Enable fm-rest-api to return json format error message

Closes-Bug: 1789979

fm-rest-api returns json format message for the correct api
access. For the invalid api access, the message is with html
format, need add wrap_app to convert it to json format.
Fix:
The implementation code is copied from upstream Ironic API,
and minor modification to pass tox pep8 check.
Also remove "x" attribute for the py file.
Test:
Pass build and multi node deploy test. Confirmed the return
message is json format.

Change-Id: I36fa89b82377d52008a467316c42c06caa65cd90
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>

fm-rest-api/fm/fm/api/app.py

@@ -23,6 +23,7 @@ from oslo_log import log
 import pecan
 
 from fm.api import config
+from fm.api import middleware
 from fm.common import policy
 from fm.common.i18n import _
 
@@ -53,6 +54,7 @@ def setup_app(config=None):
         debug=CONF.debug,
         logging=getattr(config, 'logging', {}),
         force_canonical=getattr(config.app, 'force_canonical', True),
+        wrap_app=middleware.ParsableErrorMiddleware,
         guess_content_type_from_ext=False,
         **app_conf
     )

fm-rest-api/fm/fm/api/middleware/__init__.py

@@ -3,3 +3,18 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 #
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+from fm.api.middleware import auth_token
+from fm.api.middleware import parsable_error
+
+
+ParsableErrorMiddleware = parsable_error.ParsableErrorMiddleware
+AuthTokenMiddleware = auth_token.AuthTokenMiddleware
+
+__all__ = (ParsableErrorMiddleware,
+           AuthTokenMiddleware)

fm-rest-api/fm/fm/api/middleware/parsable_error.py

@@ -0,0 +1,98 @@
+# -*- encoding: utf-8 -*-
+#
+# Copyright © 2012 New Dream Network, LLC (DreamHost)
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+"""
+Middleware to replace the plain text message body of an error
+response with one formatted so the client can parse it.
+
+Based on pecan.middleware.errordocument
+"""
+
+import json
+from xml import etree as et
+
+from oslo_log import log
+import six
+import webob
+
+from fm.common.i18n import _
+
+LOG = log.getLogger(__name__)
+
+
+class ParsableErrorMiddleware(object):
+    """Replace error body with something the client can parse."""
+    def __init__(self, app):
+        self.app = app
+
+    def __call__(self, environ, start_response):
+        # Request for this state, modified by replace_start_response()
+        # and used when an error is being reported.
+        state = {}
+
+        def replacement_start_response(status, headers, exc_info=None):
+            """Overrides the default response to make errors parsable."""
+            try:
+                status_code = int(status.split(' ')[0])
+                state['status_code'] = status_code
+            except (ValueError, TypeError):  # pragma: nocover
+                raise Exception(_(
+                    'ErrorDocumentMiddleware received an invalid '
+                    'status %s') % status)
+            else:
+                if (state['status_code'] // 100) not in (2, 3):
+                    # Remove some headers so we can replace them later
+                    # when we have the full error message and can
+                    # compute the length.
+                    headers = [(h, v)
+                               for (h, v) in headers
+                               if h not in ('Content-Length', 'Content-Type')
+                               ]
+                # Save the headers in case we need to modify them.
+                state['headers'] = headers
+                return start_response(status, headers, exc_info)
+
+        # The default output is application/json. However, Pecan will try
+        # to output HTML errors if no Accept header is provided.
+        if 'HTTP_ACCEPT' not in environ or environ['HTTP_ACCEPT'] == '*/*':
+            environ['HTTP_ACCEPT'] = 'application/json'
+
+        app_iter = self.app(environ, replacement_start_response)
+        if (state['status_code'] // 100) not in (2, 3):
+            req = webob.Request(environ)
+            if (req.accept.best_match(['application/json',
+                                       'application/xml']) == 'application/xml'):
+                try:
+                    # simple check xml is valid
+                    body = [et.ElementTree.tostring(
+                            et.ElementTree.fromstring('<error_message>' +
+                                                      '\n'.join(app_iter) +
+                                                      '</error_message>'))]
+                except et.ElementTree.ParseError as err:
+                    LOG.error('Error parsing HTTP response: %s', err)
+                    body = ['<error_message>%s' % state['status_code'] +
+                            '</error_message>']
+                state['headers'].append(('Content-Type', 'application/xml'))
+            else:
+                if six.PY3:
+                    app_iter = [i.decode('utf-8') for i in app_iter]
+                body = [json.dumps({'error_message': '\n'.join(app_iter)})]
+                if six.PY3:
+                    body = [item.encode('utf-8') for item in body]
+                state['headers'].append(('Content-Type', 'application/json'))
+            state['headers'].append(('Content-Length', str(len(body[0]))))
+        else:
+            body = app_iter
+        return body