Disallow migrations with the wrong versions

This custom Rubocop ensures that we don't end up with migrations that
are too new for Solidus's minimum required Rails version.

This has happened a few times lately. See #6192 and #6220 for more
context. It would be nice to avoid conflicts like these in an automated
way.

.rubocop.yml

@@ -5,6 +5,7 @@ inherit_from: .rubocop_todo.yml
 require:
   - rubocop-performance
   - rubocop-rails
+  - ./tasks/linting/wrong_migration_version.rb
 
 AllCops:
   Exclude:

core/lib/spree/core/version.rb

@@ -5,6 +5,8 @@ module Spree
 
   def self.solidus_version = VERSION
 
+  def self.minimum_required_rails_version = "7.0"
+
   def self.previous_solidus_minor_version = "4.5"
 
   def self.solidus_gem_version = Gem::Version.new(solidus_version)

core/solidus_core.gemspec

@@ -27,7 +27,10 @@ Gem::Specification.new do |s|
     actionmailer actionpack actionview activejob activemodel activerecord
     activestorage activesupport railties
   ].each do |rails_dep|
-    s.add_dependency rails_dep, ['>= 7.0', '< 8.1.0.beta1']
+    s.add_dependency rails_dep, [
+      ">= #{Spree.minimum_required_rails_version}",
+      "< 8.1.0.beta1"
+    ]
   end
 
   s.add_dependency 'activemerchant', '~> 1.66'

tasks/linting/wrong_migration_version.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require_relative '../../core/lib/spree/core/version'
+
+module Solidus
+  class WrongMigrationVersion < RuboCop::Cop::Base
+    MSG = "Subclasses of ActiveRecord::Migration must use a migration version " \
+      "of <= #{Spree.minimum_required_rails_version}"
+
+    def on_class(node)
+      return unless (superclass = node.parent_class)
+      return unless superclass.source&.start_with?("ActiveRecord::Migration")
+      return unless (given_version_arg = superclass.arguments.first&.value)
+
+      unless meets_solidus_minimum_required_rails_version? given_version_arg
+        add_offense node, message: MSG
+      end
+    end
+
+    private
+
+    def meets_solidus_minimum_required_rails_version?(version_argument)
+      Gem::Version.new(version_argument) <=
+        Gem::Version.new(Spree.minimum_required_rails_version)
+    end
+  end
+end

tasks/linting/wrong_migration_version_spec.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+require "rubocop"
+require "rubocop/rspec/support"
+
+require_relative "wrong_migration_version"
+
+RSpec.describe Solidus::WrongMigrationVersion do
+  include RuboCop::RSpec::ExpectOffense
+
+  subject(:cop) { described_class.new RuboCop::Config.new }
+
+  it "finds migration versions greater than the Solidus minimum required Rails version offensive" do
+    greater_than_minimum_version = Gem::Version
+      .new(Spree.minimum_required_rails_version)
+      .bump
+      .to_s
+
+    expect_offense(<<~RUBY)
+      class TestMigration < ActiveRecord::Migration[#{greater_than_minimum_version}]
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Solidus/WrongMigrationVersion: Subclasses of ActiveRecord::Migration must use a migration version of <= 7.0
+      end
+    RUBY
+  end
+
+  it "finds migration versions less than the Solidus minimum required Rails version inoffensive" do
+    expect_no_offenses(<<~RUBY)
+      class TestMigration < ActiveRecord::Migration[3.0]
+      end
+    RUBY
+  end
+
+  it "finds migration versions equal to the Solidus minimum required Rails version inoffensive" do
+    expect_no_offenses(<<~RUBY)
+      class TestMigration < ActiveRecord::Migration[#{Spree.minimum_required_rails_version}]
+      end
+    RUBY
+  end
+
+  it "finds non-migration classes inoffensive" do
+    expect_no_offenses(<<~RUBY)
+      class NotAMigration
+      end
+    RUBY
+  end
+
+  it "finds non-migration subclasses inoffensive" do
+    expect_no_offenses(<<~RUBY)
+      class SubclassOfNotAMigration < NotAMigration
+      end
+    RUBY
+  end
+end