Testing on Debian with latest trunk:
- the proposed example code works very well (I get all data).
- I added "subprocess._has_poll = False", and some (sometimes all) data
is lost. It seems that select() will return stdin even if it is not
writable.

On Windows of course, communicate() uses a blocking write, and always fail.

The proposed patch ignore the errors when EPIPE is raised, and simply
stops writing.