support both old and new postgres versions for new submission column

this migration was originally written with a backfill, but the
submissions table was large enough that the backfill was very slow. so
we changed to create the column with a default, taking advantage of
PG11+ ability to do this quickly rather than lock the table. But some
operators have not upgraded postgres versions yet, so let's support both
methods for now, while we work out a clear upgrade timeline.

Change-Id: I7e745c16ba162ab2740e9387e3138a7fca74df8c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/261779
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20210127005857_add_redo_request_to_submissions.rb

@@ -21,11 +21,17 @@ class AddRedoRequestToSubmissions < ActiveRecord::Migration[5.2]
   tag :predeploy
 
   def up
-    remove_column :submissions, :redo_request, if_exists: true
-    add_column :submissions, :redo_request, :boolean, default: false, null: false
+    if connection.postgresql_version >= 110000
+      remove_column :submissions, :redo_request, if_exists: true
+      add_column :submissions, :redo_request, :boolean, default: false, null: false
+    else
+      # backfill and default will come in a postdeploy
+      add_column :submissions, :redo_request, :boolean
+      change_column_default(:submissions, :redo_request, false)
+    end
   end
 
   def down
-    change_column_null(:submissions, :redo_request, true)
+    remove_column :submissions, :redo_request, if_exists: true
   end
 end

db/migrate/20210127005858_backfill_submissions_redo_request.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+#
+# Copyright (C) 2021 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+class BackfillSubmissionsRedoRequest < ActiveRecord::Migration[5.2]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def self.runnable?
+    connection.postgresql_version < 110000
+  end
+
+  def up
+    if Submission.columns.detect{|c| c.name == "redo_request"}&.null
+      DataFixup::BackfillNulls.run(Submission, :redo_request, default_value: false)
+      change_column_null(:submissions, :redo_request, false)
+    end
+  end
+
+  def down
+    change_column_null(:submissions, :redo_request, true)
+  end
+end