upgrades: Use only sources file to determine if backports enabled

Tests:
- Build deb and install in buster image. Manually remove backports
  sources file. Security page does not show backports notice. Updates
  page shows button to activate backports.
- Activate backports from updates page. Success message is shown and
  button to activate backports is removed. Security page shows
  backports notice.

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

actions/upgrades

@@ -13,6 +13,7 @@ import sys
 
 from plinth.action_utils import run_apt_command
 from plinth.modules.apache.components import check_url
+from plinth.modules.upgrades import is_backports_enabled, SOURCES_LIST
 
 AUTO_CONF_FILE = '/etc/apt/apt.conf.d/20auto-upgrades'
 LOG_FILE = '/var/log/unattended-upgrades/unattended-upgrades.log'
@@ -192,8 +193,7 @@ def _check_and_backports_sources():
     if os.path.exists(old_sources_list):
         os.remove(old_sources_list)
 
-    sources_list = '/etc/apt/sources.list.d/freedombox2.list'
-    if os.path.exists(sources_list):
+    if is_backports_enabled():
         print('Repositories list up-to-date. Skipping update.')
         return
 
@@ -229,7 +229,7 @@ def _check_and_backports_sources():
         return
 
     print(f'{dist}-backports is now available. Adding to sources.')
-    _add_backports_sources(sources_list, protocol, dist)
+    _add_backports_sources(SOURCES_LIST, protocol, dist)
 
 
 def _add_apt_preferences():

plinth/modules/security/templates/security.html

@@ -7,12 +7,12 @@
 {% load i18n %}
 
 {% block status %}
-  {% if backports_in_use %}
+  {% if is_backports_enabled %}
     <h3>{% trans "Security Notice" %}</h3>
     <p>
       {% blocktrans trimmed %}
-        You are using packages from Debian backports. Please note that these
-        packages do not have security support from Debian. However, they are
+        Backports are enabled. Please note that packages from the backports
+        repository do not have security support from Debian. However, they are
         maintained on a best-effort basis by contributors in Debian and
         FreedomBox community.
       {% endblocktrans %}

plinth/modules/security/views.py

@@ -9,7 +9,7 @@ from django.utils.translation import ugettext as _
 
 from plinth import action_utils, actions
 from plinth.modules import security
-from plinth.modules.upgrades import get_backports_in_use
+from plinth.modules.upgrades import is_backports_enabled
 
 from .forms import SecurityForm
 
@@ -33,7 +33,7 @@ def index(request):
         request, 'security.html', {
             'app_info': security.app.info,
             'form': form,
-            'backports_in_use': get_backports_in_use(),
+            'is_backports_enabled': is_backports_enabled(),
         })
 
 

plinth/modules/upgrades/__init__.py

@@ -3,6 +3,7 @@
 FreedomBox app for upgrades.
 """
 
+import os
 import subprocess
 
 from django.utils.translation import ugettext_lazy as _
@@ -32,6 +33,8 @@ _description = [
 
 app = None
 
+SOURCES_LIST = '/etc/apt/sources.list.d/freedombox2.list'
+
 
 class UpgradesApp(app_module.App):
     """FreedomBox app for software upgrades."""
@@ -135,22 +138,14 @@ def setup_repositories(data):
     actions.superuser_run('upgrades', ['setup-repositories'])
 
 
-def get_backports_in_use():
-    """Return whether backports packages are installed."""
-    # Only freedombox package is set to be installed from backports currently.
-    output = subprocess.check_output(['apt-cache', 'policy', 'freedombox'])
-    for line in output.decode().split('\n'):
-        if 'Installed:' in line:
-            version = line.strip().split(': ')[1]
-            if 'bpo' in version:
-                return True
-
-    return False
+def is_backports_enabled():
+    """Return whether backports are enabled."""
+    return os.path.exists(SOURCES_LIST)
 
 
 def can_activate_backports():
     """Return whether backports can be activated."""
-    if get_backports_in_use():
+    if is_backports_enabled():
         return False
 
     release = subprocess.check_output(['lsb_release', '--release',