Intermittent Magento Errors - php

Question

We are having issues with our staging machine, which is running Magento EE 1.13.1
The problem started happening Tuesday afternoon.
At that time we updated an installed php / mysql application.
On the main magento site, the browser is redirected to the magento install page.
The problem occurs only within the Magento application and is related to sessions – when the error occurs, magento throws a fatal similar to this:
(I created a small test application to try to isolate the problem).
Fatal error: Call to a member function getCode() on a non-object in /var/www/html/app/code/core/Mage/Customer/Model/Session.php on line 71
Call Stack: 0.0002 653728
1. {main}() /var/www/html/t.php:0 0.3013 8153600
2. Mage::getSingleton() /var/www/html/t.php:20 0.3013 8153912
3. Mage::getModel() /var/www/html/app/Mage.php:477 0.3013 8153912
4. Mage_Core_Model_Config->getModelInstance() /var/www/html/app/Mage.php:463 0.3016 8302032
5. Mage_Customer_Model_Session->__construct() /var/www/html/app/code/core/Mage/Core/Model/Config.php:1348
The issue only occurs intermittently, and we can temporarily fix it by stopping and starting the httpd service.
We are running magento ee 1.13.1, php 5.3.3, and httpd 2.2.15
This is happening on a staging machine. Our production machine which has the same system and code on it, it working correctly.
We have tried to turn on/off magento compiling, caching, php apc, changed the php sessions to use the disk rather than /tmp/fs, and changed magento to use the DB for its sessions.
The site fails after several minutes.
There are no errors in the system logs.
Can someone suggest how to debug this?

Answer 1

This turned out to be an issue related to the libxml_disable_entity_loader() function.
The zend framework calls this function libxml_disable_entity_loader(true) and then calls it again with (false).
There is a path through our code that was leaving this set to true.
This setting's scope is not limited to the current php script, but to the current httpd process. I.e. it's sticky, and this prevents php's simplexml_load_file function from loading external files.
I came across this article:
Intermittent simplexml_load_file(): I/O warning on local Joomla site
Which led me discover that libxml_disable_entity_loader is not thread safe, and indeed 'sticky' within httpd processes.

Answer 2

Disable Magento compilation:
$ php -f shell/compiler.php -- disable
Compiler include path disabled
There are three ways in which this can be done:
From Magento admin
Navigate to System → Tools → Compilation page and click on Disable button. Navigate to System → Cache Management screen and use Flush Cache button.
Using SFTP, by editing at includes/config.php file
To disable compilation in Magento, edit includes/config.php. Uncomment the first line and comment out the second:
define('COMPILER_INCLUDE_PATH', dirname(FILE).DIRECTORY_SEPARATOR.'src');
define('COMPILER_COLLECT_PATH', dirname(FILE).DIRECTORY_SEPARATOR.'stat');
Using SSH shell program:
php -f shell/compiler.php -- disable
php -f shell/compiler.php -- clear