Concurrent (quick reload) requests on checkout cause cart to empty - related to session_regenerate_id

[minlare]

Preconditions

1. Magento 2.2.0
2. PHP7

Steps to reproduce

1. Add product to cart
2. Navigate to checkout
3. Reload the page as quick as possible (F5 multiple times)

Expected result

1. Reload the checkout as usual
2. Cart contains session items

Actual result

1. Redirects to empty cart
2. session_id() is old session id (previous request regenerated a new session_id, but this request has old session_id)

http://php.net/manual/en/function.session-regenerate-id.php

In https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/Session/SessionManager.php#L507 you reference http://php.net/manual/en/function.session-regenerate-id.php#53480 however I believe on concurrent requests this does not work correctly...

[minlare]

I see you have re-opened the issue yourselves... Does that mean you were able to reproduce?

[magento-engcom-team]

@minlare, thank you for your report.
We've created internal ticket(s) MAGETWO-84160 to track progress on the issue.