[lxc-devel] [RFC] lxc-start: daemonize by default

[Michael Tokarev]

30.04.2014 16:26, Christian Seiler wrote:
[]
> However, from personal experience, lxc-start is not quite as user-
> friendly. In >95% of cases, I want to start a container in the
> background and keep it running. There are some cases where I want
> to have it in the foreground and get the output immediately, but
> that is the exception (I am not talking about lxc-execute here,
> just about lxc-start).

Please count my strong disagreement here.

Usually when you're start the container for the first time, when
you just configured it, you want to see some error messages
right on the terminal you start it from.  So you can fix it and
repeat, to hit another configuration error and so on.

When -d is the default, you not only see the problems immediately.
It is much worse: stderr of lxc-start is redirected to /dev/null,
so you don't even see error messages from lxc-start itself!  Such
as, cgroupfs is not mounted, or what not.

I think the better alternative, in my opinion, is to be able to
specify -d in the config file.  When you're done, write it there,
and -d becomes the default _for this container_, and you can use
-F to force foreground.

Speaking of error messages during startup, I always used the
following trick, instead of using standard daemon() function:
create a pipe, fork, and use this pipe in parent to read just
one variable: exit status from the child (close writing side
of the pipe in parent).  Now parent just sits there reading
from this pipe.  In the child, do all the necessary init
stuff, but du not close error reporting channels yet.  When
all done, write 0 into the pipe, close it, redirect stderr
and run usual background task.  In case of failure, either
just exit (parent will read 0 bytes which means failure)
or write the non-zero exit status to the pipe.

This is when you have to do all the init in a single process.
When it is posisible to do init and run in _different_
processes (across fork), it is best to do init  first, and
run daemon() only when everything is done.

Thanks,

/mjt