# Copyright 2017 Huawei Technologies Co.,LTD.
# All Rights Reserved.
#
#    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
"""

from oslo_serialization import jsonutils


class ParsableErrorMiddleware:
    """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(
                    'ParsableErrorMiddleware received an invalid '
                    'status %s' % status
                )

            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)

        app_iter = self.app(environ, replacement_start_response)

        if (state['status_code'] // 100) not in (2, 3):
            app_iter = [i.decode('utf-8') for i in app_iter]
            body = [jsonutils.dumps({'error_message': '\n'.join(app_iter)})]
            body = [i.encode('utf-8') for i in body]
            state['headers'].append(('Content-Type', 'application/json'))
            state['headers'].append(('Content-Length', str(len(body[0]))))
        else:
            body = app_iter
        return body
