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<?php
use Wikimedia\Rdbms\IDatabase;
use Wikimedia\Rdbms\DBError;
/**
* MySQL version of DBLockManager that supports shared locks.
*
* Do NOT use this on connection handles that are also being used for anything
* else as the transaction isolation will be wrong and all the other changes will
* get rolled back when the locks release!
*
* All lock servers must have the innodb table defined in maintenance/locking/filelocks.sql.
* All locks are non-blocking, which avoids deadlocks.
*
* @ingroup LockManager
*/
class MySqlLockManager extends DBLockManager {
/** @var array Mapping of lock types to the type actually used */
protected $lockTypeMap = [
self::LOCK_SH => self::LOCK_SH,
self::LOCK_UW => self::LOCK_SH,
self::LOCK_EX => self::LOCK_EX
];
public function __construct( array $config ) {
parent::__construct( $config );
$this->session = substr( $this->session, 0, 31 ); // fit to field
}
protected function initConnection( $lockDb, IDatabase $db ) {
# Let this transaction see lock rows from other transactions
$db->query( "SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;" );
# Do everything in a transaction as it all gets rolled back eventually
$db->startAtomic( __CLASS__ );
}
/**
* Get a connection to a lock DB and acquire locks on $paths.
* This does not use GET_LOCK() per https://bugs.mysql.com/bug.php?id=1118.
*
* @see DBLockManager::getLocksOnServer()
* @param string $lockSrv
* @param array $paths
* @param string $type
* @return StatusValue
*/
protected function doGetLocksOnServer( $lockSrv, array $paths, $type ) {
$status = StatusValue::newGood();
$db = $this->getConnection( $lockSrv ); // checked in isServerUp()
$keys = []; // list of hash keys for the paths
$data = []; // list of rows to insert
$checkEXKeys = []; // list of hash keys that this has no EX lock on
# Build up values for INSERT clause
foreach ( $paths as $path ) {
$key = $this->sha1Base36Absolute( $path );
$keys[] = $key;
$data[] = [ 'fls_key' => $key, 'fls_session' => $this->session ];
if ( !isset( $this->locksHeld[$path][self::LOCK_EX] ) ) {
$checkEXKeys[] = $key; // this has no EX lock on $key itself
}
}
# Block new writers (both EX and SH locks leave entries here)...
$db->insert( 'filelocks_shared', $data, __METHOD__, [ 'IGNORE' ] );
# Actually do the locking queries...
if ( $type == self::LOCK_SH ) { // reader locks
# Bail if there are any existing writers...
if ( count( $checkEXKeys ) ) {
$blocked = $db->selectField(
'filelocks_exclusive',
'1',
[ 'fle_key' => $checkEXKeys ],
__METHOD__
);
} else {
$blocked = false;
}
# Other prospective writers that haven't yet updated filelocks_exclusive
# will recheck filelocks_shared after doing so and bail due to this entry.
} else { // writer locks
$encSession = $db->addQuotes( $this->session );
# Bail if there are any existing writers...
# This may detect readers, but the safe check for them is below.
# Note: if two writers come at the same time, both bail :)
$blocked = $db->selectField(
'filelocks_shared',
'1',
[ 'fls_key' => $keys, "fls_session != $encSession" ],
__METHOD__
);
if ( !$blocked ) {
# Build up values for INSERT clause
$data = [];
foreach ( $keys as $key ) {
$data[] = [ 'fle_key' => $key ];
}
# Block new readers/writers...
$db->insert( 'filelocks_exclusive', $data, __METHOD__ );
# Bail if there are any existing readers...
$blocked = $db->selectField(
'filelocks_shared',
'1',
[ 'fls_key' => $keys, "fls_session != $encSession" ],
__METHOD__
);
}
}
if ( $blocked ) {
foreach ( $paths as $path ) {
$status->fatal( 'lockmanager-fail-acquirelock', $path );
}
}
return $status;
}
/**
* @see QuorumLockManager::releaseAllLocks()
* @return StatusValue
*/
protected function releaseAllLocks() {
$status = StatusValue::newGood();
foreach ( $this->conns as $lockDb => $db ) {
if ( $db->trxLevel() ) { // in transaction
try {
$db->rollback( __METHOD__ ); // finish transaction and kill any rows
} catch ( DBError $e ) {
$status->fatal( 'lockmanager-fail-db-release', $lockDb );
}
}
}
return $status;
}
}
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