upgrades: Treat n/a release as testing

Closes #2092

On testing and unstable systems, /etc/os-release does not contain
VERSION_ID. In this case, lsb_release will report the release as
"n/a".

For unstable, this means that backports can be enabled in development
mode. When this happens, trixie-backports will be added as an apt
repository. The repository already exists, so it does not cause any
problem.

Tests:

- In stable container, backports can be enabled.

- In stable container, dist-upgrade can be disable and enabled.

- In stable container, in development mode, dist-upgrade can be
  started.

- In testing container, backports cannot be enabled.

- In testing container, dist-upgrade cannot be enabled or started.

- In testing container, in development mode, backports can be enabled.

- In testing container, in development mode, dist-upgrade cannot be
  started.

- In unstable container, in development mode, backports can be enabled
  (as trixie-backports).

- In unstable container, in development mode, dist-upgrade cannot be
  started.

Signed-off-by: James Valleroy <jvalleroy@mailbox.org>
[sunil: Merge the case of outdated unstable distributions that return 'unstable'
        as release and newer unstable distributions that return 'n/a']
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/upgrades/__init__.py

@@ -10,8 +10,9 @@ from django.utils.translation import gettext_lazy as _
 from django.utils.translation import gettext_noop
 
 import plinth
+from plinth import action_utils
 from plinth import app as app_module
-from plinth import action_utils, cfg, glib, kvstore, menu, package
+from plinth import cfg, glib, kvstore, menu, package
 from plinth.config import DropinConfigs
 from plinth.daemon import RelatedDaemon
 from plinth.diagnostic_check import DiagnosticCheck, Result
@@ -294,17 +295,18 @@ def is_backports_current():
 
 def can_activate_backports():
     """Return whether backports can be activated."""
-    release, _ = get_current_release()
+    if cfg.develop:
-    if release == 'unstable' or (release == 'testing' and not cfg.develop):
+        return True
-        return False
 
-    return True
+    # Release will be 'n/a' in latest unstable and testing distributions.
+    release, _ = get_current_release()
+    return release not in ['unstable', 'testing', 'n/a']
 
 
 def can_enable_dist_upgrade():
     """Return whether dist upgrade can be enabled."""
     release, _ = get_current_release()
-    return release not in ['unstable', 'testing']
+    return release not in ['unstable', 'testing', 'n/a']
 
 
 def can_test_dist_upgrade():

plinth/modules/upgrades/privileged.py

@@ -257,7 +257,7 @@ def _check_and_backports_sources(develop=False):
         return
 
     release, dist = get_current_release()
-    if release == 'unstable' or (release == 'testing' and not develop):
+    if release in ['unstable', 'testing', 'n/a'] and not develop:
         logging.info(f'System release is {release}. Skip enabling backports.')
         return
 
@@ -326,7 +326,7 @@ def _check_dist_upgrade(test_upgrade=False) -> tuple[bool, str]:
 
     from plinth.modules.upgrades import get_current_release
     release, dist = get_current_release()
-    if release in ['unstable', 'testing']:
+    if release in ['unstable', 'testing', 'n/a']:
         return (False, f'already-{release}')
 
     check_dists = ['stable']