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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
@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
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