test: add tests for message unqueueing

This adds a comprehensive set of tests for the message unqueueing service.

Additionally, it improves how message databases are used for testing environments.

spec/helpers/message_db_mocking.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module GlobalMessageDB
+
+  class << self
+
+    def find_or_create
+      return @db if @db
+
+      @db = Postal::MessageDB::Database.new(1, 1, database_name: "postal-test-message-db")
+      @db.provisioner.provision
+    end
+
+    def exists?
+      !@db.nil?
+    end
+
+  end
+
+end
+
+RSpec.configure do |config|
+  config.before(:example) do
+    @mocked_message_dbs = []
+    allow_any_instance_of(Server).to receive(:message_db).and_wrap_original do |m|
+      GlobalMessageDB.find_or_create
+
+      message_db = m.call
+      @mocked_message_dbs << message_db
+      allow(message_db).to receive(:database_name).and_return("postal-test-message-db")
+      message_db
+    end
+  end
+
+  config.after(:example) do
+    if GlobalMessageDB.exists? && @mocked_message_dbs.present?
+      GlobalMessageDB.find_or_create.provisioner.clean
+      @mocked_message_dbs = []
+    end
+  end
+end

spec/helpers/message_factory.rb

@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+# This class can be used to generate a message which can be used for the purposes of
+# testing within the given server.
+class MessageFactory
+
+  def initialize(server)
+    @server = server
+  end
+
+  def incoming(route: nil, &block)
+    @message = @server.message_db.new_message
+    @message.scope = "incoming"
+    @message.rcpt_to = "test@example.com"
+    @message.mail_from = "john@example.com"
+
+    if route
+      @message.rcpt_to = route.description
+      @message.route_id = route.id
+    end
+
+    create_message(&block)
+  end
+
+  def outgoing(domain: nil, credential: nil, &block)
+    @message = @server.message_db.new_message
+    @message.scope = "outgoing"
+    @message.rcpt_to = "john@example.com"
+    @message.mail_from = "test@example.com"
+
+    if domain
+      @message.mail_from = "test@#{domain.name}"
+      @message.domain_id = domain.id
+    end
+
+    if credential
+      @message.credential_id = credential.id
+    end
+
+    create_message(&block)
+  end
+
+  class << self
+
+    def incoming(server, **kwargs, &block)
+      new(server).incoming(**kwargs, &block)
+    end
+
+    def outgoing(server, **kwargs, &block)
+      new(server).outgoing(**kwargs, &block)
+    end
+
+  end
+
+  private
+
+  def create_message
+    mail = create_mail(@message.rcpt_to, @message.mail_from)
+
+    if block_given?
+      yield @message, mail
+    end
+
+    @message.raw_message = mail.to_s
+    @message.save(queue_on_create: false)
+    @message
+  end
+
+  def create_mail(to, from)
+    mail = Mail.new
+    mail.to = to
+    mail.from = from
+    mail.subject = "An example message"
+    mail.body = "Hello world!"
+    mail
+  end
+
+end