s6: provide fake dbus-daemon executable as bash script

gdm expects to launch a dbus-daemon executable to create GNOME's session
bus

README.md

@@ -38,5 +38,10 @@ for the system and session bus, respectively.
 You are free to further tweak this scandir as you like, for example removing
 unwanted activatable services.
 
+Some programs currently launch an hardcoded `dbus-daemon` executable. An
+example is `gdm`, to spawn the user's session bus. In order to make them work, a
+fake `dbus-daemon` is provided as `tools/dbus-daemon.skel`. Fill in
+`DBUS_SERVICEREPO=` with the path to the directory generated by `s6-generator`.
+
 [dbus-broker]: https://github.com/bus1/dbus-broker
 [s6]: https://skarnet.org/software/s6

tools/dbus-daemon.skel

@@ -0,0 +1,76 @@
+#!/bin/bash
+# dbus-daemon executable for compatibility with stuff that launches dbus-daemon, such as gdm
+
+# this needs to be filled in, it's the directory generated by s6-generator
+DBUS_SERVICEREPO=
+
+warn() { echo warning: "$*" 1>&2 ; }
+die() { echo error: "$*" 1>&2 ; exit 1; }
+
+[ -z "$DBUS_SERVICEREPO" ] && die "variable DBUS_SERVICEREPO is empty"
+[ -z "$XDG_RUNTIME_DIR" ] && die "variable XDG_RUNTIME_DIR is empty"
+[ -d "$XDG_RUNTIME_DIR" ] || die "XDG_RUNTIME_DIR is not a directory"
+
+DBUS_RUNTIME_DIR=$(mktemp -d "$XDG_RUNTIME_DIR/dbus-XXXXXXXXXX")
+DBUS_SOCKET_PATH="$DBUS_RUNTIME_DIR/session_bus_socket"
+DBUS_SCANDIR="$DBUS_RUNTIME_DIR/scandir"
+
+# from /usr/share/doc/util-linux/getopt-example.bash
+TEMP=$(getopt -q -o '' --long 'nofork,print-address:,session' -- "$@")
+# NOTE: we cannot make the argument of print-address optional otherwise the '=' separator is required
+
+eval set -- "$TEMP"
+unset TEMP
+
+while true; do
+	case "$1" in
+		'--print-address')
+			# I hope that 4<&4- is a no-op... seems to be the case
+			exec 4<&${2}-
+			print_address_fd=4
+			shift 2
+			continue
+		;;
+		'--session')
+			shift
+			continue
+		;;
+		'--nofork')
+			shift
+			continue
+		;;
+		'--')
+			shift
+			break
+		;;
+		*)
+			warn "ignoring unknown argument"
+		;;
+	esac
+done
+
+cp -a ${DBUS_SERVICEREPO} ${DBUS_SCANDIR}
+
+# how do we avoid closing 4 when print_address_fd is not 4? should we care?
+DBUS_SESSION_BUS_ADDRESS="unix:path=${DBUS_SOCKET_PATH}" s6-svscan ${DBUS_SCANDIR} 4<&- &
+
+# Notification FD
+pid_fifo=$(mktemp --dry-run)
+mkfifo -m 600 "${pid_fifo}"
+exec 3<>"${pid_fifo}"
+
+rm "${pid_fifo}"
+
+dbus-controller-s6 -3 -d ${DBUS_SOCKET_PATH} -a ${DBUS_SCANDIR} dbus-broker 4<&- &
+
+read -r pid <&3
+exec 3<&-
+
+if [ -n "${print_address_fd}" ]; then
+	printf "unix:path=${DBUS_SOCKET_PATH}\n" >&${print_address_fd}
+	[ "${print_address_fd}" -eq 4 ] && exec 4<&-
+fi
+
+wait
+
+rm -rf "${DBUS_RUNTIME_DIR}"