daemon: Add new component to hold information about related daemons

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/daemon.py

@@ -107,6 +107,33 @@ class Daemon(app.LeaderComponent):
         return [testname, result]
 
 
+class RelatedDaemon(app.FollowerComponent):
+    """Component to hold information about additional systemd units handled.
+
+    Unlike a daemon described by the Daemon component which is enabled/disabled
+    when the app is enabled/disabled, the daemon described by this component is
+    unaffected by the app's enabled/disabled status. The app only has an
+    indirect interest in this daemon.
+
+    This component primarily holds information about such daemon and does
+    nothing else. This information is used to check if the app is allowed to
+    perform operations on the daemon.
+    """
+
+    def __init__(self, component_id, unit):
+        """Initialize a new related daemon component.
+
+        'component_id' must be a unique string across all apps and components
+        of a app. Conventionally starts with 'related-daemon-'.
+
+        'unit' must the name of systemd unit.
+
+        """
+        super().__init__(component_id)
+
+        self.unit = unit
+
+
 def app_is_running(app_):
     """Return whether all the daemons in the app are running."""
     for component in app_.components.values():

plinth/tests/test_daemon.py

@@ -9,8 +9,8 @@ from unittest.mock import Mock, call, patch
 import pytest
 
 from plinth.app import App, FollowerComponent
-from plinth.daemon import (Daemon, app_is_running, diagnose_netcat,
+from plinth.daemon import (Daemon, RelatedDaemon, app_is_running,
-                           diagnose_port_listening)
+                           diagnose_netcat, diagnose_port_listening)
 
 
 @pytest.fixture(name='daemon')
@@ -228,3 +228,13 @@ def test_diagnose_netcat(popen):
     result = diagnose_netcat('test-host', 3300, input='test-input',
                              negate=True)
     assert result == ['Cannot connect to test-host:3300', 'passed']
+
+
+def test_related_daemon_initialization():
+    """Test that initializing related daemon works."""
+    component = RelatedDaemon('test-component', 'test-daemon')
+    assert component.component_id == 'test-component'
+    assert component.unit == 'test-daemon'
+
+    with pytest.raises(ValueError):
+        RelatedDaemon(None, 'test-daemon')