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Diffstat (limited to 'includes/libs/objectcache/WANObjectCache.php')
| -rw-r--r-- | includes/libs/objectcache/WANObjectCache.php | 746 |
1 files changed, 746 insertions, 0 deletions
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; + } + } +} |