

Cheers,
	Simon