Mysql fixes for Rails 5

* `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

config/application.rb

@@ -5,6 +5,12 @@ require 'rails/all'
 Bundler.require(:default, Rails.env)
 
 module Gitlab
+  # This method is used for smooth upgrading from the current Rails 4.x to Rails 5.0.
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/14286
+  def self.rails5?
+    ENV["RAILS5"].in?(%w[1 true])
+  end
+
   class Application < Rails::Application
     require_dependency Rails.root.join('lib/gitlab/redis/wrapper')
     require_dependency Rails.root.join('lib/gitlab/redis/cache')
@@ -14,6 +20,11 @@ module Gitlab
     require_dependency Rails.root.join('lib/gitlab/current_settings')
     require_dependency Rails.root.join('lib/gitlab/middleware/read_only')
 
+    # This needs to be loaded before DB connection is made
+    # to make sure that all connections have NO_ZERO_DATE
+    # setting disabled
+    require_dependency Rails.root.join('lib/mysql_zero_date')
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.
@@ -211,10 +222,4 @@ module Gitlab
       Gitlab::Routing.add_helpers(MilestonesRoutingHelper)
     end
   end
-
-  # This method is used for smooth upgrading from the current Rails 4.x to Rails 5.0.
-  # https://gitlab.com/gitlab-org/gitlab-ce/issues/14286
-  def self.rails5?
-    ENV["RAILS5"].in?(%w[1 true])
-  end
 end

config/initializers/active_record_data_types.rb

@@ -65,7 +65,7 @@ elsif Gitlab::Database.mysql?
     prepend RegisterDateTimeWithTimeZone
 
     # Add the class `DateTimeWithTimeZone` so we can map `timestamp` to it.
-    class MysqlDateTimeWithTimeZone < MysqlDateTime
+    class MysqlDateTimeWithTimeZone < (Gitlab.rails5? ? ActiveRecord::Type::DateTime : MysqlDateTime)
       def type
         :datetime_with_timezone
       end

db/migrate/merge_request_diff_file_limits_to_mysql.rb

@@ -4,7 +4,7 @@ class MergeRequestDiffFileLimitsToMysql < ActiveRecord::Migration
   def up
     return unless Gitlab::Database.mysql?
 
-    change_column :merge_request_diff_files, :diff, :text, limit: 2147483647
+    change_column :merge_request_diff_files, :diff, :text, limit: 2147483647, default: nil
   end
 
   def down

lib/mysql_zero_date.rb

+# 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?