Igonore if redfish fails to logout with 401 as as response

Redfish sometimes fails to logout after scraping the data and this
end up generating the metric redfish_collector_failed that trigger
prometheus alerts that can be noise.

This can be a BMC problem that might not be properly closing
sessions.

For more info see:
DMTF/python-redfish-library#160

Closes: canonical#76

prometheus_hardware_exporter/collectors/redfish.py

@@ -7,6 +7,7 @@
 from cachetools.func import ttl_cache
 from redfish import redfish_client
 from redfish.rest.v1 import (
+    BadRequestError,
     InvalidCredentialsError,
     RetriesExhaustedError,
     SessionCreationError,
@@ -92,8 +93,16 @@ def login(self) -> None:
 
     def logout(self) -> None:
         """Logout from redfish."""
-        self.redfish_obj.logout()
-        logger.debug("(service) Logged out from redfish service ...")
+        # redfish sometimes fails to logout and this end up giving false alerts
+        # See https://github.com/DMTF/python-redfish-library/issues/160 and
+        # https://github.com/canonical/prometheus-hardware-exporter/issues/76
+        try:
+            self.redfish_obj.logout()
+            logger.debug("(service) Logged out from redfish service ...")
+        except BadRequestError as err:
+            logger.error("Failed to logout redfish: %s", str(err))
+            if "code: 401" not in str(err):
+                raise
 
     def get_sensor_data(self) -> Dict[str, List]:
         """Get sensor data.

tests/unit/test_redfish.py

@@ -5,6 +5,7 @@
 import redfish_utilities
 from parameterized import parameterized
 from redfish.rest.v1 import (
+    BadRequestError,
     InvalidCredentialsError,
     RetriesExhaustedError,
     SessionCreationError,
@@ -1095,3 +1096,28 @@ def test_discover_cache(self, mock_redfish_client):
             username="",
             password="",
         )
+
+    @patch("prometheus_hardware_exporter.collectors.redfish.redfish_client")
+    def test_redfish_logout_401(self, mock_redfish_client):
+        """Test that exception is not raised if logout fails with 401."""
+        mock_redfish_obj = Mock()
+        mock_redfish_obj.logout.side_effect = BadRequestError(
+            "Invalid session resource: /redfish/v1/SessionService/Sessions/132562, "
+            "return code: 401"
+        )
+        mock_redfish_client.return_value = mock_redfish_obj
+        with RedfishHelper(Mock()) as redfish_helper:
+            self.assertIsNone(redfish_helper.logout())
+
+    @patch("prometheus_hardware_exporter.collectors.redfish.redfish_client")
+    def test_redfish_logout_error(self, mock_redfish_client):
+        """Test that exception is raised if logout fails with response not 401."""
+        mock_redfish_obj = Mock()
+        mock_redfish_obj.logout.side_effect = BadRequestError(
+            "Invalid session resource: /redfish/v1/SessionService/Sessions/132562, "
+            "return code: 404"
+        )
+        mock_redfish_client.return_value = mock_redfish_obj
+        redfish_helper = RedfishHelper(Mock())
+        with self.assertRaises(BadRequestError):
+            redfish_helper.logout()