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Commit 80d6e5bb authored by http://jneen.net/'s avatar http://jneen.net/
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add a new DeclarativePolicy framework

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lib/declarative_policy.rb 0 → 100644
View file @ 80d6e5bb
require_dependency 'declarative_policy/cache'
require_dependency 'declarative_policy/condition'
require_dependency 'declarative_policy/dsl'
require_dependency 'declarative_policy/preferred_scope'
require_dependency 'declarative_policy/rule'
require_dependency 'declarative_policy/runner'
require_dependency 'declarative_policy/step'
require_dependency 'declarative_policy/base'
module DeclarativePolicy
class << self
def policy_for(user, subject, opts = {})
cache = opts[:cache] || {}
key = Cache.policy_key(user, subject)
cache[key] ||= class_for(subject).new(user, subject, opts)
end
def class_for(subject)
return GlobalPolicy if subject == :global
return NilPolicy if subject.nil?
subject = find_delegate(subject)
subject.class.ancestors.each do |klass|
next unless klass.name
begin
policy_class = "#{klass.name}Policy".constantize
# NOTE: the < operator here tests whether policy_class
# inherits from Base. We can't use #is_a? because that
# tests for *instances*, not *subclasses*.
return policy_class if policy_class < Base
rescue NameError
nil
end
end
raise "no policy for #{subject.class.name}"
end
private
def find_delegate(subject)
seen = Set.new
while subject.respond_to?(:declarative_policy_delegate)
raise ArgumentError, "circular delegations" if seen.include?(subject.object_id)
seen << subject.object_id
subject = subject.declarative_policy_delegate
end
subject
end
end
end
lib/declarative_policy/base.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
class Base
# A map of ability => list of rules together with :enable
# or :prevent actions. Used to look up which rules apply to
# a given ability. See Base.ability_map
class AbilityMap
attr_reader :map
def initialize(map = {})
@map = map
end
# This merge behavior is different than regular hashes - if both
# share a key, the values at that key are concatenated, rather than
# overridden.
def merge(other)
conflict_proc = proc { |key, my_val, other_val| my_val + other_val }
AbilityMap.new(@map.merge(other.map, &conflict_proc))
end
def actions(key)
@map[key] ||= []
end
def enable(key, rule)
actions(key) << [:enable, rule]
end
def prevent(key, rule)
actions(key) << [:prevent, rule]
end
end
class << self
# The `own_ability_map` vs `ability_map` distinction is used so that
# the data structure is properly inherited - with subclasses recursively
# merging their parent class.
#
# This pattern is also used for conditions, global_actions, and delegations.
def ability_map
if self == Base
own_ability_map
else
superclass.ability_map.merge(own_ability_map)
end
end
def own_ability_map
@own_ability_map ||= AbilityMap.new
end
# an inheritable map of conditions, by name
def conditions
if self == Base
own_conditions
else
superclass.conditions.merge(own_conditions)
end
end
def own_conditions
@own_conditions ||= {}
end
# a list of global actions, generated by `prevent_all`. these aren't
# stored in `ability_map` because they aren't indexed by a particular
# ability.
def global_actions
if self == Base
own_global_actions
else
superclass.global_actions + own_global_actions
end
end
def own_global_actions
@own_global_actions ||= []
end
# an inheritable map of delegations, indexed by name (which may be
# autogenerated)
def delegations
if self == Base
own_delegations
else
superclass.delegations.merge(own_delegations)
end
end
def own_delegations
@own_delegations ||= {}
end
# all the [rule, action] pairs that apply to a particular ability.
# we combine the specific ones looked up in ability_map with the global
# ones.
def configuration_for(ability)
ability_map.actions(ability) + global_actions
end
### declaration methods ###
def delegate(name = nil, &delegation_block)
if name.nil?
@delegate_name_counter ||= 0
@delegate_name_counter += 1
name = :"anonymous_#{@delegate_name_counter}"
end
name = name.to_sym
if delegation_block.nil?
delegation_block = proc { @subject.__send__(name) }
end
own_delegations[name] = delegation_block
end
# Declares a rule, constructed using RuleDsl, and returns
# a PolicyDsl which is used for registering the rule with
# this class. PolicyDsl will call back into Base.enable_when,
# Base.prevent_when, and Base.prevent_all_when.
def rule(&b)
rule = RuleDsl.new(self).instance_eval(&b)
PolicyDsl.new(self, rule)
end
# A hash in which to store calls to `desc` and `with_scope`, etc.
def last_options
@last_options ||= {}.with_indifferent_access
end
# retrieve and zero out the previously set options (used in .condition)
def last_options!
last_options.tap { @last_options = nil }
end
# Declare a description for the following condition. Currently unused,
# but opens the potential for explaining to users why they were or were
# not able to do something.
def desc(description)
last_options[:description] = description
end
def with_options(opts = {})
last_options.merge!(opts)
end
def with_scope(scope)
with_options scope: scope
end
def with_score(score)
with_options score: score
end
# Declares a condition. It gets stored in `own_conditions`, and generates
# a query method based on the condition's name.
def condition(name, opts = {}, &value)
name = name.to_sym
opts = last_options!.merge(opts)
opts[:context_key] ||= self.name
condition = Condition.new(name, opts, &value)
self.own_conditions[name] = condition
define_method(:"#{name}?") { condition(name).pass? }
end
# These next three methods are mainly called from PolicyDsl,
# and are responsible for "inverting" the relationship between
# an ability and a rule. We store in `ability_map` a map of
# abilities to rules that affect them, together with a
# symbol indicating :prevent or :enable.
def enable_when(abilities, rule)
abilities.each { |a| own_ability_map.enable(a, rule) }
end
def prevent_when(abilities, rule)
abilities.each { |a| own_ability_map.prevent(a, rule) }
end
# we store global prevents (from `prevent_all`) separately,
# so that they can be combined into every decision made.
def prevent_all_when(rule)
own_global_actions << [:prevent, rule]
end
end
# A policy object contains a specific user and subject on which
# to compute abilities. For this reason it's sometimes called
# "context" within the framework.
#
# It also stores a reference to the cache, so it can be used
# to cache computations by e.g. ManifestCondition.
attr_reader :user, :subject, :cache
def initialize(user, subject, opts = {})
@user = user
@subject = subject
@cache = opts[:cache] || {}
end
# helper for checking abilities on this and other subjects
# for the current user.
def can?(ability, new_subject = :_self)
return allowed?(ability) if new_subject == :_self
policy_for(new_subject).allowed?(ability)
end
# This is the main entry point for permission checks. It constructs
# or looks up a Runner for the given ability and asks it if it passes.
def allowed?(*abilities)
abilities.all? { |a| runner(a).pass? }
end
# The inverse of #allowed?, used mainly in specs.
def disallowed?(*abilities)
abilities.all? { |a| !runner(a).pass? }
end
# computes the given ability and prints a helpful debugging output
# showing which
def debug(ability, *a)
runner(ability).debug(*a)
end
desc "Unknown user"
condition(:anonymous, scope: :user, score: 0) { @user.nil? }
desc "By default"
condition(:default, scope: :global, score: 0) { true }
def repr
subject_repr =
if @subject.respond_to?(:id)
"#{@subject.class.name}/#{@subject.id}"
else
@subject.inspect
end
user_repr =
if @user
@user.to_reference
else
"<anonymous>"
end
"(#{user_repr} : #{subject_repr})"
end
def inspect
"#<#{self.class.name} #{repr}>"
end
# returns a Runner for the given ability, capable of computing whether
# the ability is allowed. Runners are cached on the policy (which itself
# is cached on @cache), and caches its result. This is how we perform caching
# at the ability level.
def runner(ability)
ability = ability.to_sym
@runners ||= {}
@runners[ability] ||=
begin
delegated_runners = delegated_policies.values.compact.map { |p| p.runner(ability) }
own_runner = Runner.new(own_steps(ability))
delegated_runners.inject(own_runner, &:merge_runner)
end
end
# Helpers for caching. Used by ManifestCondition in performing condition
# computation.
#
# NOTE we can't use ||= here because the value might be the
# boolean `false`
def cache(key, &b)
return @cache[key] if cached?(key)
@cache[key] = yield
end
def cached?(key)
!@cache[key].nil?
end
# returns a ManifestCondition capable of computing itself. The computation
# will use our own @cache.
def condition(name)
name = name.to_sym
@_conditions ||= {}
@_conditions[name] ||=
begin
raise "invalid condition #{name}" unless self.class.conditions.key?(name)
ManifestCondition.new(self.class.conditions[name], self)
end
end
# used in specs - returns true if there is no possible way for any action
# to be allowed, determined only by the global :prevent_all rules.
def banned?
global_steps = self.class.global_actions.map { |(action, rule)| Step.new(self, rule, action) }
!Runner.new(global_steps).pass?
end
# A list of other policies that we've delegated to (see `Base.delegate`)
def delegated_policies
@delegated_policies ||= self.class.delegations.transform_values do |block|
new_subject = instance_eval(&block)
# never delegate to nil, as that would immediately prevent_all
next if new_subject.nil?
policy_for(new_subject)
end
end
def policy_for(other_subject)
DeclarativePolicy.policy_for(@user, other_subject, cache: @cache)
end
protected
# constructs steps that come from this policy and not from any delegations
def own_steps(ability)
rules = self.class.configuration_for(ability)
rules.map { |(action, rule)| Step.new(self, rule, action) }
end
end
end
lib/declarative_policy/cache.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
module Cache
class << self
def user_key(user)
return '<anonymous>' if user.nil?
id_for(user)
end
def policy_key(user, subject)
u = user_key(user)
s = subject_key(subject)
"/dp/policy/#{u}/#{s}"
end
def subject_key(subject)
return '<nil>' if subject.nil?
return subject.inspect if subject.is_a?(Symbol)
"#{subject.class.name}:#{id_for(subject)}"
end
private
def id_for(obj)
if obj.respond_to?(:id) && obj.id
obj.id.to_s
else
"##{obj.object_id}"
end
end
end
end
end
lib/declarative_policy/condition.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
# A Condition is the data structure that is created by the
# `condition` declaration on DeclarativePolicy::Base. It is
# more or less just a struct of the data passed to that
# declaration. It holds on to the block to be instance_eval'd
# on a context (instance of Base) later, via #compute.
class Condition
attr_reader :name, :description, :scope
attr_reader :manual_score
attr_reader :context_key
def initialize(name, opts = {}, &compute)
@name = name
@compute = compute
@scope = opts.fetch(:scope, :normal)
@description = opts.delete(:description)
@context_key = opts[:context_key]
@manual_score = opts.fetch(:score, nil)
end
def compute(context)
!!context.instance_eval(&@compute)
end
def key
"#{@context_key}/#{@name}"
end
end
# In contrast to a Condition, a ManifestCondition contains
# a Condition and a context object, and is capable of calculating
# a result itself. This is the return value of Base#condition.
class ManifestCondition
def initialize(condition, context)
@condition = condition
@context = context
end
# The main entry point - does this condition pass? We reach into
# the context's cache here so that we can share in the global
# cache (often RequestStore or similar).
def pass?
@context.cache(cache_key) { @condition.compute(@context) }
end
# Whether we've already computed this condition.
def cached?
@context.cached?(cache_key)
end
# This is used to score Rule::Condition. See Rule::Condition#score
# and Runner#steps_by_score for how scores are used.
#
# The number here is intended to represent, abstractly, how
# expensive it would be to calculate this condition.
#
# See #cache_key for info about @condition.scope.
def score
# If we've been cached, no computation is necessary.
return 0 if cached?
# Use the override from condition(score: ...) if present
return @condition.manual_score if @condition.manual_score
# Global scope rules are cheap due to max cache sharing
return 2 if @condition.scope == :global
# "Normal" rules can't share caches with any other policies
return 16 if @condition.scope == :normal
# otherwise, we're :user or :subject scope, so it's 4 if
# the caller has declared a preference
return 4 if @condition.scope == DeclarativePolicy.preferred_scope
# and 8 for all other :user or :subject scope conditions.
8
end
private
# This method controls the caching for the condition. This is where
# the condition(scope: ...) option comes into play. Notice that
# depending on the scope, we may cache only by the user or only by
# the subject, resulting in sharing across different policy objects.
def cache_key
case @condition.scope
when :normal then "/dp/condition/#{@condition.key}/#{user_key},#{subject_key}"
when :user then "/dp/condition/#{@condition.key}/#{user_key}"
when :subject then "/dp/condition/#{@condition.key}/#{subject_key}"
when :global then "/dp/condition/#{@condition.key}"
else raise 'invalid scope'
end
end
def user_key
Cache.user_key(@context.user)
end
def subject_key
Cache.subject_key(@context.subject)
end
end
end
lib/declarative_policy/dsl.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
# The DSL evaluation context inside rule { ... } blocks.
# Responsible for creating and combining Rule objects.
#
# See Base.rule
class RuleDsl
def initialize(context_class)
@context_class = context_class
end
def can?(ability)
Rule::Ability.new(ability)
end
def all?(*rules)
Rule::And.make(rules)
end
def any?(*rules)
Rule::Or.make(rules)
end
def none?(*rules)
~Rule::Or.new(rules)
end
def cond(condition)
Rule::Condition.new(condition)
end
def delegate(delegate_name, condition)
Rule::DelegatedCondition.new(delegate_name, condition)
end
def method_missing(m, *a, &b)
return super unless a.size == 0 && !block_given?
if @context_class.delegations.key?(m)
DelegateDsl.new(self, m)
else
cond(m.to_sym)
end
end
end
# Used when the name of a delegate is mentioned in
# the rule DSL.
class DelegateDsl
def initialize(rule_dsl, delegate_name)
@rule_dsl = rule_dsl
@delegate_name = delegate_name
end
def method_missing(m, *a, &b)
return super unless a.size == 0 && !block_given?
@rule_dsl.delegate(@delegate_name, m)
end
end
# The return value of a rule { ... } declaration.
# Can call back to register rules with the containing
# Policy class (context_class here). See Base.rule
#
# Note that the #policy method just performs an #instance_eval,
# which is useful for multiple #enable or #prevent callse.
#
# Also provides a #method_missing proxy to the context
# class's class methods, so that helper methods can be
# defined and used in a #policy { ... } block.
class PolicyDsl
def initialize(context_class, rule)
@context_class = context_class
@rule = rule
end
def policy(&b)
instance_eval(&b)
end
def enable(*abilities)
@context_class.enable_when(abilities, @rule)
end
def prevent(*abilities)
@context_class.prevent_when(abilities, @rule)
end
def prevent_all
@context_class.prevent_all_when(@rule)
end
def method_missing(m, *a, &b)
return super unless @context_class.respond_to?(m)
@context_class.__send__(m, *a, &b)
end
def respond_to_missing?(m)
@context_class.respond_to?(m) || super
end
end
end
lib/declarative_policy/preferred_scope.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
PREFERRED_SCOPE_KEY = :"DeclarativePolicy.preferred_scope"
class << self
def with_preferred_scope(scope, &b)
Thread.current[PREFERRED_SCOPE_KEY], old_scope = scope, Thread.current[PREFERRED_SCOPE_KEY]
yield
ensure
Thread.current[PREFERRED_SCOPE_KEY] = old_scope
end
def preferred_scope
Thread.current[PREFERRED_SCOPE_KEY]
end
def user_scope(&b)
with_preferred_scope(:user, &b)
end
def subject_scope(&b)
with_preferred_scope(:subject, &b)
end
def preferred_scope=(scope)
Thread.current[PREFERRED_SCOPE_KEY] = scope
end
end
end
lib/declarative_policy/rule.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
module Rule
# A Rule is the object that results from the `rule` declaration,
# usually built using the DSL in `RuleDsl`. It is a basic logical
# combination of building blocks, and is capable of deciding,
# given a context (instance of DeclarativePolicy::Base) whether it
# passes or not. Note that this decision doesn't by itself know
# how that affects the actual ability decision - for that, a
# `Step` is used.
class Base
def self.make(*a)
new(*a).simplify
end
# true or false whether this rule passes.
# `context` is a policy - an instance of
# DeclarativePolicy::Base.
def pass?(context)
raise 'abstract'
end
# same as #pass? except refuses to do any I/O,
# returning nil if the result is not yet cached.
# used for accurately scoring And/Or
def cached_pass?(context)
raise 'abstract'
end
# abstractly, how long would it take to compute
# this rule? lower-scored rules are tried first.
def score(context)
raise 'abstract'
end
# unwrap double negatives and nested and/or
def simplify
self
end
# convenience combination methods
def or(other)
Or.make([self, other])
end
def and(other)
And.make([self, other])
end
def negate
Not.make(self)
end
alias_method :|, :or
alias_method :&, :and
alias_method :~@, :negate
def inspect
"#<Rule #{repr}>"
end
end
# A rule that checks a condition. This is the
# type of rule that results from a basic bareword
# in the rule dsl (see RuleDsl#method_missing).
class Condition < Base
def initialize(name)
@name = name
end
# we delegate scoring to the condition. See
# ManifestCondition#score.
def score(context)
context.condition(@name).score
end
# Let the ManifestCondition from the context
# decide whether we pass.
def pass?(context)
context.condition(@name).pass?
end
# returns nil unless it's already cached
def cached_pass?(context)
condition = context.condition(@name)
return nil unless condition.cached?
condition.pass?
end
def description(context)
context.class.conditions[@name].description
end
def repr
@name.to_s
end
end
# A rule constructed from DelegateDsl - using a condition from a
# delegated policy.
class DelegatedCondition < Base
# Internal use only - this is rescued each time it's raised.
MissingDelegate = Class.new(StandardError)
def initialize(delegate_name, name)
@delegate_name = delegate_name
@name = name
end
def delegated_context(context)
policy = context.delegated_policies[@delegate_name]
raise MissingDelegate if policy.nil?
policy
end
def score(context)
delegated_context(context).condition(@name).score
rescue MissingDelegate
0
end
def cached_pass?(context)
condition = delegated_context(context).condition(@name)
return nil unless condition.cached?
condition.pass?
rescue MissingDelegate
false
end
def pass?(context)
delegated_context(context).condition(@name).pass?
rescue MissingDelegate
false
end
def repr
"#{@delegate_name}.#{@name}"
end
end
# A rule constructed from RuleDsl#can?. Computes a different ability
# on the same subject.
class Ability < Base
attr_reader :ability
def initialize(ability)
@ability = ability
end
# We ask the ability's runner for a score
def score(context)
context.runner(@ability).score
end
def pass?(context)
context.allowed?(@ability)
end
def cached_pass?(context)
runner = context.runner(@ability)
return nil unless runner.cached?
runner.pass?
end
def description(context)
"User can #{@ability.inspect}"
end
def repr
"can?(#{@ability.inspect})"
end
end
# Logical `and`, containing a list of rules. Only passes
# if all of them do.
class And < Base
attr_reader :rules
def initialize(rules)
@rules = rules
end
def simplify
simplified_rules = @rules.flat_map do |rule|
simplified = rule.simplify
case simplified
when And then simplified.rules
else [simplified]
end
end
And.new(simplified_rules)
end
def score(context)
return 0 unless cached_pass?(context).nil?
# note that cached rules will have score 0 anyways.
@rules.map { |r| r.score(context) }.inject(0, :+)
end
def pass?(context)
# try to find a cached answer before
# checking in order
cached = cached_pass?(context)
return cached unless cached.nil?
@rules.all? { |r| r.pass?(context) }
end
def cached_pass?(context)
passes = @rules.map { |r| r.cached_pass?(context) }
return false if passes.any? { |p| p == false }
return true if passes.all? { |p| p == true }
nil
end
def repr
"all?(#{rules.map(&:repr).join(', ')})"
end
end
# Logical `or`. Mirrors And.
class Or < Base
attr_reader :rules
def initialize(rules)
@rules = rules
end
def pass?(context)
cached = cached_pass?(context)
return cached unless cached.nil?
@rules.any? { |r| r.pass?(context) }
end
def simplify
simplified_rules = @rules.flat_map do |rule|
simplified = rule.simplify
case simplified
when Or then simplified.rules
else [simplified]
end
end
Or.new(simplified_rules)
end
def cached_pass?(context)
passes = @rules.map { |r| r.cached_pass?(context) }
return true if passes.any? { |p| p == true }
return false if passes.all? { |p| p == false }
nil
end
def score(context)
return 0 unless cached_pass?(context).nil?
@rules.map { |r| r.score(context) }.inject(0, :+)
end
def repr
"any?(#{@rules.map(&:repr).join(', ')})"
end
end
class Not < Base
attr_reader :rule
def initialize(rule)
@rule = rule
end
def simplify
case @rule
when And then Or.new(@rule.rules.map(&:negate)).simplify
when Or then And.new(@rule.rules.map(&:negate)).simplify
when Not then @rule.rule.simplify
else Not.new(@rule.simplify)
end
end
def pass?(context)
!@rule.pass?(context)
end
def cached_pass?(context)
case @rule.cached_pass?(context)
when nil then nil
when true then false
when false then true
end
end
def score(context)
@rule.score(context)
end
def repr
"~#{@rule.repr}"
end
end
end
end
lib/declarative_policy/runner.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
class Runner
class State
def initialize
@enabled = false
@prevented = false
end
def enable!
@enabled = true
end
def enabled?
@enabled
end
def prevent!
@prevented = true
end
def prevented?
@prevented
end
def pass?
!prevented? && enabled?
end
end
# a Runner contains a list of Steps to be run.
attr_reader :steps
def initialize(steps)
@steps = steps
end
# We make sure only to run any given Runner once,
# and just continue to use the resulting @state
# that's left behind.
def cached?
!!@state
end
# used by Rule::Ability. See #steps_by_score
def score
return 0 if cached?
steps.map(&:score).inject(0, :+)
end
def merge_runner(other)
Runner.new(@steps + other.steps)
end
# The main entry point, called for making an ability decision.
# See #run and DeclarativePolicy::Base#can?
def pass?
run unless cached?
@state.pass?
end
# see DeclarativePolicy::Base#debug
def debug(out = $stderr)
run(out)
end
private
def flatten_steps!
@steps = @steps.flat_map { |s| s.flattened(@steps) }
end
# This method implements the semantic of "one enable and no prevents".
# It relies on #steps_by_score for the main loop, and updates @state
# with the result of the step.
def run(debug = nil)
@state = State.new
steps_by_score do |step, score|
passed = nil
case step.action
when :enable then
# we only check :enable actions if they have a chance of
# changing the outcome - if no other rule has enabled or
# prevented.
unless @state.enabled? || @state.prevented?
passed = step.pass?
@state.enable! if passed
end
debug << inspect_step(step, score, passed) if debug
when :prevent then
# we only check :prevent actions if the state hasn't already
# been prevented.
unless @state.prevented?
passed = step.pass?
if passed
@state.prevent!
return unless debug
end
end
debug << inspect_step(step, score, passed) if debug
else raise "invalid action #{step.action.inspect}"
end
end
@state
end
# This is the core spot where all those `#score` methods matter.
# It is critcal for performance to run steps in the correct order,
# so that we don't compute expensive conditions (potentially n times
# if we're called on, say, a large list of users).
#
# In order to determine the cheapest step to run next, we rely on
# Step#score, which returns a numerical rating of how expensive
# it would be to calculate - the lower the better. It would be
# easy enough to statically sort by these scores, but we can do
# a little better - the scores are cache-aware (conditions that
# are already in the cache have score 0), which means that running
# a step can actually change the scores of other steps.
#
# So! The way we sort here involves re-scoring at every step. This
# is by necessity quadratic, but most of the time the number of steps
# will be low. But just in case, if the number of steps exceeds 50,
# we print a warning and fall back to a static sort.
#
# For each step, we yield the step object along with the computed score
# for debugging purposes.
def steps_by_score(&b)
flatten_steps!
if @steps.size > 50
warn "DeclarativePolicy: large number of steps (#{steps.size}), falling back to static sort"
@steps.map { |s| [s.score, s] }.sort_by { |(score, _)| score }.each do |(score, step)|
yield step, score
end
return
end
steps = Set.new(@steps)
loop do
return if steps.empty?
# if the permission hasn't yet been enabled and we only have
# prevent steps left, we short-circuit the state here
@state.prevent! if !@state.enabled? && steps.all?(&:prevent?)
lowest_score = Float::INFINITY
next_step = nil
steps.each do |step|
score = step.score
if score < lowest_score
next_step = step
lowest_score = score
end
end
steps.delete(next_step)
yield next_step, lowest_score
end
end
# Formatter for debugging output.
def inspect_step(step, original_score, passed)
symbol =
case passed
when true then '+'
when false then '-'
when nil then ' '
end
"#{symbol} [#{original_score.to_i}] #{step.repr}\n"
end
end
end
lib/declarative_policy/step.rb 0 → 100644
View file @ 80d6e5bb
module DeclarativePolicy
# This object represents one step in the runtime decision of whether
# an ability is allowed. It contains a Rule and a context (instance
# of DeclarativePolicy::Base), which contains the user, the subject,
# and the cache. It also contains an "action", which is the symbol
# :prevent or :enable.
class Step
attr_reader :context, :rule, :action
def initialize(context, rule, action)
@context = context
@rule = rule
@action = action
end
# In the flattening process, duplicate steps may be generated in the
# same rule. This allows us to eliminate those (see Runner#steps_by_score
# and note its use of a Set)
def ==(other)
@context == other.context && @rule == other.rule && @action == other.action
end
# In the runner, steps are sorted dynamically by score, so that
# we are sure to compute them in close to the optimal order.
#
# See also Rule#score, ManifestCondition#score, and Runner#steps_by_score.
def score
# we slightly prefer the preventative actions
# since they are more likely to short-circuit
case @action
when :prevent
@rule.score(@context) * (7.0 / 8)
when :enable
@rule.score(@context)
end
end
def with_action(action)
Step.new(@context, @rule, action)
end
def enable?
@action == :enable
end
def prevent?
@action == :prevent
end
# This rather complex method allows us to split rules into parts so that
# they can be sorted independently for better optimization
def flattened(roots)
case @rule
when Rule::Or
# A single `Or` step is the same as each of its elements as separate steps
@rule.rules.flat_map { |r| Step.new(@context, r, @action).flattened(roots) }
when Rule::Ability
# This looks like a weird micro-optimization but it buys us quite a lot
# in some cases. If we depend on an Ability (i.e. a `can?(...)` rule),
# and that ability *only* has :enable actions (modulo some actions that
# we already have taken care of), then its rules can be safely inlined.
steps = @context.runner(@rule.ability).steps.reject { |s| roots.include?(s) }
if steps.all?(&:enable?)
# in the case that we are a :prevent step, each inlined step becomes
# an independent :prevent, even though it was an :enable in its initial
# context.
steps.map! { |s| s.with_action(:prevent) } if prevent?
steps.flat_map { |s| s.flattened(roots) }
else
[self]
end
else
[self]
end
end
def pass?
@rule.pass?(@context)
end
def repr
"#{@action} when #{@rule.repr} (#{@context.repr})"
end
end
end
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