Fix index creation to preserve comment in bulk change table on MySQL

I've found the bug when I'm refactoring index creation code in #39203.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix index creation to preserve index comment in bulk change table on MySQL.
+
+    *Ryuta Kamizono*
+
 *   Allow `unscope` to be aware of table name qualified values.
 
     It is possible to unscope only the column in the specified table.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -676,9 +676,10 @@ module ActiveRecord
         end
 
         def add_index_for_alter(table_name, column_name, options = {})
-          index_name, index_type, index_columns, _, _, index_algorithm, index_using = add_index_options(table_name, column_name, **options)
+          index_name, index_type, index_columns, _, _, index_algorithm, index_using, comment = add_index_options(table_name, column_name, **options)
           index_algorithm[0, 0] = ", " if index_algorithm.present?
-          "ADD #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})#{index_algorithm}"
+          sql = +"ADD #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})#{index_algorithm}"
+          add_sql_comment!(sql, comment)
         end
 
         def remove_index_for_alter(table_name, column_name = nil, options = {})

activerecord/test/cases/migration_test.rb

@@ -1084,7 +1084,7 @@ if ActiveRecord::Base.connection.supports_bulk_alter?
       classname = ActiveRecord::Base.connection.class.name[/[^:]*$/]
       expected_query_count = {
         "Mysql2Adapter"     => 1, # mysql2 supports creating two indexes using one statement
-        "PostgreSQLAdapter" => 2,
+        "PostgreSQLAdapter" => 3,
       }.fetch(classname) {
         raise "need an expected query count for #{classname}"
       }
@@ -1092,7 +1092,7 @@ if ActiveRecord::Base.connection.supports_bulk_alter?
       assert_queries(expected_query_count) do
         with_bulk_change_table do |t|
           t.index :username, unique: true, name: :awesome_username_index
-          t.index [:name, :age]
+          t.index [:name, :age], comment: "This is a comment"
         end
       end
 
@@ -1100,6 +1100,7 @@ if ActiveRecord::Base.connection.supports_bulk_alter?
 
       name_age_index = index(:index_delete_me_on_name_and_age)
       assert_equal ["name", "age"].sort, name_age_index.columns.sort
+      assert_equal "This is a comment", name_age_index.comment
       assert_not name_age_index.unique
 
       assert index(:awesome_username_index).unique