Fix ability to purge value of a custom field

Fixes #20002

bug_update.php

@@ -276,6 +276,12 @@
 foreach ( $t_related_custom_field_ids as $t_cf_id ) {
 	$t_cf_def = custom_field_get_definition( $t_cf_id );
 
+	# if this is not a full update action and custom field is not on the form, then don't
+	# continue with code that checks access level and validates the field.
+	if ( $f_update_type != BUG_UPDATE_TYPE_NORMAL && !custom_field_is_present( $t_cf_id ) ) {
+		continue;
+	}
+
 	if( !gpc_isset_custom_field( $t_cf_id, $t_cf_def['type'] ) ) {
 		if( $t_cf_def[$t_cf_require_check] &&
 			$f_update_type == BUG_UPDATE_TYPE_NORMAL &&
@@ -284,12 +290,6 @@
 			# no value was given by the user.
 			error_parameters( lang_get_defaulted( custom_field_get_field( $t_cf_id, 'name' ) ) );
 			trigger_error( ERROR_EMPTY_FIELD, ERROR );
-		} else {
-			# The custom field isn't compulsory and the user did
-			# not supply a value. Therefore we can just ignore this
-			# custom field completely (ie. don't attempt to update
-			# the field).
-			continue;
 		}
 	}
 

core/custom_field_api.php

@@ -1269,11 +1269,30 @@ function print_custom_field_input( array $p_field_def, $p_bug_id = null ) {
 	global $g_custom_field_type_definition;
 	if( isset( $g_custom_field_type_definition[$p_field_def['type']]['#function_print_input'] ) ) {
 		call_user_func( $g_custom_field_type_definition[$p_field_def['type']]['#function_print_input'], $p_field_def, $t_custom_field_value );
+		print_hidden_input( custom_field_presence_field_name( $p_field_def['id'] ), '1' );
 	} else {
 		trigger_error( ERROR_CUSTOM_FIELD_INVALID_DEFINITION, ERROR );
 	}
 }
 
+/**
+ * Constructs the name of a field used as a flag to indicate that a custom field is present on the form
+ * @param integer $p_custom_field_id  The custom field id to create the field name for.
+ * @return string The field name.
+ */
+function custom_field_presence_field_name( $p_custom_field_id ) {
+	return 'custom_field_' . (int)$p_custom_field_id . '_presence';
+}
+
+/**
+ * Checks the presence of a custom field on the form.
+ * @param integer $p_custom_field_id  The custom field id to check.
+ * @return bool true when field is on form, false otherwise.
+ */
+function custom_field_is_present( $p_custom_field_id ) {
+	return gpc_isset( custom_field_presence_field_name( $p_custom_field_id ) );
+}
+
 /**
  * Prepare a string containing a custom field value for display
  * @param array   $p_def      Contains the definition of the custom field.