Re: Checkpassword and FD3 - the problem

"Jason R. Mastaler" <[email protected]> wrote in message
news:[email protected]...
> Felix von Leitner <[email protected]> writes:
>
> > You could also try setting the flag yourself.  I don't know how it's
> > done in python, but in C it's
> >
> >   fcntl(fd,F_SETFD,FD_CLOEXEC);
>
> Python has a fcntl module:
>
> http://www.python.org/doc/current/lib/module-fcntl.html

Sadly this won't work... it looks like it's not that the FD is not being
closed on exec - Quite the contrary.  The pipe is closed once the
checkpassword program is actually execed.  However the real problem is that
a new pipe is created in every child every time you fork - and this pipe
cannot be altered by the user program.

This may well be a bug in the threading library - I'll have to figure out
who wrote it first and look into that.

But here's an interesting solution:

- Fork.
- In this child, create a pipe
- Close FD's 1 and 2
- Fork again, creating the grandchild
- This strange pipe now lives in FD 1 and 2, leaving FD3 free for the
grandchild to connect one end of that pipe to.
- Of course close the ends of the pipe the child and grandchild won't be
needing.
- Exec the checkpassword program in the grandchild and have the child write
the string into the pipe and wait for the grandchild to exit
- The child returns the grandchild's exit status as its own exit status
- The parent now has the result we need.

This is almost as kludgey as using /bin/sh for redirection or wrapping
python in some sort of environment where FD#3 is already open, but at least
it can be done purely in python :)

Jim