8 files changed, 1045 insertions, 134 deletions

diff --git a/includes/libs/objectcache/APCBagOStuff.php b/includes/libs/objectcache/APCBagOStuff.php
index eaf11557..0dbbaba9 100644
--- a/includes/libs/objectcache/APCBagOStuff.php
+++ b/includes/libs/objectcache/APCBagOStuff.php
@@ -27,43 +27,38 @@
  * @ingroup Cache
  */
 class APCBagOStuff extends BagOStuff {
-	public function get( $key, &$casToken = null ) {
-		$val = apc_fetch( $key );
+	/**
+	 * @var string String to append to each APC key. This may be changed
+	 *  whenever the handling of values is changed, to prevent existing code
+	 *  from encountering older values which it cannot handle.
+	 **/
+	const KEY_SUFFIX = ':1';
 
-		$casToken = $val;
+	public function get( $key, &$casToken = null, $flags = 0 ) {
+		$val = apc_fetch( $key . self::KEY_SUFFIX );
 
-		if ( is_string( $val ) ) {
-			if ( $this->isInteger( $val ) ) {
-				$val = intval( $val );
-			} else {
-				$val = unserialize( $val );
-			}
-		}
+		$casToken = $val;
 
 		return $val;
 	}
 
 	public function set( $key, $value, $exptime = 0 ) {
-		if ( !$this->isInteger( $value ) ) {
-			$value = serialize( $value );
-		}
-
-		apc_store( $key, $value, $exptime );
+		apc_store( $key . self::KEY_SUFFIX, $value, $exptime );
 
 		return true;
 	}
 
 	public function delete( $key ) {
-		apc_delete( $key );
+		apc_delete( $key . self::KEY_SUFFIX );
 
 		return true;
 	}
 
 	public function incr( $key, $value = 1 ) {
-		return apc_inc( $key, $value );
+		return apc_inc( $key . self::KEY_SUFFIX, $value );
 	}
 
 	public function decr( $key, $value = 1 ) {
-		return apc_dec( $key, $value );
+		return apc_dec( $key . self::KEY_SUFFIX, $value );
 	}
 }
diff --git a/includes/libs/objectcache/BagOStuff.php b/includes/libs/objectcache/BagOStuff.php
index 0b791e5a..ddbe8eaa 100644
--- a/includes/libs/objectcache/BagOStuff.php
+++ b/includes/libs/objectcache/BagOStuff.php
@@ -1,7 +1,5 @@
 <?php
 /**
- * Classes to cache objects in PHP accelerators, SQL database or DBA files
- *
  * Copyright © 2003-2004 Brion Vibber <brion@pobox.com>
  * https://www.mediawiki.org/
  *
@@ -37,29 +35,34 @@ use Psr\Log\NullLogger;
  * the PHP memcached client.
  *
  * backends for local hash array and SQL table included:
- * <code>
+ * @code
  *   $bag = new HashBagOStuff();
  *   $bag = new SqlBagOStuff(); # connect to db first
- * </code>
+ * @endcode
  *
  * @ingroup Cache
  */
 abstract class BagOStuff implements LoggerAwareInterface {
-	private $debugMode = false;
-
+	/** @var array[] Lock tracking */
+	protected $locks = array();
+	/** @var integer */
 	protected $lastError = self::ERR_NONE;
 
-	/**
-	 * @var LoggerInterface
-	 */
+	/** @var LoggerInterface */
 	protected $logger;
 
+	/** @var bool */
+	private $debugMode = false;
+
 	/** Possible values for getLastError() */
 	const ERR_NONE = 0; // no error
 	const ERR_NO_RESPONSE = 1; // no response
 	const ERR_UNREACHABLE = 2; // can't connect
 	const ERR_UNEXPECTED = 3; // response gave some error
 
+	/** Bitfield constants for get()/getMulti() */
+	const READ_LATEST = 1; // use latest data for replicated stores
+
 	public function __construct( array $params = array() ) {
 		if ( isset( $params['logger'] ) ) {
 			$this->setLogger( $params['logger'] );
@@ -87,9 +90,10 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	 * Get an item with the given key. Returns false if it does not exist.
 	 * @param string $key
 	 * @param mixed $casToken [optional]
+	 * @param integer $flags Bitfield; supports READ_LATEST [optional]
 	 * @return mixed Returns false on failure
 	 */
-	abstract public function get( $key, &$casToken = null );
+	abstract public function get( $key, &$casToken = null, $flags = 0 );
 
 	/**
 	 * Set an item.
@@ -109,18 +113,20 @@ abstract class BagOStuff implements LoggerAwareInterface {
 
 	/**
 	 * Merge changes into the existing cache value (possibly creating a new one).
-	 * The callback function returns the new value given the current value (possibly false),
-	 * and takes the arguments: (this BagOStuff object, cache key, current value).
+	 * The callback function returns the new value given the current value
+	 * (which will be false if not present), and takes the arguments:
+	 * (this BagOStuff, cache key, current value).
 	 *
 	 * @param string $key
 	 * @param callable $callback Callback method to be executed
 	 * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 	 * @param int $attempts The amount of times to attempt a merge in case of failure
 	 * @return bool Success
+	 * @throws InvalidArgumentException
 	 */
 	public function merge( $key, $callback, $exptime = 0, $attempts = 10 ) {
 		if ( !is_callable( $callback ) ) {
-			throw new Exception( "Got invalid callback." );
+			throw new InvalidArgumentException( "Got invalid callback." );
 		}
 
 		return $this->mergeViaLock( $key, $callback, $exptime, $attempts );
@@ -137,11 +143,17 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	 */
 	protected function mergeViaCas( $key, $callback, $exptime = 0, $attempts = 10 ) {
 		do {
+			$this->clearLastError();
 			$casToken = null; // passed by reference
 			$currentValue = $this->get( $key, $casToken );
+			if ( $this->getLastError() ) {
+				return false; // don't spam retries (retry only on races)
+			}
+
 			// Derive the new value from the old value
 			$value = call_user_func( $callback, $this, $key, $currentValue );
 
+			$this->clearLastError();
 			if ( $value === false ) {
 				$success = true; // do nothing
 			} elseif ( $currentValue === false ) {
@@ -151,6 +163,9 @@ abstract class BagOStuff implements LoggerAwareInterface {
 				// Try to update the key, failing if it gets changed in the meantime
 				$success = $this->cas( $casToken, $key, $value, $exptime );
 			}
+			if ( $this->getLastError() ) {
+				return false; // IO error; don't spam retries
+			}
 		} while ( !$success && --$attempts );
 
 		return $success;
@@ -164,6 +179,7 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	 * @param mixed $value
 	 * @param int $exptime Either an interval in seconds or a unix timestamp for expiry
 	 * @return bool Success
+	 * @throws Exception
 	 */
 	protected function cas( $casToken, $key, $value, $exptime = 0 ) {
 		throw new Exception( "CAS is not implemented in " . __CLASS__ );
@@ -183,14 +199,18 @@ abstract class BagOStuff implements LoggerAwareInterface {
 			return false;
 		}
 
+		$this->clearLastError();
 		$currentValue = $this->get( $key );
-		// Derive the new value from the old value
-		$value = call_user_func( $callback, $this, $key, $currentValue );
-
-		if ( $value === false ) {
-			$success = true; // do nothing
+		if ( $this->getLastError() ) {
+			$success = false;
 		} else {
-			$success = $this->set( $key, $value, $exptime ); // set the new value
+			// Derive the new value from the old value
+			$value = call_user_func( $callback, $this, $key, $currentValue );
+			if ( $value === false ) {
+				$success = true; // do nothing
+			} else {
+				$success = $this->set( $key, $value, $exptime ); // set the new value
+			}
 		}
 
 		if ( !$this->unlock( $key ) ) {
@@ -202,48 +222,116 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	}
 
 	/**
+	 * Acquire an advisory lock on a key string
+	 *
+	 * Note that if reentry is enabled, duplicate calls ignore $expiry
+	 *
 	 * @param string $key
-	 * @param int $timeout Lock wait timeout [optional]
-	 * @param int $expiry Lock expiry [optional]
+	 * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
+	 * @param int $expiry Lock expiry [optional]; 1 day maximum
+	 * @param string $rclass Allow reentry if set and the current lock used this value
 	 * @return bool Success
 	 */
-	public function lock( $key, $timeout = 6, $expiry = 6 ) {
+	public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
+		// Avoid deadlocks and allow lock reentry if specified
+		if ( isset( $this->locks[$key] ) ) {
+			if ( $rclass != '' && $this->locks[$key]['class'] === $rclass ) {
+				++$this->locks[$key]['depth'];
+				return true;
+			} else {
+				return false;
+			}
+		}
+
+		$expiry = min( $expiry ?: INF, 86400 );
+
 		$this->clearLastError();
 		$timestamp = microtime( true ); // starting UNIX timestamp
 		if ( $this->add( "{$key}:lock", 1, $expiry ) ) {
-			return true;
-		} elseif ( $this->getLastError() ) {
-			return false;
+			$locked = true;
+		} elseif ( $this->getLastError() || $timeout <= 0 ) {
+			$locked = false; // network partition or non-blocking
+		} else {
+			$uRTT = ceil( 1e6 * ( microtime( true ) - $timestamp ) ); // estimate RTT (us)
+			$sleep = 2 * $uRTT; // rough time to do get()+set()
+
+			$attempts = 0; // failed attempts
+			do {
+				if ( ++$attempts >= 3 && $sleep <= 5e5 ) {
+					// Exponentially back off after failed attempts to avoid network spam.
+					// About 2*$uRTT*(2^n-1) us of "sleep" happen for the next n attempts.
+					$sleep *= 2;
+				}
+				usleep( $sleep ); // back off
+				$this->clearLastError();
+				$locked = $this->add( "{$key}:lock", 1, $expiry );
+				if ( $this->getLastError() ) {
+					$locked = false; // network partition
+					break;
+				}
+			} while ( !$locked && ( microtime( true ) - $timestamp ) < $timeout );
 		}
 
-		$uRTT = ceil( 1e6 * ( microtime( true ) - $timestamp ) ); // estimate RTT (us)
-		$sleep = 2 * $uRTT; // rough time to do get()+set()
-
-		$locked = false; // lock acquired
-		$attempts = 0; // failed attempts
-		do {
-			if ( ++$attempts >= 3 && $sleep <= 5e5 ) {
-				// Exponentially back off after failed attempts to avoid network spam.
-				// About 2*$uRTT*(2^n-1) us of "sleep" happen for the next n attempts.
-				$sleep *= 2;
-			}
-			usleep( $sleep ); // back off
-			$this->clearLastError();
-			$locked = $this->add( "{$key}:lock", 1, $expiry );
-			if ( $this->getLastError() ) {
-				return false;
-			}
-		} while ( !$locked && ( microtime( true ) - $timestamp ) < $timeout );
+		if ( $locked ) {
+			$this->locks[$key] = array( 'class' => $rclass, 'depth' => 1 );
+		}
 
 		return $locked;
 	}
 
 	/**
+	 * Release an advisory lock on a key string
+	 *
 	 * @param string $key
 	 * @return bool Success
 	 */
 	public function unlock( $key ) {
-		return $this->delete( "{$key}:lock" );
+		if ( isset( $this->locks[$key] ) && --$this->locks[$key]['depth'] <= 0 ) {
+			unset( $this->locks[$key] );
+
+			return $this->delete( "{$key}:lock" );
+		}
+
+		return true;
+	}
+
+	/**
+	 * Get a lightweight exclusive self-unlocking lock
+	 *
+	 * Note that the same lock cannot be acquired twice.
+	 *
+	 * This is useful for task de-duplication or to avoid obtrusive
+	 * (though non-corrupting) DB errors like INSERT key conflicts
+	 * or deadlocks when using LOCK IN SHARE MODE.
+	 *
+	 * @param string $key
+	 * @param int $timeout Lock wait timeout; 0 for non-blocking [optional]
+	 * @param int $expiry Lock expiry [optional]; 1 day maximum
+	 * @param string $rclass Allow reentry if set and the current lock used this value
+	 * @return ScopedCallback|null Returns null on failure
+	 * @since 1.26
+	 */
+	final public function getScopedLock( $key, $timeout = 6, $expiry = 30, $rclass = '' ) {
+		$expiry = min( $expiry ?: INF, 86400 );
+
+		if ( !$this->lock( $key, $timeout, $expiry, $rclass ) ) {
+			return null;
+		}
+
+		$lSince = microtime( true ); // lock timestamp
+		// PHP 5.3: Can't use $this in a closure
+		$that = $this;
+		$logger = $this->logger;
+
+		return new ScopedCallback( function() use ( $that, $logger, $key, $lSince, $expiry ) {
+			$latency = .050; // latency skew (err towards keeping lock present)
+			$age = ( microtime( true ) - $lSince + $latency );
+			if ( ( $age + $latency ) >= $expiry ) {
+				$logger->warning( "Lock for $key held too long ($age sec)." );
+				return; // expired; it's not "safe" to delete the key
+			}
+			$that->unlock( $key );
+		} );
 	}
 
 	/**
@@ -260,14 +348,13 @@ abstract class BagOStuff implements LoggerAwareInterface {
 		return false;
 	}
 
-	/* *** Emulated functions *** */
-
 	/**
 	 * Get an associative array containing the item for each of the keys that have items.
 	 * @param array $keys List of strings
+	 * @param integer $flags Bitfield; supports READ_LATEST [optional]
 	 * @return array
 	 */
-	public function getMulti( array $keys ) {
+	public function getMulti( array $keys, $flags = 0 ) {
 		$res = array();
 		foreach ( $keys as $key ) {
 			$val = $this->get( $key );
@@ -334,7 +421,7 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	 * Decrease stored value of $key by $value while preserving its TTL
 	 * @param string $key
 	 * @param int $value
-	 * @return int
+	 * @return int|bool New value or false on failure
 	 */
 	public function decr( $key, $value = 1 ) {
 		return $this->incr( $key, - $value );
@@ -384,6 +471,24 @@ abstract class BagOStuff implements LoggerAwareInterface {
 	}
 
 	/**
+	 * Modify a cache update operation array for EventRelayer::notify()
+	 *
+	 * This is used for relayed writes, e.g. for broadcasting a change
+	 * to multiple data-centers. If the array contains a 'val' field
+	 * then the command involves setting a key to that value. Note that
+	 * for simplicity, 'val' is always a simple scalar value. This method
+	 * is used to possibly serialize the value and add any cache-specific
+	 * key/values needed for the relayer daemon (e.g. memcached flags).
+	 *
+	 * @param array $event
+	 * @return array
+	 * @since 1.26
+	 */
+	public function modifySimpleRelayEvent( array $event ) {
+		return $event;
+	}
+
+	/**
 	 * @param string $text
 	 */
 	protected function debug( $text ) {
diff --git a/includes/libs/objectcache/EmptyBagOStuff.php b/includes/libs/objectcache/EmptyBagOStuff.php
index 4ccf2707..55e84b05 100644
--- a/includes/libs/objectcache/EmptyBagOStuff.php
+++ b/includes/libs/objectcache/EmptyBagOStuff.php
@@ -27,7 +27,7 @@
  * @ingroup Cache
  */
 class EmptyBagOStuff extends BagOStuff {
-	public function get( $key, &$casToken = null ) {
+	public function get( $key, &$casToken = null, $flags = 0 ) {
 		return false;
 	}
 
diff --git a/includes/libs/objectcache/HashBagOStuff.php b/includes/libs/objectcache/HashBagOStuff.php
index 2c8b05a5..b685e41f 100644
--- a/includes/libs/objectcache/HashBagOStuff.php
+++ b/includes/libs/objectcache/HashBagOStuff.php
@@ -48,7 +48,7 @@ class HashBagOStuff extends BagOStuff {
 		return true;
 	}
 
-	public function get( $key, &$casToken = null ) {
+	public function get( $key, &$casToken = null, $flags = 0 ) {
 		if ( !isset( $this->bag[$key] ) ) {
 			return false;
 		}
@@ -68,20 +68,8 @@ class HashBagOStuff extends BagOStuff {
 	}
 
 	function delete( $key ) {
-		if ( !isset( $this->bag[$key] ) ) {
-			return false;
-		}
-
 		unset( $this->bag[$key] );
 
 		return true;
 	}
-
-	public function lock( $key, $timeout = 6, $expiry = 6 ) {
-		return true;
-	}
-
-	function unlock( $key ) {
-		return true;
-	}
 }
diff --git a/includes/libs/objectcache/ReplicatedBagOStuff.php b/includes/libs/objectcache/ReplicatedBagOStuff.php
new file mode 100644
index 00000000..9e80e9fd
--- /dev/null
+++ b/includes/libs/objectcache/ReplicatedBagOStuff.php
@@ -0,0 +1,129 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ * @author Aaron Schulz
+ */
+
+/**
+ * A cache class that directs writes to one set of servers and reads to
+ * another. This assumes that the servers used for reads are setup to slave
+ * those that writes go to. This can easily be used with redis for example.
+ *
+ * In the WAN scenario (e.g. multi-datacenter case), this is useful when
+ * writes are rare or they usually take place in the primary datacenter.
+ *
+ * @ingroup Cache
+ * @since 1.26
+ */
+class ReplicatedBagOStuff extends BagOStuff {
+	/** @var BagOStuff */
+	protected $writeStore;
+	/** @var BagOStuff */
+	protected $readStore;
+
+	/**
+	 * Constructor. Parameters are:
+	 *   - writeFactory : ObjectFactory::getObjectFromSpec parameters yeilding BagOStuff.
+	 *                    This object will be used for writes (e.g. the master DB).
+	 *   - readFactory  : ObjectFactory::getObjectFromSpec parameters yeilding BagOStuff.
+	 *                    This object will be used for reads (e.g. a slave DB).
+	 *
+	 * @param array $params
+	 * @throws InvalidArgumentException
+	 */
+	public function __construct( $params ) {
+		parent::__construct( $params );
+
+		if ( !isset( $params['writeFactory'] ) ) {
+			throw new InvalidArgumentException(
+				__METHOD__ . ': the "writeFactory" parameter is required' );
+		}
+		if ( !isset( $params['readFactory'] ) ) {
+			throw new InvalidArgumentException(
+				__METHOD__ . ': the "readFactory" parameter is required' );
+		}
+
+		$this->writeStore = ( $params['writeFactory'] instanceof BagOStuff )
+			? $params['writeFactory']
+			: ObjectFactory::getObjectFromSpec( $params['writeFactory'] );
+		$this->readStore = ( $params['readFactory'] instanceof BagOStuff )
+			? $params['readFactory']
+			: ObjectFactory::getObjectFromSpec( $params['readFactory'] );
+	}
+
+	public function setDebug( $debug ) {
+		$this->writeStore->setDebug( $debug );
+		$this->readStore->setDebug( $debug );
+	}
+
+	public function get( $key, &$casToken = null, $flags = 0 ) {
+		return ( $flags & self::READ_LATEST )
+			? $this->writeStore->get( $key, $casToken, $flags )
+			: $this->readStore->get( $key, $casToken, $flags );
+	}
+
+	public function getMulti( array $keys, $flags = 0 ) {
+		return ( $flags & self::READ_LATEST )
+			? $this->writeStore->getMulti( $keys, $flags )
+			: $this->readStore->getMulti( $keys, $flags );
+	}
+
+	public function set( $key, $value, $exptime = 0 ) {
+		return $this->writeStore->set( $key, $value, $exptime );
+	}
+
+	public function delete( $key ) {
+		return $this->writeStore->delete( $key );
+	}
+
+	public function add( $key, $value, $exptime = 0 ) {
+		return $this->writeStore->add( $key, $value, $exptime );
+	}
+
+	public function incr( $key, $value = 1 ) {
+		return $this->writeStore->incr( $key, $value );
+	}
+
+	public function decr( $key, $value = 1 ) {
+		return $this->writeStore->decr( $key, $value );
+	}
+
+	public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
+		return $this->writeStore->lock( $key, $timeout, $expiry, $rclass );
+	}
+
+	public function unlock( $key ) {
+		return $this->writeStore->unlock( $key );
+	}
+
+	public function merge( $key, $callback, $exptime = 0, $attempts = 10 ) {
+		return $this->writeStore->merge( $key, $callback, $exptime, $attempts );
+	}
+
+	public function getLastError() {
+		return ( $this->writeStore->getLastError() != self::ERR_NONE )
+			? $this->writeStore->getLastError()
+			: $this->readStore->getLastError();
+	}
+
+	public function clearLastError() {
+		$this->writeStore->clearLastError();
+		$this->readStore->clearLastError();
+	}
+}
diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
new file mode 100644
index 00000000..2d921a70
--- /dev/null
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -0,0 +1,746 @@
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Cache
+ * @author Aaron Schulz
+ */
+
+/**
+ * Multi-datacenter aware caching interface
+ *
+ * All operations go to the local cache, except the delete()
+ * and touchCheckKey(), which broadcast to all clusters.
+ * This class is intended for caching data from primary stores.
+ * If the get() method does not return a value, then the caller
+ * should query the new value and backfill the cache using set().
+ * When the source data changes, the delete() method should be called.
+ * Since delete() is expensive, it should be avoided. One can do so if:
+ *   - a) The object cached is immutable; or
+ *   - b) Validity is checked against the source after get(); or
+ *   - c) Using a modest TTL is reasonably correct and performant
+ * Consider using getWithSetCallback() instead of the get()/set() cycle.
+ *
+ * Instances of this class must be configured to point to a valid
+ * PubSub endpoint, and there must be listeners on the cache servers
+ * that subscribe to the endpoint and update the caches.
+ *
+ * Broadcasted operations like delete() and touchCheckKey() are done
+ * synchronously in the local cluster, but are relayed asynchronously.
+ * This means that callers in other datacenters will see older values
+ * for a however many milliseconds the datacenters are apart. As with
+ * any cache, this should not be relied on for cases where reads are
+ * used to determine writes to source (e.g. non-cache) data stores.
+ *
+ * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
+ * to avoid collisions with keys that are not wrapped as metadata arrays. The
+ * prefixes are as follows:
+ *   - a) "WANCache:v" : used for regular value keys
+ *   - b) "WANCache:s" : used for temporarily storing values of tombstoned keys
+ *   - c) "WANCache:t" : used for storing timestamp "check" keys
+ *
+ * @ingroup Cache
+ * @since 1.26
+ */
+class WANObjectCache {
+	/** @var BagOStuff The local cluster cache */
+	protected $cache;
+	/** @var string Cache pool name */
+	protected $pool;
+	/** @var EventRelayer */
+	protected $relayer;
+
+	/** @var int */
+	protected $lastRelayError = self::ERR_NONE;
+
+	/** Seconds to tombstone keys on delete() */
+	const HOLDOFF_TTL = 10;
+	/** Seconds to keep dependency purge keys around */
+	const CHECK_KEY_TTL = 31536000; // 1 year
+	/** Seconds to keep lock keys around */
+	const LOCK_TTL = 5;
+	/** Default remaining TTL at which to consider pre-emptive regeneration */
+	const LOW_TTL = 10;
+	/** Default TTL for temporarily caching tombstoned keys */
+	const TEMP_TTL = 5;
+
+	/** Idiom for set()/getWithSetCallback() TTL */
+	const TTL_NONE = 0;
+	/** Idiom for getWithSetCallback() callbacks to avoid calling set() */
+	const TTL_UNCACHEABLE = -1;
+
+	/** Cache format version number */
+	const VERSION = 1;
+
+	/** Fields of value holder arrays */
+	const FLD_VERSION = 0;
+	const FLD_VALUE = 1;
+	const FLD_TTL = 2;
+	const FLD_TIME = 3;
+
+	/** Possible values for getLastError() */
+	const ERR_NONE = 0; // no error
+	const ERR_NO_RESPONSE = 1; // no response
+	const ERR_UNREACHABLE = 2; // can't connect
+	const ERR_UNEXPECTED = 3; // response gave some error
+	const ERR_RELAY = 4; // relay broadcast failed
+
+	const VALUE_KEY_PREFIX = 'WANCache:v:';
+	const STASH_KEY_PREFIX = 'WANCache:s:';
+	const TIME_KEY_PREFIX = 'WANCache:t:';
+
+	const PURGE_VAL_PREFIX = 'PURGED:';
+
+	/**
+	 * @param array $params
+	 *   - cache   : BagOStuff object
+	 *   - pool    : pool name
+	 *   - relayer : EventRelayer object
+	 */
+	public function __construct( array $params ) {
+		$this->cache = $params['cache'];
+		$this->pool = $params['pool'];
+		$this->relayer = $params['relayer'];
+	}
+
+	/**
+	 * @return WANObjectCache Cache that wraps EmptyBagOStuff
+	 */
+	public static function newEmpty() {
+		return new self( array(
+			'cache'   => new EmptyBagOStuff(),
+			'pool'    => 'empty',
+			'relayer' => new EventRelayerNull( array() )
+		) );
+	}
+
+	/**
+	 * Fetch the value of a key from cache
+	 *
+	 * If passed in, $curTTL is set to the remaining TTL (current time left):
+	 *   - a) INF; if the key exists, has no TTL, and is not expired by $checkKeys
+	 *   - b) float (>=0); if the key exists, has a TTL, and is not expired by $checkKeys
+	 *   - c) float (<0); if the key is tombstoned or existing but expired by $checkKeys
+	 *   - d) null; if the key does not exist and is not tombstoned
+	 *
+	 * If a key is tombstoned, $curTTL will reflect the time since delete().
+	 *
+	 * The timestamp of $key will be checked against the last-purge timestamp
+	 * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
+	 * initialized to the current timestamp. If any of $checkKeys have a timestamp
+	 * greater than that of $key, then $curTTL will reflect how long ago $key
+	 * became invalid. Callers can use $curTTL to know when the value is stale.
+	 * The $checkKeys parameter allow mass invalidations by updating a single key:
+	 *   - a) Each "check" key represents "last purged" of some source data
+	 *   - b) Callers pass in relevant "check" keys as $checkKeys in get()
+	 *   - c) When the source data that "check" keys represent changes,
+	 *        the touchCheckKey() method is called on them
+	 *
+	 * For keys that are hot/expensive, consider using getWithSetCallback() instead.
+	 *
+	 * @param string $key Cache key
+	 * @param mixed $curTTL Approximate TTL left on the key if present [returned]
+	 * @param array $checkKeys List of "check" keys
+	 * @return mixed Value of cache key or false on failure
+	 */
+	final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
+		$curTTLs = array();
+		$values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
+		$curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$key] : null;
+
+		return isset( $values[$key] ) ? $values[$key] : false;
+	}
+
+	/**
+	 * Fetch the value of several keys from cache
+	 *
+	 * @see WANObjectCache::get()
+	 *
+	 * @param array $keys List of cache keys
+	 * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
+	 * @param array $checkKeys List of "check" keys
+	 * @return array Map of (key => value) for keys that exist
+	 */
+	final public function getMulti(
+		array $keys, &$curTTLs = array(), array $checkKeys = array()
+	) {
+		$result = array();
+		$curTTLs = array();
+
+		$vPrefixLen = strlen( self::VALUE_KEY_PREFIX );
+		$valueKeys = self::prefixCacheKeys( $keys, self::VALUE_KEY_PREFIX );
+		$checkKeys = self::prefixCacheKeys( $checkKeys, self::TIME_KEY_PREFIX );
+
+		// Fetch all of the raw values
+		$wrappedValues = $this->cache->getMulti( array_merge( $valueKeys, $checkKeys ) );
+		$now = microtime( true );
+
+		// Get/initialize the timestamp of all the "check" keys
+		$checkKeyTimes = array();
+		foreach ( $checkKeys as $checkKey ) {
+			$timestamp = isset( $wrappedValues[$checkKey] )
+				? self::parsePurgeValue( $wrappedValues[$checkKey] )
+				: false;
+			if ( !is_float( $timestamp ) ) {
+				// Key is not set or invalid; regenerate
+				$this->cache->add( $checkKey,
+					self::PURGE_VAL_PREFIX . $now, self::CHECK_KEY_TTL );
+				$timestamp = $now;
+			}
+
+			$checkKeyTimes[] = $timestamp;
+		}
+
+		// Get the main cache value for each key and validate them
+		foreach ( $valueKeys as $vKey ) {
+			if ( !isset( $wrappedValues[$vKey] ) ) {
+				continue; // not found
+			}
+
+			$key = substr( $vKey, $vPrefixLen ); // unprefix
+
+			list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
+			if ( $value !== false ) {
+				$result[$key] = $value;
+				foreach ( $checkKeyTimes as $checkKeyTime ) {
+					// Force dependant keys to be invalid for a while after purging
+					// to reduce race conditions involving stale data getting cached
+					$safeTimestamp = $checkKeyTime + self::HOLDOFF_TTL;
+					if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
+						$curTTL = min( $curTTL, $checkKeyTime - $now );
+					}
+				}
+			}
+
+			$curTTLs[$key] = $curTTL;
+		}
+
+		return $result;
+	}
+
+	/**
+	 * Set the value of a key from cache
+	 *
+	 * Simply calling this method when source data changes is not valid because
+	 * the changes do not replicate to the other WAN sites. In that case, delete()
+	 * should be used instead. This method is intended for use on cache misses.
+	 *
+	 * @param string $key Cache key
+	 * @param mixed $value
+	 * @param integer $ttl Seconds to live [0=forever]
+	 * @return bool Success
+	 */
+	final public function set( $key, $value, $ttl = 0 ) {
+		$key = self::VALUE_KEY_PREFIX . $key;
+		$wrapped = $this->wrap( $value, $ttl );
+
+		$func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
+			return ( is_string( $cWrapped ) )
+				? false // key is tombstoned; do nothing
+				: $wrapped;
+		};
+
+		return $this->cache->merge( $key, $func, $ttl, 1 );
+	}
+
+	/**
+	 * Purge a key from all clusters
+	 *
+	 * This should only be called when the underlying data (being cached)
+	 * changes in a significant way. This deletes the key and starts a hold-off
+	 * period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
+	 * This is done to avoid the following race condition:
+	 *   a) Some DB data changes and delete() is called on a corresponding key
+	 *   b) A request refills the key with a stale value from a lagged DB
+	 *   c) The stale value is stuck there until the key is expired/evicted
+	 *
+	 * This is implemented by storing a special "tombstone" value at the cache
+	 * key that this class recognizes; get() calls will return false for the key
+	 * and any set() calls will refuse to replace tombstone values at the key.
+	 * For this to always avoid writing stale values, the following must hold:
+	 *   a) Replication lag is bounded to being less than HOLDOFF_TTL; or
+	 *   b) If lag is higher, the DB will have gone into read-only mode already
+	 *
+	 * If called twice on the same key, then the last hold-off TTL takes
+	 * precedence. For idempotence, the $ttl should not vary for different
+	 * delete() calls on the same key. Also note that lowering $ttl reduces
+	 * the effective range of the 'lockTSE' parameter to getWithSetCallback().
+	 *
+	 * @param string $key Cache key
+	 * @param integer $ttl How long to block writes to the key [seconds]
+	 * @return bool True if the item was purged or not found, false on failure
+	 */
+	final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
+		$key = self::VALUE_KEY_PREFIX . $key;
+		// Avoid indefinite key salting for sanity
+		$ttl = max( $ttl, 1 );
+		// Update the local cluster immediately
+		$ok = $this->cache->set( $key, self::PURGE_VAL_PREFIX . microtime( true ), $ttl );
+		// Publish the purge to all clusters
+		return $this->relayPurge( $key, $ttl ) && $ok;
+	}
+
+	/**
+	 * Fetch the value of a timestamp "check" key
+	 *
+	 * The key will be *initialized* to the current time if not set,
+	 * so only call this method if this behavior is actually desired
+	 *
+	 * The timestamp can be used to check whether a cached value is valid.
+	 * Callers should not assume that this returns the same timestamp in
+	 * all datacenters due to relay delays.
+	 *
+	 * The level of staleness can roughly be estimated from this key, but
+	 * if the key was evicted from cache, such calculations may show the
+	 * time since expiry as ~0 seconds.
+	 *
+	 * Note that "check" keys won't collide with other regular keys
+	 *
+	 * @param string $key
+	 * @return float UNIX timestamp of the key
+	 */
+	final public function getCheckKeyTime( $key ) {
+		$key = self::TIME_KEY_PREFIX . $key;
+
+		$time = self::parsePurgeValue( $this->cache->get( $key ) );
+		if ( $time === false ) {
+			// Casting assures identical floats for the next getCheckKeyTime() calls
+			$time = (string)microtime( true );
+			$this->cache->add( $key, self::PURGE_VAL_PREFIX . $time, self::CHECK_KEY_TTL );
+			$time = (float)$time;
+		}
+
+		return $time;
+	}
+
+	/**
+	 * Purge a "check" key from all clusters, invalidating keys that use it
+	 *
+	 * This should only be called when the underlying data (being cached)
+	 * changes in a significant way, and it is impractical to call delete()
+	 * on all keys that should be changed. When get() is called on those
+	 * keys, the relevant "check" keys must be supplied for this to work.
+	 *
+	 * The "check" key essentially represents a last-modified field.
+	 * It is set in the future a few seconds when this is called, to
+	 * avoid race conditions where dependent keys get updated with a
+	 * stale value (e.g. from a DB slave).
+	 *
+	 * This is typically useful for keys with static names or some cases
+	 * dynamically generated names where a low number of combinations exist.
+	 * When a few important keys get a large number of hits, a high cache
+	 * time is usually desired as well as lockTSE logic. The resetCheckKey()
+	 * method is less appropriate in such cases since the "time since expiry"
+	 * cannot be inferred.
+	 *
+	 * Note that "check" keys won't collide with other regular keys
+	 *
+	 * @see WANObjectCache::get()
+	 *
+	 * @param string $key Cache key
+	 * @return bool True if the item was purged or not found, false on failure
+	 */
+	final public function touchCheckKey( $key ) {
+		$key = self::TIME_KEY_PREFIX . $key;
+		// Update the local cluster immediately
+		$ok = $this->cache->set( $key,
+			self::PURGE_VAL_PREFIX . microtime( true ), self::CHECK_KEY_TTL );
+		// Publish the purge to all clusters
+		return $this->relayPurge( $key, self::CHECK_KEY_TTL ) && $ok;
+	}
+
+	/**
+	 * Delete a "check" key from all clusters, invalidating keys that use it
+	 *
+	 * This is similar to touchCheckKey() in that keys using it via
+	 * getWithSetCallback() will be invalidated. The differences are:
+	 *   a) The timestamp will be deleted from all caches and lazily
+	 *      re-initialized when accessed (rather than set everywhere)
+	 *   b) Thus, dependent keys will be known to be invalid, but not
+	 *      for how long (they are treated as "just" purged), which
+	 *      effects any lockTSE logic in getWithSetCallback()
+	 * The advantage is that this does not place high TTL keys on every cache
+	 * server, making it better for code that will cache many different keys
+	 * and either does not use lockTSE or uses a low enough TTL anyway.
+	 *
+	 * This is typically useful for keys with dynamically generated names
+	 * where a high number of combinations exist.
+	 *
+	 * Note that "check" keys won't collide with other regular keys
+	 *
+	 * @see WANObjectCache::touchCheckKey()
+	 * @see WANObjectCache::get()
+	 *
+	 * @param string $key Cache key
+	 * @return bool True if the item was purged or not found, false on failure
+	 */
+	final public function resetCheckKey( $key ) {
+		$key = self::TIME_KEY_PREFIX . $key;
+		// Update the local cluster immediately
+		$ok = $this->cache->delete( $key );
+		// Publish the purge to all clusters
+		return $this->relayDelete( $key ) && $ok;
+	}
+
+	/**
+	 * Method to fetch/regenerate cache keys
+	 *
+	 * On cache miss, the key will be set to the callback result,
+	 * unless the callback returns false. The arguments supplied are:
+	 *     (current value or false, &$ttl)
+	 * The callback function returns the new value given the current
+	 * value (false if not present). Preemptive re-caching and $checkKeys
+	 * can result in a non-false current value. The TTL of the new value
+	 * can be set dynamically by altering $ttl in the callback (by reference).
+	 *
+	 * Usually, callbacks ignore the current value, but it can be used
+	 * to maintain "most recent X" values that come from time or sequence
+	 * based source data, provided that the "as of" id/time is tracked.
+	 *
+	 * Usage of $checkKeys is similar to get()/getMulti(). However,
+	 * rather than the caller having to inspect a "current time left"
+	 * variable (e.g. $curTTL, $curTTLs), a cache regeneration will be
+	 * triggered using the callback.
+	 *
+	 * The simplest way to avoid stampedes for hot keys is to use
+	 * the 'lockTSE' option in $opts. If cache purges are needed, also:
+	 *   a) Pass $key into $checkKeys
+	 *   b) Use touchCheckKey( $key ) instead of delete( $key )
+	 * Following this pattern lets the old cache be used until a
+	 * single thread updates it as needed. Also consider tweaking
+	 * the 'lowTTL' parameter.
+	 *
+	 * Example usage:
+	 * @code
+	 *     $key = wfMemcKey( 'cat-recent-actions', $catId );
+	 *     // Function that derives the new key value given the old value
+	 *     $callback = function( $cValue, &$ttl ) { ... };
+	 *     // Get the key value from cache or from source on cache miss;
+	 *     // try to only let one cluster thread manage doing cache updates
+	 *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
+	 *     $value = $cache->getWithSetCallback( $key, $callback, 60, array(), $opts );
+	 * @endcode
+	 *
+	 * Example usage:
+	 * @code
+	 *     $key = wfMemcKey( 'cat-state', $catId );
+	 *     // The "check" keys that represent things the value depends on;
+	 *     // Calling touchCheckKey() on them invalidates "cat-state"
+	 *     $checkKeys = array(
+	 *         wfMemcKey( 'water-bowls', $houseId ),
+	 *         wfMemcKey( 'food-bowls', $houseId ),
+	 *         wfMemcKey( 'people-present', $houseId )
+	 *     );
+	 *     // Function that derives the new key value
+	 *     $callback = function() { ... };
+	 *     // Get the key value from cache or from source on cache miss;
+	 *     // try to only let one cluster thread manage doing cache updates
+	 *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
+	 *     $value = $cache->getWithSetCallback( $key, $callback, 60, $checkKeys, $opts );
+	 * @endcode
+	 *
+	 * @see WANObjectCache::get()
+	 *
+	 * @param string $key Cache key
+	 * @param integer $ttl Seconds to live for key updates. Special values are:
+	 *   - WANObjectCache::TTL_NONE : Cache forever
+	 *   - WANObjectCache::TTL_UNCACHEABLE: Do not cache at all
+	 * @param callable $callback Value generation function
+	 * @param array $opts Options map:
+	 *   - checkKeys: List of "check" keys.
+	 *   - lowTTL: Consider pre-emptive updates when the current TTL (sec) of the key is less than
+	 *      this. It becomes more likely over time, becoming a certainty once the key is expired.
+	 *      Default: WANObjectCache::LOW_TTL seconds.
+	 *   - lockTSE: If the key is tombstoned or expired (by checkKeys) less than this many seconds
+	 *      ago, then try to have a single thread handle cache regeneration at any given time.
+	 *      Other threads will try to use stale values if possible. If, on miss, the time since
+	 *      expiration is low, the assumption is that the key is hot and that a stampede is worth
+	 *      avoiding. Setting this above WANObjectCache::HOLDOFF_TTL makes no difference. The
+	 *      higher this is set, the higher the worst-case staleness can be.
+	 *      Use WANObjectCache::TSE_NONE to disable this logic. Default: WANObjectCache::TSE_NONE.
+	 *   - tempTTL : TTL of the temp key used to cache values while a key is tombstoned.
+	 *      This avoids excessive regeneration of hot keys on delete() but may
+	 *      result in stale values.
+	 * @return mixed Value to use for the key
+	 */
+	final public function getWithSetCallback(
+		$key, $ttl, $callback, array $opts = array(), $oldOpts = array()
+	) {
+		// Back-compat with 1.26: Swap $ttl and $callback
+		if ( is_int( $callback ) ) {
+			$temp = $ttl;
+			$ttl = $callback;
+			$callback = $temp;
+		}
+		// Back-compat with 1.26: $checkKeys as separate parameter
+		if ( $oldOpts || ( is_array( $opts ) && isset( $opts[0] ) ) ) {
+			$checkKeys = $opts;
+			$opts = $oldOpts;
+		} else {
+			$checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : array();
+		}
+
+		$lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( self::LOW_TTL, $ttl );
+		$lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : -1;
+		$tempTTL = isset( $opts['tempTTL'] ) ? $opts['tempTTL'] : self::TEMP_TTL;
+
+		// Get the current key value
+		$curTTL = null;
+		$cValue = $this->get( $key, $curTTL, $checkKeys ); // current value
+		$value = $cValue; // return value
+
+		// Determine if a regeneration is desired
+		if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
+			return $value;
+		}
+
+		// A deleted key with a negative TTL left must be tombstoned
+		$isTombstone = ( $curTTL !== null && $value === false );
+		// Assume a key is hot if requested soon after invalidation
+		$isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
+
+		$lockAcquired = false;
+		if ( $isHot ) {
+			// Acquire a cluster-local non-blocking lock
+			if ( $this->cache->lock( $key, 0, self::LOCK_TTL ) ) {
+				// Lock acquired; this thread should update the key
+				$lockAcquired = true;
+			} elseif ( $value !== false ) {
+				// If it cannot be acquired; then the stale value can be used
+				return $value;
+			}
+		}
+
+		if ( !$lockAcquired && ( $isTombstone || $isHot ) ) {
+			// Use the stash value for tombstoned keys to reduce regeneration load.
+			// For hot keys, either another thread has the lock or the lock failed;
+			// use the stash value from the last thread that regenerated it.
+			$value = $this->cache->get( self::STASH_KEY_PREFIX . $key );
+			if ( $value !== false ) {
+				return $value;
+			}
+		}
+
+		if ( !is_callable( $callback ) ) {
+			throw new InvalidArgumentException( "Invalid cache miss callback provided." );
+		}
+
+		// Generate the new value from the callback...
+		$value = call_user_func_array( $callback, array( $cValue, &$ttl ) );
+		// When delete() is called, writes are write-holed by the tombstone,
+		// so use a special stash key to pass the new value around threads.
+		if ( $value !== false && ( $isHot || $isTombstone ) && $ttl >= 0 ) {
+			$this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
+		}
+
+		if ( $lockAcquired ) {
+			$this->cache->unlock( $key );
+		}
+
+		if ( $value !== false && $ttl >= 0 ) {
+			// Update the cache; this will fail if the key is tombstoned
+			$this->set( $key, $value, $ttl );
+		}
+
+		return $value;
+	}
+
+	/**
+	 * Get the "last error" registered; clearLastError() should be called manually
+	 * @return int ERR_* constant for the "last error" registry
+	 */
+	final public function getLastError() {
+		if ( $this->lastRelayError ) {
+			// If the cache and the relayer failed, focus on the later.
+			// An update not making it to the relayer means it won't show up
+			// in other DCs (nor will consistent re-hashing see up-to-date values).
+			// On the other hand, if just the cache update failed, then it should
+			// eventually be applied by the relayer.
+			return $this->lastRelayError;
+		}
+
+		$code = $this->cache->getLastError();
+		switch ( $code ) {
+			case BagOStuff::ERR_NONE:
+				return self::ERR_NONE;
+			case BagOStuff::ERR_NO_RESPONSE:
+				return self::ERR_NO_RESPONSE;
+			case BagOStuff::ERR_UNREACHABLE:
+				return self::ERR_UNREACHABLE;
+			default:
+				return self::ERR_UNEXPECTED;
+		}
+	}
+
+	/**
+	 * Clear the "last error" registry
+	 */
+	final public function clearLastError() {
+		$this->cache->clearLastError();
+		$this->lastRelayError = self::ERR_NONE;
+	}
+
+	/**
+	 * Do the actual async bus purge of a key
+	 *
+	 * This must set the key to "PURGED:<UNIX timestamp>"
+	 *
+	 * @param string $key Cache key
+	 * @param integer $ttl How long to keep the tombstone [seconds]
+	 * @return bool Success
+	 */
+	protected function relayPurge( $key, $ttl ) {
+		$event = $this->cache->modifySimpleRelayEvent( array(
+			'cmd' => 'set',
+			'key' => $key,
+			'val' => 'PURGED:$UNIXTIME$',
+			'ttl' => max( $ttl, 1 ),
+			'sbt' => true, // substitute $UNIXTIME$ with actual microtime
+		) );
+
+		$ok = $this->relayer->notify( "{$this->pool}:purge", $event );
+		if ( !$ok ) {
+			$this->lastRelayError = self::ERR_RELAY;
+		}
+
+		return $ok;
+	}
+
+	/**
+	 * Do the actual async bus delete of a key
+	 *
+	 * @param string $key Cache key
+	 * @return bool Success
+	 */
+	protected function relayDelete( $key ) {
+		$event = $this->cache->modifySimpleRelayEvent( array(
+			'cmd' => 'delete',
+			'key' => $key,
+		) );
+
+		$ok = $this->relayer->notify( "{$this->pool}:purge", $event );
+		if ( !$ok ) {
+			$this->lastRelayError = self::ERR_RELAY;
+		}
+
+		return $ok;
+	}
+
+	/**
+	 * Check if a key should be regenerated (using random probability)
+	 *
+	 * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
+	 * of returning true increases steadily from 0% to 100% as the $curTTL
+	 * moves from $lowTTL to 0 seconds. This handles widely varying
+	 * levels of cache access traffic.
+	 *
+	 * @param float $curTTL Approximate TTL left on the key if present
+	 * @param float $lowTTL Consider a refresh when $curTTL is less than this
+	 * @return bool
+	 */
+	protected function worthRefresh( $curTTL, $lowTTL ) {
+		if ( $curTTL >= $lowTTL ) {
+			return false;
+		} elseif ( $curTTL <= 0 ) {
+			return true;
+		}
+
+		$chance = ( 1 - $curTTL / $lowTTL );
+
+		return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
+	}
+
+	/**
+	 * Do not use this method outside WANObjectCache
+	 *
+	 * @param mixed $value
+	 * @param integer $ttl [0=forever]
+	 * @return string
+	 */
+	protected function wrap( $value, $ttl ) {
+		return array(
+			self::FLD_VERSION => self::VERSION,
+			self::FLD_VALUE => $value,
+			self::FLD_TTL => $ttl,
+			self::FLD_TIME => microtime( true )
+		);
+	}
+
+	/**
+	 * Do not use this method outside WANObjectCache
+	 *
+	 * @param array|string|bool $wrapped
+	 * @param float $now Unix Current timestamp (preferrable pre-query)
+	 * @return array (mixed; false if absent/invalid, current time left)
+	 */
+	protected function unwrap( $wrapped, $now ) {
+		// Check if the value is a tombstone
+		$purgeTimestamp = self::parsePurgeValue( $wrapped );
+		if ( is_float( $purgeTimestamp ) ) {
+			// Purged values should always have a negative current $ttl
+			$curTTL = min( -0.000001, $purgeTimestamp - $now );
+			return array( false, $curTTL );
+		}
+
+		if ( !is_array( $wrapped ) // not found
+			|| !isset( $wrapped[self::FLD_VERSION] ) // wrong format
+			|| $wrapped[self::FLD_VERSION] !== self::VERSION // wrong version
+		) {
+			return array( false, null );
+		}
+
+		if ( $wrapped[self::FLD_TTL] > 0 ) {
+			// Get the approximate time left on the key
+			$age = $now - $wrapped[self::FLD_TIME];
+			$curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
+		} else {
+			// Key had no TTL, so the time left is unbounded
+			$curTTL = INF;
+		}
+
+		return array( $wrapped[self::FLD_VALUE], $curTTL );
+	}
+
+	/**
+	 * @param array $keys
+	 * @param string $prefix
+	 * @return string[]
+	 */
+	protected static function prefixCacheKeys( array $keys, $prefix ) {
+		$res = array();
+		foreach ( $keys as $key ) {
+			$res[] = $prefix . $key;
+		}
+
+		return $res;
+	}
+
+	/**
+	 * @param string $value String like "PURGED:<timestamp>"
+	 * @return float|bool UNIX timestamp or false on failure
+	 */
+	protected static function parsePurgeValue( $value ) {
+		$m = array();
+		if ( is_string( $value ) &&
+			preg_match( '/^' . self::PURGE_VAL_PREFIX . '([^:]+)$/', $value, $m )
+		) {
+			return (float)$m[1];
+		} else {
+			return false;
+		}
+	}
+}
diff --git a/includes/libs/objectcache/WinCacheBagOStuff.php b/includes/libs/objectcache/WinCacheBagOStuff.php
index 53625746..c480aa08 100644
--- a/includes/libs/objectcache/WinCacheBagOStuff.php
+++ b/includes/libs/objectcache/WinCacheBagOStuff.php
@@ -28,15 +28,7 @@
  * @ingroup Cache
  */
 class WinCacheBagOStuff extends BagOStuff {
-
-	/**
-	 * Get a value from the WinCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @param int $casToken [optional] Cas token
-	 * @return mixed
-	 */
-	public function get( $key, &$casToken = null ) {
+	public function get( $key, &$casToken = null, $flags = 0 ) {
 		$val = wincache_ucache_get( $key );
 
 		$casToken = $val;
@@ -48,14 +40,6 @@ class WinCacheBagOStuff extends BagOStuff {
 		return $val;
 	}
 
-	/**
-	 * Store a value in the WinCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @param mixed $value Value to store
-	 * @param int $expire Expiration time
-	 * @return bool
-	 */
 	public function set( $key, $value, $expire = 0 ) {
 		$result = wincache_ucache_set( $key, serialize( $value ), $expire );
 
@@ -64,25 +48,10 @@ class WinCacheBagOStuff extends BagOStuff {
 		return ( is_array( $result ) && $result === array() ) || $result;
 	}
 
-	/**
-	 * Store a value in the WinCache object cache, race condition-safe
-	 *
-	 * @param int $casToken Cas token
-	 * @param string $key Cache key
-	 * @param int $value Object to store
-	 * @param int $exptime Expiration time
-	 * @return bool
-	 */
 	protected function cas( $casToken, $key, $value, $exptime = 0 ) {
 		return wincache_ucache_cas( $key, $casToken, serialize( $value ) );
 	}
 
-	/**
-	 * Remove a value from the WinCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @return bool
-	 */
 	public function delete( $key ) {
 		wincache_ucache_delete( $key );
 
diff --git a/includes/libs/objectcache/XCacheBagOStuff.php b/includes/libs/objectcache/XCacheBagOStuff.php
index cfee9236..9dbff6f1 100644
--- a/includes/libs/objectcache/XCacheBagOStuff.php
+++ b/includes/libs/objectcache/XCacheBagOStuff.php
@@ -28,14 +28,7 @@
  * @ingroup Cache
  */
 class XCacheBagOStuff extends BagOStuff {
-	/**
-	 * Get a value from the XCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @param mixed $casToken Cas token
-	 * @return mixed
-	 */
-	public function get( $key, &$casToken = null ) {
+	public function get( $key, &$casToken = null, $flags = 0 ) {
 		$val = xcache_get( $key );
 
 		if ( is_string( $val ) ) {
@@ -51,14 +44,6 @@ class XCacheBagOStuff extends BagOStuff {
 		return $val;
 	}
 
-	/**
-	 * Store a value in the XCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @param mixed $value Object to store
-	 * @param int $expire Expiration time
-	 * @return bool
-	 */
 	public function set( $key, $value, $expire = 0 ) {
 		if ( !$this->isInteger( $value ) ) {
 			$value = serialize( $value );
@@ -68,12 +53,6 @@ class XCacheBagOStuff extends BagOStuff {
 		return true;
 	}
 
-	/**
-	 * Remove a value from the XCache object cache
-	 *
-	 * @param string $key Cache key
-	 * @return bool
-	 */
 	public function delete( $key ) {
 		xcache_unset( $key );
 		return true;