Use function registration for policy checks

The original policy framework allowed new policy checks to be created
through inheritance.  This is somewhat clunky and unnecessary in
Python.  This change refactors policy.py to allow new policy checks
to be registered using an @register() decorator.  One consequence is
that HttpBrain is deprecated.

Care has been taken to ensure backwards compatibility; deprecation
warnings will be emitted for uses of HttpBrain or the inheritance-
based checks.

(Pull-up from openstack-common, with deprecation of HttpBrain.)

Change-Id: Ia9a6039a82fe4ebfa9b18e5eb93c21fffee90f09

nova/openstack/common/policy.py

@@ -131,13 +131,25 @@ def enforce(match_list, target_dict, credentials_dict, exc=None,
 
 class Brain(object):
     """Implements policy checking."""
+
+    _checks = {}
+
+    @classmethod
+    def _register(cls, name, func):
+        cls._checks[name] = func
+
     @classmethod
     def load_json(cls, data, default_rule=None):
         """Init a brain using json instead of a rules dictionary."""
         rules_dict = jsonutils.loads(data)
         return cls(rules=rules_dict, default_rule=default_rule)
 
     def __init__(self, rules=None, default_rule=None):
+        if self.__class__ != Brain:
+            LOG.warning(_("Inheritance-based rules are deprecated; use "
+                          "the default brain instead of %s.") %
+                        self.__class__.__name__)
+
         self.rules = rules or {}
         self.default_rule = default_rule
 
@@ -151,15 +163,24 @@ def _check(self, match, target_dict, cred_dict):
             LOG.exception(_("Failed to understand rule %(match)r") % locals())
             # If the rule is invalid, fail closed
             return False
+
+        func = None
         try:
-            f = getattr(self, '_check_%s' % match_kind)
+            old_func = getattr(self, '_check_%s' % match_kind)
         except AttributeError:
-            if not self._check_generic(match, target_dict, cred_dict):
-                return False
+            func = self._checks.get(match_kind, self._checks.get(None, None))
         else:
-            if not f(match_value, target_dict, cred_dict):
-                return False
-        return True
+            LOG.warning(_("Inheritance-based rules are deprecated; update "
+                          "_check_%s") % match_kind)
+            func = (lambda brain, kind, value, target, cred:
+                        old_func(value, target, cred))
+
+        if not func:
+            LOG.error(_("No handler for matches of kind %s") % match_kind)
+            # Fail closed
+            return False
+
+        return func(self, match_kind, match_value, target_dict, cred_dict)
 
     def check(self, match_list, target_dict, cred_dict):
         """Checks authorization of some rules against credentials.
@@ -183,58 +204,97 @@ def check(self, match_list, target_dict, cred_dict):
                 return True
         return False
 
-    def _check_rule(self, match, target_dict, cred_dict):
-        """Recursively checks credentials based on the brains rules."""
-        try:
-            new_match_list = self.rules[match]
-        except KeyError:
-            if self.default_rule and match != self.default_rule:
-                new_match_list = ('rule:%s' % self.default_rule,)
-            else:
-                return False
 
-        return self.check(new_match_list, target_dict, cred_dict)
+class HttpBrain(Brain):
+    """A brain that can check external urls for policy.
 
-    def _check_role(self, match, target_dict, cred_dict):
-        """Check that there is a matching role in the cred dict."""
-        return match.lower() in [x.lower() for x in cred_dict['roles']]
+    Posts json blobs for target and credentials.
 
-    def _check_generic(self, match, target_dict, cred_dict):
-        """Check an individual match.
+    Note that this brain is deprecated; the http check is registered
+    by default.
+    """
 
-        Matches look like:
+    pass
 
-            tenant:%(tenant_id)s
-            role:compute:admin
 
-        """
+def register(name, func=None):
+    """
+    Register a function as a policy check.
+
+    :param name: Gives the name of the check type, e.g., 'rule',
+                 'role', etc.  If name is None, a default function
+                 will be registered.
+    :param func: If given, provides the function to register.  If not
+                 given, returns a function taking one argument to
+                 specify the function to register, allowing use as a
+                 decorator.
+    """
 
-        # TODO(termie): do dict inspection via dot syntax
-        match = match % target_dict
-        key, value = match.split(':', 1)
-        if key in cred_dict:
-            return value == cred_dict[key]
-        return False
+    # Perform the actual decoration by registering the function.
+    # Returns the function for compliance with the decorator
+    # interface.
+    def decorator(func):
+        # Register the function
+        Brain._register(name, func)
+        return func
+
+    # If the function is given, do the registration
+    if func:
+        return decorator(func)
+
+    return decorator
+
+
+@register("rule")
+def _check_rule(brain, match_kind, match, target_dict, cred_dict):
+    """Recursively checks credentials based on the brains rules."""
+    try:
+        new_match_list = brain.rules[match]
+    except KeyError:
+        if brain.default_rule and match != brain.default_rule:
+            new_match_list = ('rule:%s' % brain.default_rule,)
+        else:
+            return False
 
+    return brain.check(new_match_list, target_dict, cred_dict)
 
-class HttpBrain(Brain):
-    """A brain that can check external urls for policy.
 
-    Posts json blobs for target and credentials.
+@register("role")
+def _check_role(brain, match_kind, match, target_dict, cred_dict):
+    """Check that there is a matching role in the cred dict."""
+    return match.lower() in [x.lower() for x in cred_dict['roles']]
+
+
+@register('http')
+def _check_http(brain, match_kind, match, target_dict, cred_dict):
+    """Check http: rules by calling to a remote server.
+
+    This example implementation simply verifies that the response is
+    exactly 'True'. A custom brain using response codes could easily
+    be implemented.
 
     """
+    url = 'http:' + (match % target_dict)
+    data = {'target': jsonutils.dumps(target_dict),
+            'credentials': jsonutils.dumps(cred_dict)}
+    post_data = urllib.urlencode(data)
+    f = urllib2.urlopen(url, post_data)
+    return f.read() == "True"
 
-    def _check_http(self, match, target_dict, cred_dict):
-        """Check http: rules by calling to a remote server.
 
-        This example implementation simply verifies that the response is
-        exactly 'True'. A custom brain using response codes could easily
-        be implemented.
+@register(None)
+def _check_generic(brain, match_kind, match, target_dict, cred_dict):
+    """Check an individual match.
 
-        """
-        url = match % target_dict
-        data = {'target': jsonutils.dumps(target_dict),
-                'credentials': jsonutils.dumps(cred_dict)}
-        post_data = urllib.urlencode(data)
-        f = urllib2.urlopen(url, post_data)
-        return f.read() == "True"
+    Matches look like:
+
+        tenant:%(tenant_id)s
+        role:compute:admin
+
+    """
+
+    # TODO(termie): do dict inspection via dot syntax
+    match = match % target_dict
+    if match_kind in cred_dict:
+        return match == cred_dict[match_kind]
+    return False

nova/policy.py

@@ -65,7 +65,7 @@ def init():
 
 def _set_brain(data):
     default_rule = FLAGS.policy_default_rule
-    policy.set_brain(policy.HttpBrain.load_json(data, default_rule))
+    policy.set_brain(policy.Brain.load_json(data, default_rule))
 
 
 def enforce(context, action, target):