SystemStackError reproduction script

systemstackrails.rb

# frozen_string_literal: true

## SystemStackError introduced by https://github.com/rails/rails/pull/41860

require "bundler/inline"

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

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

  # Good
  # gem "rails", github: "rails/rails", ref: "2bbb519"
  # Bad
  gem "rails", github: "rails/rails", ref: "ae56ecd"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :comments, force: true do |t|
    t.string :body
    t.integer :public
  end
end

class Comment < ActiveRecord::Base
  after_update :update_again, if: :saved_change_to_public?

  def update_again
    update_attribute :body, nil
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    comment = Comment.create(public: false, body: nil)

    comment.update(public: true)

    assert_equal 1, Comment.count
    assert Comment.first.public?
    assert_nil Comment.first.body
  end
end