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Speed up policy lookups in DeclarativePolicy

Peter Leitzen requested to merge pl-dp-class-cache into master

What does this MR do and why?

Add a global cache to cache subject's mapping to their policy classes.

This patches will be upstreamed to declarative-policy gem soonish.

See gitlab-org/ruby/gems/declarative-policy#30 and !119924 (merged).

How to set up and validate locally

Heavily inspired by !119924 (merged).

Benchmark code
# frozen_string_literal: true

require "./config/environment"

Rails.configuration.eager_load_namespaces.each(&:eager_load!)
puts "Application is loaded."

require 'benchmark/ips'

class Project2 < Project
  def self.declarative_policy_class
    :ProjectPolicy
  end
end

user = User.first
subject_1 = Project.first
subject_2 = Project2.first

Benchmark.ips do |b|
  b.report('implicit') do
    DeclarativePolicy.policy_for(user, subject_1)
  end

  b.report('explicit') do
    DeclarativePolicy.policy_for(user, subject_2)
  end

  b.compare!
end

Before

Application is loaded.
Warming up --------------------------------------
            implicit   463.000  i/100ms
            explicit    17.589k i/100ms
Calculating -------------------------------------
            implicit      4.232k (±27.7%) i/s -     18.520k in   5.073017s
            explicit    163.059k (± 6.6%) i/s -    826.683k in   5.092063s

Comparison:
            explicit:   163058.5 i/s
            implicit:     4232.2 i/s - 38.53x  slower

After

Application is loaded.
Warming up --------------------------------------
            implicit    16.562k i/100ms
            explicit    15.042k i/100ms
Calculating -------------------------------------
            implicit    139.730k (± 8.8%) i/s -    695.604k in   5.020544s
            explicit    124.751k (± 8.7%) i/s -    631.764k in   5.100487s

Comparison:
            implicit:   139730.3 i/s
            explicit:   124750.5 i/s - same-ish: difference falls within error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Peter Leitzen

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