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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|
break if !debug && @state.prevented?
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?
@state.prevent! if passed
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 critical 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
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
remaining_steps = Set.new(@steps)
remaining_enablers, remaining_preventers = remaining_steps.partition(&:enable?).map { |s| Set.new(s) }
loop do
if @state.enabled?
# Once we set this, we never need to unset it, because a single
# prevent will stop this from being enabled
remaining_steps = remaining_preventers
else
# if the permission hasn't yet been enabled and we only have
# prevent steps left, we short-circuit the state here
@state.prevent! if remaining_enablers.empty?
end
return if remaining_steps.empty?
lowest_score = Float::INFINITY
next_step = nil
remaining_steps.each do |step|
score = step.score
if score < lowest_score
next_step = step
lowest_score = score
end
break if lowest_score.zero?
end
[remaining_steps, remaining_enablers, remaining_preventers].each do |set|
set.delete(next_step)
end
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
|
