includes/filerepo/backend/lockmanager/DBLockManager.php

   1 <?php
   2
   3 /**
   4  * Version of LockManager based on using DB table locks.
   5  * This is meant for multi-wiki systems that may share files.
   6  * All locks are blocking, so it might be useful to set a small
   7  * lock-wait timeout via server config to curtail deadlocks.
   8  *
   9  * All lock requests for a resource, identified by a hash string, will map
  10  * to one bucket. Each bucket maps to one or several peer DBs, each on their
  11  * own server, all having the filelocks.sql tables (with row-level locking).
  12  * A majority of peer DBs must agree for a lock to be acquired.
  13  *
  14  * Caching is used to avoid hitting servers that are down.
  15  *
  16  * @ingroup LockManager
  17  */
  18 class DBLockManager extends LockManager {
  19         /** @var Array Map of DB names to server config */
  20         protected $dbServers; // (DB name => server config array)
  21         /** @var Array Map of bucket indexes to peer DB lists */
  22         protected $dbsByBucket; // (bucket index => (ldb1, ldb2, ...))
  23         /** @var BagOStuff */
  24         protected $statusCache;
  25
  26         protected $lockExpiry; // integer number of seconds
  27         protected $safeDelay; // integer number of seconds
  28
  29         protected $session = 0; // random integer
  30         /** @var Array Map of (locked key => lock type => count) */
  31         protected $locksHeld = array();
  32         /** @var Array Map Database connections (DB name => Database) */
  33         protected $conns = array();
  34
  35         /**
  36          * Construct a new instance from configuration.
  37          * $config paramaters include:
  38          *     'dbServers'   : Associative array of DB names to server configuration.
  39          *                     Configuration is an associative array that includes:
  40          *                     'host'        - DB server name
  41          *                     'dbname'      - DB name
  42          *                     'type'        - DB type (mysql,postgres,...)
  43          *                     'user'        - DB user
  44          *                     'password'    - DB user password
  45          *                     'tablePrefix' - DB table prefix
  46          *                     'flags'       - DB flags (see DatabaseBase)
  47          *     'dbsByBucket' : Array of 1-16 consecutive integer keys, starting from 0,
  48          *                     each having an odd-numbered list of DB names (peers) as values.
  49          *                     Any DB named 'localDBMaster' will automatically use the DB master
  50          *                     settings for this wiki (without the need for a dbServers entry).
  51          *     'lockExpiry'  : Lock timeout (seconds) for dropped connections. [optional]
  52          *                     This tells the DB server how long to wait before assuming
  53          *                     connection failure and releasing all the locks for a session.
  54          *
  55          * @param Array $config
  56          */
  57         public function __construct( array $config ) {
  58                 $this->dbServers = $config['dbServers'];
  59                 // Sanitize dbsByBucket config to prevent PHP errors
  60                 $this->dbsByBucket = array_filter( $config['dbsByBucket'], 'is_array' );
  61                 $this->dbsByBucket = array_values( $this->dbsByBucket ); // consecutive
  62
  63                 if ( isset( $config['lockExpiry'] ) ) {
  64                         $this->lockExpiry = $config['lockExpiry'];
  65                 } else {
  66                         $met = ini_get( 'max_execution_time' );
  67                         $this->lockExpiry = $met ? $met : 60; // use some sane amount if 0
  68                 }
  69                 $this->safeDelay = ( $this->lockExpiry <= 0 )
  70                         ? 60 // pick a safe-ish number to match DB timeout default
  71                         : $this->lockExpiry; // cover worst case
  72
  73                 foreach ( $this->dbsByBucket as $bucket ) {
  74                         if ( count( $bucket ) > 1 ) {
  75                                 // Tracks peers that couldn't be queried recently to avoid lengthy
  76                                 // connection timeouts. This is useless if each bucket has one peer.
  77                                 $this->statusCache = wfGetMainCache();
  78                                 break;
  79                         }
  80                 }
  81
  82                 $this->session = '';
  83                 for ( $i = 0; $i < 5; $i++ ) {
  84                         $this->session .= mt_rand( 0, 2147483647 );
  85                 }
  86                 $this->session = wfBaseConvert( sha1( $this->session ), 16, 36, 31 );
  87         }
  88
  89         protected function doLock( array $keys, $type ) {
  90                 $status = Status::newGood();
  91
  92                 $keysToLock = array();
  93                 // Get locks that need to be acquired (buckets => locks)...
  94                 foreach ( $keys as $key ) {
  95                         if ( isset( $this->locksHeld[$key][$type] ) ) {
  96                                 ++$this->locksHeld[$key][$type];
  97                         } elseif ( isset( $this->locksHeld[$key][self::LOCK_EX] ) ) {
  98                                 $this->locksHeld[$key][$type] = 1;
  99                         } else {
 100                                 $bucket = $this->getBucketFromKey( $key );
 101                                 $keysToLock[$bucket][] = $key;
 102                         }
 103                 }
 104
 105                 $lockedKeys = array(); // files locked in this attempt
 106                 // Attempt to acquire these locks...
 107                 foreach ( $keysToLock as $bucket => $keys ) {
 108                         // Try to acquire the locks for this bucket
 109                         $res = $this->doLockingQueryAll( $bucket, $keys, $type );
 110                         if ( $res === 'cantacquire' ) {
 111                                 // Resources already locked by another process.
 112                                 // Abort and unlock everything we just locked.
 113                                 $status->fatal( 'lockmanager-fail-acquirelocks', implode( ', ', $keys ) );
 114                                 $status->merge( $this->doUnlock( $lockedKeys, $type ) );
 115                                 return $status;
 116                         } elseif ( $res !== true ) {
 117                                 // Couldn't contact any DBs for this bucket.
 118                                 // Abort and unlock everything we just locked.
 119                                 $status->fatal( 'lockmanager-fail-db-bucket', $bucket );
 120                                 $status->merge( $this->doUnlock( $lockedKeys, $type ) );
 121                                 return $status;
 122                         }
 123                         // Record these locks as active
 124                         foreach ( $keys as $key ) {
 125                                 $this->locksHeld[$key][$type] = 1; // locked
 126                         }
 127                         // Keep track of what locks were made in this attempt
 128                         $lockedKeys = array_merge( $lockedKeys, $keys );
 129                 }
 130
 131                 return $status;
 132         }
 133
 134         protected function doUnlock( array $keys, $type ) {
 135                 $status = Status::newGood();
 136
 137                 foreach ( $keys as $key ) {
 138                         if ( !isset( $this->locksHeld[$key] ) ) {
 139                                 $status->warning( 'lockmanager-notlocked', $key );
 140                         } elseif ( !isset( $this->locksHeld[$key][$type] ) ) {
 141                                 $status->warning( 'lockmanager-notlocked', $key );
 142                         } else {
 143                                 --$this->locksHeld[$key][$type];
 144                                 if ( $this->locksHeld[$key][$type] <= 0 ) {
 145                                         unset( $this->locksHeld[$key][$type] );
 146                                 }
 147                                 if ( !count( $this->locksHeld[$key] ) ) {
 148                                         unset( $this->locksHeld[$key] ); // no SH or EX locks left for key
 149                                 }
 150                         }
 151                 }
 152
 153                 // Reference count the locks held and COMMIT when zero
 154                 if ( !count( $this->locksHeld ) ) {
 155                         $status->merge( $this->finishLockTransactions() );
 156                 }
 157
 158                 return $status;
 159         }
 160
 161         /**
 162          * Get a connection to a lock DB and acquire locks on $keys.
 163          * This does not use GET_LOCK() per http://bugs.mysql.com/bug.php?id=1118.
 164          *
 165          * @param $lockDb string
 166          * @param $keys Array
 167          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 168          * @return bool Resources able to be locked
 169          * @throws DBError
 170          */
 171         protected function doLockingQuery( $lockDb, array $keys, $type ) {
 172                 if ( $type == self::LOCK_EX ) { // writer locks
 173                         $db = $this->getConnection( $lockDb );
 174                         if ( !$db ) {
 175                                 return false; // bad config
 176                         }
 177                         $data = array();
 178                         foreach ( $keys as $key ) {
 179                                 $data[] = array( 'fle_key' => $key );
 180                         }
 181                         # Wait on any existing writers and block new ones if we get in
 182                         $db->insert( 'filelocks_exclusive', $data, __METHOD__ );
 183                 }
 184                 return true;
 185         }
 186
 187         /**
 188          * Attempt to acquire locks with the peers for a bucket.
 189          * This should avoid throwing any exceptions.
 190          *
 191          * @param $bucket integer
 192          * @param $keys Array List of resource keys to lock
 193          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 194          * @return bool|string One of (true, 'cantacquire', 'dberrors')
 195          */
 196         protected function doLockingQueryAll( $bucket, array $keys, $type ) {
 197                 $yesVotes = 0; // locks made on trustable DBs
 198                 $votesLeft = count( $this->dbsByBucket[$bucket] ); // remaining DBs
 199                 $quorum = floor( $votesLeft/2 + 1 ); // simple majority
 200                 // Get votes for each DB, in order, until we have enough...
 201                 foreach ( $this->dbsByBucket[$bucket] as $index => $lockDb ) {
 202                         // Check that DB is not *known* to be down
 203                         if ( $this->cacheCheckFailures( $lockDb ) ) {
 204                                 try {
 205                                         // Attempt to acquire the lock on this DB
 206                                         if ( !$this->doLockingQuery( $lockDb, $keys, $type ) ) {
 207                                                 return 'cantacquire'; // vetoed; resource locked
 208                                         }
 209                                         ++$yesVotes; // success for this peer
 210                                         if ( $yesVotes >= $quorum ) {
 211                                                 return true; // lock obtained
 212                                         }
 213                                 } catch ( DBConnectionError $e ) {
 214                                         $this->cacheRecordFailure( $lockDb );
 215                                 } catch ( DBError $e ) {
 216                                         if ( $this->lastErrorIndicatesLocked( $lockDb ) ) {
 217                                                 return 'cantacquire'; // vetoed; resource locked
 218                                         }
 219                                 }
 220                         }
 221                         $votesLeft--;
 222                         $votesNeeded = $quorum - $yesVotes;
 223                         if ( $votesNeeded > $votesLeft ) {
 224                                 // In "trust cache" mode we don't have to meet the quorum
 225                                 break; // short-circuit
 226                         }
 227                 }
 228                 // At this point, we must not have meet the quorum
 229                 return 'dberrors'; // not enough votes to ensure correctness
 230         }
 231
 232         /**
 233          * Get (or reuse) a connection to a lock DB
 234          *
 235          * @param $lockDb string
 236          * @return Database
 237          * @throws DBError
 238          */
 239         protected function getConnection( $lockDb ) {
 240                 if ( !isset( $this->conns[$lockDb] ) ) {
 241                         $db = null;
 242                         if ( $lockDb === 'localDBMaster' ) {
 243                                 $lb = wfGetLBFactory()->newMainLB();
 244                                 $db = $lb->getConnection( DB_MASTER );
 245                         } elseif ( isset( $this->dbServers[$lockDb] ) ) {
 246                                 $config = $this->dbServers[$lockDb];
 247                                 $db = DatabaseBase::factory( $config['type'], $config );
 248                         }
 249                         if ( !$db ) {
 250                                 return null; // config error?
 251                         }
 252                         $this->conns[$lockDb] = $db;
 253                         $this->conns[$lockDb]->clearFlag( DBO_TRX );
 254                         # If the connection drops, try to avoid letting the DB rollback
 255                         # and release the locks before the file operations are finished.
 256                         # This won't handle the case of DB server restarts however.
 257                         $options = array();
 258                         if ( $this->lockExpiry > 0 ) {
 259                                 $options['connTimeout'] = $this->lockExpiry;
 260                         }
 261                         $this->conns[$lockDb]->setSessionOptions( $options );
 262                         $this->initConnection( $lockDb, $this->conns[$lockDb] );
 263                 }
 264                 if ( !$this->conns[$lockDb]->trxLevel() ) {
 265                         $this->conns[$lockDb]->begin(); // start transaction
 266                 }
 267                 return $this->conns[$lockDb];
 268         }
 269
 270         /**
 271          * Do additional initialization for new lock DB connection
 272          *
 273          * @param $lockDb string
 274          * @param $db DatabaseBase
 275          * @return void
 276          * @throws DBError
 277          */
 278         protected function initConnection( $lockDb, DatabaseBase $db ) {}
 279
 280         /**
 281          * Commit all changes to lock-active databases.
 282          * This should avoid throwing any exceptions.
 283          *
 284          * @return Status
 285          */
 286         protected function finishLockTransactions() {
 287                 $status = Status::newGood();
 288                 foreach ( $this->conns as $lockDb => $db ) {
 289                         if ( $db->trxLevel() ) { // in transaction
 290                                 try {
 291                                         $db->rollback(); // finish transaction and kill any rows
 292                                 } catch ( DBError $e ) {
 293                                         $status->fatal( 'lockmanager-fail-db-release', $lockDb );
 294                                 }
 295                         }
 296                 }
 297                 return $status;
 298         }
 299
 300         /**
 301          * Check if the last DB error for $lockDb indicates
 302          * that a requested resource was locked by another process.
 303          * This should avoid throwing any exceptions.
 304          *
 305          * @param $lockDb string
 306          * @return bool
 307          */
 308         protected function lastErrorIndicatesLocked( $lockDb ) {
 309                 if ( isset( $this->conns[$lockDb] ) ) { // sanity
 310                         $db = $this->conns[$lockDb];
 311                         return ( $db->wasDeadlock() || $db->wasLockTimeout() );
 312                 }
 313                 return false;
 314         }
 315
 316         /**
 317          * Checks if the DB has not recently had connection/query errors.
 318          * This just avoids wasting time on doomed connection attempts.
 319          *
 320          * @param $lockDb string
 321          * @return bool
 322          */
 323         protected function cacheCheckFailures( $lockDb ) {
 324                 if ( $this->statusCache && $this->safeDelay > 0 ) {
 325                         $key = $this->getMissKey( $lockDb );
 326                         $misses = $this->statusCache->get( $key );
 327                         return !$misses;
 328                 }
 329                 return true;
 330         }
 331
 332         /**
 333          * Log a lock request failure to the cache
 334          *
 335          * @param $lockDb string
 336          * @return bool Success
 337          */
 338         protected function cacheRecordFailure( $lockDb ) {
 339                 if ( $this->statusCache && $this->safeDelay > 0 ) {
 340                         $key = $this->getMissKey( $lockDb );
 341                         $misses = $this->statusCache->get( $key );
 342                         if ( $misses ) {
 343                                 return $this->statusCache->incr( $key );
 344                         } else {
 345                                 return $this->statusCache->add( $key, 1, $this->safeDelay );
 346                         }
 347                 }
 348                 return true;
 349         }
 350
 351         /**
 352          * Get a cache key for recent query misses for a DB
 353          *
 354          * @param $lockDb string
 355          * @return string
 356          */
 357         protected function getMissKey( $lockDb ) {
 358                 return 'lockmanager:querymisses:' . str_replace( ' ', '_', $lockDb );
 359         }
 360
 361         /**
 362          * Get the bucket for lock key.
 363          * This should avoid throwing any exceptions.
 364          *
 365          * @param $key string (31 char hex key)
 366          * @return integer
 367          */
 368         protected function getBucketFromKey( $key ) {
 369                 $prefix = substr( $key, 0, 2 ); // first 2 hex chars (8 bits)
 370                 return intval( base_convert( $prefix, 16, 10 ) ) % count( $this->dbsByBucket );
 371         }
 372
 373         /**
 374          * Make sure remaining locks get cleared for sanity
 375          */
 376         function __destruct() {
 377                 foreach ( $this->conns as $lockDb => $db ) {
 378                         if ( $db->trxLevel() ) { // in transaction
 379                                 try {
 380                                         $db->rollback(); // finish transaction and kill any rows
 381                                 } catch ( DBError $e ) {
 382                                         // oh well
 383                                 }
 384                         }
 385                         $db->close();
 386                 }
 387         }
 388 }
 389
 390 /**
 391  * MySQL version of DBLockManager that supports shared locks.
 392  * All locks are non-blocking, which avoids deadlocks.
 393  *
 394  * @ingroup LockManager
 395  */
 396 class MySqlLockManager extends DBLockManager {
 397         /** @var Array Mapping of lock types to the type actually used */
 398         protected $lockTypeMap = array(
 399                 self::LOCK_SH => self::LOCK_SH,
 400                 self::LOCK_UW => self::LOCK_SH,
 401                 self::LOCK_EX => self::LOCK_EX
 402         );
 403
 404         protected function initConnection( $lockDb, DatabaseBase $db ) {
 405                 # Let this transaction see lock rows from other transactions
 406                 $db->query( "SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;" );
 407         }
 408
 409         protected function doLockingQuery( $lockDb, array $keys, $type ) {
 410                 $db = $this->getConnection( $lockDb );
 411                 if ( !$db ) {
 412                         return false;
 413                 }
 414                 $data = array();
 415                 foreach ( $keys as $key ) {
 416                         $data[] = array( 'fls_key' => $key, 'fls_session' => $this->session );
 417                 }
 418                 # Block new writers...
 419                 $db->insert( 'filelocks_shared', $data, __METHOD__, array( 'IGNORE' ) );
 420                 # Actually do the locking queries...
 421                 if ( $type == self::LOCK_SH ) { // reader locks
 422                         # Bail if there are any existing writers...
 423                         $blocked = $db->selectField( 'filelocks_exclusive', '1',
 424                                 array( 'fle_key' => $keys ),
 425                                 __METHOD__
 426                         );
 427                         # Prospective writers that haven't yet updated filelocks_exclusive
 428                         # will recheck filelocks_shared after doing so and bail due to our entry.
 429                 } else { // writer locks
 430                         $encSession = $db->addQuotes( $this->session );
 431                         # Bail if there are any existing writers...
 432                         # The may detect readers, but the safe check for them is below.
 433                         # Note: if two writers come at the same time, both bail :)
 434                         $blocked = $db->selectField( 'filelocks_shared', '1',
 435                                 array( 'fls_key' => $keys, "fls_session != $encSession" ),
 436                                 __METHOD__
 437                         );
 438                         if ( !$blocked ) {
 439                                 $data = array();
 440                                 foreach ( $keys as $key ) {
 441                                         $data[] = array( 'fle_key' => $key );
 442                                 }
 443                                 # Block new readers/writers...
 444                                 $db->insert( 'filelocks_exclusive', $data, __METHOD__ );
 445                                 # Bail if there are any existing readers...
 446                                 $blocked = $db->selectField( 'filelocks_shared', '1',
 447                                         array( 'fls_key' => $keys, "fls_session != $encSession" ),
 448                                         __METHOD__
 449                                 );
 450                         }
 451                 }
 452                 return !$blocked;
 453         }
 454 }