Concurrent checkouts leads to incorrect value for product stock

When two customers check out concurrently for the same product, only one of the checkouts may be reflected in the final value for product stock.

Steps to reproduce:

Start demo site, create two customers, create/pick test product.
Both customers add the product their cart and Perform a checkout concurrently, making sure both customers finish checkout (click the Submit Order button) as close to the same time as possible.

We have reproduced this behavior on a single machine, by performing the above steps with one customer in one browser window and another customer in a second browser window with a short communication delay (100 ms) between the database and application. If connecting to the database using unix sockets, the following script can act as a proxy that delays packets: https://gist.github.com/TWarszawski/a0d8dd8aea9eb5b774d64c9f826de6db

Expected Result: The final product stock value is original - 2.

Actual Result: The final product stock value is original - 1, or the application crashes. See log: stacktrace.txt

diefenbach / django-lfs

Concurrent checkouts leads to incorrect value for product stock #203