[plt-scheme] mzscheme, open-input-file and threads

> It seems that when `open-input-file' blocks, the other running threads
> starve.  In particular, `open-input-file' will block when the file it's
> trying to open is a pipe. 

Yes - this is a known bug.

I hadn't figured out how to make it work right before, specifically in
the case of `open-output-file' where no readers are ready. But as I
started to request help just now, I figured it out. So this will be
fixed in the next version.

Matthew