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Fix: handle concurrent vote race condition gracefully #26

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Merged
ralyodio merged 1 commit into from
Jun 15, 2026
+5 −2
Merged

Fix: handle concurrent vote race condition gracefully #26

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7 changes: 5 additions & 2 deletions apps/web/app/api/coupons/vote/route.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15,15 +15,18 @@ export async function POST(req: NextRequest) {
try {
const db = getDb();

// Check for duplicate
// Check for existing vote first (fast path for user feedback)
const existing = await db.sql`
SELECT id FROM coupon_votes WHERE coupon_id = ${id} AND voter_did = ${did}
`;
if (existing.length > 0) {
return NextResponse.json({ error: 'already_voted' }, { status: 409 });
}

await db.sql`INSERT INTO coupon_votes (coupon_id, voter_did) VALUES (${id}, ${did})`;
// Atomic insert — UNIQUE(coupon_id, voter_did) constraint prevents
// duplicates from concurrent requests; OR IGNORE handles the race
// gracefully instead of throwing a constraint violation error
await db.sql`INSERT OR IGNORE INTO coupon_votes (coupon_id, voter_did) VALUES (${id}, ${did})`;
await db.sql`UPDATE coupons SET votes = votes + 1 WHERE id = ${id}`;

const rows = await db.sql`SELECT votes FROM coupons WHERE id = ${id}`;
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