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<?php
/**
* Version of LockManager based on using DB table locks.
* This is meant for multi-wiki systems that may share files.
* All locks are blocking, so it might be useful to set a small
* lock-wait timeout via server config to curtail deadlocks.
*
* All lock requests for a resource, identified by a hash string, will map
* to one bucket. Each bucket maps to one or several peer DBs, each on their
* own server, all having the filelocks.sql tables (with row-level locking).
* A majority of peer DBs must agree for a lock to be acquired.
*
* Caching is used to avoid hitting servers that are down.
*
* @ingroup LockManager
* @since 1.19
*/
class DBLockManager extends LockManager {
/** @var Array Map of DB names to server config */
protected $dbServers; // (DB name => server config array)
/** @var Array Map of bucket indexes to peer DB lists */
protected $dbsByBucket; // (bucket index => (ldb1, ldb2, ...))
/** @var BagOStuff */
protected $statusCache;
protected $lockExpiry; // integer number of seconds
protected $safeDelay; // integer number of seconds
protected $session = 0; // random integer
/** @var Array Map Database connections (DB name => Database) */
protected $conns = array();
/**
* Construct a new instance from configuration.
*
* $config paramaters include:
* 'dbServers' : Associative array of DB names to server configuration.
* Configuration is an associative array that includes:
* 'host' - DB server name
* 'dbname' - DB name
* 'type' - DB type (mysql,postgres,...)
* 'user' - DB user
* 'password' - DB user password
* 'tablePrefix' - DB table prefix
* 'flags' - DB flags (see DatabaseBase)
* 'dbsByBucket' : Array of 1-16 consecutive integer keys, starting from 0,
* each having an odd-numbered list of DB names (peers) as values.
* Any DB named 'localDBMaster' will automatically use the DB master
* settings for this wiki (without the need for a dbServers entry).
* 'lockExpiry' : Lock timeout (seconds) for dropped connections. [optional]
* This tells the DB server how long to wait before assuming
* connection failure and releasing all the locks for a session.
*
* @param Array $config
*/
public function __construct( array $config ) {
$this->dbServers = isset( $config['dbServers'] )
? $config['dbServers']
: array(); // likely just using 'localDBMaster'
// Sanitize dbsByBucket config to prevent PHP errors
$this->dbsByBucket = array_filter( $config['dbsByBucket'], 'is_array' );
$this->dbsByBucket = array_values( $this->dbsByBucket ); // consecutive
if ( isset( $config['lockExpiry'] ) ) {
$this->lockExpiry = $config['lockExpiry'];
} else {
$met = ini_get( 'max_execution_time' );
$this->lockExpiry = $met ? $met : 60; // use some sane amount if 0
}
$this->safeDelay = ( $this->lockExpiry <= 0 )
? 60 // pick a safe-ish number to match DB timeout default
: $this->lockExpiry; // cover worst case
foreach ( $this->dbsByBucket as $bucket ) {
if ( count( $bucket ) > 1 ) {
// Tracks peers that couldn't be queried recently to avoid lengthy
// connection timeouts. This is useless if each bucket has one peer.
$this->statusCache = wfGetMainCache();
break;
}
}
$this->session = '';
for ( $i = 0; $i < 5; $i++ ) {
$this->session .= mt_rand( 0, 2147483647 );
}
$this->session = wfBaseConvert( sha1( $this->session ), 16, 36, 31 );
}
/**
* @see LockManager::doLock()
*/
protected function doLock( array $paths, $type ) {
$status = Status::newGood();
$pathsToLock = array();
// Get locks that need to be acquired (buckets => locks)...
foreach ( $paths as $path ) {
if ( isset( $this->locksHeld[$path][$type] ) ) {
++$this->locksHeld[$path][$type];
} elseif ( isset( $this->locksHeld[$path][self::LOCK_EX] ) ) {
$this->locksHeld[$path][$type] = 1;
} else {
$bucket = $this->getBucketFromKey( $path );
$pathsToLock[$bucket][] = $path;
}
}
$lockedPaths = array(); // files locked in this attempt
// Attempt to acquire these locks...
foreach ( $pathsToLock as $bucket => $paths ) {
// Try to acquire the locks for this bucket
$res = $this->doLockingQueryAll( $bucket, $paths, $type );
if ( $res === 'cantacquire' ) {
// Resources already locked by another process.
// Abort and unlock everything we just locked.
foreach ( $paths as $path ) {
$status->fatal( 'lockmanager-fail-acquirelock', $path );
}
$status->merge( $this->doUnlock( $lockedPaths, $type ) );
return $status;
} elseif ( $res !== true ) {
// Couldn't contact any DBs for this bucket.
// Abort and unlock everything we just locked.
$status->fatal( 'lockmanager-fail-db-bucket', $bucket );
$status->merge( $this->doUnlock( $lockedPaths, $type ) );
return $status;
}
// Record these locks as active
foreach ( $paths as $path ) {
$this->locksHeld[$path][$type] = 1; // locked
}
// Keep track of what locks were made in this attempt
$lockedPaths = array_merge( $lockedPaths, $paths );
}
return $status;
}
/**
* @see LockManager::doUnlock()
*/
protected function doUnlock( array $paths, $type ) {
$status = Status::newGood();
foreach ( $paths as $path ) {
if ( !isset( $this->locksHeld[$path] ) ) {
$status->warning( 'lockmanager-notlocked', $path );
} elseif ( !isset( $this->locksHeld[$path][$type] ) ) {
$status->warning( 'lockmanager-notlocked', $path );
} else {
--$this->locksHeld[$path][$type];
if ( $this->locksHeld[$path][$type] <= 0 ) {
unset( $this->locksHeld[$path][$type] );
}
if ( !count( $this->locksHeld[$path] ) ) {
unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
}
}
}
// Reference count the locks held and COMMIT when zero
if ( !count( $this->locksHeld ) ) {
$status->merge( $this->finishLockTransactions() );
}
return $status;
}
/**
* Get a connection to a lock DB and acquire locks on $paths.
* This does not use GET_LOCK() per http://bugs.mysql.com/bug.php?id=1118.
*
* @param $lockDb string
* @param $paths Array
* @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
* @return bool Resources able to be locked
* @throws DBError
*/
protected function doLockingQuery( $lockDb, array $paths, $type ) {
if ( $type == self::LOCK_EX ) { // writer locks
$db = $this->getConnection( $lockDb );
if ( !$db ) {
return false; // bad config
}
$keys = array_unique( array_map( 'LockManager::sha1Base36', $paths ) );
# Build up values for INSERT clause
$data = array();
foreach ( $keys as $key ) {
$data[] = array( 'fle_key' => $key );
}
# Wait on any existing writers and block new ones if we get in
$db->insert( 'filelocks_exclusive', $data, __METHOD__ );
}
return true;
}
/**
* Attempt to acquire locks with the peers for a bucket.
* This should avoid throwing any exceptions.
*
* @param $bucket integer
* @param $paths Array List of resource keys to lock
* @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
* @return bool|string One of (true, 'cantacquire', 'dberrors')
*/
protected function doLockingQueryAll( $bucket, array $paths, $type ) {
$yesVotes = 0; // locks made on trustable DBs
$votesLeft = count( $this->dbsByBucket[$bucket] ); // remaining DBs
$quorum = floor( $votesLeft/2 + 1 ); // simple majority
// Get votes for each DB, in order, until we have enough...
foreach ( $this->dbsByBucket[$bucket] as $lockDb ) {
// Check that DB is not *known* to be down
if ( $this->cacheCheckFailures( $lockDb ) ) {
try {
// Attempt to acquire the lock on this DB
if ( !$this->doLockingQuery( $lockDb, $paths, $type ) ) {
return 'cantacquire'; // vetoed; resource locked
}
++$yesVotes; // success for this peer
if ( $yesVotes >= $quorum ) {
return true; // lock obtained
}
} catch ( DBConnectionError $e ) {
$this->cacheRecordFailure( $lockDb );
} catch ( DBError $e ) {
if ( $this->lastErrorIndicatesLocked( $lockDb ) ) {
return 'cantacquire'; // vetoed; resource locked
}
}
}
--$votesLeft;
$votesNeeded = $quorum - $yesVotes;
if ( $votesNeeded > $votesLeft ) {
// In "trust cache" mode we don't have to meet the quorum
break; // short-circuit
}
}
// At this point, we must not have meet the quorum
return 'dberrors'; // not enough votes to ensure correctness
}
/**
* Get (or reuse) a connection to a lock DB
*
* @param $lockDb string
* @return Database
* @throws DBError
*/
protected function getConnection( $lockDb ) {
if ( !isset( $this->conns[$lockDb] ) ) {
$db = null;
if ( $lockDb === 'localDBMaster' ) {
$lb = wfGetLBFactory()->newMainLB();
$db = $lb->getConnection( DB_MASTER );
} elseif ( isset( $this->dbServers[$lockDb] ) ) {
$config = $this->dbServers[$lockDb];
$db = DatabaseBase::factory( $config['type'], $config );
}
if ( !$db ) {
return null; // config error?
}
$this->conns[$lockDb] = $db;
$this->conns[$lockDb]->clearFlag( DBO_TRX );
# If the connection drops, try to avoid letting the DB rollback
# and release the locks before the file operations are finished.
# This won't handle the case of DB server restarts however.
$options = array();
if ( $this->lockExpiry > 0 ) {
$options['connTimeout'] = $this->lockExpiry;
}
$this->conns[$lockDb]->setSessionOptions( $options );
$this->initConnection( $lockDb, $this->conns[$lockDb] );
}
if ( !$this->conns[$lockDb]->trxLevel() ) {
$this->conns[$lockDb]->begin(); // start transaction
}
return $this->conns[$lockDb];
}
/**
* Do additional initialization for new lock DB connection
*
* @param $lockDb string
* @param $db DatabaseBase
* @return void
* @throws DBError
*/
protected function initConnection( $lockDb, DatabaseBase $db ) {}
/**
* Commit all changes to lock-active databases.
* This should avoid throwing any exceptions.
*
* @return Status
*/
protected function finishLockTransactions() {
$status = Status::newGood();
foreach ( $this->conns as $lockDb => $db ) {
if ( $db->trxLevel() ) { // in transaction
try {
$db->rollback(); // finish transaction and kill any rows
} catch ( DBError $e ) {
$status->fatal( 'lockmanager-fail-db-release', $lockDb );
}
}
}
return $status;
}
/**
* Check if the last DB error for $lockDb indicates
* that a requested resource was locked by another process.
* This should avoid throwing any exceptions.
*
* @param $lockDb string
* @return bool
*/
protected function lastErrorIndicatesLocked( $lockDb ) {
if ( isset( $this->conns[$lockDb] ) ) { // sanity
$db = $this->conns[$lockDb];
return ( $db->wasDeadlock() || $db->wasLockTimeout() );
}
return false;
}
/**
* Checks if the DB has not recently had connection/query errors.
* This just avoids wasting time on doomed connection attempts.
*
* @param $lockDb string
* @return bool
*/
protected function cacheCheckFailures( $lockDb ) {
if ( $this->statusCache && $this->safeDelay > 0 ) {
$path = $this->getMissKey( $lockDb );
$misses = $this->statusCache->get( $path );
return !$misses;
}
return true;
}
/**
* Log a lock request failure to the cache
*
* @param $lockDb string
* @return bool Success
*/
protected function cacheRecordFailure( $lockDb ) {
if ( $this->statusCache && $this->safeDelay > 0 ) {
$path = $this->getMissKey( $lockDb );
$misses = $this->statusCache->get( $path );
if ( $misses ) {
return $this->statusCache->incr( $path );
} else {
return $this->statusCache->add( $path, 1, $this->safeDelay );
}
}
return true;
}
/**
* Get a cache key for recent query misses for a DB
*
* @param $lockDb string
* @return string
*/
protected function getMissKey( $lockDb ) {
return 'lockmanager:querymisses:' . str_replace( ' ', '_', $lockDb );
}
/**
* Get the bucket for resource path.
* This should avoid throwing any exceptions.
*
* @param $path string
* @return integer
*/
protected function getBucketFromKey( $path ) {
$prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
return intval( base_convert( $prefix, 16, 10 ) ) % count( $this->dbsByBucket );
}
/**
* Make sure remaining locks get cleared for sanity
*/
function __destruct() {
foreach ( $this->conns as $lockDb => $db ) {
if ( $db->trxLevel() ) { // in transaction
try {
$db->rollback(); // finish transaction and kill any rows
} catch ( DBError $e ) {
// oh well
}
}
$db->close();
}
}
}
/**
* MySQL version of DBLockManager that supports shared locks.
* 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 = array(
self::LOCK_SH => self::LOCK_SH,
self::LOCK_UW => self::LOCK_SH,
self::LOCK_EX => self::LOCK_EX
);
protected function initConnection( $lockDb, DatabaseBase $db ) {
# Let this transaction see lock rows from other transactions
$db->query( "SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;" );
}
protected function doLockingQuery( $lockDb, array $paths, $type ) {
$db = $this->getConnection( $lockDb );
if ( !$db ) {
return false;
}
$keys = array_unique( array_map( 'LockManager::sha1Base36', $paths ) );
# Build up values for INSERT clause
$data = array();
foreach ( $keys as $key ) {
$data[] = array( 'fls_key' => $key, 'fls_session' => $this->session );
}
# Block new writers...
$db->insert( 'filelocks_shared', $data, __METHOD__, array( 'IGNORE' ) );
# Actually do the locking queries...
if ( $type == self::LOCK_SH ) { // reader locks
# Bail if there are any existing writers...
$blocked = $db->selectField( 'filelocks_exclusive', '1',
array( 'fle_key' => $keys ),
__METHOD__
);
# Prospective writers that haven't yet updated filelocks_exclusive
# will recheck filelocks_shared after doing so and bail due to our entry.
} else { // writer locks
$encSession = $db->addQuotes( $this->session );
# Bail if there are any existing writers...
# The 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',
array( 'fls_key' => $keys, "fls_session != $encSession" ),
__METHOD__
);
if ( !$blocked ) {
# Build up values for INSERT clause
$data = array();
foreach ( $keys as $key ) {
$data[] = array( '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',
array( 'fls_key' => $keys, "fls_session != $encSession" ),
__METHOD__
);
}
}
return !$blocked;
}
}