gnome-classic: Define a gnome-session session

For one, this is simply more correct: the gnome-classic session is a
different session from the default gnome session. So let's define it as
such

For two, this enables us to directly tell gnome-shell which shell mode
to use, rather than passing it through an environment variable. In other
words, this enables us to integrate with
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3960

REUSE.toml

@@ -15,7 +15,13 @@ SPDX-FileCopyrightText = [
 SPDX-License-Identifier = "CC-BY-SA-4.0"
 
 [[annotations]]
-path = ["**.json.in", "**.desktop.in", "**.gschema.override"]
+path = [
+  "**.json.in",
+  "**.desktop.in",
+  "**.gschema.override",
+  "**.session",
+  "**.session.conf",
+]
 SPDX-FileCopyrightText = "Florian Müllner <fmuellner@gnome.org>"
 SPDX-License-Identifier = "GPL-2.0-or-later"
 

data/gnome-classic.desktop.in

@@ -1,7 +1,7 @@
 [Desktop Entry]
 Name=GNOME Classic
 Comment=This session logs you into GNOME Classic
-Exec=env GNOME_SHELL_SESSION_MODE=classic gnome-session
+Exec=gnome-session --session gnome-classic
 TryExec=gnome-session
 Type=Application
 DesktopNames=GNOME-Classic;GNOME;

data/gnome-classic.session

@@ -0,0 +1,2 @@
+[GNOME Session]
+Name=GNOME Classic

data/gnome-classic.session.conf

@@ -0,0 +1,3 @@
+[Unit]
+Requires=gnome-session-services.target
+Requires=org.gnome.Shell@classic.service

data/meson.build

@@ -31,3 +31,13 @@ configure_file(
 
 classic_override = '00_org.gnome.shell.extensions.classic.gschema.override'
 install_data(classic_override, install_dir: schemadir)
+
+install_data(
+  'gnome-classic.session',
+  install_dir: sessiondir,
+)
+
+install_data(
+  'gnome-classic.session.conf',
+  install_dir: systemd_userunitdir / 'gnome-session@gnome-classic.target.d',
+)

meson.build

@@ -25,6 +25,13 @@ schemadir = join_paths(datadir, 'glib-2.0', 'schemas')
 sessiondir = join_paths(datadir, 'gnome-session', 'sessions')
 wlsessiondir = join_paths(datadir, 'wayland-sessions')
 
+systemd_dep = dependency('systemd', required: false)
+systemd_userunitdir = systemd_dep.get_variable(
+    pkgconfig: 'systemduserunitdir',
+    pkgconfig_define: ['prefix', get_option('prefix')],
+    default_value: get_option('prefix') / 'lib' / 'systemd' / 'user',
+)
+
 ver_arr = meson.project_version().split('.')
 shell_version = ver_arr[0]