Mysql fixes for Rails 5jprovazn-rails5-mysql-datetime

* `MysqlDateTimeWithTimeZone` inherits from `ActiveRecord::Type::DateTime`
  (`MysqlDateTime` is not present in Rails 5)
* explicitly set `NULL` default value for `merge_request_diff_files`'s
  `diff` column (otherwise empty string is used in a migration) and
  empty string is not allowed for text/blob fields in Mysql
* disable NO_ZERO_DATE mode for all Mysql DB connections, otherwise
  SQL queries fail on inserting `0` value for `created_at` column

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/mysql_zero_date.rb b/lib/mysql_zero_date.rb
new file mode 100644
index 00000000000..64634f789da
--- /dev/null
+++ b/lib/mysql_zero_date.rb
@@ -0,0 +1,18 @@
+# Disable NO_ZERO_DATE mode for mysql in rails 5.
+# We use zero date as a default value
+# (config/initializers/active_record_mysql_timestamp.rb), in
+# Rails 5 using zero date fails by default (https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/75450216)
+# and NO_ZERO_DATE has to be explicitly disabled. Disabling strict mode
+# is not sufficient.
+
+require 'active_record/connection_adapters/abstract_mysql_adapter'
+
+module MysqlZeroDate
+  def configure_connection
+    super
+
+    @connection.query "SET @@SESSION.sql_mode = REPLACE(@@SESSION.sql_mode, 'NO_ZERO_DATE', '');" # rubocop:disable Gitlab/ModuleWithInstanceVariables
+  end
+end
+
+ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter.prepend(MysqlZeroDate) if Gitlab.rails5?