app: Implement API to check if app/component has diagnostics

- Use the API to skip diagnosing apps that don't implement diagnostics.

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

plinth/app.py

@@ -115,6 +115,8 @@ class App:
         Results are typically collected by diagnosing each component of the app
         and then supplementing the results with any app level diagnostic tests.
 
+        Also see :meth:`.has_diagnostics`.
+
         """
         results = []
         for component in self.components.values():
@@ -122,6 +124,36 @@ class App:
 
         return results
 
+    def has_diagnostics(self):
+        """Return whether at least one diagnostic test is implemented.
+
+        If this method returns True, a button or menu item is shown to the
+        user to run diagnostics on this app. When the action is selected by the
+        user, the :meth:`.diagnose` method is called and the results are
+        presented to the user. Additionally collection of diagnostic results of
+        all apps can be obtained by the user from the Diagnostics module in
+        System section.
+
+        If a component of this app implements a diagnostic test, this method
+        returns True.
+
+        Further, if a subclass of App overrides the :meth:`.diagnose` method,
+        it is assumed that it is for implementing diagnostic tests and this
+        method returns True for such an app. Override this method if this
+        default behavior does not fit the needs.
+
+        """
+        # App implements some diagnostics
+        if self.__class__.diagnose is not App.diagnose:
+            return True
+
+        # Any of the components implement diagnostics
+        for component in self.components.values():
+            if component.has_diagnostics():
+                return True
+
+        return False
+
 
 class Component:
     """Interface for an app component."""
@@ -148,9 +180,25 @@ class Component:
         result. The test result is a string enumeration from 'failed', 'passed'
         and 'error'.
 
+        Also see :meth:`.has_diagnostics`.
+
         """
         return []
 
+    def has_diagnostics(self):
+        """Return whether at least one diagnostic test is implemented.
+
+        If this method return True, the :meth:`App.has_diagnostics`. also
+        returns True.
+
+        If a subclass of Component overrides the :meth:`.diagnose` method, it
+        is assumed that it is for implementing diagnostic tests and this method
+        returns True for such a component. Override this method if this default
+        behavior does not fit the needs.
+
+        """
+        return self.__class__.diagnose is not Component.diagnose
+
 
 class FollowerComponent(Component):
     """Interface for an app component that follows other components.

plinth/modules/diagnostics/diagnostics.py

@@ -112,6 +112,9 @@ def run_on_all_enabled_modules():
         if not app.is_enabled():
             continue
 
+        if not app.has_diagnostics():
+            continue
+
         apps.append((app.app_id, app))
         current_results['results'][app.app_id] = None
 

plinth/tests/test_app.py

@@ -19,6 +19,7 @@ Test module for App, base class for all applications.
 """
 
 import collections
+from unittest.mock import patch
 
 import pytest
 
@@ -193,6 +194,23 @@ def test_app_diagnose(app_with_components):
                        ('test-result-test-leader-2', 'success')]
 
 
+def test_app_has_diagnostics(app_with_components):
+    """Test checking if app has diagnostics implemented."""
+    app = app_with_components
+
+    # App with components that has diagnostics
+    assert app.has_diagnostics()
+
+    # App with components that don't have diagnostics
+    app.remove('test-leader-1')
+    app.remove('test-leader-2')
+    assert not app.has_diagnostics()
+
+    # App with app-level diagnostics
+    with patch.object(AppTest, 'diagnose', return_value=[('test1', 'passed')]):
+        assert app.has_diagnostics()
+
+
 def test_component_initialization():
     """Test that component is initialized properly."""
     with pytest.raises(ValueError):
@@ -209,6 +227,15 @@ def test_component_diagnose():
     assert component.diagnose() == []
 
 
+def test_component_has_diagnostics():
+    """Test checking if component has diagnostics implemented."""
+    component = LeaderTest('test-leader-1')
+    assert component.has_diagnostics()
+
+    component = FollowerComponent('test-follower-1')
+    assert not component.has_diagnostics()
+
+
 def test_follower_component_initialization():
     """Test that follower component is initialized properly."""
     component = FollowerComponent('test-follower-1')