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 callable|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         /** Bitfield constants for get()/getMulti() */
  91         const READ_LATEST = 1; // use latest data for replicated stores
  92         const READ_VERIFIED = 2; // promise that caller can tell when keys are stale
  93         /** Bitfield constants for set()/merge() */
  94         const WRITE_SYNC = 4; // synchronously write to all locations for replicated stores
  95         const WRITE_CACHE_ONLY = 8; // Only change state of the in-memory cache
  96
  97         /**
  98          * $params include:
  99          *   - logger: Psr\Log\LoggerInterface instance
 100          *   - keyspace: Default keyspace for $this->makeKey()
 101          *   - asyncHandler: Callable to use for scheduling tasks after the web request ends.
 102          *      In CLI mode, it should run the task immediately.
 103          *   - reportDupes: Whether to emit warning log messages for all keys that were
 104          *      requested more than once (requires an asyncHandler).
 105          *   - syncTimeout: How long to wait with WRITE_SYNC in seconds.
 106          * @param array $params
 107          */
 108         public function __construct( array $params = [] ) {
 109                 $this->setLogger( $params['logger'] ?? new NullLogger() );
 110
 111                 if ( isset( $params['keyspace'] ) ) {
 112                         $this->keyspace = $params['keyspace'];
 113                 }
 114
 115                 $this->asyncHandler = $params['asyncHandler'] ?? null;
 116
 117                 if ( !empty( $params['reportDupes'] ) && is_callable( $this->asyncHandler ) ) {
 118                         $this->reportDupes = true;
 119                 }
 120
 121                 $this->syncTimeout = $params['syncTimeout'] ?? 3;
 122         }
 123
 124         /**
 125          * @param LoggerInterface $logger
 126          * @return void
 127          */
 128         public function setLogger( LoggerInterface $logger ) {
 129                 $this->logger = $logger;
 130         }
 131
 132         /**
 133          * @param bool $bool
 134          */
 135         public function setDebug( $bool ) {
 136                 $this->debugMode = $bool;
 137         }
 138
 139         /**
 140          * Get an item with the given key, regenerating and setting it if not found
 141          *
 142          * Nothing is stored nor deleted if the callback returns false
 143          *
 144          * @param string $key
 145          * @param int $ttl Time-to-live (seconds)
 146          * @param callable $callback Callback that derives the new value
 147          * @param int $flags Bitfield of BagOStuff::READ_* or BagOStuff::WRITE_* constants [optional]
 148          * @return mixed The cached value if found or the result of $callback otherwise
 149          * @since 1.27
 150          */
 151         final public function getWithSetCallback( $key, $ttl, $callback, $flags = 0 ) {
 152                 $value = $this->get( $key, $flags );
 153
 154                 if ( $value === false ) {
 155                         if ( !is_callable( $callback ) ) {
 156                                 throw new InvalidArgumentException( "Invalid cache miss callback provided." );
 157                         }
 158                         $value = call_user_func( $callback );
 159                         if ( $value !== false ) {
 160                                 $this->set( $key, $value, $ttl, $flags );
 161                         }
 162                 }
 163
 164                 return $value;
 165         }
 166
 167         /**
 168          * Get an item with the given key
 169          *
 170          * If the key includes a deterministic input hash (e.g. the key can only have
 171          * the correct value) or complete staleness checks are handled by the caller
 172          * (e.g. nothing relies on the TTL), then the READ_VERIFIED flag should be set.
 173          * This lets tiered backends know they can safely upgrade a cached value to
 174          * higher tiers using standard TTLs.
 175          *
 176          * @param string $key
 177          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 178          * @return mixed Returns false on failure or if the item does not exist
 179          */
 180         public function get( $key, $flags = 0 ) {
 181                 $this->trackDuplicateKeys( $key );
 182
 183                 return $this->doGet( $key, $flags );
 184         }
 185
 186         /**
 187          * Track the number of times that a given key has been used.
 188          * @param string $key
 189          */
 190         private function trackDuplicateKeys( $key ) {
 191                 if ( !$this->reportDupes ) {
 192                         return;
 193                 }
 194
 195                 if ( !isset( $this->duplicateKeyLookups[$key] ) ) {
 196                         // Track that we have seen this key. This N-1 counting style allows
 197                         // easy filtering with array_filter() later.
 198                         $this->duplicateKeyLookups[$key] = 0;
 199                 } else {
 200                         $this->duplicateKeyLookups[$key] += 1;
 201
 202                         if ( $this->dupeTrackScheduled === false ) {
 203                                 $this->dupeTrackScheduled = true;
 204                                 // Schedule a callback that logs keys processed more than once by get().
 205                                 call_user_func( $this->asyncHandler, function () {
 206                                         $dups = array_filter( $this->duplicateKeyLookups );
 207                                         foreach ( $dups as $key => $count ) {
 208                                                 $this->logger->warning(
 209                                                         'Duplicate get(): "{key}" fetched {count} times',
 210                                                         // Count is N-1 of the actual lookup count
 211                                                         [ 'key' => $key, 'count' => $count + 1, ]
 212                                                 );
 213                                         }
 214                                 } );
 215                         }
 216                 }
 217         }
 218
 219         /**
 220          * @param string $key
 221          * @param int $flags Bitfield of BagOStuff::READ_* constants [optional]
 222          * @param mixed|null &$casToken Token to use for check-and-set comparisons
 223          * @return mixed Returns false on failure or if the item does not exist
 224          */
 225         abstract protected function doGet( $key, $flags = 0, &$casToken = null );
 226
 227         /**
 228          * Set an item
 229          *
 230          * @param string $key
 231          * @param mixed $value
 232          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 233          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 234          * @return bool Success
 235          */
 236         abstract public function set( $key, $value, $exptime = 0, $flags = 0 );
 237
 238         /**
 239          * Delete an item
 240          *
 241          * @param string $key
 242          * @return bool True if the item was deleted or not found, false on failure
 243          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 244          */
 245         abstract public function delete( $key, $flags = 0 );
 246
 247         /**
 248          * Insert an item if it does not already exist
 249          *
 250          * @param string $key
 251          * @param mixed $value
 252          * @param int $exptime
 253          * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
 254          * @return bool Success
 255          */
 256         abstract public function add( $key, $value, $exptime = 0, $flags = 0 );
 257
 258         /**
 259          * Merge changes into the existing cache value (possibly creating a new one)
 260          *
 261          * The callback function returns the new value given the current value
 262          * (which will be false if not present), and takes the arguments:
 263          * (this BagOStuff, cache key, current value, TTL).
 264          * The TTL parameter is reference set to $exptime. It can be overriden in the callback.
 265          * Nothing is stored nor deleted if the callback returns false.
 266          *
 267          * @param string $key
 268          * @param callable $callback Callback method to be executed
 269          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 270          * @param int $attempts The amount of times to attempt a merge in case of failure
 271          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 272          * @return bool Success
 273          * @throws InvalidArgumentException
 274          */
 275         public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 276                 return $this->mergeViaCas( $key, $callback, $exptime, $attempts, $flags );
 277         }
 278
 279         /**
 280          * @see BagOStuff::merge()
 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          */
 289         protected function mergeViaCas( $key, $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 290                 do {
 291                         $casToken = null; // passed by reference
 292                         // Get the old value and CAS token from cache
 293                         $this->clearLastError();
 294                         $currentValue = $this->doGet( $key, self::READ_LATEST, $casToken );
 295                         if ( $this->getLastError() ) {
 296                                 $this->logger->warning(
 297                                         __METHOD__ . ' failed due to I/O error on get() for {key}.',
 298                                         [ 'key' => $key ]
 299                                 );
 300
 301                                 return false; // don't spam retries (retry only on races)
 302                         }
 303
 304                         // Derive the new value from the old value
 305                         $value = call_user_func( $callback, $this, $key, $currentValue, $exptime );
 306                         $hadNoCurrentValue = ( $currentValue === false );
 307                         unset( $currentValue ); // free RAM in case the value is large
 308
 309                         $this->clearLastError();
 310                         if ( $value === false ) {
 311                                 $success = true; // do nothing
 312                         } elseif ( $hadNoCurrentValue ) {
 313                                 // Try to create the key, failing if it gets created in the meantime
 314                                 $success = $this->add( $key, $value, $exptime, $flags );
 315                         } else {
 316                                 // Try to update the key, failing if it gets changed in the meantime
 317                                 $success = $this->cas( $casToken, $key, $value, $exptime, $flags );
 318                         }
 319                         if ( $this->getLastError() ) {
 320                                 $this->logger->warning(
 321                                         __METHOD__ . ' failed due to I/O error for {key}.',
 322                                         [ 'key' => $key ]
 323                                 );
 324
 325                                 return false; // IO error; don't spam retries
 326                         }
 327                 } while ( !$success && --$attempts );
 328
 329                 return $success;
 330         }
 331
 332         /**
 333          * Check and set an item
 334          *
 335          * @param mixed $casToken
 336          * @param string $key
 337          * @param mixed $value
 338          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 339          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 340          * @return bool Success
 341          * @throws Exception
 342          */
 343         protected function cas( $casToken, $key, $value, $exptime = 0, $flags = 0 ) {
 344                 if ( !$this->lock( $key, 0 ) ) {
 345                         return false; // non-blocking
 346                 }
 347
 348                 $curCasToken = null; // passed by reference
 349                 $this->doGet( $key, self::READ_LATEST, $curCasToken );
 350                 if ( $casToken === $curCasToken ) {
 351                         $success = $this->set( $key, $value, $exptime, $flags );
 352                 } else {
 353                         $this->logger->info(
 354                                 __METHOD__ . ' failed due to race condition for {key}.',
 355                                 [ 'key' => $key ]
 356                         );
 357
 358                         $success = false; // mismatched or failed
 359                 }
 360
 361                 $this->unlock( $key );
 362
 363                 return $success;
 364         }
 365
 366         /**
 367          * Change the expiration on a key if it exists
 368          *
 369          * If an expiry in the past is given then the key will immediately be expired
 370          *
 371          * @param string $key
 372          * @param int $expiry TTL or UNIX timestamp
 373          * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
 374          * @return bool Success Returns false on failure or if the item does not exist
 375          * @since 1.28
 376          */
 377         public function changeTTL( $key, $expiry = 0, $flags = 0 ) {
 378                 $found = false;
 379
 380                 $ok = $this->merge(
 381                         $key,
 382                         function ( $cache, $ttl, $currentValue ) use ( &$found ) {
 383                                 $found = ( $currentValue !== false );
 384
 385                                 return $currentValue; // nothing is written if this is false
 386                         },
 387                         $expiry,
 388                         1, // 1 attempt
 389                         $flags
 390                 );
 391
 392                 return ( $ok && $found );
 393         }
 394
 395         /**
 396          * Acquire an advisory lock on a key string
 397          *
 398          * Note that if reentry is enabled, duplicate calls ignore $expiry
 399          *
 400          * @param string $key
 401          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 402          * @param int $expiry Lock expiry [optional]; 1 day maximum
 403          * @param string $rclass Allow reentry if set and the current lock used this value
 404          * @return bool Success
 405          */
 406         public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
 407                 // Avoid deadlocks and allow lock reentry if specified
 408                 if ( isset( $this->locks[$key] ) ) {
 409                         if ( $rclass != '' && $this->locks[$key]['class'] === $rclass ) {
 410                                 ++$this->locks[$key]['depth'];
 411                                 return true;
 412                         } else {
 413                                 return false;
 414                         }
 415                 }
 416
 417                 $fname = __METHOD__;
 418                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 419                 $loop = new WaitConditionLoop(
 420                         function () use ( $key, $expiry, $fname ) {
 421                                 $this->clearLastError();
 422                                 if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
 423                                         return WaitConditionLoop::CONDITION_REACHED; // locked!
 424                                 } elseif ( $this->getLastError() ) {
 425                                         $this->logger->warning(
 426                                                 $fname . ' failed due to I/O error for {key}.',
 427                                                 [ 'key' => $key ]
 428                                         );
 429
 430                                         return WaitConditionLoop::CONDITION_ABORTED; // network partition?
 431                                 }
 432
 433                                 return WaitConditionLoop::CONDITION_CONTINUE;
 434                         },
 435                         $timeout
 436                 );
 437
 438                 $code = $loop->invoke();
 439                 $locked = ( $code === $loop::CONDITION_REACHED );
 440                 if ( $locked ) {
 441                         $this->locks[$key] = [ 'class' => $rclass, 'depth' => 1 ];
 442                 } elseif ( $code === $loop::CONDITION_TIMED_OUT ) {
 443                         $this->logger->warning(
 444                                 "$fname failed due to timeout for {key}.",
 445                                 [ 'key' => $key, 'timeout' => $timeout ]
 446                         );
 447                 }
 448
 449                 return $locked;
 450         }
 451
 452         /**
 453          * Release an advisory lock on a key string
 454          *
 455          * @param string $key
 456          * @return bool Success
 457          */
 458         public function unlock( $key ) {
 459                 if ( isset( $this->locks[$key] ) && --$this->locks[$key]['depth'] <= 0 ) {
 460                         unset( $this->locks[$key] );
 461
 462                         $ok = $this->delete( "{$key}:lock" );
 463                         if ( !$ok ) {
 464                                 $this->logger->warning(
 465                                         __METHOD__ . ' failed to release lock for {key}.',
 466                                         [ 'key' => $key ]
 467                                 );
 468                         }
 469
 470                         return $ok;
 471                 }
 472
 473                 return true;
 474         }
 475
 476         /**
 477          * Get a lightweight exclusive self-unlocking lock
 478          *
 479          * Note that the same lock cannot be acquired twice.
 480          *
 481          * This is useful for task de-duplication or to avoid obtrusive
 482          * (though non-corrupting) DB errors like INSERT key conflicts
 483          * or deadlocks when using LOCK IN SHARE MODE.
 484          *
 485          * @param string $key
 486          * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
 487          * @param int $expiry Lock expiry [optional]; 1 day maximum
 488          * @param string $rclass Allow reentry if set and the current lock used this value
 489          * @return ScopedCallback|null Returns null on failure
 490          * @since 1.26
 491          */
 492         final public function getScopedLock( $key, $timeout = 6, $expiry = 30, $rclass = '' ) {
 493                 $expiry = min( $expiry ?: INF, self::TTL_DAY );
 494
 495                 if ( !$this->lock( $key, $timeout, $expiry, $rclass ) ) {
 496                         return null;
 497                 }
 498
 499                 $lSince = $this->getCurrentTime(); // lock timestamp
 500
 501                 return new ScopedCallback( function () use ( $key, $lSince, $expiry ) {
 502                         $latency = 0.050; // latency skew (err towards keeping lock present)
 503                         $age = ( $this->getCurrentTime() - $lSince + $latency );
 504                         if ( ( $age + $latency ) >= $expiry ) {
 505                                 $this->logger->warning(
 506                                         "Lock for {key} held too long ({age} sec).",
 507                                         [ 'key' => $key, 'age' => $age ]
 508                                 );
 509                                 return; // expired; it's not "safe" to delete the key
 510                         }
 511                         $this->unlock( $key );
 512                 } );
 513         }
 514
 515         /**
 516          * Delete all objects expiring before a certain date.
 517          * @param string $date The reference date in MW format
 518          * @param callable|bool $progressCallback Optional, a function which will be called
 519          *     regularly during long-running operations with the percentage progress
 520          *     as the first parameter.
 521          *
 522          * @return bool Success, false if unimplemented
 523          */
 524         public function deleteObjectsExpiringBefore( $date, $progressCallback = false ) {
 525                 // stub
 526                 return false;
 527         }
 528
 529         /**
 530          * Get an associative array containing the item for each of the keys that have items.
 531          * @param string[] $keys List of keys
 532          * @param int $flags Bitfield; supports READ_LATEST [optional]
 533          * @return array
 534          */
 535         public function getMulti( array $keys, $flags = 0 ) {
 536                 $res = [];
 537                 foreach ( $keys as $key ) {
 538                         $val = $this->get( $key, $flags );
 539                         if ( $val !== false ) {
 540                                 $res[$key] = $val;
 541                         }
 542                 }
 543
 544                 return $res;
 545         }
 546
 547         /**
 548          * Batch insertion/replace
 549          * @param mixed[] $data Map of (key => value)
 550          * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 551          * @param int $flags Bitfield of BagOStuff::WRITE_* constants (since 1.33)
 552          * @return bool Success
 553          * @since 1.24
 554          */
 555         public function setMulti( array $data, $exptime = 0, $flags = 0 ) {
 556                 $res = true;
 557                 foreach ( $data as $key => $value ) {
 558                         if ( !$this->set( $key, $value, $exptime, $flags ) ) {
 559                                 $res = false;
 560                         }
 561                 }
 562
 563                 return $res;
 564         }
 565
 566         /**
 567          * Batch deletion
 568          * @param string[] $keys List of keys
 569          * @param int $flags Bitfield of BagOStuff::WRITE_* constants
 570          * @return bool Success
 571          * @since 1.33
 572          */
 573         public function deleteMulti( array $keys, $flags = 0 ) {
 574                 $res = true;
 575                 foreach ( $keys as $key ) {
 576                         $res = $this->delete( $key, $flags ) && $res;
 577                 }
 578
 579                 return $res;
 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) [optional]
 586          * @return int|bool New value or false on failure
 587          */
 588         abstract public function incr( $key, $value = 1 );
 589
 590         /**
 591          * Decrease stored value of $key by $value while preserving its TTL
 592          * @param string $key
 593          * @param int $value Value to subtract from $key (default: 1) [optional]
 594          * @return int|bool New value or false on failure
 595          */
 596         public function decr( $key, $value = 1 ) {
 597                 return $this->incr( $key, - $value );
 598         }
 599
 600         /**
 601          * Increase stored value of $key by $value while preserving its TTL
 602          *
 603          * This will create the key with value $init and TTL $ttl instead if not present
 604          *
 605          * @param string $key
 606          * @param int $ttl
 607          * @param int $value
 608          * @param int $init
 609          * @return int|bool New value or false on failure
 610          * @since 1.24
 611          */
 612         public function incrWithInit( $key, $ttl, $value = 1, $init = 1 ) {
 613                 $this->clearLastError();
 614                 $newValue = $this->incr( $key, $value );
 615                 if ( $newValue === false && !$this->getLastError() ) {
 616                         // No key set; initialize
 617                         $newValue = $this->add( $key, (int)$init, $ttl ) ? $init : false;
 618                         if ( $newValue === false && !$this->getLastError() ) {
 619                                 // Raced out initializing; increment
 620                                 $newValue = $this->incr( $key, $value );
 621                         }
 622                 }
 623
 624                 return $newValue;
 625         }
 626
 627         /**
 628          * Get the "last error" registered; clearLastError() should be called manually
 629          * @return int ERR_* constant for the "last error" registry
 630          * @since 1.23
 631          */
 632         public function getLastError() {
 633                 return $this->lastError;
 634         }
 635
 636         /**
 637          * Clear the "last error" registry
 638          * @since 1.23
 639          */
 640         public function clearLastError() {
 641                 $this->lastError = self::ERR_NONE;
 642         }
 643
 644         /**
 645          * Set the "last error" registry
 646          * @param int $err ERR_* constant
 647          * @since 1.23
 648          */
 649         protected function setLastError( $err ) {
 650                 $this->lastError = $err;
 651         }
 652
 653         /**
 654          * Let a callback be run to avoid wasting time on special blocking calls
 655          *
 656          * The callbacks may or may not be called ever, in any particular order.
 657          * They are likely to be invoked when something WRITE_SYNC is used used.
 658          * They should follow a caching pattern as shown below, so that any code
 659          * using the work will get it's result no matter what happens.
 660          * @code
 661          *     $result = null;
 662          *     $workCallback = function () use ( &$result ) {
 663          *         if ( !$result ) {
 664          *             $result = ....
 665          *         }
 666          *         return $result;
 667          *     }
 668          * @endcode
 669          *
 670          * @param callable $workCallback
 671          * @since 1.28
 672          */
 673         public function addBusyCallback( callable $workCallback ) {
 674                 $this->busyCallbacks[] = $workCallback;
 675         }
 676
 677         /**
 678          * @param string $text
 679          */
 680         protected function debug( $text ) {
 681                 if ( $this->debugMode ) {
 682                         $this->logger->debug( "{class} debug: $text", [
 683                                 'class' => static::class,
 684                         ] );
 685                 }
 686         }
 687
 688         /**
 689          * @param int $exptime
 690          * @return bool
 691          */
 692         protected function expiryIsRelative( $exptime ) {
 693                 return ( $exptime != 0 && $exptime < ( 10 * self::TTL_YEAR ) );
 694         }
 695
 696         /**
 697          * Convert an optionally relative time to an absolute time
 698          * @param int $exptime
 699          * @return int
 700          */
 701         protected function convertToExpiry( $exptime ) {
 702                 if ( $this->expiryIsRelative( $exptime ) ) {
 703                         return (int)$this->getCurrentTime() + $exptime;
 704                 } else {
 705                         return $exptime;
 706                 }
 707         }
 708
 709         /**
 710          * Convert an optionally absolute expiry time to a relative time. If an
 711          * absolute time is specified which is in the past, use a short expiry time.
 712          *
 713          * @param int $exptime
 714          * @return int
 715          */
 716         protected function convertToRelative( $exptime ) {
 717                 if ( $exptime >= ( 10 * self::TTL_YEAR ) ) {
 718                         $exptime -= (int)$this->getCurrentTime();
 719                         if ( $exptime <= 0 ) {
 720                                 $exptime = 1;
 721                         }
 722                         return $exptime;
 723                 } else {
 724                         return $exptime;
 725                 }
 726         }
 727
 728         /**
 729          * Check if a value is an integer
 730          *
 731          * @param mixed $value
 732          * @return bool
 733          */
 734         protected function isInteger( $value ) {
 735                 return ( is_int( $value ) || ctype_digit( $value ) );
 736         }
 737
 738         /**
 739          * Construct a cache key.
 740          *
 741          * @since 1.27
 742          * @param string $keyspace
 743          * @param array $args
 744          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 745          */
 746         public function makeKeyInternal( $keyspace, $args ) {
 747                 $key = $keyspace;
 748                 foreach ( $args as $arg ) {
 749                         $key .= ':' . str_replace( ':', '%3A', $arg );
 750                 }
 751                 return strtr( $key, ' ', '_' );
 752         }
 753
 754         /**
 755          * Make a global cache key.
 756          *
 757          * @since 1.27
 758          * @param string $class Key class
 759          * @param string|null $component [optional] Key component (starting with a key collection name)
 760          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 761          */
 762         public function makeGlobalKey( $class, $component = null ) {
 763                 return $this->makeKeyInternal( 'global', func_get_args() );
 764         }
 765
 766         /**
 767          * Make a cache key, scoped to this instance's keyspace.
 768          *
 769          * @since 1.27
 770          * @param string $class Key class
 771          * @param string|null $component [optional] Key component (starting with a key collection name)
 772          * @return string Colon-delimited list of $keyspace followed by escaped components of $args
 773          */
 774         public function makeKey( $class, $component = null ) {
 775                 return $this->makeKeyInternal( $this->keyspace, func_get_args() );
 776         }
 777
 778         /**
 779          * @param int $flag ATTR_* class constant
 780          * @return int QOS_* class constant
 781          * @since 1.28
 782          */
 783         public function getQoS( $flag ) {
 784                 return $this->attrMap[$flag] ?? self::QOS_UNKNOWN;
 785         }
 786
 787         /**
 788          * Merge the flag maps of one or more BagOStuff objects into a "lowest common denominator" map
 789          *
 790          * @param BagOStuff[] $bags
 791          * @return int[] Resulting flag map (class ATTR_* constant => class QOS_* constant)
 792          */
 793         protected function mergeFlagMaps( array $bags ) {
 794                 $map = [];
 795                 foreach ( $bags as $bag ) {
 796                         foreach ( $bag->attrMap as $attr => $rank ) {
 797                                 if ( isset( $map[$attr] ) ) {
 798                                         $map[$attr] = min( $map[$attr], $rank );
 799                                 } else {
 800                                         $map[$attr] = $rank;
 801                                 }
 802                         }
 803                 }
 804
 805                 return $map;
 806         }
 807
 808         /**
 809          * @return float UNIX timestamp
 810          * @codeCoverageIgnore
 811          */
 812         protected function getCurrentTime() {
 813                 return $this->wallClockOverride ?: microtime( true );
 814         }
 815
 816         /**
 817          * @param float|null &$time Mock UNIX timestamp for testing
 818          * @codeCoverageIgnore
 819          */
 820         public function setMockTime( &$time ) {
 821                 $this->wallClockOverride =& $time;
 822         }
 823 }