[lxc-devel] fixing restoring of tty mode after lxc-start

[Michel Normand]

Michael Tokarev a e'crit :
> Hello.
> 
> While trying to fix the tty mode after lxc-start, I come across
> lxc_close_all_inherited_fd() routine.  Which closes all fds which
> were inherited when lxc-start started.
> 
> It's called this way:
> 
>   lxc_start.c:main():
>     lxc_start():
>      lxc_spawn();
>      lxc_close_all_inherited_fd()
>      lxc_poll()
> 
> The question is why lxc_close_all_inherited() is needed here.
> And why it is ever needed _outside_ of the container to start
> with.
> 
> This call to close_all_inherited_fd() is the reason why tty
> modes are not restored after lxc-start: because stdin is now
> closed and hence nothing to restore.
> 
> It looks like the whole inherited_fd thing is only needed in
> lxc_init helper binary, 

In fact the lxc_close_all_inherited_fd function was defined
to cover the case of a user appli creating pipes before to fork
the lxc-start, and to use these pipes to communicate with the appli
started inside the container.
For that purpose it was required for lxc to not keep a reference
on these fds while lxc code is waiting for the appli to run up to its end.
This is the reason why this function is called in two places
in lxc_init.c and start.c

Probably it would be possible to modify this function to not
close inherited fds if they are tty.

---
Michel

> there's no need to use it anywhere else.
> And even there it can be avoided, by iterating over /proc/self/fd
> and closing them on the way.
> 
> But maybe there's some other for this housekeeping somewhere, so
> I'm asking.
> 
> Thanks!
> 
> /mjt
>