csm-systemd: wait for logind's PrepareForShutdown before quitting

[leigh123linux]

cinnamon-session quit immediately after requesting PowerOff/Reboot via
logind, without confirming logind had actually accepted the request. If
another process held a delay inhibitor, cinnamon-session would tear
down and hand back to the display manager greeter well before the
machine actually powered off, leaving the session in a misleading
half-terminated state for the duration of the delay.

Take our own delay inhibitor before requesting shutdown/restart, drop
any held block inhibitor (which would make logind refuse the request
outright), and wait for logind's PrepareForShutdown signal to confirm
shutdown is genuinely proceeding before quitting -- matching
gnome-session's gsm_systemd_prepare_shutdown()/complete_shutdown().

Also remove the MDM display-manager fallback path (quit_request_failed,
mdm_set_logout_action calls, the CSM_MANAGER_LOGOUT_*_MDM enum values,
and mdm.c/mdm.h) since it's dead: Linux Mint dropped MDM for LightDM
years ago, and the fallback was never actually reachable once
csm_manager_quit() started quitting unconditionally after issuing the
shutdown/restart request.

Fixes: #214

[Copilot]

🟡 Changes recommended

There are confirmed failure-path issues that can crash initialization or leave the session stuck in a non-running phase when shutdown/reboot isn’t confirmed.

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Pull request overview

This PR improves cinnamon-session’s shutdown/reboot flow when using logind by taking its own delay inhibitor, dropping conflicting block inhibitors, and waiting for logind’s PrepareForShutdown signal before quitting; it also removes the long-dead MDM display-manager fallback code paths.

Changes:

• Add a shutdown-prepared signal and complete_shutdown() hook to the CsmSystem interface, and implement delay-inhibitor handling + PrepareForShutdown listening in the systemd backend.
• Update CsmManager to quit only after shutdown is confirmed (and remove the old request-failed → MDM fallback).
• Remove mdm.c/mdm.h and update the build to stop compiling MDM logout-action support.

File summaries

File	Description
cinnamon-session/meson.build	Removes mdm.c from the build sources.
cinnamon-session/mdm.h	Deletes legacy MDM logout-action API header.
cinnamon-session/mdm.c	Deletes legacy MDM protocol implementation.
cinnamon-session/csm-systemd.c	Adds delay inhibitor + waits for PrepareForShutdown, emits shutdown-prepared, implements complete_shutdown().
cinnamon-session/csm-system.h	Extends CsmSystemInterface with complete_shutdown().
cinnamon-session/csm-system.c	Adds shutdown-prepared signal and csm_system_complete_shutdown() wrapper.
cinnamon-session/csm-manager.c	Removes MDM fallback and waits for shutdown-prepared before quitting.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 2
• Review effort level: Lite

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[mtwebster]

Don't we still need to csm_quit() regardless? csd_manager_quit() is called from CSM_MANAGER_PHASE_EXIT - everything has already terminated, the session is done. There's no point in leaving a blank/broken desktop either way.

[mtwebster]

I don't know if we need csm_system_complete_shutdown() either - we could just csm_quit() on this callback and print an error if !success.

csm_system_complete_shutdown() would release the inhibitor here, while cinnamon-session is still running. If you just csm_quit() the process exits and the inhibitor gets dropped automatically (fd closes).

You could remove the whole complete_shutdown vfunc in csm-system.*, etc... and just rename it to a static function in csm-systemd.c:

static void
drop_shutdown_inhibitor (CsmSystemd *manager)
{
        if (manager->priv->shutdown_inhibit_fd != -1) {
                g_debug ("Dropping shutdown inhibitor");
                close (manager->priv->shutdown_inhibit_fd);
                manager->priv->shutdown_inhibit_fd = -1;
        }
}

so it can still be called by reboot_or_poweroff_done() there.