Two Buyers, One Lamp in Go

The shop has one lamp left, and two customers press Buy at the same moment. A handler that first reads the stock and then writes stock - 1 can be fooled: Go's server runs each request in its own goroutine, side by side, so both requests can read 1 before either one writes. Both customers get the lamp, and the stock ends at -1. This is a race condition: the result depends on which request gets there first.

One statement: let the database check and change the value in a single UPDATE, with the check in its WHERE. The database runs one write at a time, so nobody can slip in between the check and the change. Afterwards, res.RowsAffected() gives the number of rows the UPDATE changed: 1 if it worked, 0 if not. Here is the idea for a coupon with a limited number of uses:

res := runExec("UPDATE coupons SET uses_left = uses_left - 1 WHERE code = ? AND uses_left > 0", code)
changed, _ := res.RowsAffected()
if changed == 0 {
	writeJSON(w, 409, map[string]string{"error": "Coupon used up"})
	return
}

409 Conflict says the request is fine, but the current state says no. An UPDATE also changes no row when the id doesn't exist, so when nothing changed, look the product up to choose between 409 and 404.

POST /products/1/buy   →  200  {"bought": 1, "id": 1}
GET /products/1        →  200  {"category": "kitchen", "id": 1, "name": "Mug", "price": 12, "stock": 9}
POST /products/3/buy   →  409  {"error": "Out of stock"}   (another customer bought the last lamp first)
GET /products/3        →  200  {"category": "home", "id": 3, "name": "Lamp", "price": 29, "stock": 0}
POST /products/2/buy   →  409  {"error": "Out of stock"}   (the notebook's stock is 0)
POST /products/99/buy  →  404  {"error": "Product not found"}
POST /products/8/buy   →  200  {"bought": 1, "id": 8}

Your task: add a handler for POST /products/{id}/buy. Take one item off the stock with a single UPDATE … WHERE id = ? AND stock > 0 run through runExec, keep res.RowsAffected() in changed, and answer {"id": …, "bought": 1}. If no row changed, answer 404 Product not found when there is no such product, and 409 Out of stock otherwise.

Assignment
Follow the Coding Tutorial to build a buy handler that never sells more than the shop has.

Hint
Don't read the stock first. Keep the result of runExec in res and check changed == 0. Only then run SELECT id FROM products WHERE id = ? with runQuery, to tell a missing product from a sold-out one. End each error answer with return, or the handler runs on.