Make dbus-broker our default D-Bus daemon

Summary

Make dbus-broker our default D-Bus daemon, while still allowing the venerable dbus-daemon to be used and minimizing the disruption of existing installations.

Motivation

dbus-broker provides better performance and higher reliability than dbus-daemon, with per-user accounting of resources in the broker. For more information, see the dbus-broker wiki.

It integrates better with systemd, asking the manager to start transient services when D-Bus services define systemd unit to be activated, avoiding launching services into the D-Bus daemon's cgroup.

The service's environment is the manager's environment. This replaces the distinction between dbus-daemon's launch environment and systemd's environment.

Fedora 30 switched to dbus-broker in 2019. Their change proposal also includes a detailed rationale.

Specification

• Move dbus-broker to [core].

• Add dbus-broker-units and dbus-daemon-units containing the respective dbus.service system and user units for the implementation, each providing dbus-units and conflicting with the other.

• Make systemd depend on dbus-units to ensure a provider is installed.

• Make a news post informing users about the change:

  We are making `dbus-broker` our default implementation of D-Bus, for
  improved performance, reliability and integration with systemd.
  
  For the foreseeable future we will still support the use of `dbus-daemon`,
  the previous implementation. Pacman will ask you whether to install
  `dbus-broker-units` or `dbus-daemon-units`. We recommend to pick the
  default.
  
  For a more detailed rationale, please see our [rfc0025][1].
  
  [1]: https://gitlab.archlinux.org/archlinux/rfcs/-/blob/master/rfcs/0025-dbus-broker-default.rst
  

Drawbacks

We force the user to select an implementation on upgrade and on each new installation. However, the default selection (when pressing Enter) will be dbus-broker-units because of the lexicographic ordering.

Unresolved Questions

None.

Alternatives Considered

Making dbus-units an empty package depending on dbus-broker-units and setting up replaces to install either dbus-broker-units or dbus-daemon-units on existing systems. This would avoid asking the user on a new installation, but I could not get the replaces to work properly.

The status quo is that dbus-broker has to be installed and its services enabled manually.