action_utils: Implement methods to get/set the systemd boot target

- To be used in GNOME app.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/action_utils.py

@@ -29,6 +29,24 @@ def is_systemd_running():
     return os.path.exists('/run/systemd')
 
 
+def systemd_get_default() -> str:
+    """Return the default target that systemd will boot into."""
+    process = subprocess.run(['systemctl', 'get-default'],
+                             stdout=subprocess.PIPE, check=True)
+    return process.stdout.decode().strip()
+
+
+def systemd_set_default(target: str, isolate: bool = True):
+    """Set the default target that systemd will boot into.
+
+    If 'isolate' is True, then immediate switch the current system target to
+    newly set target resulting in starting/stopping of services.
+    """
+    subprocess.run(['systemctl', 'set-default', target], check=True)
+    if isolate:
+        subprocess.run(['systemctl', 'isolate', target], check=True)
+
+
 def service_daemon_reload():
     """Reload systemd to ensure that newer unit files are read."""
     subprocess.run(['systemctl', 'daemon-reload'], check=True,

plinth/privileged/__init__.py

@@ -5,12 +5,13 @@ from .config import dropin_is_valid, dropin_link, dropin_unlink
 from .packages import (filter_conffile_packages, install,
                        is_package_manager_busy, remove, update)
 from .service import (disable, enable, is_enabled, is_running, mask, reload,
-                      restart, start, stop, try_reload_or_restart, try_restart,
+                      restart, start, stop, systemd_set_default,
-                      unmask)
+                      try_reload_or_restart, try_restart, unmask)
 
 __all__ = [
     'filter_conffile_packages', 'install', 'is_package_manager_busy', 'remove',
-    'update', 'disable', 'enable', 'is_enabled', 'is_running', 'mask',
+    'update', 'systemd_set_default', 'disable', 'enable', 'is_enabled',
-    'reload', 'restart', 'start', 'stop', 'try_reload_or_restart',
+    'is_running', 'mask', 'reload', 'restart', 'start', 'stop',
-    'try_restart', 'unmask', 'dropin_is_valid', 'dropin_link', 'dropin_unlink'
+    'try_reload_or_restart', 'try_restart', 'unmask', 'dropin_is_valid',
+    'dropin_link', 'dropin_unlink'
 ]

plinth/privileged/service.py

@@ -8,6 +8,15 @@ from plinth.actions import privileged
 from plinth.daemon import Daemon, RelatedDaemon
 
 
+@privileged
+def systemd_set_default(target: str, isolate: bool = True):
+    """Set the default target that systemd will boot into."""
+    if target not in ['graphical.target', 'multi-user.target']:
+        raise ValueError('Invalid target')
+
+    action_utils.systemd_set_default(target, isolate)
+
+
 @privileged
 def start(service: str):
     """Start a service."""