Description:
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... under not yet identified descriptions.

This is part of our efforts to get Drupal 8 green on PHP 7, see https://www.drupal.org/node/2454439.

At some point a very strange bug was introduced in PHP 7, this is a relatively new bug. Many tests have fails, I haven't checked yet if it's the same bug for all of them.

You can reproduce by running MemoryBackendUnitTest. https://bugs.php.net/bug.php?id=69371 for example has instructions on how to run a test.

The problematic code is in MemoryBackend::prepareItem():

    $prepared = clone $cache;
    $prepared->data = unserialize($prepared->data);

This is called repeatedly and on the second call, $cache->data is already unserialized which results in a notice. So it looks like the clone doesn't fully work. $cache is a stdClass object.

I tried to extract it into a standalone script but that doesn't fail.

When debugging, I noticed that both cloned object have the same hash (spl_object_hash()) and that both $prepared->data and $cache->data are already unserialized.

Expected result:
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No error

Actual result:
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unserialize(): Error at offset 0 of 6 bytes
unserialize('foobar')
Drupal\Core\Cache\MemoryBackend->prepareItem(Object, 1)
Drupal\Core\Cache\MemoryBackend->get('test3', 1)
Drupal\system\Tests\Cache\GenericCacheBackendUnitTestBase->testSetGet()
Drupal\simpletest\TestBase->run()
_simpletest_batch_operation(Array, '7', Array)
_batch_process()
_batch_do()
_batch_page(Object)
Drupal\system\Controller\BatchController->batchPage(Object)
call_user_func_array(Array, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)