allow additional columns to be ignored through Consul

refs AE-124

When running pre-deploy migrations that add a new column, if an application server loads the schema on a shard where the migration has already run, and tries to run queries on a shard where the migration has not already run, it can lead to errors. Fix this by allowing ignored_columns to append to its value dynamically through Consul.

Change-Id: I3b13e0fd2ac066e1439d3d314d71f2ebd8938e0c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/310561
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>

config/initializers/active_record.rb

@@ -2219,3 +2219,13 @@ if Rails.version >= "6.1"
   # rubocop:enable Lint/RescueException
   # rubocop:enable Naming/RescuedExceptionsVariableName
 end
+
+module AdditionalIgnoredColumns
+  def ignored_columns
+    return super unless superclass <= ActiveRecord::Base && !abstract_class?
+
+    columns_to_ignore = DynamicSettings.find("activerecord/ignored_columns", tree: :store)[table_name]&.split(",") || []
+    super + columns_to_ignore
+  end
+end
+ActiveRecord::Base.singleton_class.include(AdditionalIgnoredColumns)

lib/tasks/db.rake

@@ -11,3 +11,35 @@ end
     MultiCache.delete("schema_cache")
   end
 end
+
+namespace :db do
+  desc "Clear columns to be ignored for each model"
+  task :clear_ignored_columns, [:table_name] => :environment do |_t, args|
+    Diplomat::Kv.delete(
+      "store/canvas/#{DynamicSettings.environment}/activerecord/ignored_columns/#{args[:table_name]}"
+    )
+
+    MultiCache.delete("schema_cache")
+  end
+
+  desc "Get columns to be ignored for each model"
+  task :get_ignored_columns, [:table_name] => :environment do |_t, args|
+    ignored_columns = Diplomat::Kv.get(
+      "store/canvas/#{DynamicSettings.environment}/activerecord/ignored_columns/#{args[:table_name]}"
+    )
+
+    puts "Ignored Columns: #{ignored_columns}"
+  rescue Diplomat::KeyNotFound
+    puts "Ignored Columns: -"
+  end
+
+  desc "Set columns to be ignored for each model"
+  task :set_ignored_columns, [:table_name, :columns] => :environment do |_t, args|
+    Diplomat::Kv.put(
+      "store/canvas/#{DynamicSettings.environment}/activerecord/ignored_columns/#{args[:table_name]}",
+      args[:columns]
+    )
+
+    MultiCache.delete("schema_cache")
+  end
+end

spec/initializers/active_record_spec.rb

@@ -338,6 +338,31 @@ module ActiveRecord
           .to eq [user]
       end
     end
+
+    describe ".ignored_columns" do
+      it "ignores additional columns specified in Consul" do
+        # If this test is the first one to run that requires User - preload User so that the correct
+        # accessors (getters / setters) already exist since the "ensure" block won't create them. In
+        # a real situation, we would first perform a rolling restart after having unset this key and
+        # finished pre-deploy migrations everywhere.
+        User.create!(name: "user u1")
+
+        allow(DynamicSettings).to receive(:find).with(any_args).and_call_original
+        allow(DynamicSettings).to receive(:find).with("activerecord/ignored_columns", tree: :store).and_return(
+          {
+            "users" => "name"
+          }
+        )
+
+        User.reset_column_information
+        expect { User.create!(name: "user u2") }.to raise_exception(ActiveModel::UnknownAttributeError)
+      ensure
+        allow(DynamicSettings).to receive(:find).with("activerecord/ignored_columns", tree: :store).and_call_original
+
+        User.reset_column_information
+        User.create!(name: "user u2")
+      end
+    end
   end
 
   describe ".asset_string" do