Added RuboCop cops for checking DB migrations

There are currently two cops for this:

* Migration/AddIndex: checks if indexes are added concurrently
* Migration/ColumnWithDefault: checks if columns with default values are
  added in a concurrent manner

Both cops are fairly simple and make no attempt at correcting the code
as this is quite hard to do (e.g. modifications may need to be applied
in various places in the same file). They however should provide enough
to catch people ignoring the comments in generated migrations telling
them to use add_concurrent_index or add_column_with_default.

.rubocop.yml

-require: rubocop-rspec
+require:
+  - rubocop-rspec
+  - ./rubocop/rubocop
 
 AllCops:
   TargetRubyVersion: 2.1

rubocop/cop/migration/add_index.rb

+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if indexes are added in a concurrent manner.
+      class AddIndex < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'add_index requires downtime, use add_concurrent_index instead'
+
+        def on_def(node)
+          return unless in_migration?(node)
+
+          new_tables = []
+
+          node.each_descendant(:send) do |send_node|
+            first_arg = first_argument(send_node)
+
+            # The first argument of "create_table" / "add_index" is the table
+            # name.
+            new_tables << first_arg if create_table?(send_node)
+
+            next if method_name(send_node) != :add_index
+
+            # Using "add_index" is fine for newly created tables as there's no
+            # data in these tables yet.
+            next if new_tables.include?(first_arg)
+
+            add_offense(send_node, :selector)
+          end
+        end
+
+        def create_table?(node)
+          method_name(node) == :create_table
+        end
+
+        def method_name(node)
+          node.children[1]
+        end
+
+        def first_argument(node)
+          node.children[2] ? node.children[0] : nil
+        end
+      end
+    end
+  end
+end

rubocop/cop/migration/column_with_default.rb

+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if columns are added in a way that doesn't require
+      # downtime.
+      class ColumnWithDefault < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        WHITELISTED_TABLES = [:application_settings]
+
+        MSG = 'add_column with a default value requires downtime, ' \
+          'use add_column_with_default instead'
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          return unless name == :add_column
+
+          # Ignore whitelisted tables.
+          return if table_whitelisted?(node.children[2])
+
+          opts = node.children.last
+
+          return unless opts && opts.type == :hash
+
+          opts.each_node(:pair) do |pair|
+            if hash_key_type(pair) == :sym && hash_key_name(pair) == :default
+              add_offense(node, :selector)
+            end
+          end
+        end
+
+        def table_whitelisted?(symbol)
+          symbol && symbol.type == :sym &&
+            WHITELISTED_TABLES.include?(symbol.children[0])
+        end
+
+        def hash_key_type(pair)
+          pair.children[0].type
+        end
+
+        def hash_key_name(pair)
+          pair.children[0].children[0]
+        end
+      end
+    end
+  end
+end

rubocop/migration_helpers.rb

+module RuboCop
+  # Module containing helper methods for writing migration cops.
+  module MigrationHelpers
+    # Returns true if the given node originated from the db/migrate directory.
+    def in_migration?(node)
+      File.dirname(node.location.expression.source_buffer.name).
+        end_with?('db/migrate')
+    end
+  end
+end

rubocop/rubocop.rb

+require_relative 'migration_helpers'
+require_relative 'cop/migration/add_index'
+require_relative 'cop/migration/column_with_default'