Ensure Action Mailbox processes an email only once when received multiple times

This also adds a new column, message_checksum, to the action_mailbox_inbound_emails table
for storing SHA1 digest of the email source. Additionally, it makes generating the missing
message id deterministic and adds a unique index on message_checksum and message_id to
detect duplicate emails.

actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb

@@ -9,30 +9,30 @@
 module ActionMailbox::InboundEmail::MessageId
   extend ActiveSupport::Concern
 
-  included do
-    before_save :generate_missing_message_id
-  end
-
   class_methods do
     # Create a new +InboundEmail+ from the raw +source+ of the email, which be uploaded as a Active Storage
     # attachment called +raw_email+. Before the upload, extract the Message-ID from the +source+ and set
     # it as an attribute on the new +InboundEmail+.
     def create_and_extract_message_id!(source, **options)
-      create! options.merge(message_id: extract_message_id(source)) do |inbound_email|
+      message_checksum = Digest::SHA1.hexdigest(source)
+      message_id = extract_message_id(source) || generate_missing_message_id(message_checksum)
+
+      create! options.merge(message_id: message_id, message_checksum: message_checksum) do |inbound_email|
         inbound_email.raw_email.attach io: StringIO.new(source), filename: "message.eml", content_type: "message/rfc822"
       end
+    rescue ActiveRecord::RecordNotUnique
+      nil
     end
 
     private
       def extract_message_id(source)
         Mail.from_source(source).message_id rescue nil
       end
-  end
 
-  private
-    def generate_missing_message_id
-      self.message_id ||= Mail::MessageIdField.new.message_id.tap do |message_id|
-        logger.warn "Message-ID couldn't be parsed or is missing. Generated a new Message-ID: #{message_id}"
+      def generate_missing_message_id(message_checksum)
+        Mail::MessageIdField.new("<#{message_checksum}@#{::Socket.gethostname}.mail>").message_id.tap do |message_id|
+          logger.warn "Message-ID couldn't be parsed or is missing. Generated a new Message-ID: #{message_id}"
+        end
       end
-    end
+  end
 end

actionmailbox/db/migrate/20180917164000_create_action_mailbox_tables.rb

@@ -2,10 +2,13 @@ class CreateActionMailboxTables < ActiveRecord::Migration[6.0]
   def change
     create_table :action_mailbox_inbound_emails do |t|
       t.integer :status, default: 0, null: false
-      t.string  :message_id
+      t.string  :message_id, null: false
+      t.string  :message_checksum, null: false
 
       t.datetime :created_at, precision: 6, null: false
       t.datetime :updated_at, precision: 6, null: false
+
+      t.index [ :message_id, :message_checksum ], unique: true
     end
   end
 end

actionmailbox/test/dummy/db/migrate/20180208205311_create_action_mailroom_tables.rb

@@ -2,10 +2,13 @@ class CreateActionMailboxTables < ActiveRecord::Migration[6.0]
   def change
     create_table :action_mailbox_inbound_emails do |t|
       t.integer :status, default: 0, null: false
-      t.string  :message_id
+      t.string  :message_id, null: false
+      t.string  :message_checksum, null: false
 
       t.datetime :created_at, precision: 6
       t.datetime :updated_at, precision: 6
+
+      t.index [ :message_id, :message_checksum ], unique: true
     end
   end
 end

actionmailbox/test/dummy/db/schema.rb

@@ -15,8 +15,10 @@ ActiveRecord::Schema.define(version: 2018_02_12_164506) do
   create_table "action_mailbox_inbound_emails", force: :cascade do |t|
     t.integer "status", default: 0, null: false
     t.string "message_id"
+    t.string "message_checksum"
     t.datetime "created_at", precision: 6
     t.datetime "updated_at", precision: 6
+    t.index ["message_id", "message_checksum"], name: "index_action_mailbox_inbound_emails_uniqueness", unique: true
   end
 
   create_table "active_storage_attachments", force: :cascade do |t|

actionmailbox/test/unit/inbound_email_test.rb

@@ -11,5 +11,27 @@ module ActionMailbox
     test "source returns the contents of the raw email" do
       assert_equal file_fixture("welcome.eml").read, create_inbound_email_from_fixture("welcome.eml").source
     end
+
+    test "email with message id is processed only once when received multiple times" do
+      mail = Mail.from_source(file_fixture("welcome.eml").read)
+      assert mail.message_id
+
+      inbound_email_1 = create_inbound_email_from_source(mail.to_s)
+      assert inbound_email_1
+
+      inbound_email_2 = create_inbound_email_from_source(mail.to_s)
+      assert_nil inbound_email_2
+    end
+
+    test "email with missing message id is processed only once when received multiple times" do
+      mail = Mail.from_source("Date: Fri, 28 Sep 2018 11:08:55 -0700\r\nTo: a@example.com\r\nMime-Version: 1.0\r\nContent-Type: text/plain\r\nContent-Transfer-Encoding: 7bit\r\n\r\nHello!")
+      assert_nil mail.message_id
+
+      inbound_email_1 = create_inbound_email_from_source(mail.to_s)
+      assert inbound_email_1
+
+      inbound_email_2 = create_inbound_email_from_source(mail.to_s)
+      assert_nil inbound_email_2
+    end
   end
 end

actionmailbox/test/unit/mailbox/routing_test.rb

@@ -15,11 +15,10 @@ end
 class ActionMailbox::Base::RoutingTest < ActiveSupport::TestCase
   setup do
     $processed = false
-    @inbound_email = create_inbound_email_from_fixture("welcome.eml")
   end
 
   test "string routing" do
-    ApplicationMailbox.route @inbound_email
+    ApplicationMailbox.route create_inbound_email_from_fixture("welcome.eml")
     assert_equal "Discussion: Let's debate these attachments", $processed
   end