[Haskell-cafe] Trouble with forkProcess

[Brandon Allbery]

On Fri, Feb 20, 2015 at 7:58 PM, Rob Leslie <rob at mars.org> wrote:

> My experience with forkProcess so far is that sometimes it works
> brilliantly, and other times it just deadlocks. I’m at a loss to understand
> what the problem is, but the deadlock seems to occur before the new process
> gets any control -- certainly before it has started to access any of the
> shared state.
>

I'd be using strace / truss / dtruss / platform equivalent to see what's
going on in that case.


> I’m aware of the giant warning in the forkProcess documentation, but I’m
> not sure if or how it could explain the behavior I’m seeing. Has anyone
> else used forkProcess successfully?
>

xmonad uses it extensively with no problems except an oddity on FreeBSD
that appears to be some nonportable behavior inside GHC's I/O manager. (The
person who ran into that was asked to submit a GHC bug report.) That said,
it doesn't do any significant processing in the child process; the most
common use is a double-fork with the grandchild doing executeFile. It is
conceivable that the I/O manager is not handling some use cases other than
that one properly; since most uses of forkProcess are followed by
executeFile, it's entirely possible that some bug has crept in that only
shows up in the few cases where that isn't done.

It’s possible too that I’m running into something related to the way I’m
> setting up a pipe to receive messages from the forked process. Here is what
> I’m doing:
>

The basic pipe handling looks correct to me, for what it's worth.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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