Web Setup Wizard 500 error when session storage is configured to use memcache #9633

Preconditions

1. Magento Version: 2.1.6
2. PHP-FPM - Specific pool is currently using 7.0.17
3. Apache 2.2.31

Steps to reproduce

1. Perform a fresh installation of Magento 2.1.6
2. Switch and configure session storage method to memcache in env.php
3. Log into admin and navigate to System > Tools > Web Setup Wizard

Expected result

1. The Setup Wizard page loading successfully

Actual result

1. Page returns an HTTP ERROR 500. Additionally here is the error from the PHP logs when this occurs:

[15-May-2017 07:22:26 UTC] PHP Fatal error: Uncaught Exception: Warning: SessionHandler::read(): open(unix:///var/tmp/memcached.magetwot.216.mage2test.com_sessions.sock?persistent=1&weight=2&timeout=10&retry_interval=0/sess_cvud2fpo9dbmofdhld3fap3682, O_RDWR) failed: No such file or directory (2) in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/Session/SaveHandler/Native.php on line 22 in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/App/ErrorHandler.php:61 Stack trace: #0 [internal function]: Magento\Framework\App\ErrorHandler->handler(2, 'SessionHandler:...', '/chroot/home/ma...', 22, Array) #1 /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/Session/SaveHandler/Native.php(22): SessionHandler->read('cvud2fpo9dbmofd...') #2 /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/Session/SaveHandler.php(93): Magento\Framework\Session\SaveHandler\Native->read('cvud2fpo9dbmofd...') #3 [internal function]: Magento\Framework\Session\SaveHandler->read('cvud2fpo9dbmofd...') #4 in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/App/ErrorHandler.php on line 61

[15-May-2017 07:22:26 UTC] PHP Fatal error: Uncaught Exception: Warning: SessionHandler::write(): open(unix:///var/tmp/memcached.magetwot.216.mage2test.com_sessions.sock?persistent=1&weight=2&timeout=10&retry_interval=0/sess_cvud2fpo9dbmofdhld3fap3682, O_RDWR) failed: No such file or directory (2) in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/Session/SaveHandler.php on line 105 in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/App/ErrorHandler.php:61 Stack trace: #0 [internal function]: Magento\Framework\App\ErrorHandler->handler(2, 'SessionHandler:...', '/chroot/home/ma...', 105, Array) #1 /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/Session/SaveHandler.php(105): SessionHandler->write('cvud2fpo9dbmofd...', '') #2 [internal function]: Magento\Framework\Session\SaveHandler->write('cvud2fpo9dbmofd...', '') #3 {main} thrown in /chroot/home/magetwot/216.mage2test.com/html/vendor/magento/framework/App/ErrorHandler.php on line 61

As the title states. When the session storage method is set to memcache an 500 error happens when navigating to the Web Setup Wizard. With the session method set to files it loads as intended. Based off the error it seems like it's still trying to write/read the session as a file even though magento is configured to use the memcache socket.

I've found as a workaround if i specifically set php_value[session.save_handler] = memcache in the PHP-FPM pool configuration file for this pool then the Setup Wizard page loads successfully. In the past this change has not been needed. Simply configuring memcache in env.php was enough.