X startup and authorization

[tom fogal]

Hi Alan, thanks for your reply.  I hit a snag + worked around it, and
so I wanted to report back to the list for posterity.

Comments/suggestions of course still welcome.

Alan Coopersmith <alan.coopersmith at oracle.com> writes:
> tom fogal wrote:
> > The first issue is startup: we fork and then exec `xinit' in the
> > child; the parent sets the appropriate DISPLAY and renders into
> > it.  The issue we have is that the parent does not know when the
> > X server is "ready" for rendering.  Our current "solution" is to
> > sleep a bit, which obviously isn't great.  How can I tell when a
> > server has initialized?
>
> The xserver notifies its parent - xinit/xdm/etc. - via a signal, at
> which point they start clients - presumably you'd put something in
> the .xinitrc to in turn signal your parent process that called xinit.
>
> You could of course take xinit out of the middle and exec the X
> server from your software so it gets the signal or error return
> directly - you can see from the source it's not that complicated.

FWIW, I gave this a try, and spent a while banging my head against the
desk trying to figure out what was wrong.  It seems like my distro
inserts an extra layer -- Xwrapper.config(5) -- which comes between
/usr/bin/X and actually getting an X server.  I imagine this wrapper
was the one getting the signal, because my code would always lock up at
the sigsuspend and just wait forever.

*sigh*.  I imagine other distros do something similar, and I of course
don't want it to be "broken by default" from a user's point of view.
So I went back to an xinit approach, specifically:

  xinit sleep 28800 -- display ...

The `sleep' client is for a couple reasons: first, if my process dies
(segfaults or something), then there's an as-root X server which in
some cases can be impossible to kill; at least it dies *eventually*
in this approach. Secondly, I really don't *want* ~/.xinitrc to get
sourced -- it's a source of unanticipated behavior.

To detect errors, I've got a loop checking XOpenDisplay.  That alone
will never let you know that the X server failed startup, though, so I
save the PID and do a waitpid(... WNOHANG) in the loop and make sure
the X server hasn't died yet.

Haven't gotten to the auth stuff yet, but your directions/pointers
seemed straightforward -- thanks.

-tom