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.

lib/migration/base_dropper.rb

@@ -22,7 +22,11 @@ module Migration
       SQL
     end
 
-    def self.readonly_function_name(table_name, column_name = nil)
+    def self.drop_readonly_function(table_name, column_name = nil)
+      DB.exec("DROP FUNCTION IF EXISTS #{readonly_function_name(table_name, column_name)} CASCADE")
+    end
+
+    def self.readonly_function_name(table_name, column_name = nil, with_schema: true)
       function_name = [
         "raise",
         table_name,
@@ -30,12 +34,7 @@ module Migration
         "readonly()"
       ].compact.join("_")
 
-      if DB.exec(<<~SQL).to_s == '1'
-         SELECT schema_name
-         FROM information_schema.schemata
-         WHERE schema_name = '#{FUNCTION_SCHEMA_NAME}'
-         SQL
-
+      if with_schema && function_schema_exists?
         "#{FUNCTION_SCHEMA_NAME}.#{function_name}"
       else
         function_name
@@ -51,5 +50,21 @@ module Migration
     def self.readonly_trigger_name(table_name, column_name = nil)
       [table_name, column_name, "readonly"].compact.join("_")
     end
+
+    def self.function_schema_exists?
+      DB.exec(<<~SQL).to_s == '1'
+        SELECT schema_name
+        FROM information_schema.schemata
+        WHERE schema_name = '#{FUNCTION_SCHEMA_NAME}'
+      SQL
+    end
+
+    def self.existing_discourse_function_names
+      DB.query_single(<<~SQL)
+        SELECT routine_name
+        FROM information_schema.routines
+        WHERE routine_type = 'FUNCTION' AND specific_schema = '#{FUNCTION_SCHEMA_NAME}'
+      SQL
+    end
   end
 end

lib/migration/column_dropper.rb

@@ -28,13 +28,11 @@ module Migration
       end
     end
 
-    def self.drop_readonly(table, column)
-      DB.exec <<~SQL
-        DROP FUNCTION IF EXISTS #{BaseDropper.readonly_function_name(table, column)} CASCADE;
-        -- Backward compatibility for old functions created in the public
-        -- schema
-        DROP FUNCTION IF EXISTS #{BaseDropper.old_readonly_function_name(table, column)} CASCADE;
-      SQL
+    def self.drop_readonly(table_name, column_name)
+      BaseDropper.drop_readonly_function(table_name, column_name)
+
+      # Backward compatibility for old functions created in the public schema
+      DB.exec("DROP FUNCTION IF EXISTS #{BaseDropper.old_readonly_function_name(table_name, column_name)} CASCADE")
     end
   end
 end