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 = $params['asyncHandler'] ?? null;
 126
 127                 if ( !empty( $params['reportDupes'] ) && is_callable( $this->asyncHandler ) ) {
 128                         $this->reportDupes = true;
 129                 }
 130
 131                 $this->syncTimeout = $params['syncTimeout'] ?? 3;
 132         }
 133
 134         /**
 135          * @param LoggerInterface $logger
 136          * @return null
 137          */
 138         public function setLogger( LoggerInterface $logger ) {
 139                 $this->logger = $logger;
 140         }
 141
 142         /**
 143          * @param bool $bool
 144          */
 145         public function setDebug( $bool ) {
 146                 $this->debugMode = $bool;
 147         }
 148
 149         /**
 150          * Get an item with the given key, regenerating and setting it if not found
 151          *
 152          * If the callback returns false, then nothing is stored.
 153          *
 154          * @param string $key
 155          * @param int $ttl Time-to-live (seconds)
 156          * @param callable $callback Callback that derives the new value
 157          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 158          * @return mixed The cached value if found or the result of $callback otherwise
 159          * @since 1.27
 160          */
 161         final public function getWithSetCallback( $key, $ttl, $callback, $flags = 0 ) {
 162                 $value = $this->get( $key, $flags );
 163
 164                 if ( $value === false ) {
 165                         if ( !is_callable( $callback ) ) {
 166                                 throw new InvalidArgumentException( "Invalid cache miss callback provided." );
 167                         }
 168                         $value = call_user_func( $callback );
 169                         if ( $value !== false ) {
 170                                 $this->set( $key, $value, $ttl );
 171                         }
 172                 }
 173
 174                 return $value;
 175         }
 176
 177         /**
 178          * Get an item with the given key
 179          *
 180          * If the key includes a deterministic input hash (e.g. the key can only have
 181          * the correct value) or complete staleness checks are handled by the caller
 182          * (e.g. nothing relies on the TTL), then the READ_VERIFIED flag should be set.
 183          * This lets tiered backends know they can safely upgrade a cached value to
 184          * higher tiers using standard TTLs.
 185          *
 186          * @param string $key
 187          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 188          * @param int|null $oldFlags [unused]
 189          * @return mixed Returns false on failure and if the item does not exist
 190          */
 191         public function get( $key, $flags = 0, $oldFlags = null ) {
 192                 // B/C for ( $key, &$casToken = null, $flags = 0 )
 193                 $flags = is_int( $oldFlags ) ? $oldFlags : $flags;
 194
 195                 $this->trackDuplicateKeys( $key );
 196
 197                 return $this->doGet( $key, $flags );
 198         }
 199
 200         /**
 201          * Track the number of times that a given key has been used.
 202          * @param string $key
 203          */
 204         private function trackDuplicateKeys( $key ) {
 205                 if ( !$this->reportDupes ) {
 206                         return;
 207                 }
 208
 209                 if ( !isset( $this->duplicateKeyLookups[$key] ) ) {
 210                         // Track that we have seen this key. This N-1 counting style allows
 211                         // easy filtering with array_filter() later.
 212                         $this->duplicateKeyLookups[$key] = 0;
 213                 } else {
 214                         $this->duplicateKeyLookups[$key] += 1;
 215
 216                         if ( $this->dupeTrackScheduled === false ) {
 217                                 $this->dupeTrackScheduled = true;
 218                                 // Schedule a callback that logs keys processed more than once by get().
 219                                 call_user_func( $this->asyncHandler, function () {
 220                                         $dups = array_filter( $this->duplicateKeyLookups );
 221                                         foreach ( $dups as $key => $count ) {
 222                                                 $this->logger->warning(
 223                                                         'Duplicate get(): "{key}" fetched {count} times',
 224                                                         // Count is N-1 of the actual lookup count
 225                                                         [ 'key' => $key, 'count' => $count + 1, ]
 226                                                 );
 227                                         }
 228                                 } );
 229                         }
 230                 }
 231         }
 232
 233         /**
 234          * @param string $key
 235          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 236          * @return mixed Returns false on failure and if the item does not exist
 237          */
 238         abstract protected function doGet( $key, $flags = 0 );
 239
 240         /**
 241          * @note: This method is only needed if merge() uses mergeViaCas()
 242          *
 243          * @param string $key
 244          * @param mixed &$casToken
 245          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 246          * @return mixed Returns false on failure and if the item does not exist
 247          * @throws Exception
 248          */
 249         protected function getWithToken( $key, &$casToken, $flags = 0 ) {
 250                 throw new Exception( __METHOD__ . ' not implemented.' );
 251         }
 252
 253         /**
 254          * Set an item
 255          *
 256          * @param string $key
 257          * @param mixed $value
 258          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 259          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 260          * @return bool Success
 261          */
 262         abstract public function set( $key, $value, $exptime = 0, $flags = 0 );
 263
 264         /**
 265          * Delete an item
 266          *
 267          * @param string $key
 268          * @return bool True if the item was deleted or not found, false on failure
 269          */
 270         abstract public function delete( $key );
 271
 272         /**
 273          * Merge changes into the existing cache value (possibly creating a new one)
 274          *
 275          * The callback function returns the new value given the current value
 276          * (which will be false if not present), and takes the arguments:
 277          * (this BagOStuff, cache key, current value, TTL).
 278          * The TTL parameter is reference set to $exptime. It can be overriden in the callback.
 279          *
 280          * @param string $key
 281          * @param callable $callback Callback method to be executed
 282          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 283          * @param int $attempts The amount of times to attempt a merge in case of failure
 284          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 285          * @return bool Success
 286          * @throws InvalidArgumentException
 287          */
 288         public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 289                 return $this->mergeViaLock( $key, $callback, $exptime, $attempts, $flags );
 290         }
 291
 292         /**
 293          * @see BagOStuff::merge()
 294          *
 295          * @param string $key
 296          * @param callable $callback Callback method to be executed
 297          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 298          * @param int $attempts The amount of times to attempt a merge in case of failure
 299          * @return bool Success
 300          */
 301         protected function mergeViaCas( $key, $callback, $exptime = 0, $attempts = 10 ) {
 302                 do {
 303                         $this->clearLastError();
 304                         $reportDupes = $this->reportDupes;
 305                         $this->reportDupes = false;
 306                         $casToken = null; // passed by reference
 307                         $currentValue = $this->getWithToken( $key, $casToken, self::READ_LATEST );
 308                         $this->reportDupes = $reportDupes;
 309
 310                         if ( $this->getLastError() ) {
 311                                 return false; // don't spam retries (retry only on races)
 312                         }
 313
 314                         // Derive the new value from the old value
 315                         $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
 316
 317                         $this->clearLastError();
 318                         if ( $value === false ) {
 319                                 $success = true; // do nothing
 320                         } elseif ( $currentValue === false ) {
 321                                 // Try to create the key, failing if it gets created in the meantime
 322                                 $success = $this->add( $key, $value, $exptime );
 323                         } else {
 324                                 // Try to update the key, failing if it gets changed in the meantime
 325                                 $success = $this->cas( $casToken, $key, $value, $exptime );
 326                         }
 327                         if ( $this->getLastError() ) {
 328                                 return false; // IO error; don't spam retries
 329                         }
 330                 } while ( !$success && --$attempts );
 331
 332                 return $success;
 333         }
 334
 335         /**
 336          * Check and set an item
 337          *
 338          * @param mixed $casToken
 339          * @param string $key
 340          * @param mixed $value
 341          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 342          * @return bool Success
 343          * @throws Exception
 344          */
 345         protected function cas( $casToken, $key, $value, $exptime = 0 ) {
 346                 if ( !$this->lock( $key, 0 ) ) {
 347                         return false; // non-blocking
 348                 }
 349
 350                 $curCasToken = null; // passed by reference
 351                 $this->getWithToken( $key, $curCasToken, self::READ_LATEST );
 352                 if ( $casToken === $curCasToken ) {
 353                         $success = $this->set( $key, $value, $exptime );
 354                 } else {
 355                         $success = false; // mismatched or failed
 356                 }
 357
 358                 $this->unlock( $key );
 359
 360                 return $success;
 361         }
 362
 363         /**
 364          * @see BagOStuff::merge()
 365          *
 366          * @param string $key
 367          * @param callable $callback Callback method to be executed
 368          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 369          * @param int $attempts The amount of times to attempt a merge in case of failure
 370          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 371          * @return bool Success
 372          */
 373         protected function mergeViaLock( $key, $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 374                 if ( $attempts <= 1 ) {
 375                         $timeout = 0; // clearly intended to be "non-blocking"
 376                 } else {
 377                         $timeout = 3;
 378                 }
 379
 380                 if ( !$this->lock( $key, $timeout ) ) {
 381                         return false;
 382                 }
 383
 384                 $this->clearLastError();
 385                 $reportDupes = $this->reportDupes;
 386                 $this->reportDupes = false;
 387                 $currentValue = $this->get( $key, self::READ_LATEST );
 388                 $this->reportDupes = $reportDupes;
 389
 390                 if ( $this->getLastError() ) {
 391                         $success = false;
 392                 } else {
 393                         // Derive the new value from the old value
 394                         $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
 395                         if ( $value === false ) {
 396                                 $success = true; // do nothing
 397                         } else {
 398                                 $success = $this->set( $key, $value, $exptime, $flags ); // set the new value
 399                         }
 400                 }
 401
 402                 if ( !$this->unlock( $key ) ) {
 403                         // this should never happen
 404                         trigger_error( "Could not release lock for key '$key'." );
 405                 }
 406
 407                 return $success;
 408         }
 409
 410         /**
 411          * Reset the TTL on a key if it exists
 412          *
 413          * @param string $key
 414          * @param int $expiry
 415          * @return bool Success Returns false if there is no key
 416          * @since 1.28
 417          */
 418         public function changeTTL( $key, $expiry = 0 ) {
 419                 $value = $this->get( $key );
 420
 421                 return ( $value === false ) ? false : $this->set( $key, $value, $expiry );
 422         }
 423
 424         /**
 425          * Acquire an advisory lock on a key string
 426          *
 427          * Note that if reentry is enabled, duplicate calls ignore $expiry
 428          *
 429          * @param string $key
 430          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 431          * @param int $expiry Lock expiry [optional]; 1 day maximum
 432          * @param string $rclass Allow reentry if set and the current lock used this value
 433          * @return bool Success
 434          */
 435         public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
 436                 // Avoid deadlocks and allow lock reentry if specified
 437                 if ( isset( $this->locks[$key] ) ) {
 438                         if ( $rclass != '' && $this->locks[$key]['class'] === $rclass ) {
 439                                 ++$this->locks[$key]['depth'];
 440                                 return true;
 441                         } else {
 442                                 return false;
 443                         }
 444                 }
 445
 446                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 447                 $loop = new WaitConditionLoop(
 448                         function () use ( $key, $timeout, $expiry ) {
 449                                 $this->clearLastError();
 450                                 if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
 451                                         return true; // locked!
 452                                 } elseif ( $this->getLastError() ) {
 453                                         return WaitConditionLoop::CONDITION_ABORTED; // network partition?
 454                                 }
 455
 456                                 return WaitConditionLoop::CONDITION_CONTINUE;
 457                         },
 458                         $timeout
 459                 );
 460
 461                 $locked = ( $loop->invoke() === $loop::CONDITION_REACHED );
 462                 if ( $locked ) {
 463                         $this->locks[$key] = [ 'class' => $rclass, 'depth' => 1 ];
 464                 }
 465
 466                 return $locked;
 467         }
 468
 469         /**
 470          * Release an advisory lock on a key string
 471          *
 472          * @param string $key
 473          * @return bool Success
 474          */
 475         public function unlock( $key ) {
 476                 if ( isset( $this->locks[$key] ) && --$this->locks[$key]['depth'] <= 0 ) {
 477                         unset( $this->locks[$key] );
 478
 479                         return $this->delete( "{$key}:lock" );
 480                 }
 481
 482                 return true;
 483         }
 484
 485         /**
 486          * Get a lightweight exclusive self-unlocking lock
 487          *
 488          * Note that the same lock cannot be acquired twice.
 489          *
 490          * This is useful for task de-duplication or to avoid obtrusive
 491          * (though non-corrupting) DB errors like INSERT key conflicts
 492          * or deadlocks when using LOCK IN SHARE MODE.
 493          *
 494          * @param string $key
 495          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 496          * @param int $expiry Lock expiry [optional]; 1 day maximum
 497          * @param string $rclass Allow reentry if set and the current lock used this value
 498          * @return ScopedCallback|null Returns null on failure
 499          * @since 1.26
 500          */
 501         final public function getScopedLock( $key, $timeout = 6, $expiry = 30, $rclass = '' ) {
 502                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 503
 504                 if ( !$this->lock( $key, $timeout, $expiry, $rclass ) ) {
 505                         return null;
 506                 }
 507
 508                 $lSince = $this->getCurrentTime(); // lock timestamp
 509
 510                 return new ScopedCallback( function () use ( $key, $lSince, $expiry ) {
 511                         $latency = 0.050; // latency skew (err towards keeping lock present)
 512                         $age = ( $this->getCurrentTime() - $lSince + $latency );
 513                         if ( ( $age + $latency ) >= $expiry ) {
 514                                 $this->logger->warning( "Lock for $key held too long ($age sec)." );
 515                                 return; // expired; it's not "safe" to delete the key
 516                         }
 517                         $this->unlock( $key );
 518                 } );
 519         }
 520
 521         /**
 522          * Delete all objects expiring before a certain date.
 523          * @param string $date The reference date in MW format
 524          * @param callable|bool $progressCallback Optional, a function which will be called
 525          *     regularly during long-running operations with the percentage progress
 526          *     as the first parameter.
 527          *
 528          * @return bool Success, false if unimplemented
 529          */
 530         public function deleteObjectsExpiringBefore( $date, $progressCallback = false ) {
 531                 // stub
 532                 return false;
 533         }
 534
 535         /**
 536          * Get an associative array containing the item for each of the keys that have items.
 537          * @param array $keys List of strings
 538          * @param int $flags Bitfield; supports READ_LATEST [optional]
 539          * @return array
 540          */
 541         public function getMulti( array $keys, $flags = 0 ) {
 542                 $res = [];
 543                 foreach ( $keys as $key ) {
 544                         $val = $this->get( $key );
 545                         if ( $val !== false ) {
 546                                 $res[$key] = $val;
 547                         }
 548                 }
 549                 return $res;
 550         }
 551
 552         /**
 553          * Batch insertion
 554          * @param array $data $key => $value assoc array
 555          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 556          * @return bool Success
 557          * @since 1.24
 558          */
 559         public function setMulti( array $data, $exptime = 0 ) {
 560                 $res = true;
 561                 foreach ( $data as $key => $value ) {
 562                         if ( !$this->set( $key, $value, $exptime ) ) {
 563                                 $res = false;
 564                         }
 565                 }
 566                 return $res;
 567         }
 568
 569         /**
 570          * @param string $key
 571          * @param mixed $value
 572          * @param int $exptime
 573          * @return bool Success
 574          */
 575         public function add( $key, $value, $exptime = 0 ) {
 576                 if ( $this->get( $key ) === false ) {
 577                         return $this->set( $key, $value, $exptime );
 578                 }
 579                 return false; // key already set
 580         }
 581
 582         /**
 583          * Increase stored value of $key by $value while preserving its TTL
 584          * @param string $key Key to increase
 585          * @param int $value Value to add to $key (Default 1)
 586          * @return int|bool New value or false on failure
 587          */
 588         public function incr( $key, $value = 1 ) {
 589                 if ( !$this->lock( $key ) ) {
 590                         return false;
 591                 }
 592                 $n = $this->get( $key );
 593                 if ( $this->isInteger( $n ) ) { // key exists?
 594                         $n += intval( $value );
 595                         $this->set( $key, max( 0, $n ) ); // exptime?
 596                 } else {
 597                         $n = false;
 598                 }
 599                 $this->unlock( $key );
 600
 601                 return $n;
 602         }
 603
 604         /**
 605          * Decrease stored value of $key by $value while preserving its TTL
 606          * @param string $key
 607          * @param int $value
 608          * @return int|bool New value or false on failure
 609          */
 610         public function decr( $key, $value = 1 ) {
 611                 return $this->incr( $key, - $value );
 612         }
 613
 614         /**
 615          * Increase stored value of $key by $value while preserving its TTL
 616          *
 617          * This will create the key with value $init and TTL $ttl instead if not present
 618          *
 619          * @param string $key
 620          * @param int $ttl
 621          * @param int $value
 622          * @param int $init
 623          * @return int|bool New value or false on failure
 624          * @since 1.24
 625          */
 626         public function incrWithInit( $key, $ttl, $value = 1, $init = 1 ) {
 627                 $newValue = $this->incr( $key, $value );
 628                 if ( $newValue === false ) {
 629                         // No key set; initialize
 630                         $newValue = $this->add( $key, (int)$init, $ttl ) ? $init : false;
 631                 }
 632                 if ( $newValue === false ) {
 633                         // Raced out initializing; increment
 634                         $newValue = $this->incr( $key, $value );
 635                 }
 636
 637                 return $newValue;
 638         }
 639
 640         /**
 641          * Get the "last error" registered; clearLastError() should be called manually
 642          * @return int ERR_* constant for the "last error" registry
 643          * @since 1.23
 644          */
 645         public function getLastError() {
 646                 return $this->lastError;
 647         }
 648
 649         /**
 650          * Clear the "last error" registry
 651          * @since 1.23
 652          */
 653         public function clearLastError() {
 654                 $this->lastError = self::ERR_NONE;
 655         }
 656
 657         /**
 658          * Set the "last error" registry
 659          * @param int $err ERR_* constant
 660          * @since 1.23
 661          */
 662         protected function setLastError( $err ) {
 663                 $this->lastError = $err;
 664         }
 665
 666         /**
 667          * Let a callback be run to avoid wasting time on special blocking calls
 668          *
 669          * The callbacks may or may not be called ever, in any particular order.
 670          * They are likely to be invoked when something WRITE_SYNC is used used.
 671          * They should follow a caching pattern as shown below, so that any code
 672          * using the word will get it's result no matter what happens.
 673          * @code
 674          *     $result = null;
 675          *     $workCallback = function () use ( &$result ) {
 676          *         if ( !$result ) {
 677          *             $result = ....
 678          *         }
 679          *         return $result;
 680          *     }
 681          * @endcode
 682          *
 683          * @param callable $workCallback
 684          * @since 1.28
 685          */
 686         public function addBusyCallback( callable $workCallback ) {
 687                 $this->busyCallbacks[] = $workCallback;
 688         }
 689
 690         /**
 691          * Modify a cache update operation array for EventRelayer::notify()
 692          *
 693          * This is used for relayed writes, e.g. for broadcasting a change
 694          * to multiple data-centers. If the array contains a 'val' field
 695          * then the command involves setting a key to that value. Note that
 696          * for simplicity, 'val' is always a simple scalar value. This method
 697          * is used to possibly serialize the value and add any cache-specific
 698          * key/values needed for the relayer daemon (e.g. memcached flags).
 699          *
 700          * @param array $event
 701          * @return array
 702          * @since 1.26
 703          */
 704         public function modifySimpleRelayEvent( array $event ) {
 705                 return $event;
 706         }
 707
 708         /**
 709          * @param string $text
 710          */
 711         protected function debug( $text ) {
 712                 if ( $this->debugMode ) {
 713                         $this->logger->debug( "{class} debug: $text", [
 714                                 'class' => static::class,
 715                         ] );
 716                 }
 717         }
 718
 719         /**
 720          * Convert an optionally relative time to an absolute time
 721          * @param int $exptime
 722          * @return int
 723          */
 724         protected function convertExpiry( $exptime ) {
 725                 if ( $exptime != 0 && $exptime < ( 10 * self::TTL_YEAR ) ) {
 726                         return (int)$this->getCurrentTime() + $exptime;
 727                 } else {
 728                         return $exptime;
 729                 }
 730         }
 731
 732         /**
 733          * Convert an optionally absolute expiry time to a relative time. If an
 734          * absolute time is specified which is in the past, use a short expiry time.
 735          *
 736          * @param int $exptime
 737          * @return int
 738          */
 739         protected function convertToRelative( $exptime ) {
 740                 if ( $exptime >= ( 10 * self::TTL_YEAR ) ) {
 741                         $exptime -= (int)$this->getCurrentTime();
 742                         if ( $exptime <= 0 ) {
 743                                 $exptime = 1;
 744                         }
 745                         return $exptime;
 746                 } else {
 747                         return $exptime;
 748                 }
 749         }
 750
 751         /**
 752          * Check if a value is an integer
 753          *
 754          * @param mixed $value
 755          * @return bool
 756          */
 757         protected function isInteger( $value ) {
 758                 return ( is_int( $value ) || ctype_digit( $value ) );
 759         }
 760
 761         /**
 762          * Construct a cache key.
 763          *
 764          * @since 1.27
 765          * @param string $keyspace
 766          * @param array $args
 767          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 768          */
 769         public function makeKeyInternal( $keyspace, $args ) {
 770                 $key = $keyspace;
 771                 foreach ( $args as $arg ) {
 772                         $arg = str_replace( ':', '%3A', $arg );
 773                         $key = $key . ':' . $arg;
 774                 }
 775                 return strtr( $key, ' ', '_' );
 776         }
 777
 778         /**
 779          * Make a global cache key.
 780          *
 781          * @since 1.27
 782          * @param string $class Key class
 783          * @param string|null $component [optional] Key component (starting with a key collection name)
 784          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 785          */
 786         public function makeGlobalKey( $class, $component = null ) {
 787                 return $this->makeKeyInternal( 'global', func_get_args() );
 788         }
 789
 790         /**
 791          * Make a cache key, scoped to this instance's keyspace.
 792          *
 793          * @since 1.27
 794          * @param string $class Key class
 795          * @param string|null $component [optional] Key component (starting with a key collection name)
 796          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 797          */
 798         public function makeKey( $class, $component = null ) {
 799                 return $this->makeKeyInternal( $this->keyspace, func_get_args() );
 800         }
 801
 802         /**
 803          * @param int $flag ATTR_* class constant
 804          * @return int QOS_* class constant
 805          * @since 1.28
 806          */
 807         public function getQoS( $flag ) {
 808                 return $this->attrMap[$flag] ?? self::QOS_UNKNOWN;
 809         }
 810
 811         /**
 812          * Merge the flag maps of one or more BagOStuff objects into a "lowest common denominator" map
 813          *
 814          * @param BagOStuff[] $bags
 815          * @return int[] Resulting flag map (class ATTR_* constant => class QOS_* constant)
 816          */
 817         protected function mergeFlagMaps( array $bags ) {
 818                 $map = [];
 819                 foreach ( $bags as $bag ) {
 820                         foreach ( $bag->attrMap as $attr => $rank ) {
 821                                 if ( isset( $map[$attr] ) ) {
 822                                         $map[$attr] = min( $map[$attr], $rank );
 823                                 } else {
 824                                         $map[$attr] = $rank;
 825                                 }
 826                         }
 827                 }
 828
 829                 return $map;
 830         }
 831
 832         /**
 833          * @return float UNIX timestamp
 834          * @codeCoverageIgnore
 835          */
 836         protected function getCurrentTime() {
 837                 return $this->wallClockOverride ?: microtime( true );
 838         }
 839
 840         /**
 841          * @param float|null &$time Mock UNIX timestamp for testing
 842          * @codeCoverageIgnore
 843          */
 844         public function setMockTime( &$time ) {
 845                 $this->wallClockOverride =& $time;
 846         }
 847 }