include i3.service systemd user unit (#6547)

On Linux systems using systemd, we should activate graphical-session.target.

We conceptually need our i3.desktop file to do a
blockingly-run-via-systemd action (this is what scopes the activation of
graphical-session.target to the X session), and shipping an i3.service
is the best way to define the systemd unit we use for that.

Based on https://github.com/i3/i3/pull/5591 by David Sansome.

fixes https://github.com/i3/i3/issues/5186

libexec/i3-session.in

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+# is-enabled seems like an elegant way to check for i3.service at first,
+# but on NixOS it returns non-zero ('linked-runtime').
+if [ "$(systemctl --user show -P CanStart @UNIT@ 2>/dev/null)" = "yes" ]
+then
+  # Ensure the X11 environment variables are available
+  # to the systemd user session (required on some distros).
+  systemctl --user import-environment DISPLAY XAUTHORITY
+
+  if command -v dbus-update-activation-environment >/dev/null 2>&1; then
+    dbus-update-activation-environment DISPLAY XAUTHORITY
+  fi
+
+  systemctl --user --wait start @UNIT@
+else
+  i3
+fi

meson.build

@@ -325,6 +325,8 @@ pangocairo_dep = dependency('pangocairo', method: 'pkg-config')
 glib_dep = dependency('glib-2.0', method: 'pkg-config')
 gobject_dep = dependency('gobject-2.0', method: 'pkg-config')
 
+systemd_dep = dependency('systemd', required: false)
+
 ev_dep = cc.find_library('ev')
 
 inc = include_directories('include')
@@ -599,11 +601,75 @@ install_subdir(
 )
 
 install_subdir(
-  'share/',
+  'share/applications',
   strip_directory: true,
-  install_dir: get_option('datadir'),
+  install_dir: join_paths(get_option('datadir'), 'applications'),
 )
 
+xsessions = join_paths(get_option('datadir'), 'xsessions')
+xsession_cdata = configuration_data()
+xsession_cdata.set('NAME', 'i3')
+xsession_cdata.set('UNIT', 'i3.service')
+xsession_cdata.set('EXEC', join_paths(get_option('prefix'), get_option('libexecdir'), 'i3-session'))
+configure_file(
+  input: 'share/xsessions/i3.desktop.in',
+  output: 'i3.desktop',
+  configuration: xsession_cdata,
+  install: true,
+  install_dir: xsessions,
+)
+configure_file(
+  input: 'libexec/i3-session.in',
+  output: 'i3-session',
+  configuration: xsession_cdata,
+  install: true,
+  install_dir: get_option('libexecdir'),
+)
+xsession_log_cdata = configuration_data()
+xsession_log_cdata.set('NAME', 'i3-with-shmlog')
+xsession_log_cdata.set('UNIT', 'i3-with-shmlog.service')
+xsession_log_cdata.set('EXEC', join_paths(get_option('prefix'), get_option('libexecdir'), 'i3-session-with-shmlog'))
+configure_file(
+  input: 'share/xsessions/i3.desktop.in',
+  output: 'i3-with-shmlog.desktop',
+  configuration: xsession_log_cdata,
+  install: true,
+  install_dir: xsessions,
+)
+configure_file(
+  input: 'libexec/i3-session.in',
+  output: 'i3-session-with-shmlog',
+  configuration: xsession_log_cdata,
+  install: true,
+  install_dir: get_option('libexecdir'),
+)
+
+if systemd_dep.found()
+  userunitdir = systemd_dep.get_pkgconfig_variable('systemduserunitdir')
+
+  # The global cdata uses set_quoted (for C header files),
+  # but for systemd unit files, we must not use quoting.
+  systemd_cdata = configuration_data()
+  systemd_cdata.set('BINDIR', join_paths(get_option('prefix'), get_option('bindir')))
+  systemd_cdata.set('EXECSTART', join_paths(get_option('prefix'), get_option('bindir'), 'i3'))
+  configure_file(
+    input: 'systemd/i3.service.in',
+    output: 'i3.service',
+    configuration: systemd_cdata,
+    install_dir: userunitdir,
+  )
+
+  systemd_shmlog_cdata = configuration_data()
+  systemd_shmlog_cdata.merge_from(systemd_cdata)
+  systemd_shmlog_cdata.set('EXECSTART', join_paths(get_option('prefix'), get_option('bindir'), 'i3-with-shmlog'))
+  configure_file(
+    input: 'systemd/i3.service.in',
+    output: 'i3-with-shmlog.service',
+    configuration: systemd_shmlog_cdata,
+    install_dir: userunitdir,
+  )
+endif
+
 install_headers(
   'include/i3/ipc.h',
   subdir: 'i3',

share/xsessions/i3-with-shmlog.desktop

@@ -1,6 +0,0 @@
-[Desktop Entry]
-Name=i3 (with debug log)
-Comment=improved dynamic tiling window manager
-Exec=i3-with-shmlog
-Type=Application
-Keywords=tiling;wm;windowmanager;window;manager;

share/xsessions/i3.desktop.in

@@ -1,8 +1,8 @@
 [Desktop Entry]
-Name=i3
+Name=@NAME@
 Comment=improved dynamic tiling window manager
-Exec=i3
-TryExec=i3
+Exec=@EXEC@
+TryExec=@EXEC@
 Type=XSession
 X-LightDM-DesktopName=i3
 DesktopNames=i3

systemd/i3.service.in

@@ -0,0 +1,25 @@
+[Unit]
+Description=i3 window manager
+
+# For more details, see the systemd.special man page:
+# https://www.freedesktop.org/software/systemd/man/latest/systemd.special.html
+
+# "The graphical-session-pre target contains services which set up
+# the environment or global configuration of a graphical session,
+# such as SSH/GPG agents (which need to export an environment variable
+# into all desktop processes)."
+Requires=graphical-session-pre.target
+After=graphical-session-pre.target
+
+# When i3.service is activated, activate graphical-session.target, too.
+# When graphical-session.target deactivates, i3.service is deactivated.
+# When i3.service deactivates, nothing keeps graphical-session.target
+# alive anymore, so due to its StopWhenUnneeded property, it deactivates.
+BindsTo=graphical-session.target
+# Only consider graphical-session.target active once i3 has started.
+Before=graphical-session.target
+
+[Service]
+Type=notify
+ExecStart=@EXECSTART@
+ExecReload=@BINDIR@/i3-msg reload