Add default: support for ActiveSupport::CurrentAttributes.attribute

[seanpdoyle]

Detail

Extend the .attribute class method to accept a :default option for its list of attributes:

class Current < ActiveSupport::CurrentAttributes
  attribute :counter, default: 0
end

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[rafaelfranca]

Does this decrease the performance of the creation of those objects in any meaningful way?

[seanpdoyle]

@rafaelfranca these are the benchmark results:

Warming up --------------------------------------
          no default    76.698k i/100ms
  default from value    79.369k i/100ms
   default from proc    79.499k i/100ms
Calculating -------------------------------------
          no default    767.078k (± 0.4%) i/s -      3.912M in   5.099426s
  default from value    778.670k (± 3.4%) i/s -      3.968M in   5.102782s
   default from proc    779.728k (± 1.9%) i/s -      3.975M in   5.099794s

Comparison:
   default from proc:   779727.8 i/s
  default from value:   778670.0 i/s - same-ish: difference falls within error
          no default:   767078.2 i/s - same-ish: difference falls within error

source:

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "activesupport", path: "."

  gem "benchmark-ips"
end

require "active_support/code_generator"
require "active_support/current_attributes"

class Current < ActiveSupport::CurrentAttributes
  attribute :counter
end

class CurrentWithDefaultValue < ActiveSupport::CurrentAttributes
  attribute :counter, default: 0
end

class CurrentWithDefaultProc < ActiveSupport::CurrentAttributes
  attribute :counter, default: -> { 0 }
end

Benchmark.ips do |x|
  x.report("no default") do
    instance = Current.new
    instance.counter = 0
    instance.counter += 1
    instance.reset
  end
  x.report("default from value") do
    instance = CurrentWithDefaultValue.new
    instance.counter += 1
    instance.reset
  end
  x.report("default from proc") do
    instance = CurrentWithDefaultProc.new
    instance.counter += 1
    instance.reset
  end
  x.compare!
end

[rafaelfranca]

Amazing. Can you rebase please?

[seanpdoyle]

To compare to prior art, this is the code that Active Model uses when determining the default:

rails/activemodel/lib/active_model/attribute/user_provided_default.rb

Lines 17 to 23 in beb6214

	def value_before_type_cast
	if user_provided_value.is_a?(Proc)
	@memoized_value_before_type_cast ||= user_provided_value.call
	else
	@user_provided_value
	end
	end