Add debconf question for display-manager, based on gdm3

debian/control

@@ -17,4 +17,5 @@ Homepage: https://github.com/pop-os/cosmic-greeter
 Package: cosmic-greeter
 Architecture: amd64 arm64
 Depends: cage, greetd, ${misc:Depends}, ${shlibs:Depends}
+Provides: x-display-manager
 Description: Cosmic Greeter

debian/cosmic-greeter.config

@@ -0,0 +1,43 @@
+#!/bin/sh
+# Debian cosmic-greeter package configuration script
+# Copyright 2000-2001 Branden Robinson.
+# Licensed under the GNU General Public License, version 2.  See the file
+# /usr/share/common-licenses/GPL or <http://www.gnu.org/copyleft/gpl.txt>.
+
+set -e
+
+# source debconf library
+. /usr/share/debconf/confmodule
+
+THIS_PACKAGE=cosmic-greeter
+DEFAULT_DISPLAY_MANAGER_FILE=/etc/X11/default-display-manager
+
+# set default display manager
+
+db_get shared/default-x-display-manager
+OLD_DEFAULT="$RET"
+
+db_metaget shared/default-x-display-manager owners
+OWNERS="$RET"
+db_metaget shared/default-x-display-manager choices
+CHOICES="$RET"
+
+if [ "$OWNERS" != "$CHOICES" ]; then
+  db_subst shared/default-x-display-manager choices $OWNERS
+  db_fset shared/default-x-display-manager seen false
+fi
+
+db_input high shared/default-x-display-manager || true
+db_go
+
+# using this display manager?
+db_get shared/default-x-display-manager
+CURRENT_DEFAULT="$RET"
+# set a flag to indicate to postinst that we need to update from debconf
+if [ "$OLD_DEFAULT" != "$CURRENT_DEFAULT" ]; then
+  DEFAULT_DISPLAY_MANAGER_DIR=$(dirname $DEFAULT_DISPLAY_MANAGER_FILE)
+  test -e $DEFAULT_DISPLAY_MANAGER_DIR || mkdir -p $DEFAULT_DISPLAY_MANAGER_DIR
+  touch $DEFAULT_DISPLAY_MANAGER_FILE.debconf-update
+fi
+
+exit 0

debian/cosmic-greeter.postinst

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+set -e
+
+. /usr/share/debconf/confmodule
+
+THIS_PACKAGE=cosmic-greeter
+DEFAULT_DISPLAY_MANAGER_FILE=/etc/X11/default-display-manager
+
+# debconf is not a registry, so we only fiddle with the default file if 
+# the configure script requested an update
+if [ -e $DEFAULT_DISPLAY_MANAGER_FILE.debconf-update ]; then
+  rm -f $DEFAULT_DISPLAY_MANAGER_FILE.debconf-update
+  if db_get shared/default-x-display-manager; then
+    # workaround debconf passthru bug (#379198)
+    if [ -z "$RET" ]; then
+      RET="$THIS_PACKAGE"
+    fi
+    if [ "$THIS_PACKAGE" != "$RET" ]; then
+      echo "Please be sure to run \"dpkg --configure $RET\"."
+    fi
+    if db_get "$RET"/daemon_name; then
+      echo "$RET" > $DEFAULT_DISPLAY_MANAGER_FILE
+    fi
+  fi
+fi
+
+DEFAULT_SERVICE=/etc/systemd/system/display-manager.service
+# set default-display-manager systemd service link according to our config
+if [ "$1" = configure ] && [ -d /etc/systemd/system/ ]; then
+  if [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ]; then
+    SERVICE=/lib/systemd/system/$(basename $(cat "$DEFAULT_DISPLAY_MANAGER_FILE")).service
+    if [ -h "$DEFAULT_SERVICE" ] && [ $(readlink "$DEFAULT_SERVICE") = /dev/null ]; then
+      echo "Display manager service is masked" >&2
+    elif [ -e "$SERVICE" ]; then
+      ln -sf "$SERVICE" "$DEFAULT_SERVICE"
+    else
+      echo "WARNING: $SERVICE is the selected default display manager but does not exist" >&2
+      rm -f "$DEFAULT_SERVICE"
+    fi
+  else
+    rm -f "$DEFAULT_SERVICE"
+  fi
+fi
+
+# debconf hangs if cosmic-greeter gets started below without this
+db_stop || true
+
+#DEBHELPER#

debian/cosmic-greeter.prerm

@@ -0,0 +1,64 @@
+#!/bin/bash
+# Debian gdm package pre-removal script
+# Copyright 2001 Branden Robinson.
+# Licensed under the GNU General Public License, version 2.  See the file
+# /usr/share/common-licenses/GPL or <http://www.gnu.org/copyleft/gpl.txt>.
+# Acknowlegements to Stephen Early, Mark Eichin, and Manoj Srivastava.
+
+set -e
+
+THIS_PACKAGE=cosmic-greeter
+DEFAULT_DISPLAY_MANAGER_FILE=/etc/X11/default-display-manager
+
+if [ "$1" = "remove" -o "$1" = "deconfigure" ]; then
+  if [ -e /usr/share/debconf/confmodule ]; then
+    . /usr/share/debconf/confmodule
+    # disown this question
+    db_unregister shared/default-x-display-manager || true
+    # does the question still exist?
+    if db_get shared/default-x-display-manager; then
+      db_metaget shared/default-x-display-manager owners
+      db_subst shared/default-x-display-manager choices "$RET"
+      db_get shared/default-x-display-manager
+      # are we removing the currently selected display manager?
+      if [ "$THIS_PACKAGE" = "$RET" ]; then
+        if [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ]; then
+          db_get "$RET"/daemon_name
+          if [ "$(cat $DEFAULT_DISPLAY_MANAGER_FILE)" = "$RET" ]; then
+            rm "$DEFAULT_DISPLAY_MANAGER_FILE"
+          fi
+        fi
+        # ask the user to choose a new default
+        db_fset shared/default-x-display-manager seen false
+        db_input critical shared/default-x-display-manager || true
+        db_go
+        # if the display manager file doesn't exist, write it with the path
+        # to the new default display manager
+        if [ ! -e $DEFAULT_DISPLAY_MANAGER_FILE ]; then
+          db_get shared/default-x-display-manager
+          echo "Please be sure to run \"dpkg-reconfigure $RET\"."
+          db_get "$RET"/daemon_name
+          echo "$RET" > "$DEFAULT_DISPLAY_MANAGER_FILE"
+        fi
+      fi
+    fi
+  fi
+
+  DEFAULT_SERVICE=/etc/systemd/system/display-manager.service
+  # set default-display-manager systemd service link according to our config
+  if [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ]; then
+    SERVICE=/lib/systemd/system/$(basename $(cat "$DEFAULT_DISPLAY_MANAGER_FILE")).service
+    if [ -h "$DEFAULT_SERVICE" ] && [ $(readlink "$DEFAULT_SERVICE") = /dev/null ]; then
+      echo "Display manager service is masked" >&2
+    elif [ -e "$SERVICE" ]; then
+      ln -sf "$SERVICE" "$DEFAULT_SERVICE"
+    else
+      echo "WARNING: $SERVICE is the selected default display manager but does not exist" >&2
+      rm -f "$DEFAULT_SERVICE"
+    fi
+  else
+    rm -f "$DEFAULT_SERVICE"
+  fi
+fi
+
+#DEBHELPER#

debian/cosmic-greeter.service

@@ -16,5 +16,6 @@ RestartSec=1
 StartLimitBurst=5
 StartLimitInterval=30
 
-[Install]
-Alias=display-manager.service
+# Managed by debconf
+#[Install]
+#Alias=display-manager.service

debian/cosmic-greeter.templates

@@ -0,0 +1,20 @@
+Template: cosmic-greeter/daemon_name
+Type: string
+Default: /usr/bin/cosmic-greeter
+Description: for internal use only
+
+Template: shared/default-x-display-manager
+Type: select
+Choices: ${choices}
+_Description: Default display manager:
+ A display manager is a program that provides graphical login capabilities for
+ the X Window System.
+ .
+ Only one display manager can manage a given X server, but multiple display
+ manager packages are installed. Please select which display manager should
+ run by default.
+ .
+ Multiple display managers can run simultaneously if they are configured to
+ manage different servers; to achieve this, configure the display managers
+ accordingly, edit each of their init scripts in /etc/init.d, and disable the
+ check for a default display manager.