Review a pull request: coupons and withdrawals

Not solved

Money-moving code that is correct one request at a time. Find what breaks when requests arrive together.

src/services/wallet.ts0/2 found

		import { db } from '../db'

	+	export async function redeemCoupon(userId: string, code: string) {
	+	const coupon = await db.one('SELECT * FROM coupons WHERE code = $1', [code])
	+	if (!coupon) throw new HttpError(404, 'Unknown coupon')
	+	if (coupon.redeemed_at) throw new HttpError(409, 'Coupon already used')
	+	await credit(userId, coupon.amount)
	+	await db.query('UPDATE coupons SET redeemed_at = now(), redeemed_by = $2 WHERE id = $1', [coupon.id, userId])
	+	}
	+
	+	export async function withdraw(userId: string, amount: number) {
	+	if (!Number.isInteger(amount) \|\| amount <= 0) throw new HttpError(400, 'Invalid amount')
	+	const wallet = await db.one('SELECT balance FROM wallets WHERE user_id = $1', [userId])
	+	if (wallet.balance < amount) throw new HttpError(402, 'Insufficient funds')
	+	await db.query('UPDATE wallets SET balance = $2 WHERE user_id = $1', [userId, wallet.balance - amount])
	+	await payouts.send(userId, amount)
	+	}

		async function credit(userId: string, amount: number) {
		await db.query('UPDATE wallets SET balance = balance + $2 WHERE user_id = $1', [userId, amount])
		}

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