ghci and ghc -threaded broken with pipes & forking

[Simon Marlow]

John Goerzen wrote:
> On 2007-03-02, Simon Marlow <simonmarhaskell at gmail.com> wrote:
> 
>> Regarding your shell: I would suggest trying forkIO for the Haskell "processes" 
>> (not forkOS unless for some reason you really need another OS thread).  However, 
>>   I can imagine that it might be hard to implement job control and signal 
>> handling in that system.  You could also consider using 
>> System.Process.runInteractiveProcess, for portability.
> 
> Thinking about forkIO seems that it could be fairly complex to just drop
> in.  The problem lies around file descriptors.  When you fork off a new
> process, and then close file descriptors in the parent, they stay open
> in the child, and vice versa.  Proper management of file descriptors is
> a critical part of a shell, and it's vital to close the proper set of
> FDs at the proper time in the parent and the child, or else bad things
> like pipes never closing could easily lead to deadlock.
> 
> Of course, it is possible to work around this, but I fear that it could
> make the program very complex.

Admittedly I haven't completely thought this through, but my intuition was that 
you would be able to use forkIO at a higher level.  That is, instead of just 
trying to replace forkProcess with forkIO, you replace forkProcess + pipes + FD 
handling with forkIO + lazy streams, for Haskell processes.

So the way in which data is fed between processes depends on the process: 
Haskell processes talk to each other using lazy streams, external processes talk 
to each other over pipes, and at a boundary between the two you need a pipe with 
another Haskell thread to feed the pipe from a lazy stream, or vice-versa.

Cheers,
	Simon