Setup xinitrc (startx) and connect to DBUS (set DBUS_SESSION_BUS_ADDRESS) on Debian 11

I recently installed Debian 11 on a workstation and the .xinitrc I usually use on Gentoo (with openrc) and Alpine (also openrc) did not properly setup dbus ($DBUS_SESSION_BUS_ADDRESS env variable was empty). I tried putting export $(dbus-launch) in my .xinitrc as stackexchange recommended but this did not fix the dbus environmental variables being unset. $DBUS_SESSION_BUS_ADDRESS not being set is problematic as pinentry-gnome3 will not launch.

The fix comes from examining the example xinitrc with cat /etc/X11/xinit/xinitrc on Debian:

#!/bin/sh

# /etc/X11/xinit/xinitrc
#
# global xinitrc file, used by all X sessions started by xinit (startx)

# invoke global X session script
. /etc/X11/Xsession

The global X session script runs a number of scripts located in /etc/X11/Xsession.d/. Notably, to set the dbus environmental variables, /etc/X11/Xsession runs /etc/X11/Xsession.d/20dbus_xdg-runtime:

# vim:set ft=sh sw=2 sts=2 et:

if [ -z "$DBUS_SESSION_BUS_ADDRESS" ] && [ -n "$XDG_RUNTIME_DIR" ] && \
    [ "$XDG_RUNTIME_DIR" = "/run/user/`id -u`" ] && \
    [ -S "$XDG_RUNTIME_DIR/bus" ]; then
  # We are under systemd-logind or something remarkably similar, and
  # a user-session socket has already been set up.
  #
  # Be nice to non-libdbus, non-sd-bus implementations by using
  # that as the session bus address in the environment. The check for
  # XDG_RUNTIME_DIR = "/run/user/`id -u`" is because we know that
  # form of the address, from systemd-logind, doesn't need escaping,
  # whereas arbitrary addresses might.
  DBUS_SESSION_BUS_ADDRESS="unix:path=$XDG_RUNTIME_DIR/bus"
  export DBUS_SESSION_BUS_ADDRESS
fi

if [ -x "/usr/bin/dbus-update-activation-environment" ]; then
  # tell dbus-daemon --session (and systemd --user, if running)
  # to put a minimal subset of the Xsession's environment in activated
  # services' environments
  dbus-update-activation-environment --verbose --systemd \
    DBUS_SESSION_BUS_ADDRESS DISPLAY XAUTHORITY
fi

dbus-update-activation-environment uses systemd to set the environmental variables:

–systemd Set environment variables for systemd user services as well as for traditional D-Bus session services.

Source: dbus-update-activation-environment manpage

To recap, if you dont have a display manager and want to use startx, you should use the example xinitrc (/etc/X11/xinit/xinitrc) that will setup a lot of stuff that you probably need in a Xorg session. But, you likely have some custom programs that you need to launch before your window manager. The default Xsession script allows for user defined script to ran (grep XSESSION /etc/X11/Xsession):

USERXSESSION=$HOME/.xsession
USERXSESSIONRC=$HOME/.xsessionrc
ALTUSERXSESSION=$HOME/.Xsession

mv $HOME/.xinitrc $HOME/.xsession