Add support for before and after request functions

In flask it is possible to register functions that will run before
and after every request, these are useful for ensuring certain
things occur (setting up environments) and protecting against
cases such as un-enforced API calls not explicitly white-listed.

Partial-Bug: #1776504
Change-Id: I0138a4676c1e257cd0c238bbe11a520446c299cb

keystone/server/flask/common.py

@@ -90,6 +90,8 @@ class APIBase(object):
     def __init__(self, blueprint_url_prefix='', api_url_prefix='',
                  default_mediatype='application/json', decorators=None,
                  errors=None):
+        self.__before_request_functions_added = False
+        self.__after_request_functions_added = False
         self._blueprint_url_prefix = blueprint_url_prefix
         self._default_mediatype = default_mediatype
         self._api_url_prefix = api_url_prefix
@@ -102,6 +104,15 @@ class APIBase(object):
             decorators=decorators, errors=errors)
         self._add_resources()
 
+        # Apply Before and After request functions
+        self._register_before_request_functions()
+        self._register_after_request_functions()
+        # Assert is intended to ensure code works as expected in development,
+        # it is fine to optimize out with python -O
+        msg = '%s_request functions not registered'
+        assert self.__before_request_functions_added, msg % 'before'  # nosec
+        assert self.__after_request_functions_added, msg % 'after'  # nosec
+
     def _add_resources(self):
         for r in self.resource_mapping:
             LOG.debug(
@@ -110,6 +121,66 @@ class APIBase(object):
                 {'name': self._name, 'urls': r.urls, 'kwargs': r.kwargs})
             self._blueprint.add_resource(r.resource, *r.urls, **r.kwargs)
 
+    def _register_before_request_functions(self, functions=None):
+        """Register functions to be executed in the `before request` phase.
+
+        Override this method and pass in via "super" any additional functions
+        that should be registered. It is assumed that any override will also
+        accept a "functions" list and append the passed in values to it's
+        list prior to calling super.
+
+        Each function will be called with no arguments and expects a NoneType
+        return. If the function returns a value, that value will be returned
+        as the response to the entire request, no further processing will
+        happen.
+
+        :param functions: list of functions that will be run in the
+                          `before_request` phase.
+        :type functions: list
+        """
+        functions = functions or []
+        # Assert is intended to ensure code works as expected in development,
+        # it is fine to optimize out with python -O
+        msg = 'before_request functions already registered'
+        assert not self.__before_request_functions_added, msg  # nosec
+        # register global before request functions
+        # e.g. self.__blueprint.before_request(function)
+
+        # Add passed-in functions
+        for f in functions:
+            self.__blueprint.before_request(f)
+        self.__before_request_functions_added = True
+
+    def _register_after_request_functions(self, functions=None):
+        """Register functions to be executed in the `after request` phase.
+
+        Override this method and pass in via "super" any additional functions
+        that should be registered. It is assumed that any override will also
+        accept a "functions" list and append the passed in values to it's
+        list prior to calling super.
+
+        Each function will be called with a single argument of the Response
+        class type. The function must return either the passed in Response or
+        a new Response. NOTE: As of flask 0.7, these functions may not be
+        executed in the case of an unhandled exception.
+
+        :param functions: list of functions that will be run in the
+                          `after_request` phase.
+        :type functions: list
+        """
+        functions = functions or []
+        # Assert is intended to ensure code works as expected in development,
+        # it is fine to optimize out with python -O
+        msg = 'after_request functions already registered'
+        assert not self.__after_request_functions_added, msg  # nosec
+        # register global after request functions
+        # e.g. self.__blueprint.after_request(function)
+
+        # Add Passed-In Functions
+        for f in functions:
+            self.__blueprint.after_request(f)
+        self.__after_request_functions_added = True
+
     @classmethod
     def instantiate_and_register_to_app(cls, flask_app):
         """Build the API object and register to the passed in flask_app.