FIX: Create readonly functions during backup

Temporarily recreate already dropped functions in the discourse_functions schema in order to allow restoring of backups which still reference dropped functions.

spec/integrity/coding_style_spec.rb

@@ -32,4 +32,36 @@ describe 'Coding style' do
       MSG
     end
   end
+
+  describe 'Post Migrations' do
+    def check_offenses(files, method_name, constant_name)
+      method_name_regex = /#{Regexp.escape(method_name)}/
+      constant_name_regex = /#{Regexp.escape(constant_name)}/
+      offenses = files.reject { |file| is_valid?(file, method_name_regex, constant_name_regex) }
+
+      expect(offenses).to be_empty, <<~MSG
+        You need to use the constant #{constant_name} when you use
+        #{method_name} in order to help with restoring backups.
+
+        Please take a look at existing migrations to see how to use it correctly.
+
+        Offenses:
+        #{offenses.join("\n")}
+      MSG
+    end
+
+    def is_valid?(file, method_name_regex, constant_name_regex)
+      contains_method_name = File.open(file).grep(method_name_regex).any?
+      contains_constant_name = File.open(file).grep(constant_name_regex).any?
+
+      contains_method_name ? contains_constant_name : true
+    end
+
+    it 'ensures dropped tables and columns are stored in constants' do
+      migration_files = list_files('db/post_migrate', '**/*.rb')
+
+      check_offenses(migration_files, "ColumnDropper.execute_drop", "DROPPED_COLUMNS")
+      check_offenses(migration_files, "TableDropper.execute_drop", "DROPPED_TABLES")
+    end
+  end
 end