Fix compatibility with Bundler 1.6

Stop using `Bundler.settings.with`, because it will be removed from
Bundler 1.6.

Also, as described in #7653, we could use `Bundler.settings[:with]`, but
that would be internal API again, so it probably breaks again in the
future. That's why I added a `BundlerHelper` module to parse the
required optional group from our config files, without the use of any
internal Bundler API.

Fixes #7653

config/application.rb

@@ -4,9 +4,11 @@ require_relative 'boot'
 
 require 'rails/all'
 
+require_relative "bundler_helper"
+
 # Require the gems listed in Gemfile, including any gems
 # you've limited to :test, :development, or :production.
-Bundler.require(*Rails.groups(*Bundler.settings.with))
+Bundler.require(*Rails.groups(BundlerHelper.database))
 
 # Do not dump the limit of boolean fields on MySQL,
 # since that generates a db/schema.rb that's incompatible

config/bundler_helper.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "yaml"
+
+module BundlerHelper
+  def self.rails_env
+    @rails_env ||= ENV["RAILS_ENV"] ||
+      parse_value_from_file("diaspora.yml", "configuration", "server", "rails_environment") ||
+      parse_value_from_file("defaults.yml", "defaults", "server", "rails_environment")
+  end
+
+  def self.database
+    @adapter ||= parse_value_from_file("database.yml", rails_env, "adapter")
+
+    raise "No database adapter found, please fix your config/database.yml!" unless @adapter
+
+    @adapter.sub("mysql2", "mysql")
+  end
+
+  private_class_method def self.parse_value_from_file(file, *keys)
+    path = File.join(__dir__, file)
+    return YAML.load_file(path).dig(*keys) if File.file?(path)
+
+    puts "Configuration file #{path} not found, ensure it's present" # rubocop:disable Rails/Output
+  end
+end