Race Condition Lab: Redeem a One-Time Coupon Twice

DealStore's WELCOME10 coupon is good for $10 off, once. The code reads 'has this coupon been used?', sees no, applies the discount, then writes 'used'. Those are three separate steps — and if two requests arrive at the same instant, both can read 'no' before either writes 'used'. Lab at a glance: Advanced · 100 XP · about 20 minutes · runs in your browser, no signup or setup. Try it yourself first; the full solution is behind a spoiler…

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