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| 1 | <?php |
| 2 | |
| 3 | namespace MediaWiki\Settings\Cache; |
| 4 | |
| 5 | use BagOStuff; |
| 6 | use MediaWiki\Settings\SettingsBuilderException; |
| 7 | use MediaWiki\Settings\Source\SettingsIncludeLocator; |
| 8 | use MediaWiki\Settings\Source\SettingsSource; |
| 9 | use Wikimedia\WaitConditionLoop; |
| 10 | |
| 11 | /** |
| 12 | * Provides a caching layer for a {@link CacheableSource}. |
| 13 | * |
| 14 | * @newable |
| 15 | * @since 1.38 |
| 16 | */ |
| 17 | class CachedSource implements SettingsSource, SettingsIncludeLocator { |
| 18 | /** |
| 19 | * Cached source generation timeout (in seconds). |
| 20 | */ |
| 21 | private const TIMEOUT = 2; |
| 22 | |
| 23 | /** @var BagOStuff */ |
| 24 | private $cache; |
| 25 | |
| 26 | /** @var CacheableSource */ |
| 27 | private $source; |
| 28 | |
| 29 | /** |
| 30 | * Constructs a new CachedSource using an instantiated cache and |
| 31 | * {@link CacheableSource}. |
| 32 | * |
| 33 | * @stable to call |
| 34 | * |
| 35 | * @param BagOStuff $cache |
| 36 | * @param CacheableSource $source |
| 37 | */ |
| 38 | public function __construct( |
| 39 | BagOStuff $cache, |
| 40 | CacheableSource $source |
| 41 | ) { |
| 42 | $this->cache = $cache; |
| 43 | $this->source = $source; |
| 44 | } |
| 45 | |
| 46 | /** |
| 47 | * Queries cache for source contents and performs loading/caching of the |
| 48 | * source contents on miss. |
| 49 | * |
| 50 | * If the load fails but the source implements {@link |
| 51 | * CacheableSource::allowsStaleLoad()} as <code>true</code>, stale results |
| 52 | * may be returned if still present in the cache store. |
| 53 | * |
| 54 | * @return array |
| 55 | */ |
| 56 | public function load(): array { |
| 57 | $key = $this->cache->makeGlobalKey( |
| 58 | __CLASS__, |
| 59 | $this->source->getHashKey() |
| 60 | ); |
| 61 | |
| 62 | $result = null; |
| 63 | $loop = new WaitConditionLoop( |
| 64 | function () use ( $key, &$result ) { |
| 65 | $item = $this->cache->get( $key ); |
| 66 | |
| 67 | if ( $this->isValidHit( $item ) ) { |
| 68 | if ( $this->isExpired( $item ) ) { |
| 69 | // The cached item is stale but use it as a default in |
| 70 | // case of failure if the source allows that |
| 71 | if ( $this->source->allowsStaleLoad() ) { |
| 72 | $result = $item['value']; |
| 73 | } |
| 74 | } else { |
| 75 | $result = $item['value']; |
| 76 | return WaitConditionLoop::CONDITION_REACHED; |
| 77 | } |
| 78 | } |
| 79 | |
| 80 | if ( $this->cache->lock( $key, 0, self::TIMEOUT ) ) { |
| 81 | try { |
| 82 | $result = $this->loadAndCache( $key ); |
| 83 | } catch ( SettingsBuilderException $e ) { |
| 84 | if ( $result === null ) { |
| 85 | // We have a failure and no stale result to fall |
| 86 | // back on, so throw |
| 87 | throw $e; |
| 88 | } |
| 89 | } finally { |
| 90 | $this->cache->unlock( $key ); |
| 91 | } |
| 92 | } |
| 93 | |
| 94 | return $result === null |
| 95 | ? WaitConditionLoop::CONDITION_CONTINUE |
| 96 | : WaitConditionLoop::CONDITION_REACHED; |
| 97 | }, |
| 98 | self::TIMEOUT |
| 99 | ); |
| 100 | |
| 101 | if ( $loop->invoke() !== WaitConditionLoop::CONDITION_REACHED ) { |
| 102 | throw new SettingsBuilderException( |
| 103 | 'Exceeded {timeout}s timeout attempting to load and cache source {source}', |
| 104 | [ |
| 105 | 'timeout' => self::TIMEOUT, |
| 106 | 'source' => $this->source, |
| 107 | ] |
| 108 | |
| 109 | ); |
| 110 | } |
| 111 | |
| 112 | // @phan-suppress-next-line PhanTypeMismatchReturn WaitConditionLoop throws or value set |
| 113 | return $result; |
| 114 | } |
| 115 | |
| 116 | /** |
| 117 | * Returns the string representation of the encapsulated source. |
| 118 | * |
| 119 | * @return string |
| 120 | */ |
| 121 | public function __toString(): string { |
| 122 | return $this->source->__toString(); |
| 123 | } |
| 124 | |
| 125 | /** |
| 126 | * Whether the given cache item is considered a cache hit, in other words: |
| 127 | * - it is not a falsey value |
| 128 | * - it has the correct type and structure for this cache implementation |
| 129 | * |
| 130 | * @param mixed $item Cache item. |
| 131 | * |
| 132 | * @return bool |
| 133 | */ |
| 134 | private function isValidHit( $item ): bool { |
| 135 | return $item && |
| 136 | is_array( $item ) && |
| 137 | isset( $item['expiry'] ) && |
| 138 | isset( $item['generation'] ) && |
| 139 | isset( $item['value'] ); |
| 140 | } |
| 141 | |
| 142 | /** |
| 143 | * Whether the given cache item is considered expired, in other words: |
| 144 | * - its expiry timestamp has passed |
| 145 | * - it is deemed to expire early so as to mitigate cache stampedes |
| 146 | * |
| 147 | * @param array $item Cache item. |
| 148 | * |
| 149 | * @return bool |
| 150 | */ |
| 151 | private function isExpired( $item ): bool { |
| 152 | return $item['expiry'] < microtime( true ) || |
| 153 | $this->expiresEarly( $item, $this->source->getExpiryWeight() ); |
| 154 | } |
| 155 | |
| 156 | /** |
| 157 | * Decide whether the cached source should be expired early according to a |
| 158 | * probabilistic calculation that becomes more likely as the normal expiry |
| 159 | * approaches. |
| 160 | * |
| 161 | * In other words, we're going to pretend we're a bit further into the |
| 162 | * future than we are so that we might expire and regenerate the cached |
| 163 | * settings before other threads attempt to the do the same. The number of |
| 164 | * threads that will pretend to be far into the future (and thus will |
| 165 | * concurrently reload/cache the settings) will most probably be so |
| 166 | * exponentially fewer than the number of threads pretending to be near |
| 167 | * into the future that it will approach optimal stampede protection |
| 168 | * without the use of an exclusive lock. |
| 169 | * |
| 170 | * @param array $item Cached source with expiry metadata. |
| 171 | * @param float $weight Coefficient used to increase/decrease the |
| 172 | * likelihood of early expiration. |
| 173 | * |
| 174 | * @link https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration |
| 175 | * |
| 176 | * @return bool |
| 177 | */ |
| 178 | private function expiresEarly( array $item, float $weight ): bool { |
| 179 | if ( $weight == 0 || !isset( $item['expiry'] ) || !isset( $item['generation'] ) ) { |
| 180 | return false; |
| 181 | } |
| 182 | |
| 183 | // Calculate a negative expiry offset using generation time, expiry |
| 184 | // weight, and a random number within the exponentially distributed |
| 185 | // range of log n where n: (0, 1] (which is always negative) |
| 186 | $expiryOffset = |
| 187 | $item['generation'] * |
| 188 | $weight * |
| 189 | log( random_int( 1, PHP_INT_MAX ) / PHP_INT_MAX ); |
| 190 | |
| 191 | return ( $item['expiry'] + $expiryOffset ) <= microtime( true ); |
| 192 | } |
| 193 | |
| 194 | /** |
| 195 | * Loads the source and caches the result. |
| 196 | * |
| 197 | * @param string $key |
| 198 | * |
| 199 | * @return array |
| 200 | */ |
| 201 | private function loadAndCache( string $key ): array { |
| 202 | $ttl = |
| 203 | $this->source->allowsStaleLoad() |
| 204 | ? BagOStuff::TTL_INDEFINITE |
| 205 | : $this->source->getExpiryTtl(); |
| 206 | |
| 207 | $item = $this->loadWithMetadata(); |
| 208 | $this->cache->set( $key, $item, $ttl ); |
| 209 | return $item['value']; |
| 210 | } |
| 211 | |
| 212 | /** |
| 213 | * Wraps cached source with the metadata needed to perform probabilistic |
| 214 | * early expiration to help mitigate cache stampedes. |
| 215 | * |
| 216 | * @return array |
| 217 | */ |
| 218 | private function loadWithMetadata(): array { |
| 219 | $start = microtime( true ); |
| 220 | $value = $this->source->load(); |
| 221 | $finish = microtime( true ); |
| 222 | |
| 223 | return [ |
| 224 | 'value' => $value, |
| 225 | 'expiry' => $start + $this->source->getExpiryTtl(), |
| 226 | 'generation' => $finish - $start, |
| 227 | ]; |
| 228 | } |
| 229 | |
| 230 | public function locateInclude( string $location ): string { |
| 231 | if ( $this->source instanceof SettingsIncludeLocator ) { |
| 232 | return $this->source->locateInclude( $location ); |
| 233 | } else { |
| 234 | // Just return the location as-is |
| 235 | return $location; |
| 236 | } |
| 237 | } |
| 238 | } |