bug#14752: sort fails to fork() + execlp(compress_program) if overcommit limit is reached

[Petros Aggelatos]

Following up on this issue it seems that posix_spawn() cannot be used,
at least not trivially. The issue is that posix_spawn decides if it will
spawn the new process with fork() of vfork() based on some conditions,
one of which is if file_actions is NULL.

http://repo.or.cz/w/glibc.git/blob/HEAD:/sysdeps/posix/spawni.c#l105

For the temp compression to work it is nessesary to pass the file
descriptors of the pipe from the parent to the child. I'm not sure how
to proceed, I found this relevant thread that proposes to relax the
restrictions and use vfork more often:

http://sourceware.org/bugzilla/show_bug.cgi?id=10354

And this thread http://sourceware.org/ml/libc-help/2010-10/msg00001.html
of someone having the same problem and proposing two solutions. Solution
#1 seems to me that adds a lot of complexity. Solution #2 is hacky, and
I'm not aware if there are unwanted sideffects of using the enviroment
to transfer the FDs.