upgrades: Check that backports is for current release

- If backports is for older release, then it can be activated again to
  upgrade to latest release. (Plan is to make this automatic, but
  leave the manual option as a fallback.)

- Security notice still shown if older backports are enabled.

Tests:

- On Buster system, change distribution in
  /etc/apt/sources.list.d/freedombox2.list to
  stretch-backports. Updates page shows button to activate backports
  again. Activate and check the source list to confirm that it has
  buster-backports again.

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

actions/upgrades

@@ -13,7 +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
+from plinth.modules.upgrades import is_backports_current, SOURCES_LIST
 
 AUTO_CONF_FILE = '/etc/apt/apt.conf.d/20auto-upgrades'
 LOG_FILE = '/var/log/unattended-upgrades/unattended-upgrades.log'
@@ -193,7 +193,7 @@ def _check_and_backports_sources():
     if os.path.exists(old_sources_list):
         os.remove(old_sources_list)
 
-    if is_backports_enabled():
+    if is_backports_current():
         print('Repositories list up-to-date. Skipping update.')
         return
 

plinth/modules/upgrades/__init__.py

@@ -3,6 +3,7 @@
 FreedomBox app for upgrades.
 """
 
+from aptsources import sourceslist
 import os
 import subprocess
 
@@ -143,9 +144,24 @@ def is_backports_enabled():
     return os.path.exists(SOURCES_LIST)
 
 
+def is_backports_current():
+    """Return whether backports are enabled for the current release."""
+    if not is_backports_enabled:
+        return False
+
+    dist = subprocess.check_output(['lsb_release', '--codename', '--short'
+                                    ]).decode().strip() + '-backports'
+    sources = sourceslist.SourcesList()
+    for source in sources:
+        if source.dist == dist:
+            return True
+
+    return False
+
+
 def can_activate_backports():
     """Return whether backports can be activated."""
-    if is_backports_enabled():
+    if is_backports_current():
         return False
 
     release = subprocess.check_output(['lsb_release', '--release',