Re: g_spawn_async_with_pipes is not thread safe

[Alan Coopersmith <alan coopersmith oracle com>]

On 06/ 3/16 08:31 AM, Chris Vine wrote:
> On Fri, 3 Jun 2016 12:30:04 +0100
> Simon McVittie <simon mcvittie collabora co uk> wrote:
> > There's a gap here between the theory (GLib mostly has only POSIX
> > requirements) and the practice (glibc malloc is designed to allow
> > things POSIX doesn't, and this useful functionality would be
> > difficult to achieve without that). In principle GLib could call
> > opendir() on /proc/$pid before forking, then read it in the child,
> > but readdir() isn't async-signal-safe either. Doing the reading in
> > the parent isn't a solution, because the parent is potentially
> > multi-threaded, so it could be opening and closing file descriptors
> > in other threads (racing with the readdir()).
> >
> > In principle the parent could read the new child's fd table and write
> > a list of fds into a pipe, for the child to close them all or set
> > them all close-on-exec, while the child waits for EOF on that pipe
> > before proceeding with the close and exec operations... but that
> > seems fairly horrible.
> >
> > I think what we really want here is *BSD closefrom(), which at least
> > FreeBSD documents as async-signal-safe. Unfortunately, Linux doesn't
> > have that system call. libdbus would have the same issue on platforms
> > that don't have closefrom().
>
> For the g_spawn* family of functions, glib has in effect adopted (on
> linux platforms) an unspoken glibc dependency.  I do not know what the
> position is on windows and solaris.  (Possibly solaris doesn't count
> any more.)

I'd like to think Solaris still counts - certainly we're trying to support
modern glib & gtk3 applications in Solaris 12.   We do have closefrom()
though ours isn't documented as async-signal-safe or MT-safe.

I've wondered before how hard it would be to wrap the g_spawn* functions
around the posix_spawn API's we prefer, but have been too busy getting
our packages updated to more current versions to work on that.  We do
have a MT-safe posix_spawn_file_actions_addclosefrom_np() in libc we
could then use.

--
        -Alan Coopersmith-              alan coopersmith oracle com
         Oracle Solaris Engineering - http://blogs.oracle.com/alanc