`ActiveStorage::Attached::Many#attach` may erase files uploaded concurrently

[smt116]

Steps to reproduce

First setup the sample application:

1. gem install rails -v6.1.4

2. rails new ConcurrentAttachTestApp --skip-action-mailer --skip-action-mailbox --skip-action-text --skip-action-cable --skip-sprockets --skip-spring --skip-listen --skip-javascript --skip-turbolinks --skip-jbuilder --skip-test --skip-system-test --skip-bootsnap

3. apply the following diff:

   diff --git a/Gemfile b/Gemfile
   index 01a705b..25b3e19 100644
   --- a/Gemfile
   +++ b/Gemfile
   @@ -15,6 +15,8 @@ gem 'puma', '~> 5.0'
    # Use Active Storage variant
    # gem 'image_processing', '~> 1.2'
   
   +gem 'pg'
   +
    group :development, :test do
      # Call 'byebug' anywhere in the code to stop execution and get a debugger console
      gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
   @@ -26,6 +28,8 @@ group :development do
      # Display performance information such as SQL time and flame graphs for each request in your browser.
      # Can be configured to work on production as well see: https://github.com/MiniProfiler/rack-mini-profiler/blob/master/README.md
      gem 'rack-mini-profiler', '~> 2.0'
   +
   +  gem 'parallel'
    end
   
    # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
   diff --git a/app/models/user.rb b/app/models/user.rb
   new file mode 100644
   index 0000000..2db75b6
   --- /dev/null
   +++ b/app/models/user.rb
   @@ -0,0 +1,3 @@
   +class User < ApplicationRecord
   +  has_many_attached :files
   +end
   diff --git a/config/database.yml b/config/database.yml
   index 4a8a1b2..78ac63a 100644
   --- a/config/database.yml
   +++ b/config/database.yml
   @@ -10,8 +10,12 @@ default: &default
      timeout: 5000
   
    development:
   -  <<: *default
   -  database: db/development.sqlite3
   +  adapter: postgresql
   +  database: concurrent_attach_development
   +  encoding: unicode
   +  host: localhost
   +  pool: 5
   +  username: postgres
   
    # Warning: The database defined as "test" will be erased and
    # re-generated from your development database when you run "rake".
   diff --git a/db/migrate/20210804080852_create_active_storage_tables.active_storage.rb b/db/migrate/20210804080852_create_active_storage_tables.active_storage.rb
   new file mode 100644
   index 0000000..8779826
   --- /dev/null
   +++ b/db/migrate/20210804080852_create_active_storage_tables.active_storage.rb
   @@ -0,0 +1,36 @@
   +# This migration comes from active_storage (originally 20170806125915)
   +class CreateActiveStorageTables < ActiveRecord::Migration[5.2]
   +  def change
   +    create_table :active_storage_blobs do |t|
   +      t.string   :key,          null: false
   +      t.string   :filename,     null: false
   +      t.string   :content_type
   +      t.text     :metadata
   +      t.string   :service_name, null: false
   +      t.bigint   :byte_size,    null: false
   +      t.string   :checksum,     null: false
   +      t.datetime :created_at,   null: false
   +
   +      t.index [ :key ], unique: true
   +    end
   +
   +    create_table :active_storage_attachments do |t|
   +      t.string     :name,     null: false
   +      t.references :record,   null: false, polymorphic: true, index: false
   +      t.references :blob,     null: false
   +
   +      t.datetime :created_at, null: false
   +
   +      t.index [ :record_type, :record_id, :name, :blob_id ], name: "index_active_storage_attachments_uniqueness", unique: true
   +      t.foreign_key :active_storage_blobs, column: :blob_id
   +    end
   +
   +    create_table :active_storage_variant_records do |t|
   +      t.belongs_to :blob, null: false, index: false
   +      t.string :variation_digest, null: false
   +
   +      t.index %i[ blob_id variation_digest ], name: "index_active_storage_variant_records_uniqueness", unique: true
   +      t.foreign_key :active_storage_blobs, column: :blob_id
   +    end
   +  end
   +end
   diff --git a/db/schema.rb b/db/schema.rb
   new file mode 100644
   index 0000000..dbb533c
   --- /dev/null
   +++ b/db/schema.rb
   @@ -0,0 +1,51 @@
   +# This file is auto-generated from the current state of the database. Instead
   +# of editing this file, please use the migrations feature of Active Record to
   +# incrementally modify your database, and then regenerate this schema definition.
   +#
   +# This file is the source Rails uses to define your schema when running `bin/rails
   +# db:schema:load`. When creating a new database, `bin/rails db:schema:load` tends to
   +# be faster and is potentially less error prone than running all of your
   +# migrations from scratch. Old migrations may fail to apply correctly if those
   +# migrations use external dependencies or application code.
   +#
   +# It's strongly recommended that you check this file into your version control system.
   +
   +ActiveRecord::Schema.define(version: 2021_08_04_080852) do
   +
   +  # These are extensions that must be enabled in order to support this database
   +  enable_extension "plpgsql"
   +
   +  create_table "active_storage_attachments", force: :cascade do |t|
   +    t.string "name", null: false
   +    t.string "record_type", null: false
   +    t.bigint "record_id", null: false
   +    t.bigint "blob_id", null: false
   +    t.datetime "created_at", null: false
   +    t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
   +    t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
   +  end
   +
   +  create_table "active_storage_blobs", force: :cascade do |t|
   +    t.string "key", null: false
   +    t.string "filename", null: false
   +    t.string "content_type"
   +    t.text "metadata"
   +    t.string "service_name", null: false
   +    t.bigint "byte_size", null: false
   +    t.string "checksum", null: false
   +    t.datetime "created_at", null: false
   +    t.index ["key"], name: "index_active_storage_blobs_on_key", unique: true
   +  end
   +
   +  create_table "active_storage_variant_records", force: :cascade do |t|
   +    t.bigint "blob_id", null: false
   +    t.string "variation_digest", null: false
   +    t.index ["blob_id", "variation_digest"], name: "index_active_storage_variant_records_uniqueness", unique: true
   +  end
   +
   +  create_table "users", force: :cascade do |t|
   +  end
   +
   +  add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"
   +  add_foreign_key "active_storage_variant_records", "active_storage_blobs", column: "blob_id"
   +end
   

4. bundle install

5. bin/rails active_storage:install

6. bin/rails db:setup

Now try the following code in the bin/rails console:

User.create

Parallel.each(1..10) do
  ActiveRecord::Base.connection_pool.with_connection do
    User.first.files.attach(io: File.open("README.md"), filename: "README.md")
  end
end

User.first.files.count

Note that User.first.files.count may return a smaller number than 10.

Expected behavior

Each Object#attach appends the new file and does not touch existing attachments.

Actual behavior

Object#attach may purge files that are uploaded concurrently.

System configuration

Rails version: 6.1.4

Ruby version: 3.0.1