performance: Fix web client link to Cockpit

Closes: #2022.

- Differently URL based on version of Cockpit used.

- Lazily evaluate when necessary.

- Cache the result.

[sunil: Use subprocess.run() instead of getoutput() for security]
[sunil: Use long form arguments in command invocation for readability]
[sunil: Perform isort]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/performance/manifest.py

@@ -3,12 +3,36 @@
 FreedomBox app for System Monitoring (cockpit-pcp) in ‘System’.
 """
 
+import subprocess
+from functools import lru_cache
+
+from django.utils.functional import lazy
 from django.utils.translation import ugettext_lazy as _
 
+from plinth.utils import Version
+
+
+@lru_cache
+def _get_url():
+    """Return the web client URL based on Cockpit version."""
+    process = subprocess.run(
+        ['dpkg-query', '--showformat=${Version}', '--show', 'cockpit'],
+        stdout=subprocess.PIPE)
+    cockpit_version = process.stdout.decode()
+    if Version(cockpit_version) >= Version('235'):
+        url = '/_cockpit/metrics'
+    else:
+        url = '/_cockpit/system/graphs'
+
+    return url
+
+
+get_url = lazy(_get_url, str)
+
 clients = [{
     'name': _('Cockpit'),
     'platforms': [{
         'type': 'web',
-        'url': '/_cockpit/system/graphs'
+        'url': get_url()
     }]
 }]