ActiveStorage bug w/ multiple `#attach` calls in a transaction

bug.rb

# 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 "rails", "6.1.3"
  gem "sqlite3"
end

require "active_record/railtie"
require "active_storage/engine"
require "tmpdir"

class TestApp < Rails::Application
  config.root = __dir__
  config.hosts << "example.org"
  config.eager_load = false
  config.session_store :cookie_store, key: "cookie_store_key"
  secrets.secret_key_base = "secret_key_base"

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  config.active_storage.service = :local
  config.active_storage.service_configurations = {
    local: {
      root: Dir.tmpdir,
      service: "Disk"
    }
  }
end

ENV["DATABASE_URL"] = "sqlite3::memory:"

Rails.application.initialize!

require ActiveStorage::Engine.root.join("db/migrate/20170806125915_create_active_storage_tables.rb").to_s

ActiveRecord::Schema.define do
  CreateActiveStorageTables.new.change

  create_table :users, force: true
end

class User < ActiveRecord::Base
  has_many_attached :attachments
end

require "minitest/autorun"

class BugTest < Minitest::Test
  # Passes
  def test_attach_files_together_no_transaction
    user = User.create!

    user.attachments.attach([
      {
        content_type: "text/plain",
        filename: "dummy.txt",
        io: ::StringIO.new("dummy"),
      },
      {
        content_type: "text/plain",
        filename: "dummy2.txt",
        io: ::StringIO.new("dummy2"),
      }
    ])

    assert_equal 2, user.attachments.count
    assert user.attachments.first.service.exist?(user.attachments.first.key)
    assert user.attachments.second.service.exist?(user.attachments.second.key)
  end

  # Passes
  def test_attach_files_separately_no_transaction
    user = User.create!

    user.attachments.attach({
      content_type: "text/plain",
      filename: "dummy.txt",
      io: ::StringIO.new("dummy"),
    })
    user.attachments.attach({
      content_type: "text/plain",
      filename: "dummy2.txt",
      io: ::StringIO.new("dummy2"),
    })

    assert_equal 2, user.attachments.count
    assert user.attachments.first.service.exist?(user.attachments.first.key)
    assert user.attachments.second.service.exist?(user.attachments.second.key)
  end

  # Passes
  def test_attach_files_together_in_transaction
    user = User.create!

    ActiveRecord::Base.transaction do
      user.attachments.attach([
        {
          content_type: "text/plain",
          filename: "dummy.txt",
          io: ::StringIO.new("dummy"),
        },
        {
          content_type: "text/plain",
          filename: "dummy2.txt",
          io: ::StringIO.new("dummy2"),
        }
      ])
    end

    assert_equal 2, user.attachments.count
    assert user.attachments.first.service.exist?(user.attachments.first.key)
    assert user.attachments.second.service.exist?(user.attachments.second.key)
  end

  # Fails
  def test_attach_files_separately_in_transaction
    user = User.create!

    ActiveRecord::Base.transaction do
      user.attachments.attach({
        content_type: "text/plain",
        filename: "dummy.txt",
        io: ::StringIO.new("dummy"),
      })
      user.attachments.attach({
        content_type: "text/plain",
        filename: "dummy2.txt",
        io: ::StringIO.new("dummy2"),
      })
    end

    assert_equal 2, user.attachments.count
    assert user.attachments.first.service.exist?(user.attachments.first.key)  # Fails
    assert user.attachments.second.service.exist?(user.attachments.second.key)
  end
end