diff --git a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
index 1a460ad..278a823 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileReadOnlyStorage.php
@@ -34,19 +34,21 @@ public function __construct(array $configuration) {
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
+   * {@inheritdoc}
    */
-  public function exists($name) {
-    return file_exists($this->getFullPath($name));
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
+    // The FALSE returned on failure is enough for the caller to handle this,
+    // we do not want a warning too. Also, no race conditions in read only
+    // storage so ignore all those parameters.
+    return (@include_once $this->getFullPath($name)) !== FALSE;
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::load().
+   * {@inheritdoc}
    */
-  public function load($name) {
-    // The FALSE returned on failure is enough for the caller to handle this,
-    // we do not want a warning too.
-    return (@include_once $this->getFullPath($name)) !== FALSE;
+  public function loadClass($name, $class_name, $force = FALSE, $wait = 200000, $retry = 5) {
+    // No race conditions in read only storage, huzzah!
+    return $this->load($name);
   }
 
   /**
diff --git a/core/lib/Drupal/Component/PhpStorage/FileStorage.php b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
index 71ff3f2..bef7a05 100644
--- a/core/lib/Drupal/Component/PhpStorage/FileStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/FileStorage.php
@@ -33,18 +33,60 @@ public function __construct(array $configuration) {
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
+   * {@inheritdoc}
    */
-  public function exists($name) {
-    return file_exists($this->getFullPath($name));
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
+    if (!isset($success)) {
+      // Without a success callback, the only way to be safe is to include the
+      // file once.
+      return $force ? $this->doInclude($name) : $this->doIncludeOnce($name);
+    }
+    if (!$force && $success()) {
+      return TRUE;
+    }
+    // If the success callback fails after loading the file a race condition is
+    // very likely so wait some random time allowing the racing process to
+    // finish.
+    while ($this->doInclude($name) && !($return = $success()) && $retry--) {
+      usleep(mt_rand($wait, 2 * $wait));
+    }
+    // If the include failed then return is not set but then returning FALSE
+    // is correct.
+    return !empty($return);
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::load().
+   * {@inheritdoc}
    */
-  public function load($name) {
+  public function loadClass($name, $class_name = NULL, $force = FALSE, $wait = 200000, $retry = 5) {
     // The FALSE returned on failure is enough for the caller to handle this,
     // we do not want a warning too.
+    return $this->load($name, function () use($class_name) { return class_exists($class_name, FALSE); }, $wait, $retry);
+  }
+
+  /**
+   * Run the actual include.
+   *
+   * @param $name
+   *   The name of the file.
+   *
+   * @return bool
+   *   TRUE on success, FALSE otherwise.
+   */
+  protected function doInclude($name) {
+    return (@include $this->getFullPath($name)) !== FALSE;
+  }
+
+  /**
+   * Run the actual include_once.
+   *
+   * @param $name
+   *   The name of the file.
+   *
+   * @return bool
+   *   TRUE on success, FALSE otherwise.
+   */
+  protected function doIncludeOnce($name) {
     return (@include_once $this->getFullPath($name)) !== FALSE;
   }
 
diff --git a/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php b/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
index 573e23c..ba0f66f 100644
--- a/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
+++ b/core/lib/Drupal/Component/PhpStorage/MTimeProtectedFileStorage.php
@@ -36,21 +36,25 @@
 class MTimeProtectedFileStorage extends MTimeProtectedFastFileStorage {
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::load().
+   * {@inheritdoc}
    */
-  public function load($name) {
+  protected function doInclude($name) {
     if (($filename = $this->checkFile($name)) !== FALSE) {
-      // Inline parent::load() to avoid an expensive getFullPath() call.
-      return (@include_once $filename) !== FALSE;
+      // Inline parent::doInclude() to avoid an expensive getFullPath() call.
+      return (@include $filename) !== FALSE;
     }
     return FALSE;
   }
 
   /**
-   * Implements Drupal\Component\PhpStorage\PhpStorageInterface::exists().
+   * {@inheritdoc}
    */
-  public function exists($name) {
-    return $this->checkFile($name) !== FALSE;
+  protected function doIncludeOnce($name) {
+    if (($filename = $this->checkFile($name)) !== FALSE) {
+      // Inline parent::doIncludeOnce() to avoid an expensive getFullPath() call.
+      return (@include_once $filename) !== FALSE;
+    }
+    return FALSE;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
index 1f6a682..587c56d 100644
--- a/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
+++ b/core/lib/Drupal/Component/PhpStorage/PhpStorageInterface.php
@@ -19,29 +19,65 @@
 interface PhpStorageInterface {
 
   /**
-   * Checks whether the PHP code exists in storage.
-   *
-   * @param string $name
-   *   The virtual file name. Can be a relative path.
-   *
-   * @return bool
-   *   TRUE if the virtual file exists, FALSE otherwise.
-   */
-  public function exists($name);
-
-  /**
    * Loads PHP code from storage.
    *
-   * Depending on storage implementation, exists() checks can be expensive, so
-   * this function may be called for a file that doesn't exist, and that should
-   * not result in errors. This function does not return anything, so it is
-   * up to the caller to determine if any code was loaded (for example, check
-   * class_exists() or function_exists() for what was expected in the code).
+   * This function may be called for a file that doesn't exist, and that should
+   * not result in errors.
+   *
+   * It is safe to call this function multiple times: the relevant code will
+   * only be loaded once.
    *
    * @param string $name
    *   The virtual file name. Can be a relative path.
+   * @param callable $success
+   *   A race condition might cause the file to exist but be empty. If this
+   *   parameter is specified the system will retry loading if the callback
+   *   return FALSE to allow the racing process to finish. It is strongly
+   *   recommended to supply this argument or use loadClass() instead to load
+   *   a class. Otherwise, if the same file is written by anotehr process,
+   *   a false positive might occur.
+   * @param bool $force
+   *   Override the safety guarantee and force loading. For example, the caller
+   *   might've already determined the code does not exist before calling.
+   * @param int $wait
+   *   Wait a random amount of time between $wait and 2 * $wait microseconds
+   *   between retries.
+   * @param int $retry
+   *   The number of times to retry.
+   *
+   * @return
+   *   TRUE on success, FALSE otherwise.
    */
-  public function load($name);
+  public function load($name, callable $success = NULL, $force = FALSE, $wait = 200000, $retry = 5);
+
+  /**
+   * Loads a class from storage.
+   *
+   * This function may be called for a file that doesn't exist, and that should
+   * not result in errors.
+   *
+   * It is safe to call this function multiple times: the class will only be
+   * loaded once.
+   *
+   * @param string $name
+   *   The virtual file name. Can be a relative path.
+   * @param string $class_name
+   *   A race condition might cause the file to exist but be empty. The system
+   *   will retry loading if the class does not exist to allow the racing
+   *   process to finish.
+   * @param bool $force
+   *   Override the safety guarantee and force loading. For example, the caller
+   *   might've already determined the class does not exist before calling.
+   * @param int $wait
+   *   Wait a random amount of time between $wait and 2 * $wait microseconds
+   *   between retries.
+   * @param int $retry
+   *   The number of times to retry.
+   *
+   * @return
+   *   TRUE on success, FALSE otherwise.
+   */
+  public function loadClass($name, $class_name, $force = FALSE, $wait = 200000, $retry = 5);
 
   /**
    * Saves PHP code to storage.
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 8f052ba..03c6a7f 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -742,12 +742,11 @@ protected function initializeContainer($rebuild = FALSE) {
     // If the module list hasn't already been set in updateModules and we are
     // not forcing a rebuild, then try and load the container from the disk.
     if (empty($this->moduleList) && !$rebuild) {
-      $fully_qualified_class_name = '\\' . $this->getClassNamespace() . '\\' . $this->getClassName();
+      $class_name = $this->getClassName();
+      $fully_qualified_class_name = '\\' . $this->getClassNamespace() . '\\' . $class_name;
 
       // First, try to load from storage.
-      if (!class_exists($fully_qualified_class_name, FALSE)) {
-        $this->storage()->load($this->getClassName() . '.php');
-      }
+      $this->storage()->loadClass($class_name . '.php', $fully_qualified_class_name);
       // If the load succeeded or the class already existed, use it.
       if (class_exists($fully_qualified_class_name, FALSE)) {
         $container = new $fully_qualified_class_name;
diff --git a/core/lib/Drupal/Core/Template/TwigEnvironment.php b/core/lib/Drupal/Core/Template/TwigEnvironment.php
index 9225712..c30ca2e 100644
--- a/core/lib/Drupal/Core/Template/TwigEnvironment.php
+++ b/core/lib/Drupal/Core/Template/TwigEnvironment.php
@@ -127,9 +127,9 @@ public function loadTemplate($name, $index = NULL) {
           $this->updateCompiledTemplate($cache_filename, $name);
         }
 
-        if (!$this->storage()->load($cache_filename)) {
+        if (!$this->storage()->loadClass($cache_filename, $cls, TRUE)) {
           $this->updateCompiledTemplate($cache_filename, $name);
-          $this->storage()->load($cache_filename);
+          $this->storage()->loadClass($cache_filename, $cls, TRUE);
         }
       }
     }
diff --git a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
index 36cc48d..0ab9773 100644
--- a/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigSettingsTest.php
@@ -105,7 +105,7 @@ function testTwigCacheOverride() {
 
     // Navigate to the page and make sure the template gets cached.
     $this->drupalGet('theme-test/template-test');
-    $this->assertTrue(PhpStorageFactory::get('twig')->exists($cache_filename), 'Cached Twig template found.');
+    $this->assertTrue(PhpStorageFactory::get('twig')->load($cache_filename), 'Cached Twig template found.');
 
     // Disable the Twig cache and rebuild the service container.
     $parameters = $this->container->getParameter('twig.config');
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
index 8cf9642..2516465 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageReadOnlyTest.php
@@ -71,7 +71,7 @@ public function testReadOnly() {
 
     // If the file was successfully loaded, it must also exist, but ensure the
     // exists() method returns that correctly.
-    $this->assertSame($php_read->exists($name), TRUE);
+    $this->assertSame($php_read->load($name), TRUE);
     // Saving and deleting should always fail.
     $this->assertFalse($php_read->save($name, $code));
     $this->assertFalse($php_read->delete($name));
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php
index e0a7d81..024b566 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/FileStorageTest.php
@@ -40,7 +40,6 @@ protected function setUp() {
    *
    * @covers ::load
    * @covers ::save
-   * @covers ::exists
    * @covers ::delete
    */
   public function testCRUD() {
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
index 5667e42..86041e1 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/MTimeProtectedFileStorageBase.php
@@ -54,7 +54,6 @@ protected function setUp() {
    * @covers ::load
    * @covers ::save
    * @covers ::delete
-   * @covers ::exists
    */
   public function testCRUD() {
     $php = new $this->storageClass($this->settings);
@@ -71,6 +70,7 @@ public function testCRUD() {
    * @medium
    */
   public function testSecurity() {
+    /** @var \Drupal\Component\PhpStorage\PhpStorageInterface $php */
     $php = new $this->storageClass($this->settings);
     $name = 'simpletest.php';
     $php->save($name, '<?php');
@@ -120,7 +120,6 @@ public function testSecurity() {
       chmod($expected_filename, 0400);
       chmod($expected_directory, 0100);
       $this->assertSame(file_get_contents($expected_filename), $untrusted_code);
-      $this->assertSame($php->exists($name), $this->expected[$i]);
       $this->assertSame($php->load($name), $this->expected[$i]);
       $this->assertSame($GLOBALS['hacked'], $this->expected[$i]);
     }
diff --git a/core/tests/Drupal/Tests/Component/PhpStorage/PhpStorageTestBase.php b/core/tests/Drupal/Tests/Component/PhpStorage/PhpStorageTestBase.php
index fabf301..535c272 100644
--- a/core/tests/Drupal/Tests/Component/PhpStorage/PhpStorageTestBase.php
+++ b/core/tests/Drupal/Tests/Component/PhpStorage/PhpStorageTestBase.php
@@ -46,16 +46,12 @@ public function assertCRUD($php) {
     $code = "<?php\n\$GLOBALS[$random] = TRUE;";
     $success = $php->save($name, $code);
     $this->assertTrue($success, 'Saved php file');
-    $php->load($name);
+    $this->assertTrue($php->load($name), 'Load works correctly');
     $this->assertTrue($GLOBALS[$random], 'File saved correctly with correct value');
 
-    // If the file was successfully loaded, it must also exist, but ensure the
-    // exists() method returns that correctly.
-    $this->assertTrue($php->exists($name), 'Exists works correctly');
-
     // Delete the file, and then ensure exists() returns FALSE.
     $this->assertTrue($php->delete($name), 'Delete suceeded');
-    $this->assertFalse($php->exists($name), 'Delete deleted file');
+    $this->assertFalse($php->load($name), 'Delete deleted file');
 
     // Ensure delete() can be called on a non-existing file. It should return
     // FALSE, but not trigger errors.