[Cache] Prevent stampede at warmup using flock()

symfony · fabpot · Jun 18, 2018 · Jun 14, 2018 · Jun 18, 2018 · 0ac2777ad11db51008efd4933f11898ff59b42a9
commit 0ac2777ad11db51008efd4933f11898ff59b42a9
diff --git a/src/Symfony/Component/Cache/LockRegistry.php b/src/Symfony/Component/Cache/LockRegistry.php
@@ -0,0 +1,130 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Cache;
+
+use Psr\Cache\CacheItemInterface;
+use Psr\Cache\CacheItemPoolInterface;
+
+/**
+ * LockRegistry is used internally by existing adapters to protect against cache stampede.
+ *
+ * It does so by wrapping the computation of items in a pool of locks.
+ * Foreach each apps, there can be at most 20 concurrent processes that
+ * compute items at the same time and only one per cache-key.
+ *
+ * @author Nicolas Grekas <[email protected]>
+ */
+class LockRegistry
+{
+    private static $save;
+    private static $openedFiles = array();
+    private static $lockedFiles = array();
+
+    /**
+     * The number of items in this list controls the max number of concurrent processes.
+     */
+    private static $files = array(
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'AbstractAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'AdapterInterface.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'ApcuAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'ArrayAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'ChainAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'DoctrineAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'FilesystemAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'MemcachedAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'NullAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'PdoAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'PhpArrayAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'PhpFilesAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'ProxyAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'RedisAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'SimpleCacheAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'TagAwareAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'TagAwareAdapterInterface.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'TraceableAdapter.php',
+        __DIR__.\DIRECTORY_SEPARATOR.'Adapter'.\DIRECTORY_SEPARATOR.'TraceableTagAwareAdapter.php',
+    );
+
+    /**
+     * Defines a set of existing files that will be used as keys to acquire locks.
+     *
+     * @return array The previously defined set of files
+     */
+    public static function setFiles(array $files): array
+    {
+        $previousFiles = self::$files;
+        self::$files = $files;
+
+        foreach (self::$openedFiles as $k => $file) {
+            flock($file, LOCK_UN);
+            fclose($file);
+        }
+        self::$openedFiles = self::$lockedFiles = array();
+
+        return $previousFiles;
+    }
+
+    /**
+     * @internal
+     */
+    public static function save(string $key, CacheItemPoolInterface $pool, CacheItemInterface $item, callable $callback, float $startTime, &$value): bool
+    {
+        self::$save = self::$save ?? \Closure::bind(
+            function (CacheItemPoolInterface $pool, CacheItemInterface $item, $value, float $startTime) {
+                if ($item instanceof CacheItem && $startTime && $item->expiry > $endTime = microtime(true)) {
+                    $item->newMetadata[CacheItem::METADATA_EXPIRY] = $item->expiry;
+                    $item->newMetadata[CacheItem::METADATA_CTIME] = 1000 * (int) ($endTime - $startTime);
+                }
+                $pool->save($item->set($value));
+
+                return $value;
+            },
+            null,
+            CacheItem::class
+        );
+
+        $key = self::$files ? crc32($key) % \count(self::$files) : -1;
+
+        if ($key < 0 || (self::$lockedFiles[$key] ?? false) || !$lock = self::open($key)) {
+            $value = (self::$save)($pool, $item, $callback($item), $startTime);
+
+            return true;
+        }
+
+        try {
+            // race to get the lock in non-blocking mode
+            if (flock($lock, LOCK_EX | LOCK_NB)) {
+                self::$lockedFiles[$key] = true;
+                $value = (self::$save)($pool, $item, $callback($item), $startTime);
+
+                return true;
+            }
+            // if we failed the race, retry locking in blocking mode to wait for the winner
+            flock($lock, LOCK_SH);
+        } finally {
+            flock($lock, LOCK_UN);
+            self::$lockedFiles[$key] = false;
+        }
+
+        return false;
+    }
+
+    private static function open(int $key)
+    {
+        if ($h = self::$openedFiles[$key] ?? null) {
+            return $h;
+        }
+        if ($h = fopen(self::$files[$key], 'rb')) {
+            return self::$openedFiles[$key] = $h;
+        }
+    }
+}
diff --git a/src/Symfony/Component/Cache/Tests/LockRegistryTest.php b/src/Symfony/Component/Cache/Tests/LockRegistryTest.php
@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Cache\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Cache\LockRegistry;
+
+class LockRegistryTest extends TestCase
+{
+    public function testFiles()
+    {
+        $lockFiles = LockRegistry::setFiles(array());
+        LockRegistry::setFiles($lockFiles);
+        $expected = array_map('realpath', glob(__DIR__.'/../Adapter/*'));
+        $this->assertSame($expected, $lockFiles);
+    }
+}
diff --git a/src/Symfony/Component/Cache/Traits/GetTrait.php b/src/Symfony/Component/Cache/Traits/GetTrait.php
@@ -11,9 +11,9 @@
 
 namespace Symfony\Component\Cache\Traits;
 
-use Psr\Cache\CacheItemInterface;
 use Psr\Cache\CacheItemPoolInterface;
 use Symfony\Component\Cache\CacheItem;
+use Symfony\Component\Cache\LockRegistry;
 
 /**
  * An implementation for CacheInterface that provides stampede protection via probabilistic early expiration.
@@ -36,6 +36,7 @@ public function get(string $key, callable $callback, float $beta = null)
 
     private function doGet(CacheItemPoolInterface $pool, string $key, callable $callback, float $beta)
     {
+        retry:
         $t = 0;
         $item = $pool->getItem($key);
         $recompute = !$item->isHit() || INF === $beta;
@@ -63,24 +64,11 @@ private function doGet(CacheItemPoolInterface $pool, string $key, callable $call
             return $item->get();
         }
 
-        static $save = null;
-
-        if (null === $save) {
-            $save = \Closure::bind(
-                function (CacheItemPoolInterface $pool, CacheItemInterface $item, $value, float $startTime) {
-                    if ($item instanceof CacheItem && $startTime && $item->expiry > $endTime = microtime(true)) {
-                        $item->newMetadata[CacheItem::METADATA_EXPIRY] = $item->expiry;
-                        $item->newMetadata[CacheItem::METADATA_CTIME] = 1000 * (int) ($endTime - $startTime);
-                    }
-                    $pool->save($item->set($value));
-
-                    return $value;
-                },
-                null,
-                CacheItem::class
-            );
+        if (!LockRegistry::save($key, $pool, $item, $callback, $t, $value)) {
+            $beta = 0;
+            goto retry;
         }
 
-        return $save($pool, $item, $callback($item), $t);
+        return $value;
     }
 }