Treating POSIX signals as exceptions?

[Simon Marlow]

> I was wondering why System.Posix.Signals is as it is, and 
> whether it could
> be rewritten to use exceptions, which seems like the obvious 
> way to handle
> signals.  Of course, that requires asynchronous exceptions, but that
> doesn't seem like a severe problem.
> 
> What I currently do is to use the following function
> 
> withSig :: Signal -> IO a -> IO a -> IO a
> withSig s handler job = do
>     id <- myThreadId
>     installHandler s (Catch $ throwDynTo id s) Nothing
>     job `catchDyn` catcher
>         where catcher s' | s' == s = handler
>               catcher s' = throwDyn s'

Hmm, there is clearly a choice between having a signal spawn a new
thread or raise an asynchronous exception, and it's not at all clear
which is best.

But, since we can implement either using the other one, it doesn't
really matter too much which one is primitive.  I've attached a more
robust implementation of your withSig.  This one:

 - doesn't suffer from race conditions around the point at which
   the signal handler is installed

 - it uses a datatype for the signal exception (Typeable can be
   derived these days)

 - it allows several threads to request notification of the same
   signal

 - it doesn't support nesting use of withSig with the same signal.
   (that could be added, but I'm not sure it's worth it).


> Is there any reason all of System.Posix.Signals couldn't be 
> done in this
> manner? Have the RTS always add a handler for each signal, which just
> throws an exception.  Off the top of my head, all we'd need 
> is one function
> 
> acceptSignals :: IO () -- tells RTS to send signal exceptions to this
>                        -- thread.

You can implement this too using the module I attached.

I guess we should provide this in the POSIX library.  Would you like to
play around with the API and submit it for inclusion?

Cheers,
	Simon


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