Replace using cache record for locking with MySQL GET_LOCK function.

Author: colinmollenhour

app/code/core/Mage/Core/Model/App.php

@@ -432,14 +432,21 @@ protected function _initCache(array $cacheInitOptions = [])
     protected function _initModules()
     {
         if (!$this->_config->loadModulesCache()) {
-            $this->_config->loadModules();
-            if ($this->_config->isLocalConfigLoaded() && !$this->_shouldSkipProcessModulesUpdates()) {
-                Varien_Profiler::start('mage::app::init::apply_db_schema_updates');
-                Mage_Core_Model_Resource_Setup::applyAllUpdates();
-                Varien_Profiler::stop('mage::app::init::apply_db_schema_updates');
+            try {
+                $this->_config->getCacheSaveLock();
+                if (!$this->_config->loadModulesCache()) {
+                    $this->_config->loadModules();
+                    if ($this->_config->isLocalConfigLoaded() && !$this->_shouldSkipProcessModulesUpdates()) {
+                        Varien_Profiler::start('mage::app::init::apply_db_schema_updates');
+                        Mage_Core_Model_Resource_Setup::applyAllUpdates();
+                        Varien_Profiler::stop('mage::app::init::apply_db_schema_updates');
+                    }
+                    $this->_config->loadDb();
+                    $this->_config->saveCache();
+                }
+            } finally {
+                $this->_config->releaseCacheSaveLock();
             }
-            $this->_config->loadDb();
-            $this->_config->saveCache();
         }
         return $this;
     }

app/code/core/Mage/Core/Model/Config.php

@@ -313,16 +313,19 @@ public function init($options = [])
         $this->setOptions($options);
         $this->loadBase();
 
-        $cacheLoad = $this->loadModulesCache();
-        if ($cacheLoad) {
-            return $this;
+        if (!$this->loadModulesCache()) {
+            try {
+                $this->getCacheSaveLock();
+                if (!$this->loadModulesCache()) {
+                    $this->_useCache = false;
+                    $this->loadModules();
+                    $this->loadDb();
+                    $this->saveCache();
+                }
+            } finally {
+                $this->releaseCacheSaveLock();
+            }
         }
-
-        $this->_useCache = false;
-
-        $this->loadModules();
-        $this->loadDb();
-        $this->saveCache();
         return $this;
     }
 
@@ -354,15 +357,13 @@ public function loadBase()
      */
     public function loadModulesCache()
     {
-        if (Mage::isInstalled(['etc_dir' => $this->getOptions()->getEtcDir()])) {
-            if ($this->_canUseCacheForInit()) {
-                Varien_Profiler::start('mage::app::init::config::load_cache');
-                $loaded = $this->loadCache();
-                Varien_Profiler::stop('mage::app::init::config::load_cache');
-                if ($loaded) {
-                    $this->_useCache = true;
-                    return true;
-                }
+        if ($this->_canUseCacheForInit()) {
+            Varien_Profiler::start('mage::app::init::config::load_cache');
+            $loaded = $this->loadCache();
+            Varien_Profiler::stop('mage::app::init::config::load_cache');
+            if ($loaded) {
+                $this->_useCache = true;
+                return true;
             }
         }
         return false;
@@ -474,20 +475,9 @@ protected function _canUseLocalModules()
      */
     protected function _canUseCacheForInit()
     {
-        if (Mage::app()->useCache('config') && $this->_allowCacheForInit) {
-            $retries = 10;
-            do {
-                if ($this->_loadCache($this->_getCacheLockId())) {
-                    if ($retries) {
-                        usleep(500000); // 0.5 seconds
-                    }
-                } else {
-                    return true;
-                }
-            } while ($retries--);
-        }
-
-        return false;
+        return $this->_allowCacheForInit
+            && Mage::isInstalled(['etc_dir' => $this->getOptions()->getEtcDir()])
+            && Mage::app()->useCache('config');
     }
 
     /**
@@ -501,13 +491,43 @@ public function getCache()
     }
 
     /**
-     * Get lock flag cache identifier
+     * Call before building and saving cache to ensure only one process can save the cache
      *
-     * @return string
+     * If failed to get cache lock:
+     *   - CLI: throws exception
+     *   - Other: 503 error
+     *
+     * @return void
+     * @throws Exception
      */
-    protected function _getCacheLockId()
+    public function getCacheSaveLock()
     {
-        return $this->getCacheId() . '.lock';
+        if ( ! Mage::app()->useCache('config')) {
+            return;
+        }
+        $connection = Mage::getSingleton('core/resource')->getConnection('core_write');
+        if (!$connection->fetchOne("SELECT GET_LOCK('core_config_cache_save_lock', ?)", [PHP_SAPI === 'cli' ? 60 : 3])) {
+            if (PHP_SAPI === 'cli') {
+                throw new Exception('Could not get lock on cache save operation.');
+            } else {
+                require_once Mage::getBaseDir() . DS . 'errors' . DS . '503.php';
+                die();
+            }
+        }
+    }
+
+    /**
+     * Release the cache saving lock after it is saved or no longer needed
+     *
+     * @return void
+     */
+    public function releaseCacheSaveLock()
+    {
+        if ( ! Mage::app()->useCache('config')) {
+            return;
+        }
+        $connection = Mage::getSingleton('core/resource')->getConnection('core_write');
+        $connection->fetchOne("SELECT RELEASE_LOCK('core_config_cache_save_lock')");
     }
 
     /**
@@ -524,12 +544,6 @@ public function saveCache($tags = [])
         if (!in_array(self::CACHE_TAG, $tags)) {
             $tags[] = self::CACHE_TAG;
         }
-        $cacheLockId = $this->_getCacheLockId();
-        if ($this->_loadCache($cacheLockId)) {
-            return $this;
-        }
-
-        $this->_saveCache(time(), $cacheLockId, [], 60);
 
         if (!empty($this->_cacheSections)) {
             $xml = clone $this->_xml;
@@ -540,15 +554,13 @@ public function saveCache($tags = [])
             $this->_cachePartsForSave[$this->getCacheId()] = $xml->asNiceXml('', false);
         } else {
             parent::saveCache($tags);
-            $this->_removeCache($cacheLockId);
             return $this;
         }
 
         foreach ($this->_cachePartsForSave as $cacheId => $cacheData) {
             $this->_saveCache($cacheData, $cacheId, $tags, $this->getCacheLifetime());
         }
         unset($this->_cachePartsForSave);
-        $this->_removeCache($cacheLockId);
 
         return $this;
     }