gnu-social/lib/schema.php

907 lines
25 KiB
PHP
Raw Normal View History

2009-07-21 00:04:48 +01:00
<?php
/**
2009-09-23 14:20:04 +01:00
* StatusNet, the distributed open-source microblogging tool
2009-07-21 00:04:48 +01:00
*
* Database schema utilities
*
* PHP version 5
*
* LICENCE: This program 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, either version 3 of the License, or
* (at your option) any later version.
*
* This program 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/>.
*
* @category Database
2009-09-23 14:20:04 +01:00
* @package StatusNet
* @author Evan Prodromou <evan@status.net>
* @copyright 2009 StatusNet, Inc.
2009-07-21 00:04:48 +01:00
* @license http://www.fsf.org/licensing/licenses/agpl-3.0.html GNU Affero General Public License version 3.0
2009-09-23 14:20:04 +01:00
* @link http://status.net/
2009-07-21 00:04:48 +01:00
*/
2009-09-23 14:20:04 +01:00
if (!defined('STATUSNET')) {
2009-07-21 00:04:48 +01:00
exit(1);
}
/**
* Class representing the database schema
*
* A class representing the database schema. Can be used to
* manipulate the schema -- especially for plugins and upgrade
* utilities.
*
* @category Database
2009-09-23 14:20:04 +01:00
* @package StatusNet
* @author Evan Prodromou <evan@status.net>
* @author Brion Vibber <brion@status.net>
2009-07-21 00:04:48 +01:00
* @license http://www.fsf.org/licensing/licenses/agpl-3.0.html GNU Affero General Public License version 3.0
2009-09-23 14:20:04 +01:00
* @link http://status.net/
2009-07-21 00:04:48 +01:00
*/
class Schema
{
static $_static = null;
protected $conn = null;
2009-07-21 00:04:48 +01:00
2009-10-02 20:02:33 +01:00
/**
* Constructor. Only run once for singleton object.
*/
protected function __construct($conn = null)
2009-07-21 00:04:48 +01:00
{
if (is_null($conn)) {
// XXX: there should be an easier way to do this.
$user = new User();
$conn = $user->getDatabaseConnection();
$user->free();
unset($user);
}
2009-10-02 20:02:33 +01:00
$this->conn = $conn;
}
2009-07-21 00:04:48 +01:00
2009-10-02 20:02:33 +01:00
/**
* Main public entry point. Use this to get
* the schema object.
2009-10-02 20:02:33 +01:00
*
* @return Schema the Schema object for the connection
2009-10-02 20:02:33 +01:00
*/
static function get($conn = null)
{
if (is_null($conn)) {
$key = 'default';
} else {
$key = md5(serialize($conn->dsn));
}
$type = common_config('db', 'type');
if (empty(self::$_static[$key])) {
$schemaClass = ucfirst($type).'Schema';
self::$_static[$key] = new $schemaClass($conn);
}
return self::$_static[$key];
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Gets a ColumnDef object for a single column.
*
* Throws an exception if the table is not found.
*
* @param string $table name of the table
* @param string $column name of the column
*
* @return ColumnDef definition of the column or null
* if not found.
*/
2009-07-21 00:04:48 +01:00
public function getColumnDef($table, $column)
{
2009-09-24 03:24:35 +01:00
$td = $this->getTableDef($table);
foreach ($td->columns as $cd) {
if ($cd->name == $column) {
return $cd;
}
}
return null;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Creates a table with the given names and columns.
*
* @param string $name Name of the table
* @param array $columns Array of ColumnDef objects
* for new table.
*
* @return boolean success flag
*/
public function createTable($name, $columns)
2009-07-21 00:04:48 +01:00
{
$statements = $this->buildCreateTable($tableName, $def);
return $this->runSqlSet($statements);
}
2009-09-24 03:24:35 +01:00
/**
* Build a set of SQL statements to create a table with the given
* name and columns.
*
* @param string $name Name of the table
* @param array $def Table definition array
*
* @return boolean success flag
*/
public function buildCreateTable($name, $def)
{
$def = $this->filterDef($def);
$sql = array();
2009-09-24 03:24:35 +01:00
foreach ($def['fields'] as $col => $colDef) {
$this->appendColumnDef($sql, $col, $colDef);
}
2009-09-24 03:24:35 +01:00
// Primary and unique keys are constraints, so go within
// the CREATE TABLE statement normally.
if (!empty($def['primary key'])) {
$this->appendPrimaryKeyDef($sql, $def['primary key']);
}
2009-09-24 03:24:35 +01:00
if (!empty($def['unique keys'])) {
foreach ($def['unique keys'] as $col => $colDef) {
$this->appendUniqueKeyDef($sql, $col, $colDef);
2009-09-24 03:24:35 +01:00
}
}
2009-09-24 03:24:35 +01:00
// Multi-value indexes are advisory and for best portability
// should be created as separate statements.
$statements = array();
$statements[] = $this->startCreateTable($name, $def) . "\n" .
implode($sql, ",\n") . "\n" .
$this->endCreateTable($name, $def);
if (!empty($def['indexes'])) {
foreach ($def['indexes'] as $col => $colDef) {
2010-10-12 03:18:47 +01:00
$this->appendCreateIndex($statements, $name, $col, $colDef);
2009-09-24 03:24:35 +01:00
}
}
return $statements;
}
2009-09-24 03:24:35 +01:00
/**
* Set up a 'create table' SQL statement.
*
* @param string $name table name
* @param array $def table definition
* @param $string
*/
function startCreateTable($name, array $def)
{
return 'CREATE TABLE ' . $this->quoteIdentifier($name) . ' (';
}
2009-09-24 03:24:35 +01:00
/**
* Close out a 'create table' SQL statement.
*
* @param string $name table name
* @param array $def table definition
* @return string
*/
function endCreateTable($name, array $def)
{
return ')';
}
2009-09-24 03:24:35 +01:00
/**
* Append an SQL fragment with a column definition in a CREATE TABLE statement.
*
* @param array $sql
* @param string $name
* @param array $def
*/
function appendColumnDef(array &$sql, $name, array $def)
{
$sql[] = "$name " . $this->columnSql($def);
}
2009-09-24 03:24:35 +01:00
/**
* Append an SQL fragment with a constraint definition for a primary
* key in a CREATE TABLE statement.
*
* @param array $sql
* @param array $def
*/
function appendPrimaryKeyDef(array &$sql, array $def)
{
$sql[] = "PRIMARY KEY " . $this->buildIndexList($def);
}
2009-09-24 03:24:35 +01:00
/**
* Append an SQL fragment with a constraint definition for a primary
* key in a CREATE TABLE statement.
*
* @param array $sql
* @param string $name
* @param array $def
*/
function appendUniqueKeyDef(array &$sql, $name, array $def)
{
$sql[] = "UNIQUE $name " . $this->buildIndexList($def);
}
2009-09-24 03:24:35 +01:00
/**
* Append an SQL statement with an index definition for an advisory
* index over one or more columns on a table.
*
* @param array $statements
* @param string $table
* @param string $name
* @param array $def
*/
function appendCreateIndex(array &$statements, $table, $name, array $def)
{
$statements[] = "CREATE INDEX $name ON $table " . $this->buildIndexList($def);
}
function buildIndexList(array $def)
{
// @fixme
return '(' . implode(',', array_map(array($this, 'buildIndexItem'), $def)) . ')';
}
function buildIndexItem($def)
{
if (is_array($def)) {
list($name, $size) = $def;
return $this->quoteIdentifier($name) . '(' . intval($size) . ')';
}
return $this->quoteIdentifier($def);
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Drops a table from the schema
*
* Throws an exception if the table is not found.
*
* @param string $name Name of the table to drop
*
* @return boolean success flag
*/
2009-07-21 00:04:48 +01:00
public function dropTable($name)
{
$res = $this->conn->query("DROP TABLE $name");
2009-09-24 03:24:35 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Adds an index to a table.
*
* If no name is provided, a name will be made up based
* on the table name and column names.
*
* Throws an exception on database error, esp. if the table
* does not exist.
*
* @param string $table Name of the table
* @param array $columnNames Name of columns to index
* @param string $name (Optional) name of the index
*
* @return boolean success flag
*/
public function createIndex($table, $columnNames, $name=null)
2009-07-21 00:04:48 +01:00
{
2009-10-01 20:00:54 +01:00
if (!is_array($columnNames)) {
$columnNames = array($columnNames);
}
if (empty($name)) {
$name = "{$table}_".implode("_", $columnNames)."_idx";
2009-10-01 20:00:54 +01:00
}
$res = $this->conn->query("ALTER TABLE $table ".
2009-10-02 20:02:33 +01:00
"ADD INDEX $name (".
implode(",", $columnNames).")");
2009-10-01 20:00:54 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Drops a named index from a table.
*
* @param string $table name of the table the index is on.
* @param string $name name of the index
*
* @return boolean success flag
*/
2009-10-01 20:00:54 +01:00
public function dropIndex($table, $name)
2009-07-21 00:04:48 +01:00
{
$res = $this->conn->query("ALTER TABLE $table DROP INDEX $name");
2009-10-01 20:00:54 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Adds a column to a table
*
* @param string $table name of the table
* @param ColumnDef $columndef Definition of the new
* column.
*
* @return boolean success flag
*/
2009-07-21 00:04:48 +01:00
public function addColumn($table, $columndef)
{
2009-10-01 20:00:54 +01:00
$sql = "ALTER TABLE $table ADD COLUMN " . $this->_columnSql($columndef);
$res = $this->conn->query($sql);
2009-10-01 20:00:54 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
}
2009-10-02 20:02:33 +01:00
/**
* Modifies a column in the schema.
*
* The name must match an existing column and table.
*
* @param string $table name of the table
* @param ColumnDef $columndef new definition of the column.
*
* @return boolean success flag
*/
2009-10-01 20:00:54 +01:00
public function modifyColumn($table, $columndef)
{
2009-10-02 20:02:33 +01:00
$sql = "ALTER TABLE $table MODIFY COLUMN " .
$this->_columnSql($columndef);
2009-10-01 20:00:54 +01:00
$res = $this->conn->query($sql);
2009-10-01 20:00:54 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
}
2009-10-02 20:02:33 +01:00
/**
* Drops a column from a table
*
* The name must match an existing column.
*
* @param string $table name of the table
* @param string $columnName name of the column to drop
*
* @return boolean success flag
*/
2009-10-01 20:00:54 +01:00
public function dropColumn($table, $columnName)
{
$sql = "ALTER TABLE $table DROP COLUMN $columnName";
$res = $this->conn->query($sql);
2009-10-01 20:00:54 +01:00
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
return true;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Ensures that a table exists with the given
* name and the given column definitions.
*
* If the table does not yet exist, it will
* create the table. If it does exist, it will
* alter the table to match the column definitions.
*
* @param string $tableName name of the table
* @param array $def Table definition array
2009-10-02 20:02:33 +01:00
*
* @return boolean success flag
*/
public function ensureTable($tableName, $def)
2009-07-21 00:04:48 +01:00
{
$statements = $this->buildEnsureTable($tableName, $def);
return $this->runSqlSet($statements);
}
2009-10-01 20:00:54 +01:00
/**
* Run a given set of SQL commands on the connection in sequence.
* Empty input is ok.
*
* @fixme if multiple statements, wrap in a transaction?
* @param array $statements
* @return boolean success flag
*/
function runSqlSet(array $statements)
{
$ok = true;
foreach ($statements as $sql) {
$res = $this->conn->query($sql);
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
}
return $ok;
}
/**
* Check a table's status, and if needed build a set
* of SQL statements which change it to be consistent
* with the given table definition.
*
* If the table does not yet exist, statements will
* be returned to create the table. If it does exist,
* statements will be returned to alter the table to
* match the column definitions.
*
* @param string $tableName name of the table
* @param array $columns array of ColumnDef
* objects for the table
*
* @return array of SQL statements
*/
function buildEnsureTable($tableName, $def)
{
2009-10-01 20:00:54 +01:00
try {
$old = $this->getTableDef($tableName);
2009-10-01 20:00:54 +01:00
} catch (Exception $e) {
// @fixme this is a terrible check :D
2009-10-01 20:00:54 +01:00
if (preg_match('/no such table/', $e->getMessage())) {
return $this->buildCreateTable($tableName, $def);
2009-10-01 20:00:54 +01:00
} else {
throw $e;
}
}
$old = $this->filterDef($old);
$def = $this->filterDef($def);
// @fixme check if not present
$fields = $this->diffArrays($old, $def, 'fields', array($this, 'columnsEqual'));
$uniques = $this->diffArrays($old, $def, 'unique keys');
$indexes = $this->diffArrays($old, $def, 'indexes');
2009-10-01 20:00:54 +01:00
$total = $fields['count'] + $uniques['count'] + $indexes['count'];
if ($total == 0) {
2009-10-01 20:00:54 +01:00
// nothing to do
return array();
2009-10-01 20:00:54 +01:00
}
// For efficiency, we want this all in one
// query, instead of using our methods.
$phrase = array();
foreach ($uniques['del'] + $uniques['mod'] as $keyName) {
$this->appendAlterDropUnique($phrase, $keyName);
}
foreach ($fields['add'] as $columnName) {
$this->appendAlterAddColumn($phrase, $columnName,
$def['fields'][$columnName]);
2009-10-01 20:00:54 +01:00
}
foreach ($fields['mod'] as $columnName) {
$this->appendAlterModifyColumn($phrase, $columnName,
$old['fields'][$columnName],
$def['fields'][$columnName]);
2009-10-01 20:00:54 +01:00
}
foreach ($fields['del'] as $columnName) {
$this->appendAlterDropColumn($phrase, $columnName);
2009-10-01 20:00:54 +01:00
}
foreach ($uniques['mod'] + $uniques['add'] as $keyName) {
$this->appendAlterAddUnique($phrase, $keyName, $def['unique keys'][$keyName]);
}
2009-10-01 20:00:54 +01:00
$sql = 'ALTER TABLE ' . $tableName . ' ' . implode(",\n", $phrase);
2009-10-01 20:00:54 +01:00
return array($sql);
}
2009-10-01 20:00:54 +01:00
function diffArrays($oldDef, $newDef, $section, $compareCallback=null)
{
$old = isset($oldDef[$section]) ? $oldDef[$section] : array();
$new = isset($newDef[$section]) ? $newDef[$section] : array();
$oldKeys = array_keys($old);
$newKeys = array_keys($new);
$toadd = array_diff($newKeys, $oldKeys);
$todrop = array_diff($oldKeys, $newKeys);
$same = array_intersect($newKeys, $oldKeys);
$tomod = array();
$tokeep = array();
// Find which fields have actually changed definition
// in a way that we need to tweak them for this DB type.
foreach ($same as $name) {
if ($compareCallback) {
$same = call_user_func($compareCallback, $old[$name], $new[$name]);
} else {
$same = ($old[$name] != $new[$name]);
}
if ($same) {
$tokeep[] = $name;
continue;
}
$tomod[] = $name;
}
return array('add' => $toadd,
'del' => $todrop,
'mod' => $tomod,
'keep' => $tokeep,
'count' => count($toadd) + count($todrop) + count($tomod));
2009-07-21 00:04:48 +01:00
}
/**
* Append phrase(s) to an array of partial ALTER TABLE chunks in order
* to add the given column definition to the table.
*
* @param array $phrase
* @param string $columnName
* @param array $cd
*/
function appendAlterAddColumn(array &$phrase, $columnName, array $cd)
{
$phrase[] = 'ADD COLUMN ' .
$this->quoteIdentifier($columnName) .
' ' .
$this->columnSql($cd);
}
/**
* Append phrase(s) to an array of partial ALTER TABLE chunks in order
* to alter the given column from its old state to a new one.
*
* @param array $phrase
* @param string $columnName
* @param array $old previous column definition as found in DB
* @param array $cd current column definition
*/
function appendAlterModifyColumn(array &$phrase, $columnName, array $old, array $cd)
{
$phrase[] = 'MODIFY COLUMN ' .
$this->quoteIdentifier($columnName) .
' ' .
$this->columnSql($cd);
}
/**
* Append phrase(s) to an array of partial ALTER TABLE chunks in order
* to drop the given column definition from the table.
*
* @param array $phrase
* @param string $columnName
*/
function appendAlterDropColumn(array &$phrase, $columnName)
{
$phrase[] = 'DROP COLUMN ' . $this->quoteIdentifier($columnName);
}
function appendAlterAddUnique(array &$phrase, $keyName, array $def)
{
$sql = array();
$sql[] = 'ADD';
$this->appendUniqueKeyDef($sql, $keyName, $def);
$phrase[] = implode(' ', $sql);'ADD CONSTRAINT ' . $keyName;
}
function appendAlterDropUnique(array &$phrase, $keyName)
{
$phrase[] = 'DROP CONSTRAINT ' . $keyName;
}
/**
* Quote a db/table/column identifier if necessary.
*
* @param string $name
* @return string
*/
function quoteIdentifier($name)
{
return $name;
}
function quoteDefaultValue($cd)
{
if ($cd['type'] == 'datetime' && $cd['default'] == 'CURRENT_TIMESTAMP') {
return $cd['default'];
} else {
return $this->quoteValue($cd['default']);
}
}
function quoteValue($val)
{
2010-10-12 03:17:21 +01:00
if (is_int($val) || is_float($val) || is_double($val)) {
return strval($val);
} else {
return '"' . $this->conn->escapeSimple($val) . '"';
}
}
/**
* Check if two column definitions are equivalent.
* The default implementation checks _everything_ but in many cases
* you may be able to discard a bunch of equivalencies.
*
* @param array $a
* @param array $b
* @return boolean
*/
function columnsEqual(array $a, array $b)
{
return !array_diff_assoc($a, $b) && !array_diff_assoc($b, $a);
}
2009-10-02 20:02:33 +01:00
/**
* Returns the array of names from an array of
* ColumnDef objects.
*
* @param array $cds array of ColumnDef objects
*
* @return array strings for name values
*/
protected function _names($cds)
2009-07-21 00:04:48 +01:00
{
2009-10-01 20:00:54 +01:00
$names = array();
foreach ($cds as $cd) {
$names[] = $cd->name;
}
return $names;
2009-07-21 00:04:48 +01:00
}
2009-10-02 20:02:33 +01:00
/**
* Get a ColumnDef from an array matching
* name.
*
* @param array $cds Array of ColumnDef objects
* @param string $name Name of the column
*
* @return ColumnDef matching item or null if no match.
*/
protected function _byName($cds, $name)
2009-07-21 00:04:48 +01:00
{
2009-10-01 20:00:54 +01:00
foreach ($cds as $cd) {
if ($cd->name == $name) {
return $cd;
}
2009-07-21 00:04:48 +01:00
}
2009-10-01 20:00:54 +01:00
return null;
2009-07-21 00:04:48 +01:00
}
2009-09-24 03:24:35 +01:00
2009-10-02 20:02:33 +01:00
/**
* Return the proper SQL for creating or
* altering a column.
*
* Appropriate for use in CREATE TABLE or
* ALTER TABLE statements.
*
* @param ColumnDef $cd column to create
*
* @return string correct SQL for that column
*/
function columnSql(array $cd)
2009-09-24 03:24:35 +01:00
{
$line = array();
$line[] = $this->typeAndSize($cd);
if (isset($cd['default'])) {
$line[] = 'default';
$line[] = $this->quoteDefaultValue($cd);
} else if (!empty($cd['not null'])) {
// Can't have both not null AND default!
$line[] = 'not null';
2009-09-24 03:24:35 +01:00
}
return implode(' ', $line);
}
/**
*
* @param string $column canonical type name in defs
* @return string native DB type name
*/
function mapType($column)
{
return $column;
}
2009-09-24 03:24:35 +01:00
function typeAndSize($column)
{
$type = $this->mapType($column);
$lengths = array();
if ($column['type'] == 'numeric') {
if (isset($column['precision'])) {
$lengths[] = $column['precision'];
if (isset($column['scale'])) {
$lengths[] = $column['scale'];
}
}
} else if (isset($column['length'])) {
$lengths[] = $column['length'];
}
if ($lengths) {
return $type . '(' . implode(',', $lengths) . ')';
} else {
return $type;
}
2009-09-24 03:24:35 +01:00
}
/**
* Map a native type back to an independent type + size
*
* @param string $type
* @return array ($type, $size) -- $size may be null
*/
protected function reverseMapType($type)
{
return array($type, null);
}
/**
* Convert an old-style set of ColumnDef objects into the current
* Drupal-style schema definition array, for backwards compatibility
* with plugins written for 0.9.x.
*
* @param string $tableName
* @param array $defs
* @return array
*/
function oldToNew($tableName, $defs)
{
$table = array();
$prefixes = array(
'tiny',
'small',
'medium',
'big',
);
foreach ($defs as $cd) {
$cd->addToTableDef($table);
$column = array();
$column['type'] = $cd->type;
foreach ($prefixes as $prefix) {
if (substr($cd->type, 0, strlen($prefix)) == $prefix) {
$column['type'] = substr($cd->type, strlen($prefix));
$column['size'] = $prefix;
break;
}
}
if ($cd->size) {
if ($cd->type == 'varchar' || $cd->type == 'char') {
$column['length'] = $cd->size;
}
}
if (!$cd->nullable) {
$column['not null'] = true;
}
if ($cd->autoincrement) {
$column['type'] = 'serial';
}
if ($cd->default) {
$column['default'] = $cd->default;
}
$table['fields'][$cd->name] = $column;
if ($cd->key == 'PRI') {
// If multiple columns are defined as primary key,
// we'll pile them on in sequence.
if (!isset($table['primary key'])) {
$table['primary key'] = array();
}
$table['primary key'][] = $cd->name;
} else if ($cd->key == 'MUL') {
// Individual multiple-value indexes are only per-column
// using the old ColumnDef syntax.
$idx = "{$tableName}_{$cd->name}_idx";
$table['indexes'][$idx] = array($cd->name);
} else if ($cd->key == 'UNI') {
// Individual unique-value indexes are only per-column
// using the old ColumnDef syntax.
$idx = "{$tableName}_{$cd->name}_idx";
$table['unique keys'][$idx] = array($cd->name);
}
}
return $table;
}
2010-10-08 00:49:49 +01:00
/**
* Filter the given table definition array to match features available
* in this database.
*
* This lets us strip out unsupported things like comments, foreign keys,
* or type variants that we wouldn't get back from getTableDef().
*
* @param array $tableDef
*/
function filterDef(array $tableDef)
{
return $tableDef;
}
2010-10-08 00:49:49 +01:00
function isNumericType($type)
{
$type = strtolower($type);
$known = array('int', 'serial', 'numeric');
return in_array($type, $known);
}
/**
* Pull info from the query into a fun-fun array of dooooom
*
* @param string $sql
* @return array of arrays
*/
protected function fetchQueryData($sql)
{
$res = $this->conn->query($sql);
if (PEAR::isError($res)) {
throw new Exception($res->getMessage());
}
$out = array();
$row = array();
while ($res->fetchInto($row, DB_FETCHMODE_ASSOC)) {
$out[] = $row;
}
$res->free();
return $out;
}
2009-07-21 00:04:48 +01:00
}
class SchemaTableMissingException extends Exception
{
// no-op
}