Handle EE edge cases in issues.closed_at migration

EE seems to have had an outdated schema at some point, leading to some
environments not having the right columns in place. This adjusts the
migration for `issues.closed_at` so it takes care of those cases,
ensuring data can be migrated properly.

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/4803

db/migrate/20180201145907_migrate_remaining_issues_closed_at.rb

@@ -18,12 +18,21 @@ class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration
     Gitlab::BackgroundMigration.steal('CopyColumn')
     Gitlab::BackgroundMigration.steal('CleanupConcurrentTypeChange')
 
-    # It's possible the cleanup job was killed which means we need to manually
-    # migrate any remaining rows.
-    migrate_remaining_rows if migrate_column_type?
+    if migrate_column_type?
+      if closed_at_for_type_change_exists?
+        migrate_remaining_rows
+      else
+        # Due to some EE merge problems some environments may not have the
+        # "closed_at_for_type_change" column. If this is the case we have no
+        # other option than to migrate the data _right now_.
+        change_column_type_concurrently(:issues, :closed_at, :datetime_with_timezone)
+        cleanup_concurrent_column_type_change(:issues, :closed_at)
+      end
+    end
   end
 
   def down
+    # Previous migrations already revert the changes made here.
   end
 
   def migrate_remaining_rows
@@ -39,4 +48,8 @@ class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration
     # migration, thus we don't need to migrate those environments again.
     column_for('issues', 'closed_at').type == :datetime # rubocop:disable Migration/Datetime
   end
+
+  def closed_at_for_type_change_exists?
+    columns('issues').any? { |col| col.name == 'closed_at_for_type_change' }
+  end
 end