Describe the bug

After running nixos-rebuild switch my graphical session crashed. The Sway process was technically still running but I was back to the console and could see various error messages.

This probably applies to other Wayland compositors as well and maybe even X11.

To Reproduce

Tested the following and it it indeed reproducible:

1. Switch to the old revision: e89b215
2. Switch to the new revision: 0ee0489
3. Now Sway should've "crashed".

Expected behavior

The system upgrades normally without Sway crashing.

Screenshots

TODO

Additional context

One of the following systemd units that where stopped should be responsible for this:

'dbus.service', 'dbus.socket', 'pcscd.service', 'pcscd.socket',
'systemd-coredump.service', 'systemd-coredump.socket',
'systemd-initctl.service', 'systemd-initctl.socket',
'systemd-journald-audit.service', 'systemd-journald-audit.socket',
'systemd-journald-dev-log.service', 'systemd-journald-dev-log.socket',
'systemd-journald.service', 'systemd-journald.socket', 'systemd-rfkill.service',
'systemd-rfkill.socket', 'systemd-udevd-control.service',
'systemd-udevd-kernel.service'

Probably due to D-Bus or udev.

Update: systemctl stop dbus crashes Sway, so that was probably the reason. I also don't think we did ever restart (stop + start) D-Bus in the past (only reloaded it). AFAIK restarting D-Bus will kick all clients and is therefore a bad idea (but haven't looked much into it).

Update2: Apparently we still have the following (which is very important):

      # Don't restart dbus-daemon. Bad things tend to happen if we do.
      reloadIfChanged = true;

Not sure why nixos-rebuild switch did restart dbus.service then (actually it did stop, start, and reload the unit...). Maybe we have a bigger regression in nixos-rebuild.

Update3: 89806e9 could be the problem, I'll investigate.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.0, NixOS, 20.03.git.ee376d4 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): ``
 - nixpkgs: `/var/nixpkgs`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: sway
# a list of nixos modules affected by the problem
module: sway