problem.rb

# INFO: works well with Ruby 3.0.1
# hangs on any Ruby including https://github.com/ruby/ruby/commit/f9196de1dee6f5ab8b6fe115070b92775a3500fe
# since the yielding code in each method is running in different Fiber (due to be wrapped into Enumerator and next is used)

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 "activerecord", "~> 7.0.0"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"
require 'csv'

# 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 :users, force: true do |t|
    t.string :name
  end
end

class User < ActiveRecord::Base
end

class Exporter
  attr_reader :scope

  def initialize(scope)
    @scope = scope
  end

  def each
    return enum_for(__method__) unless block_given?

    yield CSV.generate_line(["name"])

    scope.in_batches(of: 10).each_record do |user|
      yield CSV.generate_line([user.name])
    end
  end
end

class LockTest < Minitest::Test
  def test_my_problem
    User.insert_all(100.times.map { |i| {name: "User ##{i}"} })

    User.transaction do
      # simulate something affecting data to be visible only in this transaction
      sleep 2

      # and "streamed" export
      enum = Exporter.new(User.all).each

      # dummy example of consuming iterator
      # in real code it is wrapped into IO-like API and consumed as IO using buffered read
      # output is being zipped and uploaded in chunks to GCS
      #
      # for IO wrapper approach similar to https://github.com/lautis/piperator/blob/master/lib/piperator/io.rb is used
      #
      # iteration happens only inside of the transaction, enum is not leaked outside
      File.open('/tmp/export.csv', 'w+') do |file|
        loop do
          data = enum.next
          sleep 0.05 # simulate more work in the "streaming" chain
          file.write(data)
        rescue StopIteration
          break
        end
      end
    end
  end
end