How can I know how much to read from a subprocess

[spam.noam at gmail.com]

Ok, I could have researched this before posting, but here's an
explanation how to do it with twisted:

http://unpythonic.blogspot.com/2007/08/spawning-subprocess-with-pygtk-using.html

It seems that another solution is gobject.io_add_watch, but I don't
see how it tells me how much I can read from the file - if I don't
know that, I won't know the argument to give to the read() method in
order to get all the data:

http://www.pygtk.org/docs/pygobject/gobject-functions.html#function-gobject--io-add-watch

Noam