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# vim: tabstop=4 shiftwidth=4 softtabstop=4

# Copyright (c) 2012 OpenStack Foundation.

# 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.

"""

Common Policy Engine Implementation

Policies can be expressed in one of two forms: A list of lists, or a

string written in the new policy language.

In the list-of-lists representation, each check inside the innermost

list is combined as with an "and" conjunction--for that check to pass,

all the specified checks must pass. These innermost lists are then

combined as with an "or" conjunction. This is the original way of

expressing policies, but there now exists a new way: the policy

language.

In the policy language, each check is specified the same way as in the

list-of-lists representation: a simple "a:b" pair that is matched to

the correct code to perform that check. However, conjunction

operators are available, allowing for more expressiveness in crafting

policies.

As an example, take the following rule, expressed in the list-of-lists

representation::

[["role:admin"], ["project_id:%(project_id)s", "role:projectadmin"]]

In the policy language, this becomes::

role:admin or (project_id:%(project_id)s and role:projectadmin)

The policy language also has the "not" operator, allowing a richer

policy rule::

project_id:%(project_id)s and not role:dunce

Finally, two special policy checks should be mentioned; the policy

check "@" will always accept an access, and the policy check "!" will

always reject an access. (Note that if a rule is either the empty

list ("[]") or the empty string, this is equivalent to the "@" policy

check.) Of these, the "!" policy check is probably the most useful,

as it allows particular rules to be explicitly disabled.

"""

import abc

import re

import urllib

import urllib2

from nova.openstack.common.gettextutils import _

from nova.openstack.common import jsonutils

from nova.openstack.common import log as logging

LOG = logging.getLogger(__name__)

_rules = None

_checks = {}

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class Rules(dict):

"""

A store for rules. Handles the

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    def load_json(cls, data, default_rule=None):

        """

        Allow loading of JSON rule data.

        """

        # Suck in the JSON data and parse the rules

        rules = dict((k, parse_rule(v)) for k, v in

                     jsonutils.loads(data).items())

        return cls(rules, default_rule)

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    def __init__(self, rules=None, default_rule=None):

        """Initialize the Rules store."""

        super(Rules, self).__init__(rules or {})

        self.default_rule = default_rule

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    def __missing__(self, key):

        """Implements the default rule handling."""

        # If the default rule isn't actually defined, do something

        # reasonably intelligent

        if not self.default_rule or self.default_rule not in self:

            raise KeyError(key)

        return self[self.default_rule]

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    def __str__(self):

        """Dumps a string representation of the rules."""

        # Start by building the canonical strings for the rules

        out_rules = {}

        for key, value in self.items():

            # Use empty string for singleton TrueCheck instances

            if isinstance(value, TrueCheck):

                out_rules[key] = ''

            else:

                out_rules[key] = str(value)

        # Dump a pretty-printed JSON representation

        return jsonutils.dumps(out_rules, indent=4)

# Really have to figure out a way to deprecate this

def set_rules(rules):

    """Set the rules in use for policy checks."""

    global _rules

    _rules = rules

# Ditto

def reset():

    """Clear the rules used for policy checks."""

    global _rules

    _rules = None

def check(rule, target, creds, exc=None, *args, **kwargs):

    """

    Checks authorization of a rule against the target and credentials.

    :param rule: The rule to evaluate.

    :param target: As much information about the object being operated

                   on as possible, as a dictionary.

    :param creds: As much information about the user performing the

                  action as possible, as a dictionary.

    :param exc: Class of the exception to raise if the check fails.

                Any remaining arguments passed to check() (both

                positional and keyword arguments) will be passed to

                the exception class.  If exc is not provided, returns

                False.

    :return: Returns False if the policy does not allow the action and

             exc is not provided; otherwise, returns a value that

             evaluates to True.  Note: for rules using the "case"

             expression, this True value will be the specified string

             from the expression.

    """

    # Allow the rule to be a Check tree

    if isinstance(rule, BaseCheck):

        result = rule(target, creds)

    elif not _rules:

        # No rules to reference means we're going to fail closed

        result = False

    else:

        try:

            # Evaluate the rule

            result = _rules[rule](target, creds)

        except KeyError:

            # If the rule doesn't exist, fail closed

            result = False

    # If it is False, raise the exception if requested

    if exc and result is False:

        raise exc(*args, **kwargs)

    return result

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def set_rules(rules):

    """Set the rules in use for policy checks."""

    global _rules

    _rules = rules

# Ditto

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def reset():

    """Clear the rules used for policy checks."""

    global _rules

    _rules = None

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def check(rule, target, creds, exc=None, *args, **kwargs):

    """

    Checks authorization of a rule against the target and credentials.

    :param rule: The rule to evaluate.

    :param target: As much information about the object being operated

                   on as possible, as a dictionary.

    :param creds: As much information about the user performing the

                  action as possible, as a dictionary.

    :param exc: Class of the exception to raise if the check fails.

                Any remaining arguments passed to check() (both

                positional and keyword arguments) will be passed to

                the exception class.  If exc is not provided, returns

                False.

    :return: Returns False if the policy does not allow the action and

             exc is not provided; otherwise, returns a value that

             evaluates to True.  Note: for rules using the "case"

             expression, this True value will be the specified string

             from the expression.

    """

    # Allow the rule to be a Check tree

    if isinstance(rule, BaseCheck):

        result = rule(target, creds)

    elif not _rules:

        # No rules to reference means we're going to fail closed

        result = False

    else:

        try:

            # Evaluate the rule

            result = _rules[rule](target, creds)

        except KeyError:

            # If the rule doesn't exist, fail closed

            result = False

    # If it is False, raise the exception if requested

    if exc and result is False:

        raise exc(*args, **kwargs)

    return result

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class BaseCheck(object):

"""

Abstract base class for Check classes.

"""

__metaclass__ = abc.ABCMeta

@abc.abstractmethod

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    def __str__(self):

        """

        Retrieve a string representation of the Check tree rooted at

        this node.

        """

        pass

    @abc.abstractmethod

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    def __call__(self, target, cred):

        """

        Perform the check.  Returns False to reject the access or a

        true value (not necessary True) to accept the access.

        """

        pass

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class FalseCheck(BaseCheck):

"""

A policy check that always returns False (disallow).

"""

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    def __str__(self):

        """Return a string representation of this check."""

        return "!"

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    def __call__(self, target, cred):

        """Check the policy."""

        return False

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class TrueCheck(BaseCheck):

"""

A policy check that always returns True (allow).

"""

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    def __str__(self):

        """Return a string representation of this check."""

        return "@"

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    def __call__(self, target, cred):

        """Check the policy."""

        return True

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class Check(BaseCheck):

"""

A base class to allow for user-

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    def __init__(self, kind, match):

        """

        :param kind: The kind of the check, i.e., the field before the

                     ':'.

        :param match: The match of the check, i.e., the field after

                      the ':'.

        """

        self.kind = kind

        self.match = match

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    def __str__(self):

        """Return a string representation of this check."""

        return "%s:%s" % (self.kind, self.match)

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class NotCheck(BaseCheck):

"""

A policy check that inverts the result of another policy check.

Implements the "not" operator.

"""

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    def __init__(self, rule):

        """

        Initialize the 'not' check.

        :param rule: The rule to negate.  Must be a Check.

        """

        self.rule = rule

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    def __str__(self):

        """Return a string representation of this check."""

        return "not %s" % self.rule

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    def __call__(self, target, cred):

        """

        Check the policy.  Returns the logical inverse of the wrapped

        check.

        """

        return not self.rule(target, cred)

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class AndCheck(BaseCheck):

"""

A policy check that requires that a list of other checks all

return True. Implements the "and" operator.

"""

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    def __init__(self, rules):

        """

        Initialize the 'and' check.

        :param rules: A list of rules that will be tested.

        """

        self.rules = rules

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    def __str__(self):

        """Return a string representation of this check."""

        return "(%s)" % ' and '.join(str(r) for r in self.rules)

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    def __call__(self, target, cred):

        """

        Check the policy.  Requires that all rules accept in order to

        return True.

        """

        for rule in self.rules:

            if not rule(target, cred):

                return False

        return True

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    def add_check(self, rule):

        """

        Allows addition of another rule to the list of rules that will

        be tested.  Returns the AndCheck object for convenience.

        """

        self.rules.append(rule)

        return self

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class OrCheck(BaseCheck):

"""

A policy check that requires that at least one of a list of other

checks returns True. Implements the "or" operator.

"""

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    def __init__(self, rules):

        """

        Initialize the 'or' check.

        :param rules: A list of rules that will be tested.

        """

        self.rules = rules

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    def __str__(self):

        """Return a string representation of this check."""

        return "(%s)" % ' or '.join(str(r) for r in self.rules)

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    def __call__(self, target, cred):

        """

        Check the policy.  Requires that at least one rule accept in

        order to return True.

        """

        for rule in self.rules:

            if rule(target, cred):

                return True

        return False

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    def add_check(self, rule):

        """

        Allows addition of another rule to the list of rules that will

        be tested.  Returns the OrCheck object for convenience.

        """

        self.rules.append(rule)

        return self

def _parse_check(rule):

    """

    Parse a single base check rule into an appropriate Check object.

    """

    # Handle the special checks

    if rule == '!':

        return FalseCheck()

    elif rule == '@':

        return TrueCheck()

    try:

        kind, match = rule.split(':', 1)

    except Exception:

        LOG.exception(_("Failed to understand rule %(rule)s") % locals())

        # If the rule is invalid, we'll fail closed

        return FalseCheck()

    # Find what implements the check

    if kind in _checks:

        return _checks[kind](kind, match)

    elif None in _checks:

        return _checks[None](kind, match)

    else:

        LOG.error(_("No handler for matches of kind %s") % kind)

        return FalseCheck()

def _parse_list_rule(rule):

    """

    Provided for backwards compatibility.  Translates the old

    list-of-lists syntax into a tree of Check objects.

    """

    # Empty rule defaults to True

    if not rule:

        return TrueCheck()

    # Outer list is joined by "or"; inner list by "and"

    or_list = []

    for inner_rule in rule:

        # Elide empty inner lists

        if not inner_rule:

            continue

        # Handle bare strings

        if isinstance(inner_rule, basestring):

            inner_rule = [inner_rule]

        # Parse the inner rules into Check objects

        and_list = [_parse_check(r) for r in inner_rule]

        # Append the appropriate check to the or_list

        if len(and_list) == 1:

            or_list.append(and_list[0])

        else:

            or_list.append(AndCheck(and_list))

    # If we have only one check, omit the "or"

    if len(or_list) == 0:

        return FalseCheck()

    elif len(or_list) == 1:

        return or_list[0]

    return OrCheck(or_list)

# Used for tokenizing the policy language

_tokenize_re = re.compile(r'\s+')

def _parse_tokenize(rule):

    """

    Tokenizer for the policy language.

    Most of the single-character tokens are specified in the

    _tokenize_re; however, parentheses need to be handled specially,

    because they can appear inside a check string.  Thankfully, those

    parentheses that appear inside a check string can never occur at

    the very beginning or end ("%(variable)s" is the correct syntax).

    """

    for tok in _tokenize_re.split(rule):

        # Skip empty tokens

        if not tok or tok.isspace():

            continue

        # Handle leading parens on the token

        clean = tok.lstrip('(')

        for i in range(len(tok) - len(clean)):

            yield '(', '('

        # If it was only parentheses, continue

        if not clean:

            continue

        else:

            tok = clean

        # Handle trailing parens on the token

        clean = tok.rstrip(')')

        trail = len(tok) - len(clean)

        # Yield the cleaned token

        lowered = clean.lower()

        if lowered in ('and', 'or', 'not'):

            # Special tokens

            yield lowered, clean

        elif clean:

            # Not a special token, but not composed solely of ')'

            if len(tok) >= 2 and ((tok[0], tok[-1]) in

                                  [('"', '"'), ("'", "'")]):

                # It's a quoted string

                yield 'string', tok[1:-1]

            else:

                yield 'check', _parse_check(clean)

        # Yield the trailing parens

        for i in range(trail):

            yield ')', ')'

**** CubicPower OpenStack Study ****

def _parse_check(rule):

    """

    Parse a single base check rule into an appropriate Check object.

    """

    # Handle the special checks

    if rule == '!':

        return FalseCheck()

    elif rule == '@':

        return TrueCheck()

    try:

        kind, match = rule.split(':', 1)

    except Exception:

        LOG.exception(_("Failed to understand rule %(rule)s") % locals())

        # If the rule is invalid, we'll fail closed

        return FalseCheck()

    # Find what implements the check

    if kind in _checks:

        return _checks[kind](kind, match)

    elif None in _checks:

        return _checks[None](kind, match)

    else:

        LOG.error(_("No handler for matches of kind %s") % kind)

        return FalseCheck()

**** CubicPower OpenStack Study ****

def _parse_list_rule(rule):

    """

    Provided for backwards compatibility.  Translates the old

    list-of-lists syntax into a tree of Check objects.

    """

    # Empty rule defaults to True

    if not rule:

        return TrueCheck()

    # Outer list is joined by "or"; inner list by "and"

    or_list = []

    for inner_rule in rule:

        # Elide empty inner lists

        if not inner_rule:

            continue

        # Handle bare strings

        if isinstance(inner_rule, basestring):

            inner_rule = [inner_rule]

        # Parse the inner rules into Check objects

        and_list = [_parse_check(r) for r in inner_rule]

        # Append the appropriate check to the or_list

        if len(and_list) == 1:

            or_list.append(and_list[0])

        else:

            or_list.append(AndCheck(and_list))

    # If we have only one check, omit the "or"

    if len(or_list) == 0:

        return FalseCheck()

    elif len(or_list) == 1:

        return or_list[0]

    return OrCheck(or_list)

# Used for tokenizing the policy language

_tokenize_re = re.compile(r'\s+')

**** CubicPower OpenStack Study ****

def _parse_tokenize(rule):

    """

    Tokenizer for the policy language.

    Most of the single-character tokens are specified in the

    _tokenize_re; however, parentheses need to be handled specially,

    because they can appear inside a check string.  Thankfully, those

    parentheses that appear inside a check string can never occur at

    the very beginning or end ("%(variable)s" is the correct syntax).

    """

    for tok in _tokenize_re.split(rule):

        # Skip empty tokens

        if not tok or tok.isspace():

            continue

        # Handle leading parens on the token

        clean = tok.lstrip('(')

        for i in range(len(tok) - len(clean)):

            yield '(', '('

        # If it was only parentheses, continue

        if not clean:

            continue

        else:

            tok = clean

        # Handle trailing parens on the token

        clean = tok.rstrip(')')

        trail = len(tok) - len(clean)

        # Yield the cleaned token

        lowered = clean.lower()

        if lowered in ('and', 'or', 'not'):

            # Special tokens

            yield lowered, clean

        elif clean:

            # Not a special token, but not composed solely of ')'

            if len(tok) >= 2 and ((tok[0], tok[-1]) in

                                  [('"', '"'), ("'", "'")]):

                # It's a quoted string

                yield 'string', tok[1:-1]

            else:

                yield 'check', _parse_check(clean)

        # Yield the trailing parens

        for i in range(trail):

            yield ')', ')'

**** CubicPower OpenStack Study ****

class ParseStateMeta(type):

"""

Metaclass for the ParseState class. Facilitates identifying

reduction methods.

"""

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    def __new__(mcs, name, bases, cls_dict):

        """

        Create the class.  Injects the 'reducers' list, a list of

        tuples matching token sequences to the names of the

        corresponding reduction methods.

        """

        reducers = []

        for key, value in cls_dict.items():

            if not hasattr(value, 'reducers'):

                continue

            for reduction in value.reducers:

                reducers.append((reduction, key))

        cls_dict['reducers'] = reducers

        return super(ParseStateMeta, mcs).__new__(mcs, name, bases, cls_dict)

def reducer(*tokens):

    """

    Decorator for reduction methods.  Arguments are a sequence of

    tokens, in order, which should trigger running this reduction

    method.

    """

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def reducer(*tokens):

    """

    Decorator for reduction methods.  Arguments are a sequence of

    tokens, in order, which should trigger running this reduction

    method.

    """

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    def decorator(func):

        # Make sure we have a list of reducer sequences

        if not hasattr(func, 'reducers'):

            func.reducers = []

        # Add the tokens to the list of reducer sequences

        func.reducers.append(list(tokens))

        return func

    return decorator

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class ParseState(object):

"""

Implement the core of parsing the policy language. Uses a greedy

reduction algorithm to reduce a sequence of tokens into a single

terminal, the value of which will be the root of the Check tree.

Note: error reporting is rather lacking. The best we can get with

this parser formulation is an overall "parse failed" error.

Fortunately, the policy language is simple enough that this

shouldn't be that big a problem.

"""

__metaclass__ = ParseStateMeta

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    def __init__(self):

        """Initialize the ParseState."""

        self.tokens = []

        self.values = []

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    def reduce(self):

        """

        Perform a greedy reduction of the token stream.  If a reducer

        method matches, it will be executed, then the reduce() method

        will be called recursively to search for any more possible

        reductions.

        """

        for reduction, methname in self.reducers:

            if (len(self.tokens) >= len(reduction) and

                    self.tokens[-len(reduction):] == reduction):

                # Get the reduction method

                meth = getattr(self, methname)

                # Reduce the token stream

                results = meth(*self.values[-len(reduction):])

                # Update the tokens and values

                self.tokens[-len(reduction):] = [r[0] for r in results]

                self.values[-len(reduction):] = [r[1] for r in results]

                # Check for any more reductions

                return self.reduce()

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    def shift(self, tok, value):

        """Adds one more token to the state.  Calls reduce()."""

        self.tokens.append(tok)

        self.values.append(value)

        # Do a greedy reduce...

        self.reduce()

    @property

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    def result(self):

        """

        Obtain the final result of the parse.  Raises ValueError if

        the parse failed to reduce to a single result.

        """

        if len(self.values) != 1:

            raise ValueError("Could not parse rule")

        return self.values[0]

    @reducer('(', 'check', ')')

    @reducer('(', 'and_expr', ')')

    @reducer('(', 'or_expr', ')')

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    def _wrap_check(self, _p1, check, _p2):

        """Turn parenthesized expressions into a 'check' token."""

        return [('check', check)]

    @reducer('check', 'and', 'check')

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    def _make_and_expr(self, check1, _and, check2):

        """

        Create an 'and_expr' from two checks joined by the 'and'

        operator.

        """

        return [('and_expr', AndCheck([check1, check2]))]

    @reducer('and_expr', 'and', 'check')

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    def _extend_and_expr(self, and_expr, _and, check):

        """

        Extend an 'and_expr' by adding one more check.

        """

        return [('and_expr', and_expr.add_check(check))]

    @reducer('check', 'or', 'check')

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    def _make_or_expr(self, check1, _or, check2):

        """

        Create an 'or_expr' from two checks joined by the 'or'

        operator.

        """

        return [('or_expr', OrCheck([check1, check2]))]

    @reducer('or_expr', 'or', 'check')

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    def _extend_or_expr(self, or_expr, _or, check):

        """

        Extend an 'or_expr' by adding one more check.

        """

        return [('or_expr', or_expr.add_check(check))]

    @reducer('not', 'check')

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    def _make_not_expr(self, _not, check):

        """Invert the result of another check."""

        return [('check', NotCheck(check))]

def _parse_text_rule(rule):

    """

    Translates a policy written in the policy language into a tree of

    Check objects.

    """

    # Empty rule means always accept

    if not rule:

        return TrueCheck()

    # Parse the token stream

    state = ParseState()

    for tok, value in _parse_tokenize(rule):

        state.shift(tok, value)

    try:

        return state.result

    except ValueError:

        # Couldn't parse the rule

        LOG.exception(_("Failed to understand rule %(rule)r") % locals())

        # Fail closed

        return FalseCheck()

def parse_rule(rule):

    """

    Parses a policy rule into a tree of Check objects.

    """

    # If the rule is a string, it's in the policy language

    if isinstance(rule, basestring):

        return _parse_text_rule(rule)

    return _parse_list_rule(rule)

def register(name, func=None):

    """

    Register a function or Check class as a policy check.

    :param name: Gives the name of the check type, e.g., 'rule',

                 'role', etc.  If name is None, a default check type

                 will be registered.

    :param func: If given, provides the function or class to register.

                 If not given, returns a function taking one argument

                 to specify the function or class to register,

                 allowing use as a decorator.

    """

    # Perform the actual decoration by registering the function or

    # class.  Returns the function or class for compliance with the

    # decorator interface.

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def _parse_text_rule(rule):

    """

    Translates a policy written in the policy language into a tree of

    Check objects.

    """

    # Empty rule means always accept

    if not rule:

        return TrueCheck()

    # Parse the token stream

    state = ParseState()

    for tok, value in _parse_tokenize(rule):

        state.shift(tok, value)

    try:

        return state.result

    except ValueError:

        # Couldn't parse the rule

        LOG.exception(_("Failed to understand rule %(rule)r") % locals())

        # Fail closed

        return FalseCheck()

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def parse_rule(rule):

    """

    Parses a policy rule into a tree of Check objects.

    """

    # If the rule is a string, it's in the policy language

    if isinstance(rule, basestring):

        return _parse_text_rule(rule)

    return _parse_list_rule(rule)

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def register(name, func=None):

    """

    Register a function or Check class as a policy check.

    :param name: Gives the name of the check type, e.g., 'rule',

                 'role', etc.  If name is None, a default check type

                 will be registered.

    :param func: If given, provides the function or class to register.

                 If not given, returns a function taking one argument

                 to specify the function or class to register,

                 allowing use as a decorator.

    """

    # Perform the actual decoration by registering the function or

    # class.  Returns the function or class for compliance with the

    # decorator interface.

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    def decorator(func):

        _checks[name] = func

        return func

    # If the function or class is given, do the registration

    if func:

        return decorator(func)

    return decorator

@register("rule")

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class RuleCheck(Check):

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    def __call__(self, target, creds):

        """

        Recursively checks credentials based on the defined rules.

        """

        try:

            return _rules[self.match](target, creds)

        except KeyError:

            # We don't have any matching rule; fail closed

            return False

@register("role")

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class RoleCheck(Check):

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    def __call__(self, target, creds):

        """Check that there is a matching role in the cred dict."""

        return self.match.lower() in [x.lower() for x in creds['roles']]

@register('http')

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class HttpCheck(Check):

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    def __call__(self, target, creds):

        """

        Check http: rules by calling to a remote server.

        This example implementation simply verifies that the response

        is exactly 'True'.

        """

        url = ('http:' + self.match) % target

        data = {'target': jsonutils.dumps(target),

                'credentials': jsonutils.dumps(creds)}

        post_data = urllib.urlencode(data)

        f = urllib2.urlopen(url, post_data)

        return f.read() == "True"

@register(None)

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class GenericCheck(Check):

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    def __call__(self, target, creds):

        """

        Check an individual match.

        Matches look like:

            tenant:%(tenant_id)s

            role:compute:admin

        """

        # TODO(termie): do dict inspection via dot syntax

        match = self.match % target

        if self.kind in creds:

            return match == unicode(creds[self.kind])

        return False