Use a background migration for issues.closed_atchange-issues-closed-at-background-migration

In a previous attempt (rolled back in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16021) we tried
to migrate `issues.closed_at` from timestamp to timestamptz using a
regular migration. This has a bad impact on GitLab.com and as such was
rolled back.

This commit re-implements the original migrations using generic
background migrations, allowing us to still migrate the data in a single
release but without a negative impact on availability.

To ensure the database schema is up to date the background migrations
are performed inline in development and test environments. We also make
sure to not migrate that that doesn't need migrating in the first place
or has already been migrated.

2 files changed, 46 insertions, 1 deletions

diff --git a/db/post_migrate/20171221140220_schedule_issues_closed_at_type_change.rb b/db/post_migrate/20171221140220_schedule_issues_closed_at_type_change.rb
new file mode 100644
index 00000000000..be18c5866ae
--- /dev/null
+++ b/db/post_migrate/20171221140220_schedule_issues_closed_at_type_change.rb
@@ -0,0 +1,45 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ScheduleIssuesClosedAtTypeChange < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class Issue < ActiveRecord::Base
+    self.table_name = 'issues'
+    include EachBatch
+
+    def self.to_migrate
+      where('closed_at IS NOT NULL')
+    end
+  end
+
+  def up
+    return unless migrate_column_type?
+
+    change_column_type_using_background_migration(
+      Issue.to_migrate,
+      :closed_at,
+      :datetime_with_timezone
+    )
+  end
+
+  def down
+    return if migrate_column_type?
+
+    change_column_type_using_background_migration(
+      Issue.to_migrate,
+      :closed_at,
+      :datetime
+    )
+  end
+
+  def migrate_column_type?
+    # Some environments may have already executed the previous version of this
+    # migration, thus we don't need to migrate those environments again.
+    column_for('issues', 'closed_at').type == :datetime
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 42715d5e1e8..cd1f5d12fef 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171220191323) do
+ActiveRecord::Schema.define(version: 20171221140220) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"