Buying same product simultaneously in drupal commerce Case: There is a product in stock with 1 quantity. Now two or more buyers try to buy that product simultaneously. 

I want to restrict any one from buying that product on first come first serve basis.

Is there any hook available ?

Please help.
Check out Commerce Stock - https://www.drupal.org/project/commerce_stock. The module itself will expose hooks for controlling stock. Or there are sub modules it provides - Simple Stock and Simple Stock Rules - which add an integer field and use Rules to validate purchases.

This runs on Add to Cart, Can Checkout, and prior to purchase.