[RFE] add a way to run in a new systemd scope automatically

[keszybz]

With systemd 230 we switched to a default in which user processes started as part of a login session are terminated when the session exists (KillUserProcesses=yes). See https://github.com/systemd/systemd/blob/master/NEWS#L3, the commit that made the change: systemd/systemd@97e5530, and bug reports which gave the impetus for the change: https://bugs.freedesktop.org/show_bug.cgi?id=94508, systemd/systemd#2900.

Unfortunately this means starting tmux in the usual way is not effective, because it will be killed upon logout. There are a few option to avoid that, the best being:

systemd-run --scope --user tmux

This starts tmux as a scope unit under the systemd --user instance. It would be great if tmux could do this automatically. Probably the best way to do this would be to make the dbus call to org.freedesktop.systemd1.Manager.StartTransientUnit directly from tmux. See https://github.com/systemd/systemd/blob/master/src/run/run.c#L907 for how systemd-run does it, and https://www.freedesktop.org/wiki/Software/systemd/dbus/ for the description of the API.

[nicm]

tmux is not going to use dbus. It sounds like this should be done as part of daemon(3).

[keszybz]

daemon(3) is an old BSD interface, I'm afraid it's infeasible to change it.

I think tmux is the the right place to add such functionality, because it's one of the mechanisms that people use to starts long-running processes from a user session. daemon(3) is called from daemons, which certainly should not start their own scope.

In case it wasn't clear, this dbus call should be compile time and run time optional, i.e. it should probably be only compiled in on Linux systems, and should be disableable with a switch (assuming that the default would be yes).

[nicm]

I am not suggesting you change the daemon(3) API, and I don't see why you would need to. tmux wants to leave itself running in the background - daemon() does that, in a way that is vaguely portable and works similarly on all platforms. If you want to change how processes daemonize, why don't you change how daemon() is implemented instead of adding a new API?

[keszybz]

Because I don't want to change how processes daemonize in general, just a few specific processes: tmux, screen, x2go was also mentioned.

There has been a request to add a library function to wrap the dbus call. So far I haven't considered that, but indeed it would make things easier. Would that work for you?

[nicm]

Well, tmux uses daemon() so if it needs to change so will everything else that uses daemon() (or does fork/setsid itself) and expects to stay running in the background.

An alternative libc function would be simpler, then we could just use it instead of daemon() on Linux.

[rfrancoise]

FWIW, speaking as the Debian maintainer I'm okay with adding a dependency on libsystemd to the package, but the code needs to be upstream in tmux. There's a precedent with the utempter support.

[nicm]

Well, I wouldn't say utempter is a precedent. We have it because there is no way to achieve the same functionality without making tmux setgid. I don't think we should use it as an excuse to add other dependencies.

My concern is that we have a little function, daemon(), that does a simple little procedure to make a daemon that has worked basically unchanged across multiple platforms for maybe, what, 30 years? Now to do the same thing we need to add 150 lines of new, Linux-only code AND a library dependency.

Sure, maybe the fork/setsid procedure needs to change for whatever reason. But why not make daemon() do all the new stuff for us? That's what it's there for, to hide these details.

If that won't fly, then a similar new API would be a sensible addition.

[mathstuf]

I think there is, alternatively, a way to start a new session through PAM so that systemd knows to leave tmux alone. I had a thread on the ML a few years ago about it, but PAM calls were deemed as not suitable to be in touch itself (similar to the D-Bus calls requested here). I also don't know if the PAM dance is still a valid way to do this though.

[mathstuf]

Here's the thread: http://thread.gmane.org/gmane.comp.terminal-emulators.tmux.user/1849/focus=1856

[nicm]

The PAM API is certainly more compact.

Anyway, if we need to do this, the way to do it is to add osdep_daemon to all the osdep*.c files (like we have osdep_event_init) and call it instead of daemon (for most platforms it can just forward directly to daemon() itself).

[nicm]

OS X could probably also use it too. It seems they did fiddle with daemon(3) but caused a different class of problems, that needs some other code to fix.

[joshtriplett]

This is not something that all daemons should do; most daemons should not run in a new scope, but as part of the existing user scope. For instance, gpg-daemon and ssh-agent should not do this.

This is only something that programs like tmux, screen, a VNC or remote desktop daemon, or some other program that represents a distinct "interactive user session" from the program that launches it should do. (And they should only do so when starting the background service, not in the client process that connects to an existing server.)

I do think PAM sessions are the correct mechanism to use here. tmux should start a new user session using PAM, which will do the right thing not only with logind but also with any other PAM session modules that the admin might configure.

[rain-1]

@keszybv
why don't you fix systemd instead of forcing other programs to add systemd specific code?

[mathstuf]

How would systemd know that tmux is supposed to be special? And tmux and screen are different than your normal daemons (since they are essentially nested login sessions).

[lmachucab]

FWIW, speaking as the Debian maintainer I'm okay with adding a dependency on libsystemd to the package,

Why? Why should something like tmux depend on systemd? Should stuff like wget too?

Adding a Suggests, yeah, maybe. A hard Depends, no reason to.

[nicm]

I have thought about it further and I don't think I will accept a
dependency on systemd or on PAM, such large dependencies are excessive for
this. If a simpler API is provided I will reconsider at that point.
On 28 May 2016 8:44 p.m., "Luis Machuca Bezzaza" notifications@github.com
wrote:

FWIW, speaking as the Debian maintainer I'm okay with adding a dependency
on libsystemd to the package,

Why? Why should something like tmux depend on systemd? Should stuff like
wget too?

Adding a Suggests, yeah, maybe. A hard Depends, no reason to.

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[mathstuf]

Would you mind me opening a PR with my patches here so that folks interested can easily find it to at least try out?

[shadowcat-mst]

Or somebody could go find the actual problem @keszybz saw here - systemd/systemd#3005 - which is:

In particular, for my gnome session, if I log out, without KillUserProcesses=yes I get some processes which are obviously mistakes. Even if I log in again, I'm much better starting those again cleanly.

fix that, and stop trying to make systemd break the world because somebody's gnome session doesn't currently exit cleanly.

[joshtriplett]

On Sat, May 28, 2016 at 01:37:50PM -0700, Nicholas Marriott wrote:

I have thought about it further and I don't think I will accept a
dependency on systemd or on PAM, such large dependencies are excessive for
this. If a simpler API is provided I will reconsider at that point.

I certainly wouldn't suggest adding systemd-specific code; a systemd
transient unit/scope is the wrong solution for this anyway, as this isn't a
daemon, it's a user session.

However, PAM is supposed to be the correct way to start a user session.
Integrating with it helps with more than just systemd. For instance,
depending on user configuration, starting a new PAM session would avoid
the problem of tmux using an ssh-agent or keyring that doesn't last as
long as the tmux session (because PAM would associate the agent with
tmux).

If you're concerned about the overhead of PAM support, would you
consider providing optional integration with PAM, such as via
--enable-pam-session? If someone supplied a patch for optional PAM
session support (off by default if that help), would you consider
merging it?

[tsypa]

loginctl enable-linger

[natermer]

How would systemd know that tmux is supposed to be special?

There are probably a couple decent ways that systemd can be told these are 'special'. First way is to tell systemd these are special via a service file. Tell people that want to use tmux/screen/etc to start them then as 'systemctl --user start tmux'. Second way is by wrapping the tmux/etc commands in a simple shell script that runs 'systemd-run' in a way to indicate not to kill this process.

I am hoping that there is a 'lingering' option that can be set per-user service file or 'systemd-run' to allow this to happen. Being able to set a 'linger' per-service rather then for all user's programs via 'loginctl' seems like a better solution because then it wouldn't be a 'all or nothing' choice.

I feel this is better because it won't require software authors to code special PAM support just to deal with systemd + Linux. It can be used by distributions to maintain expected behavior for these classes of programs. And, finally, it will make life easier for users/developers that wish to make new programs of these types and have things 'just work' by providing a suitable service file in their source code.

[ap]

So then we get to the next iteration where the “tell systemd I want to stay running in the background” step has become part of everyone’s dæmonization recipe, and now you’re back to square 1 where a user exits their session but a bunch of unwanted processes stick around.

Result: a long period of making everything in world depend on systemd, followed by nothing gained for all the pain.

[kentfredric]

IMHO, This sounds more like there needs to be a user_daemon() interface so processes can voluntarily state "yes, kill me when the user logs out please". ( And have autoconf just fall-back to using daemon() instead on systems that don't support user_daemon() ).

Changing semantics of existing functionality however seems unwise.

(And then maybe, people who have a foot-to-gun-ratio greater than 2 can tell systemd to prefer one instead of the other, instead of covering earth with a foot assassination squad)

I half expected such a mechanism to already exist, but my reading of daemon(3) and setsid(2) indicates there isn't.

That seems to be why we're where we are now, because everyone who wants "exit on user vanishes" has to manually implement painful mechanics ... so they opt not to and just reach for a full daemon.

[hashworks]

Honestly, I don't see why processes should be able to decide that they should run forever. I'm completely fine with starting specific processes with systemd user scope. This is just something new for people who use systemd what they need to learn.

[eekee]

I haven't had time to participate in open source for a few years. Now I'm coming back to it, I'm a bit shocked by this. Persistent screen and tmux sessions were common enough many years before systemd was released. If systemd won't make it easy for users to set up and manage these sessions, that's systemd's problem, not tmux's.

[lilydjwg]

I've met this kind of problem already: I was using ssh.socket on Debian and every time I logged out, I found my tmux session disappeared....After some debugging I finally learned to set ServiceKillMode=process for ssh@.service.d.

[joshtriplett]

@lilydjwg That's a different problem. You need to have SSH configured to use PAM (typically the default), and have pam_systemd installed. That will then start a new user session for SSH, rather than leaving the logged-in user processes as part of the SSH socket unit's scope.

[ThomasAdam]

Any chance you can shift this discussion elsewhere? It's not anything to do
with tmux.
On 29 May 2016 1:57 p.m., "Josh Triplett" notifications@github.com wrote:

@lilydjwg https://github.com/lilydjwg That's a different problem. You
need to have SSH configured to use PAM (typically the default), and have
pam_systemd installed. That will then start a new user session for SSH,
rather than leaving the logged-in user processes as part of the SSH socket
unit's scope.

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[joshtriplett]

@ThomasAdam @nicm
At this point, I would suggest closing this issue to comments; there isn't any new discussion happening related to the actual problem reported, and judging by the number of unproductive comments and the 👎s and 👍s on comments, a link to the issue most likely wound up somewhere that has little to do with actually solving the problem. The proposal to run in a new systemd scope isn't the right solution to the problem regardless, so this issue as originally reported should remain closed. Someone interested in working on PAM support may want to file a separate pull request with patches implementing it.

[wisq]

Yeah, it's making the rounds on Twitter, as a "wtf systemd" sort of thing. Closing comments might be best.

[shadowcat-mst]

Hey, to anybody who say this because I tweeted it in a fit of aggravation - please use the thumbs up or whatever thing to add 1 to one of the numbers underneath my comment rather than making another comment that doesn't need to be here on an already-closed-as-reject-issue. (@wisq - more part 3 of 3 of a wtf systemd tweet stream of mine, I suspect, but that doesn't help, of course)