Make dbus-broker our default D-Bus daemon

Rendered: https://gitlab.archlinux.org/heftig/rfcs/-/blob/dbus-broker/rfcs/0025-dbus-broker-default.rst

rfcs/0025-dbus-broker-default.rst

+=========================================
+Make dbus-broker our default D-Bus daemon
+=========================================
+
+- Date proposed: 2023-12-08
+- RFC MR: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/25
+
+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.
+
+.. _`dbus-broker wiki`: https://github.com/bus1/dbus-broker/wiki
+.. _`change proposal`: https://fedoraproject.org/wiki/Changes/DbusBrokerAsTheDefaultDbusImplementation
+
+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:
+
+  .. code:: markdown
+
+    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.