Fix local RPC IPv6 detection to use socket binding instead of file checks

The local RPC code previously checked for IPv6 availability by examining
the existence of /proc/sys/net/ipv6/conf/lo/disable_ipv6 and reading its
contents. This approach was unreliable as it depended on filesystem
checks rather than testing actual IPv6 functionality.

The fix replaces the file-based check with an actual socket binding test
to ::1 using a context manager for proper resource management. Socket
reuse is enabled to prevent port conflicts. Debug logging is added when
IPv6 is unavailable. Unit tests are updated to provide comprehensive
coverage of the new implementation.

Change-Id: I1e3afabc78f1382ff5248707ff2ca8114d10dd90
Generated-By: Claude Code Sonnet 4
Signed-off-by: Dmitry Tantsur <dtantsur@protonmail.com>

ironic/conductor/local_rpc.py

@@ -11,8 +11,8 @@
 # limitations under the License.
 
 import atexit
-import os
 import secrets
+import socket
 import tempfile
 
 from keystoneauth1 import loading as ks_loading
@@ -34,10 +34,15 @@ _USERNAME = 'ironic'
 
 
 def _lo_has_ipv6():
-    return (
-        os.path.exists("/proc/sys/net/ipv6/conf/lo/disable_ipv6")
-        and open("/proc/sys/net/ipv6/conf/lo/disable_ipv6").read() != "1"
-    )
+    """Check if IPv6 is available by attempting to bind to ::1."""
+    try:
+        with socket.socket(socket.AF_INET6, socket.SOCK_STREAM) as sock:
+            sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+            sock.bind(('::1', 0))
+            return True
+    except (OSError, socket.error) as e:
+        LOG.debug('IPv6 is not available on localhost: %s', e)
+        return False
 
 
 def _create_tls_files(ip):

ironic/tests/unit/conductor/test_local_rpc.py

@@ -12,6 +12,7 @@
 
 import ipaddress
 import os
+import socket
 from unittest import mock
 
 import bcrypt
@@ -95,3 +96,74 @@ class ConfigureTestCase(tests_base.TestCase):
         self.assertEqual('127.0.0.1', CONF.json_rpc.host_ip)
         self._verify_password()
         self._verify_tls(ipv6=False)
+
+
+@mock.patch('socket.socket', autospec=True)
+class LoHasIpv6TestCase(tests_base.TestCase):
+
+    def test_ipv6_available(self, mock_socket):
+        # Mock successful IPv6 socket creation and bind
+        mock_sock = mock.Mock()
+        mock_sock.__enter__ = mock.Mock(return_value=mock_sock)
+        mock_sock.__exit__ = mock.Mock(return_value=False)
+        mock_socket.return_value = mock_sock
+
+        result = local_rpc._lo_has_ipv6()
+
+        # Verify socket operations
+        mock_socket.assert_called_once_with(socket.AF_INET6,
+                                            socket.SOCK_STREAM)
+        mock_sock.setsockopt.assert_called_once_with(socket.SOL_SOCKET,
+                                                     socket.SO_REUSEADDR,
+                                                     1)
+        mock_sock.bind.assert_called_once_with(('::1', 0))
+        self.assertTrue(result)
+
+    def test_ipv6_not_available_os_error(self, mock_socket):
+        # Mock failed IPv6 socket bind (IPv6 not available)
+        mock_sock = mock.Mock()
+        mock_sock.__enter__ = mock.Mock(return_value=mock_sock)
+        mock_sock.__exit__ = mock.Mock(return_value=False)
+        mock_socket.return_value = mock_sock
+        mock_sock.bind.side_effect = OSError("Cannot assign requested address")
+
+        result = local_rpc._lo_has_ipv6()
+
+        # Verify socket operations attempted
+        mock_socket.assert_called_once_with(socket.AF_INET6,
+                                            socket.SOCK_STREAM)
+        mock_sock.setsockopt.assert_called_once_with(socket.SOL_SOCKET,
+                                                     socket.SO_REUSEADDR,
+                                                     1)
+        mock_sock.bind.assert_called_once_with(('::1', 0))
+        self.assertFalse(result)
+
+    def test_ipv6_not_available_socket_error(self, mock_socket):
+        # Mock socket.error during bind
+        mock_sock = mock.Mock()
+        mock_sock.__enter__ = mock.Mock(return_value=mock_sock)
+        mock_sock.__exit__ = mock.Mock(return_value=False)
+        mock_socket.return_value = mock_sock
+        mock_sock.bind.side_effect = socket.error("Network unreachable")
+
+        result = local_rpc._lo_has_ipv6()
+
+        # Verify socket operations attempted
+        mock_socket.assert_called_once_with(socket.AF_INET6,
+                                            socket.SOCK_STREAM)
+        mock_sock.setsockopt.assert_called_once_with(socket.SOL_SOCKET,
+                                                     socket.SO_REUSEADDR,
+                                                     1)
+        mock_sock.bind.assert_called_once_with(('::1', 0))
+        self.assertFalse(result)
+
+    def test_ipv6_not_available_socket_creation_fails(self, mock_socket):
+        # Mock socket creation failure
+        mock_socket.side_effect = OSError("Address family not supported")
+
+        result = local_rpc._lo_has_ipv6()
+
+        # Verify socket creation attempted
+        mock_socket.assert_called_once_with(socket.AF_INET6,
+                                            socket.SOCK_STREAM)
+        self.assertFalse(result)