[Haskell-cafe] chroot a single thread

[Jeremy Shaw]

Hello,

I am working on an application where I would like to chroot a thread,
but I am not seeing a way to do it.

I already have code which can run an IO action inside a chroot. The
type signature is:

fchroot :: FilePath -> IO a -> IO a

The first argument is the new root, the second argument is the action
to run with that root. 

You can see the implementation here if you are curious:

http://src.seereason.com/build-env/Chroot.hs

The problem with that function is that chroot affects the root of the
whole process. In a single-threaded program this is (possibly) ok,
because the original root is restored after the IO action completes
(though with unsafeInterleaveIO, perhaps bad stuff will happen). In a
multithreaded program it can be disasterous.

As far as I know, the only solution would be to implement a function
like fchroot which looks more like:

pchroot :: FilePath -> IO () -> IO ExitCode

Here the IO action would be forked off and run in a whole new process
so that changing its root does not affect other 'threads'. Of course,
you also can't use any of the nifty Haskell intra-thread communication
stuff. Basically you can pass some values in and get back an exit
code.

However, I am not even sure how to implement a useful version of
pchroot. In theory, I can just use forkProcess, but, the Giant Warning
for forkProcess indicates that this will not be very useful in
practice:

    forkProcess comes with a giant warning: since any other running
    threads are not copied into the child process, it's easy to go
    wrong: e.g. by accessing some shared resource that was held by
    another thread in the parent. Another example is the I/O manager
    thread: since the I/O manager isn't running in the child,
    attempting to do any Handle-based I/O will deadlock.

Anyone have an ideas ?

Thanks!
- jeremy