Race Condition
优惠券偶尔会被多领，单独跑一次又看不出问题。用并发时序复现竞态，修掉它，再反复验证。
T1: read x=0 T2: read x=0 T1: x = x+1 → write 1 T2: x = x+1 → write 1 ← lost update
race-condition · 图文示意