Adding HTTP Method Overrides
============================

Some HTTP proxies do not support arbitrary HTTP methods or newer HTTP
methods (such as PATCH). In that case it's possible to "proxy" HTTP
methods through another HTTP method in total violation of the protocol.

The way this works is by letting the client do an HTTP POST request and
set the ``X-HTTP-Method-Override`` header. Then the method is replaced
with the header value before being passed to Flask.

This can be accomplished with an HTTP middleware::

    class HTTPMethodOverrideMiddleware(object):
        allowed_methods = frozenset([
            'GET',
            'HEAD',
            'POST',
            'DELETE',
            'PUT',
            'PATCH',
            'OPTIONS'
        ])
        bodyless_methods = frozenset(['GET', 'HEAD', 'OPTIONS', 'DELETE'])

        def __init__(self, app):
            self.app = app

        def __call__(self, environ, start_response):
            method = environ.get('HTTP_X_HTTP_METHOD_OVERRIDE', '').upper()
            if method in self.allowed_methods:
                environ['REQUEST_METHOD'] = method
            if method in self.bodyless_methods:
                environ['CONTENT_LENGTH'] = '0'
            return self.app(environ, start_response)

To use this with Flask, wrap the app object with the middleware::

    from flask import Flask

    app = Flask(__name__)
    app.wsgi_app = HTTPMethodOverrideMiddleware(app.wsgi_app)