Below I'll explain how I've replicated racing condition. Feel free to use other preferred method if needed.

in Admin Console:

• create a tangible product
• create an Incomplete order (can be without products as well)
• go to Configuration → E-commerce → General section
• confirm, that Comm_Next_Order_Number system setting value is 2 (if you've created orders before it will be last order number plus one)
• try entering larger value (e.g. 5)
• confirm, that value is saved
• try entering lower value (lower, than last created order number)
• confirm, that validation error is shown exposing actual minimal value that can be entered in that field

in PhpStorm

• enable debugger
• make sure that parallel debug is enabled:
  
• put breakpoint on the line where LOCK ... statement is in changed code

on Front-End

1. open 2 browser tabs (one normal and one private so that cookies are not shared)
2. in each tab:
   • go to category category where product as added
   • enable Xdebug
   • press Buy Now button
3. confirm, that 2 debug connections were created in PhpStorm
4. allow the LOCK ... sql to be executed in 1st debug session
5. allow the LOCK ... sql to be executed in 2nd debug session
6. confirm, that in 2nd debug session debugging is impossible due attempt of lock tables, that are already locked
7. confirm, that you can open any of Configuration section (that read from SystemSettings table) in the Admin Console
8. let code from both debug sessions be executed till the end
9. confirm, that:
   • both created orders have different Number column value
   • the value of Comm_Next_Order_Number system setting (in Configuration → E-commerce → General section) has next available order number (+1 then last used one)