Fixing the 500 HTTP code in the metadata service if Nova is down

If the Nova metadata service is unavailable, the requests.request()
function may raise a ConnectionError. This results in the upper code
returning a 500 HTTP status code to the user along with a traceback.
Let's handle this scenario and instead return a 503 HTTP status code
(service unavailable).

If the Nova service is down and is behind another proxy (such as
Nginx), then instead of a ConnectionError, the request may result in
receiving a 502 or 503 HTTP status code. Let's also consider this
situation and add support for an additional 504 code.

Closes-Bug: #2059032
Change-Id: I16be18c46a6796224b0793dc385b0ddec01739c4

neutron/agent/metadata/agent.py

@@ -246,12 +246,18 @@ class MetadataProxyHandler(object):
             client_cert = (self.conf.nova_client_cert,
                            self.conf.nova_client_priv_key)
 
-        resp = requests.request(method=req.method, url=url,
-                                headers=headers,
-                                data=req.body,
-                                cert=client_cert,
-                                verify=verify_cert,
-                                timeout=60)
+        try:
+            resp = requests.request(method=req.method, url=url,
+                                    headers=headers,
+                                    data=req.body,
+                                    cert=client_cert,
+                                    verify=verify_cert,
+                                    timeout=60)
+        except requests.ConnectionError:
+            msg = _('The remote metadata server is temporarily unavailable. '
+                    'Please try again later.')
+            explanation = str(msg)
+            return webob.exc.HTTPServiceUnavailable(explanation=explanation)
 
         if resp.status_code == 200:
             req.response.content_type = resp.headers['content-type']
@@ -264,12 +270,6 @@ class MetadataProxyHandler(object):
                 'response usually occurs when shared secrets do not match.'
             )
             return webob.exc.HTTPForbidden()
-        elif resp.status_code == 400:
-            return webob.exc.HTTPBadRequest()
-        elif resp.status_code == 404:
-            return webob.exc.HTTPNotFound()
-        elif resp.status_code == 409:
-            return webob.exc.HTTPConflict()
         elif resp.status_code == 500:
             msg = _(
                 'Remote metadata server experienced an internal server error.'
@@ -277,6 +277,9 @@ class MetadataProxyHandler(object):
             LOG.warning(msg)
             explanation = str(msg)
             return webob.exc.HTTPInternalServerError(explanation=explanation)
+        elif resp.status_code in (400, 404, 409, 502, 503, 504):
+            webob_exc_cls = webob.exc.status_map.get(resp.status_code)
+            return webob_exc_cls()
         else:
             raise Exception(_('Unexpected response code: %s') %
                             resp.status_code)

neutron/agent/ovn/metadata/server.py

@@ -168,12 +168,18 @@ class MetadataProxyHandler(object):
             client_cert = (self.conf.nova_client_cert,
                            self.conf.nova_client_priv_key)
 
-        resp = requests.request(method=req.method, url=url,
-                                headers=headers,
-                                data=req.body,
-                                cert=client_cert,
-                                verify=verify_cert,
-                                timeout=60)
+        try:
+            resp = requests.request(method=req.method, url=url,
+                                    headers=headers,
+                                    data=req.body,
+                                    cert=client_cert,
+                                    verify=verify_cert,
+                                    timeout=60)
+        except requests.ConnectionError:
+            msg = _('The remote metadata server is temporarily unavailable. '
+                    'Please try again later.')
+            explanation = str(msg)
+            return webob.exc.HTTPServiceUnavailable(explanation=explanation)
 
         if resp.status_code == 200:
             req.response.content_type = resp.headers['content-type']
@@ -186,12 +192,6 @@ class MetadataProxyHandler(object):
                 'response usually occurs when shared secrets do not match.'
             )
             return webob.exc.HTTPForbidden()
-        elif resp.status_code == 400:
-            return webob.exc.HTTPBadRequest()
-        elif resp.status_code == 404:
-            return webob.exc.HTTPNotFound()
-        elif resp.status_code == 409:
-            return webob.exc.HTTPConflict()
         elif resp.status_code == 500:
             msg = _(
                 'Remote metadata server experienced an internal server error.'
@@ -199,6 +199,9 @@ class MetadataProxyHandler(object):
             LOG.warning(msg)
             explanation = str(msg)
             return webob.exc.HTTPInternalServerError(explanation=explanation)
+        elif resp.status_code in (400, 404, 409, 502, 503, 504):
+            webob_exc_cls = webob.exc.status_map.get(resp.status_code)
+            return webob_exc_cls()
         else:
             raise Exception(_('Unexpected response code: %s') %
                             resp.status_code)

neutron/tests/unit/agent/metadata/test_agent.py

@@ -17,6 +17,7 @@ from unittest import mock
 import ddt
 import netaddr
 from neutron_lib import constants as n_const
+import requests
 import testtools
 import webob
 
@@ -469,10 +470,30 @@ class _TestMetadataProxyHandlerCacheMixin(object):
         self.assertIsInstance(self._proxy_request_test_helper(500),
                               webob.exc.HTTPInternalServerError)
 
+    def test_proxy_request_502(self):
+        self.assertIsInstance(self._proxy_request_test_helper(502),
+                              webob.exc.HTTPBadGateway)
+
+    def test_proxy_request_503(self):
+        self.assertIsInstance(self._proxy_request_test_helper(503),
+                              webob.exc.HTTPServiceUnavailable)
+
+    def test_proxy_request_504(self):
+        self.assertIsInstance(self._proxy_request_test_helper(504),
+                              webob.exc.HTTPGatewayTimeout)
+
     def test_proxy_request_other_code(self):
         with testtools.ExpectedException(Exception):
             self._proxy_request_test_helper(302)
 
+    def test_proxy_request_conenction_error(self):
+        req = mock.Mock(path_info='/the_path', query_string='', headers={},
+                        method='GET', body='')
+        with mock.patch('requests.request') as mock_request:
+            mock_request.side_effect = requests.ConnectionError()
+            retval = self.handler._proxy_request('the_id', 'tenant_id', req)
+            self.assertIsInstance(retval, webob.exc.HTTPServiceUnavailable)
+
 
 class TestMetadataProxyHandlerNewCache(TestMetadataProxyHandlerBase,
                                        _TestMetadataProxyHandlerCacheMixin):

neutron/tests/unit/agent/ovn/metadata/test_server.py

@@ -18,6 +18,7 @@ from unittest import mock
 from oslo_config import cfg
 from oslo_config import fixture as config_fixture
 from oslo_utils import fileutils
+import requests
 import testtools
 import webob
 
@@ -232,10 +233,30 @@ class TestMetadataProxyHandler(base.BaseTestCase):
         self.assertIsInstance(self._proxy_request_test_helper(500),
                               webob.exc.HTTPInternalServerError)
 
+    def test_proxy_request_502(self):
+        self.assertIsInstance(self._proxy_request_test_helper(502),
+                              webob.exc.HTTPBadGateway)
+
+    def test_proxy_request_503(self):
+        self.assertIsInstance(self._proxy_request_test_helper(503),
+                              webob.exc.HTTPServiceUnavailable)
+
+    def test_proxy_request_504(self):
+        self.assertIsInstance(self._proxy_request_test_helper(504),
+                              webob.exc.HTTPGatewayTimeout)
+
     def test_proxy_request_other_code(self):
         with testtools.ExpectedException(Exception):
             self._proxy_request_test_helper(302)
 
+    def test_proxy_request_conenction_error(self):
+        req = mock.Mock(path_info='/the_path', query_string='', headers={},
+                        method='GET', body='')
+        with mock.patch('requests.request') as mock_request:
+            mock_request.side_effect = requests.ConnectionError()
+            retval = self.handler._proxy_request('the_id', 'tenant_id', req)
+            self.assertIsInstance(retval, webob.exc.HTTPServiceUnavailable)
+
 
 class TestUnixDomainMetadataProxy(base.BaseTestCase):
     def setUp(self):

releasenotes/notes/metadata-proxy-enhance-error-handling-3655404d44249097.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Enhance error handling in the Neutron metadata service for cases when the
+    Nova metadata service is unavailable, ensuring correct HTTP status codes
+    are returned.