diagnostics: fix IPv6 failures

- Curl: if an IPv6 zone index is specified, use --interface parameter.
  This fixes two issues:
  - Curl in Debian Buster doesn't support an IPv6 zone index in URL
  - Curl in Debian Bullseye doesn't redirect properly if a zone index is in URL.
- Disable IPv6 diagnostics on daemons that don't listen on an IPv6 address:
  plinth, deluge, transmission

Closes #1519

Signed-off-by: Veiko Aasa <veiko17@disroot.org>
Reviewed-by: Joseph Nuthalapati <njoseph@riseup.net>

plinth/action_utils.py

@@ -20,6 +20,7 @@ Python action utility functions.
 
 import logging
 import os
+import re
 import shutil
 import socket
 import subprocess
@@ -351,7 +352,16 @@ def _check_port(port, kind='tcp', listen_address=None):
 def check_url(url, kind=None, env=None, check_certificate=True,
               extra_options=None, wrapper=None, expected_output=None):
     """Check whether a URL is accessible."""
-    command = ['curl', '--location', '-f', '-w', '%{response_code}', url]
+    command = ['curl', '--location', '-f', '-w', '%{response_code}']
+
+    if kind == '6':
+        # extract zone index
+        match = re.match(r'(.*://)\[(.*)%(?P<zone>.*)\](.*)', url)
+        if match:
+            command = command + ['--interface', match.group('zone')]
+            url = '{0}[{1}]{2}'.format(*match.group(1, 2, 4))
+
+    command.append(url)
 
     if wrapper:
         command.insert(0, wrapper)

plinth/modules/deluge/__init__.py

@@ -111,7 +111,6 @@ def diagnose():
     results = []
 
     results.append(action_utils.diagnose_port_listening(8112, 'tcp4'))
-    results.append(action_utils.diagnose_port_listening(8112, 'tcp6'))
     results.extend(
         action_utils.diagnose_url_on_all('https://{host}/deluge',
                                          check_certificate=False))

plinth/modules/diagnostics/__init__.py

@@ -67,7 +67,6 @@ def diagnose():
     """Run diagnostics and return the results."""
     results = []
     results.append(action_utils.diagnose_port_listening(8000, 'tcp4'))
-    results.append(action_utils.diagnose_port_listening(8000, 'tcp6'))
     results.extend(
         action_utils.diagnose_url_on_all('http://{host}/plinth/',
                                          check_certificate=False))

plinth/modules/transmission/__init__.py

@@ -122,7 +122,6 @@ def diagnose():
     results = []
 
     results.append(action_utils.diagnose_port_listening(9091, 'tcp4'))
-    results.append(action_utils.diagnose_port_listening(9091, 'tcp6'))
     results.extend(
         action_utils.diagnose_url_on_all('https://{host}/transmission',
                                          check_certificate=False))