includes/libs/objectcache/BagOStuff.php

   1 <?php
   2 /**
   3  * Copyright © 2003-2004 Brion Vibber <brion@pobox.com>
   4  * https://www.mediawiki.org/
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License along
  17  * with this program; if not, write to the Free Software Foundation, Inc.,
  18  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  19  * http://www.gnu.org/copyleft/gpl.html
  20  *
  21  * @file
  22  * @ingroup Cache
  23  */
  24
  25 /**
  26  * @defgroup Cache Cache
  27  */
  28
  29 use Psr\Log\LoggerAwareInterface;
  30 use Psr\Log\LoggerInterface;
  31 use Psr\Log\NullLogger;
  32 use Wikimedia\ScopedCallback;
  33 use Wikimedia\WaitConditionLoop;
  34
  35 /**
  36  * Class representing a cache/ephemeral data store
  37  *
  38  * This interface is intended to be more or less compatible with the PHP memcached client.
  39  *
  40  * Instances of this class should be created with an intended access scope, such as:
  41  *   - a) A single PHP thread on a server (e.g. stored in a PHP variable)
  42  *   - b) A single application server (e.g. stored in APC or sqlite)
  43  *   - c) All application servers in datacenter (e.g. stored in memcached or mysql)
  44  *   - d) All application servers in all datacenters (e.g. stored via mcrouter or dynomite)
  45  *
  46  * Callers should use the proper factory methods that yield BagOStuff instances. Site admins
  47  * should make sure the configuration for those factory methods matches their access scope.
  48  * BagOStuff subclasses have widely varying levels of support for replication features.
  49  *
  50  * For any given instance, methods like lock(), unlock(), merge(), and set() with WRITE_SYNC
  51  * should semantically operate over its entire access scope; any nodes/threads in that scope
  52  * should serialize appropriately when using them. Likewise, a call to get() with READ_LATEST
  53  * from one node in its access scope should reflect the prior changes of any other node its access
  54  * scope. Any get() should reflect the changes of any prior set() with WRITE_SYNC.
  55  *
  56  * @ingroup Cache
  57  */
  58 abstract class BagOStuff implements IExpiringStore, LoggerAwareInterface {
  59         /** @var array[] Lock tracking */
  60         protected $locks = [];
  61         /** @var int ERR_* class constant */
  62         protected $lastError = self::ERR_NONE;
  63         /** @var string */
  64         protected $keyspace = 'local';
  65         /** @var LoggerInterface */
  66         protected $logger;
  67         /** @var callback|null */
  68         protected $asyncHandler;
  69         /** @var int Seconds */
  70         protected $syncTimeout;
  71
  72         /** @var bool */
  73         private $debugMode = false;
  74         /** @var array */
  75         private $duplicateKeyLookups = [];
  76         /** @var bool */
  77         private $reportDupes = false;
  78         /** @var bool */
  79         private $dupeTrackScheduled = false;
  80
  81         /** @var callable[] */
  82         protected $busyCallbacks = [];
  83
  84         /** @var float|null */
  85         private $wallClockOverride;
  86
  87         /** @var int[] Map of (ATTR_* class constant => QOS_* class constant) */
  88         protected $attrMap = [];
  89
  90         /** Possible values for getLastError() */
  91         const ERR_NONE = 0; // no error
  92         const ERR_NO_RESPONSE = 1; // no response
  93         const ERR_UNREACHABLE = 2; // can't connect
  94         const ERR_UNEXPECTED = 3; // response gave some error
  95
  96         /** Bitfield constants for get()/getMulti() */
  97         const READ_LATEST = 1; // use latest data for replicated stores
  98         const READ_VERIFIED = 2; // promise that caller can tell when keys are stale
  99         /** Bitfield constants for set()/merge() */
 100         const WRITE_SYNC = 1; // synchronously write to all locations for replicated stores
 101         const WRITE_CACHE_ONLY = 2; // Only change state of the in-memory cache
 102
 103         /**
 104          * $params include:
 105          *   - logger: Psr\Log\LoggerInterface instance
 106          *   - keyspace: Default keyspace for $this->makeKey()
 107          *   - asyncHandler: Callable to use for scheduling tasks after the web request ends.
 108          *      In CLI mode, it should run the task immediately.
 109          *   - reportDupes: Whether to emit warning log messages for all keys that were
 110          *      requested more than once (requires an asyncHandler).
 111          *   - syncTimeout: How long to wait with WRITE_SYNC in seconds.
 112          * @param array $params
 113          */
 114         public function __construct( array $params = [] ) {
 115                 if ( isset( $params['logger'] ) ) {
 116                         $this->setLogger( $params['logger'] );
 117                 } else {
 118                         $this->setLogger( new NullLogger() );
 119                 }
 120
 121                 if ( isset( $params['keyspace'] ) ) {
 122                         $this->keyspace = $params['keyspace'];
 123                 }
 124
 125                 $this->asyncHandler = isset( $params['asyncHandler'] )
 126                         ? $params['asyncHandler']
 127                         : null;
 128
 129                 if ( !empty( $params['reportDupes'] ) && is_callable( $this->asyncHandler ) ) {
 130                         $this->reportDupes = true;
 131                 }
 132
 133                 $this->syncTimeout = isset( $params['syncTimeout'] ) ? $params['syncTimeout'] : 3;
 134         }
 135
 136         /**
 137          * @param LoggerInterface $logger
 138          * @return null
 139          */
 140         public function setLogger( LoggerInterface $logger ) {
 141                 $this->logger = $logger;
 142         }
 143
 144         /**
 145          * @param bool $bool
 146          */
 147         public function setDebug( $bool ) {
 148                 $this->debugMode = $bool;
 149         }
 150
 151         /**
 152          * Get an item with the given key, regenerating and setting it if not found
 153          *
 154          * If the callback returns false, then nothing is stored.
 155          *
 156          * @param string $key
 157          * @param int $ttl Time-to-live (seconds)
 158          * @param callable $callback Callback that derives the new value
 159          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 160          * @return mixed The cached value if found or the result of $callback otherwise
 161          * @since 1.27
 162          */
 163         final public function getWithSetCallback( $key, $ttl, $callback, $flags = 0 ) {
 164                 $value = $this->get( $key, $flags );
 165
 166                 if ( $value === false ) {
 167                         if ( !is_callable( $callback ) ) {
 168                                 throw new InvalidArgumentException( "Invalid cache miss callback provided." );
 169                         }
 170                         $value = call_user_func( $callback );
 171                         if ( $value !== false ) {
 172                                 $this->set( $key, $value, $ttl );
 173                         }
 174                 }
 175
 176                 return $value;
 177         }
 178
 179         /**
 180          * Get an item with the given key
 181          *
 182          * If the key includes a deterministic input hash (e.g. the key can only have
 183          * the correct value) or complete staleness checks are handled by the caller
 184          * (e.g. nothing relies on the TTL), then the READ_VERIFIED flag should be set.
 185          * This lets tiered backends know they can safely upgrade a cached value to
 186          * higher tiers using standard TTLs.
 187          *
 188          * @param string $key
 189          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 190          * @param int $oldFlags [unused]
 191          * @return mixed Returns false on failure and if the item does not exist
 192          */
 193         public function get( $key, $flags = 0, $oldFlags = null ) {
 194                 // B/C for ( $key, &$casToken = null, $flags = 0 )
 195                 $flags = is_int( $oldFlags ) ? $oldFlags : $flags;
 196
 197                 $this->trackDuplicateKeys( $key );
 198
 199                 return $this->doGet( $key, $flags );
 200         }
 201
 202         /**
 203          * Track the number of times that a given key has been used.
 204          * @param string $key
 205          */
 206         private function trackDuplicateKeys( $key ) {
 207                 if ( !$this->reportDupes ) {
 208                         return;
 209                 }
 210
 211                 if ( !isset( $this->duplicateKeyLookups[$key] ) ) {
 212                         // Track that we have seen this key. This N-1 counting style allows
 213                         // easy filtering with array_filter() later.
 214                         $this->duplicateKeyLookups[$key] = 0;
 215                 } else {
 216                         $this->duplicateKeyLookups[$key] += 1;
 217
 218                         if ( $this->dupeTrackScheduled === false ) {
 219                                 $this->dupeTrackScheduled = true;
 220                                 // Schedule a callback that logs keys processed more than once by get().
 221                                 call_user_func( $this->asyncHandler, function () {
 222                                         $dups = array_filter( $this->duplicateKeyLookups );
 223                                         foreach ( $dups as $key => $count ) {
 224                                                 $this->logger->warning(
 225                                                         'Duplicate get(): "{key}" fetched {count} times',
 226                                                         // Count is N-1 of the actual lookup count
 227                                                         [ 'key' => $key, 'count' => $count + 1, ]
 228                                                 );
 229                                         }
 230                                 } );
 231                         }
 232                 }
 233         }
 234
 235         /**
 236          * @param string $key
 237          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 238          * @return mixed Returns false on failure and if the item does not exist
 239          */
 240         abstract protected function doGet( $key, $flags = 0 );
 241
 242         /**
 243          * @note: This method is only needed if merge() uses mergeViaCas()
 244          *
 245          * @param string $key
 246          * @param mixed &$casToken
 247          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 248          * @return mixed Returns false on failure and if the item does not exist
 249          * @throws Exception
 250          */
 251         protected function getWithToken( $key, &$casToken, $flags = 0 ) {
 252                 throw new Exception( __METHOD__ . ' not implemented.' );
 253         }
 254
 255         /**
 256          * Set an item
 257          *
 258          * @param string $key
 259          * @param mixed $value
 260          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 261          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 262          * @return bool Success
 263          */
 264         abstract public function set( $key, $value, $exptime = 0, $flags = 0 );
 265
 266         /**
 267          * Delete an item
 268          *
 269          * @param string $key
 270          * @return bool True if the item was deleted or not found, false on failure
 271          */
 272         abstract public function delete( $key );
 273
 274         /**
 275          * Merge changes into the existing cache value (possibly creating a new one)
 276          *
 277          * The callback function returns the new value given the current value
 278          * (which will be false if not present), and takes the arguments:
 279          * (this BagOStuff, cache key, current value, TTL).
 280          * The TTL parameter is reference set to $exptime. It can be overriden in the callback.
 281          *
 282          * @param string $key
 283          * @param callable $callback Callback method to be executed
 284          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 285          * @param int $attempts The amount of times to attempt a merge in case of failure
 286          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 287          * @return bool Success
 288          * @throws InvalidArgumentException
 289          */
 290         public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 291                 return $this->mergeViaLock( $key, $callback, $exptime, $attempts, $flags );
 292         }
 293
 294         /**
 295          * @see BagOStuff::merge()
 296          *
 297          * @param string $key
 298          * @param callable $callback Callback method to be executed
 299          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 300          * @param int $attempts The amount of times to attempt a merge in case of failure
 301          * @return bool Success
 302          */
 303         protected function mergeViaCas( $key, $callback, $exptime = 0, $attempts = 10 ) {
 304                 do {
 305                         $this->clearLastError();
 306                         $reportDupes = $this->reportDupes;
 307                         $this->reportDupes = false;
 308                         $casToken = null; // passed by reference
 309                         $currentValue = $this->getWithToken( $key, $casToken, self::READ_LATEST );
 310                         $this->reportDupes = $reportDupes;
 311
 312                         if ( $this->getLastError() ) {
 313                                 return false; // don't spam retries (retry only on races)
 314                         }
 315
 316                         // Derive the new value from the old value
 317                         $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
 318
 319                         $this->clearLastError();
 320                         if ( $value === false ) {
 321                                 $success = true; // do nothing
 322                         } elseif ( $currentValue === false ) {
 323                                 // Try to create the key, failing if it gets created in the meantime
 324                                 $success = $this->add( $key, $value, $exptime );
 325                         } else {
 326                                 // Try to update the key, failing if it gets changed in the meantime
 327                                 $success = $this->cas( $casToken, $key, $value, $exptime );
 328                         }
 329                         if ( $this->getLastError() ) {
 330                                 return false; // IO error; don't spam retries
 331                         }
 332                 } while ( !$success && --$attempts );
 333
 334                 return $success;
 335         }
 336
 337         /**
 338          * Check and set an item
 339          *
 340          * @param mixed $casToken
 341          * @param string $key
 342          * @param mixed $value
 343          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 344          * @return bool Success
 345          * @throws Exception
 346          */
 347         protected function cas( $casToken, $key, $value, $exptime = 0 ) {
 348                 throw new Exception( "CAS is not implemented in " . __CLASS__ );
 349         }
 350
 351         /**
 352          * @see BagOStuff::merge()
 353          *
 354          * @param string $key
 355          * @param callable $callback Callback method to be executed
 356          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 357          * @param int $attempts The amount of times to attempt a merge in case of failure
 358          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 359          * @return bool Success
 360          */
 361         protected function mergeViaLock( $key, $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 362                 if ( !$this->lock( $key, 6 ) ) {
 363                         return false;
 364                 }
 365
 366                 $this->clearLastError();
 367                 $reportDupes = $this->reportDupes;
 368                 $this->reportDupes = false;
 369                 $currentValue = $this->get( $key, self::READ_LATEST );
 370                 $this->reportDupes = $reportDupes;
 371
 372                 if ( $this->getLastError() ) {
 373                         $success = false;
 374                 } else {
 375                         // Derive the new value from the old value
 376                         $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
 377                         if ( $value === false ) {
 378                                 $success = true; // do nothing
 379                         } else {
 380                                 $success = $this->set( $key, $value, $exptime, $flags ); // set the new value
 381                         }
 382                 }
 383
 384                 if ( !$this->unlock( $key ) ) {
 385                         // this should never happen
 386                         trigger_error( "Could not release lock for key '$key'." );
 387                 }
 388
 389                 return $success;
 390         }
 391
 392         /**
 393          * Reset the TTL on a key if it exists
 394          *
 395          * @param string $key
 396          * @param int $expiry
 397          * @return bool Success Returns false if there is no key
 398          * @since 1.28
 399          */
 400         public function changeTTL( $key, $expiry = 0 ) {
 401                 $value = $this->get( $key );
 402
 403                 return ( $value === false ) ? false : $this->set( $key, $value, $expiry );
 404         }
 405
 406         /**
 407          * Acquire an advisory lock on a key string
 408          *
 409          * Note that if reentry is enabled, duplicate calls ignore $expiry
 410          *
 411          * @param string $key
 412          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 413          * @param int $expiry Lock expiry [optional]; 1 day maximum
 414          * @param string $rclass Allow reentry if set and the current lock used this value
 415          * @return bool Success
 416          */
 417         public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
 418                 // Avoid deadlocks and allow lock reentry if specified
 419                 if ( isset( $this->locks[$key] ) ) {
 420                         if ( $rclass != '' && $this->locks[$key]['class'] === $rclass ) {
 421                                 ++$this->locks[$key]['depth'];
 422                                 return true;
 423                         } else {
 424                                 return false;
 425                         }
 426                 }
 427
 428                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 429                 $loop = new WaitConditionLoop(
 430                         function () use ( $key, $timeout, $expiry ) {
 431                                 $this->clearLastError();
 432                                 if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
 433                                         return true; // locked!
 434                                 } elseif ( $this->getLastError() ) {
 435                                         return WaitConditionLoop::CONDITION_ABORTED; // network partition?
 436                                 }
 437
 438                                 return WaitConditionLoop::CONDITION_CONTINUE;
 439                         },
 440                         $timeout
 441                 );
 442
 443                 $locked = ( $loop->invoke() === $loop::CONDITION_REACHED );
 444                 if ( $locked ) {
 445                         $this->locks[$key] = [ 'class' => $rclass, 'depth' => 1 ];
 446                 }
 447
 448                 return $locked;
 449         }
 450
 451         /**
 452          * Release an advisory lock on a key string
 453          *
 454          * @param string $key
 455          * @return bool Success
 456          */
 457         public function unlock( $key ) {
 458                 if ( isset( $this->locks[$key] ) && --$this->locks[$key]['depth'] <= 0 ) {
 459                         unset( $this->locks[$key] );
 460
 461                         return $this->delete( "{$key}:lock" );
 462                 }
 463
 464                 return true;
 465         }
 466
 467         /**
 468          * Get a lightweight exclusive self-unlocking lock
 469          *
 470          * Note that the same lock cannot be acquired twice.
 471          *
 472          * This is useful for task de-duplication or to avoid obtrusive
 473          * (though non-corrupting) DB errors like INSERT key conflicts
 474          * or deadlocks when using LOCK IN SHARE MODE.
 475          *
 476          * @param string $key
 477          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 478          * @param int $expiry Lock expiry [optional]; 1 day maximum
 479          * @param string $rclass Allow reentry if set and the current lock used this value
 480          * @return ScopedCallback|null Returns null on failure
 481          * @since 1.26
 482          */
 483         final public function getScopedLock( $key, $timeout = 6, $expiry = 30, $rclass = '' ) {
 484                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 485
 486                 if ( !$this->lock( $key, $timeout, $expiry, $rclass ) ) {
 487                         return null;
 488                 }
 489
 490                 $lSince = $this->getCurrentTime(); // lock timestamp
 491
 492                 return new ScopedCallback( function () use ( $key, $lSince, $expiry ) {
 493                         $latency = 0.050; // latency skew (err towards keeping lock present)
 494                         $age = ( $this->getCurrentTime() - $lSince + $latency );
 495                         if ( ( $age + $latency ) >= $expiry ) {
 496                                 $this->logger->warning( "Lock for $key held too long ($age sec)." );
 497                                 return; // expired; it's not "safe" to delete the key
 498                         }
 499                         $this->unlock( $key );
 500                 } );
 501         }
 502
 503         /**
 504          * Delete all objects expiring before a certain date.
 505          * @param string $date The reference date in MW format
 506          * @param callable|bool $progressCallback Optional, a function which will be called
 507          *     regularly during long-running operations with the percentage progress
 508          *     as the first parameter.
 509          *
 510          * @return bool Success, false if unimplemented
 511          */
 512         public function deleteObjectsExpiringBefore( $date, $progressCallback = false ) {
 513                 // stub
 514                 return false;
 515         }
 516
 517         /**
 518          * Get an associative array containing the item for each of the keys that have items.
 519          * @param array $keys List of strings
 520          * @param int $flags Bitfield; supports READ_LATEST [optional]
 521          * @return array
 522          */
 523         public function getMulti( array $keys, $flags = 0 ) {
 524                 $res = [];
 525                 foreach ( $keys as $key ) {
 526                         $val = $this->get( $key );
 527                         if ( $val !== false ) {
 528                                 $res[$key] = $val;
 529                         }
 530                 }
 531                 return $res;
 532         }
 533
 534         /**
 535          * Batch insertion
 536          * @param array $data $key => $value assoc array
 537          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 538          * @return bool Success
 539          * @since 1.24
 540          */
 541         public function setMulti( array $data, $exptime = 0 ) {
 542                 $res = true;
 543                 foreach ( $data as $key => $value ) {
 544                         if ( !$this->set( $key, $value, $exptime ) ) {
 545                                 $res = false;
 546                         }
 547                 }
 548                 return $res;
 549         }
 550
 551         /**
 552          * @param string $key
 553          * @param mixed $value
 554          * @param int $exptime
 555          * @return bool Success
 556          */
 557         public function add( $key, $value, $exptime = 0 ) {
 558                 if ( $this->get( $key ) === false ) {
 559                         return $this->set( $key, $value, $exptime );
 560                 }
 561                 return false; // key already set
 562         }
 563
 564         /**
 565          * Increase stored value of $key by $value while preserving its TTL
 566          * @param string $key Key to increase
 567          * @param int $value Value to add to $key (Default 1)
 568          * @return int|bool New value or false on failure
 569          */
 570         public function incr( $key, $value = 1 ) {
 571                 if ( !$this->lock( $key ) ) {
 572                         return false;
 573                 }
 574                 $n = $this->get( $key );
 575                 if ( $this->isInteger( $n ) ) { // key exists?
 576                         $n += intval( $value );
 577                         $this->set( $key, max( 0, $n ) ); // exptime?
 578                 } else {
 579                         $n = false;
 580                 }
 581                 $this->unlock( $key );
 582
 583                 return $n;
 584         }
 585
 586         /**
 587          * Decrease stored value of $key by $value while preserving its TTL
 588          * @param string $key
 589          * @param int $value
 590          * @return int|bool New value or false on failure
 591          */
 592         public function decr( $key, $value = 1 ) {
 593                 return $this->incr( $key, - $value );
 594         }
 595
 596         /**
 597          * Increase stored value of $key by $value while preserving its TTL
 598          *
 599          * This will create the key with value $init and TTL $ttl instead if not present
 600          *
 601          * @param string $key
 602          * @param int $ttl
 603          * @param int $value
 604          * @param int $init
 605          * @return int|bool New value or false on failure
 606          * @since 1.24
 607          */
 608         public function incrWithInit( $key, $ttl, $value = 1, $init = 1 ) {
 609                 $newValue = $this->incr( $key, $value );
 610                 if ( $newValue === false ) {
 611                         // No key set; initialize
 612                         $newValue = $this->add( $key, (int)$init, $ttl ) ? $init : false;
 613                 }
 614                 if ( $newValue === false ) {
 615                         // Raced out initializing; increment
 616                         $newValue = $this->incr( $key, $value );
 617                 }
 618
 619                 return $newValue;
 620         }
 621
 622         /**
 623          * Get the "last error" registered; clearLastError() should be called manually
 624          * @return int ERR_* constant for the "last error" registry
 625          * @since 1.23
 626          */
 627         public function getLastError() {
 628                 return $this->lastError;
 629         }
 630
 631         /**
 632          * Clear the "last error" registry
 633          * @since 1.23
 634          */
 635         public function clearLastError() {
 636                 $this->lastError = self::ERR_NONE;
 637         }
 638
 639         /**
 640          * Set the "last error" registry
 641          * @param int $err ERR_* constant
 642          * @since 1.23
 643          */
 644         protected function setLastError( $err ) {
 645                 $this->lastError = $err;
 646         }
 647
 648         /**
 649          * Let a callback be run to avoid wasting time on special blocking calls
 650          *
 651          * The callbacks may or may not be called ever, in any particular order.
 652          * They are likely to be invoked when something WRITE_SYNC is used used.
 653          * They should follow a caching pattern as shown below, so that any code
 654          * using the word will get it's result no matter what happens.
 655          * @code
 656          *     $result = null;
 657          *     $workCallback = function () use ( &$result ) {
 658          *         if ( !$result ) {
 659          *             $result = ....
 660          *         }
 661          *         return $result;
 662          *     }
 663          * @endcode
 664          *
 665          * @param callable $workCallback
 666          * @since 1.28
 667          */
 668         public function addBusyCallback( callable $workCallback ) {
 669                 $this->busyCallbacks[] = $workCallback;
 670         }
 671
 672         /**
 673          * Modify a cache update operation array for EventRelayer::notify()
 674          *
 675          * This is used for relayed writes, e.g. for broadcasting a change
 676          * to multiple data-centers. If the array contains a 'val' field
 677          * then the command involves setting a key to that value. Note that
 678          * for simplicity, 'val' is always a simple scalar value. This method
 679          * is used to possibly serialize the value and add any cache-specific
 680          * key/values needed for the relayer daemon (e.g. memcached flags).
 681          *
 682          * @param array $event
 683          * @return array
 684          * @since 1.26
 685          */
 686         public function modifySimpleRelayEvent( array $event ) {
 687                 return $event;
 688         }
 689
 690         /**
 691          * @param string $text
 692          */
 693         protected function debug( $text ) {
 694                 if ( $this->debugMode ) {
 695                         $this->logger->debug( "{class} debug: $text", [
 696                                 'class' => static::class,
 697                         ] );
 698                 }
 699         }
 700
 701         /**
 702          * Convert an optionally relative time to an absolute time
 703          * @param int $exptime
 704          * @return int
 705          */
 706         protected function convertExpiry( $exptime ) {
 707                 if ( $exptime != 0 && $exptime < ( 10 * self::TTL_YEAR ) ) {
 708                         return (int)$this->getCurrentTime() + $exptime;
 709                 } else {
 710                         return $exptime;
 711                 }
 712         }
 713
 714         /**
 715          * Convert an optionally absolute expiry time to a relative time. If an
 716          * absolute time is specified which is in the past, use a short expiry time.
 717          *
 718          * @param int $exptime
 719          * @return int
 720          */
 721         protected function convertToRelative( $exptime ) {
 722                 if ( $exptime >= ( 10 * self::TTL_YEAR ) ) {
 723                         $exptime -= (int)$this->getCurrentTime();
 724                         if ( $exptime <= 0 ) {
 725                                 $exptime = 1;
 726                         }
 727                         return $exptime;
 728                 } else {
 729                         return $exptime;
 730                 }
 731         }
 732
 733         /**
 734          * Check if a value is an integer
 735          *
 736          * @param mixed $value
 737          * @return bool
 738          */
 739         protected function isInteger( $value ) {
 740                 return ( is_int( $value ) || ctype_digit( $value ) );
 741         }
 742
 743         /**
 744          * Construct a cache key.
 745          *
 746          * @since 1.27
 747          * @param string $keyspace
 748          * @param array $args
 749          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 750          */
 751         public function makeKeyInternal( $keyspace, $args ) {
 752                 $key = $keyspace;
 753                 foreach ( $args as $arg ) {
 754                         $arg = str_replace( ':', '%3A', $arg );
 755                         $key = $key . ':' . $arg;
 756                 }
 757                 return strtr( $key, ' ', '_' );
 758         }
 759
 760         /**
 761          * Make a global cache key.
 762          *
 763          * @since 1.27
 764          * @param string $class Key class
 765          * @param string $component [optional] Key component (starting with a key collection name)
 766          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 767          */
 768         public function makeGlobalKey( $class, $component = null ) {
 769                 return $this->makeKeyInternal( 'global', func_get_args() );
 770         }
 771
 772         /**
 773          * Make a cache key, scoped to this instance's keyspace.
 774          *
 775          * @since 1.27
 776          * @param string $class Key class
 777          * @param string $component [optional] Key component (starting with a key collection name)
 778          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 779          */
 780         public function makeKey( $class, $component = null ) {
 781                 return $this->makeKeyInternal( $this->keyspace, func_get_args() );
 782         }
 783
 784         /**
 785          * @param int $flag ATTR_* class constant
 786          * @return int QOS_* class constant
 787          * @since 1.28
 788          */
 789         public function getQoS( $flag ) {
 790                 return isset( $this->attrMap[$flag] ) ? $this->attrMap[$flag] : self::QOS_UNKNOWN;
 791         }
 792
 793         /**
 794          * Merge the flag maps of one or more BagOStuff objects into a "lowest common denominator" map
 795          *
 796          * @param BagOStuff[] $bags
 797          * @return int[] Resulting flag map (class ATTR_* constant => class QOS_* constant)
 798          */
 799         protected function mergeFlagMaps( array $bags ) {
 800                 $map = [];
 801                 foreach ( $bags as $bag ) {
 802                         foreach ( $bag->attrMap as $attr => $rank ) {
 803                                 if ( isset( $map[$attr] ) ) {
 804                                         $map[$attr] = min( $map[$attr], $rank );
 805                                 } else {
 806                                         $map[$attr] = $rank;
 807                                 }
 808                         }
 809                 }
 810
 811                 return $map;
 812         }
 813
 814         /**
 815          * @return float UNIX timestamp
 816          * @codeCoverageIgnore
 817          */
 818         protected function getCurrentTime() {
 819                 return $this->wallClockOverride ?: microtime( true );
 820         }
 821
 822         /**
 823          * @param float|null &$time Mock UNIX timestamp for testing
 824          * @codeCoverageIgnore
 825          */
 826         public function setMockTime( &$time ) {
 827                 $this->wallClockOverride =& $time;
 828         }
 829 }