Moheeb ZaraandClaude Opus 4.8
2ee929658e
Redeeming an invite read uses_count, checked the cap in JS, then did a non-atomic uses_count = uses_count + 1 write AFTER creating the membership. Two concurrent redemptions of a max_uses=1 invite both passed the check and both created a member, and the counter under-counted. Fix: claim a use ATOMICALLY before creating the membership via a compare-and-swap on uses_count (lib/invite-claim.ts) — the UPDATE only writes when uses_count still equals the value the cap decision was based on, so of two racing claims exactly one wins. The loser re-reads and retries while capacity remains (multi-use invites unaffected). On a failed membership insert the claim is rolled back so a slot is not burned. Extracted to lib/invite-claim.ts (out of the 'use server' surface, unit-tested). Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
48 lines
1.7 KiB
TypeScript
48 lines
1.7 KiB
TypeScript
import type { createAdminClient } from '@/lib/supabase/admin'
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type Admin = ReturnType<typeof createAdminClient>
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/**
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* Atomically claim one use of a multi-code invite via compare-and-swap on
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* `uses_count`, closing the check-then-increment TOCTOU where two concurrent
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* redemptions of a single-use invite could both succeed.
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*
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* The UPDATE only writes when `uses_count` still equals the value we based our
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* cap decision on, so of two racing claims exactly one wins; the loser sees a
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* 0-row result, re-reads, and retries while capacity remains. `uses_count` only
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* ever increases, so the CAS has no ABA hazard.
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*
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* Returns the new `uses_count` on success (the caller stores it to roll the
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* claim back if the subsequent membership insert fails), or `null` if the cap
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* is reached.
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*/
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export async function claimInviteUse(
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admin: Admin,
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inviteId: string,
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knownCount: number,
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maxUses: number | null,
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maxAttempts = 5,
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): Promise<number | null> {
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let current = knownCount
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for (let attempt = 0; attempt < maxAttempts; attempt++) {
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if (maxUses !== null && current >= maxUses) return null
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const next = current + 1
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const { data } = await admin
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.from('space_invites')
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.update({ uses_count: next })
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.eq('id', inviteId)
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.eq('uses_count', current) // compare-and-swap guard
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.select('id')
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if (data && data.length > 0) return next
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// Lost the race (or the row changed): re-read and retry while under cap.
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const { data: fresh } = await admin
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.from('space_invites')
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.select('uses_count')
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.eq('id', inviteId)
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.maybeSingle()
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if (!fresh) return null
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current = (fresh as { uses_count: number }).uses_count
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}
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return null
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}
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