Idempotency Keys for batch_payout

Overview

The batch_payout_idempotent and batch_payout_idempotent_by entrypoints let callers attach a caller-supplied idempotency key to every batch payout. If the backend retries a payout (e.g. after a network timeout), the contract detects the duplicate key and returns the current state without transferring funds again.

This is the primary defence against double-payment caused by retry storms or at-least-once delivery semantics in the Grainlify backend.

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Entrypoints

batch_payout_idempotent

pub fn batch_payout_idempotent(
    env: Env,
    idempotency_key: String,   // unique caller-supplied key
    recipients: Vec<Address>,
    amounts: Vec<i128>,
) -> ProgramData

batch_payout_idempotent_by (delegate variant)

pub fn batch_payout_idempotent_by(
    env: Env,
    idempotency_key: String,
    caller: Address,           // delegate address
    recipients: Vec<Address>,
    amounts: Vec<i128>,
) -> ProgramData

Both functions share identical idempotency semantics.

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Behaviour

Scenario	Outcome
Key is new	Payout executes; key is stored; BatchPayoutEvent emitted
Key was already consumed	No funds transferred; BatchPayoutReplayedEvent emitted; current ProgramData returned

Key lifecycle

1. On the first call the key is absent from storage → payout proceeds.
2. The key is written to persistent storage only after all transfers succeed (write-after-success prevents a failed payout from consuming the key).
3. On any subsequent call with the same key the contract emits BatchPayoutReplayedEvent and returns immediately.

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Audit Events

BatchPayoutEvent (normal payout)

Emitted on every successful first-time execution.

pub struct BatchPayoutEvent {
    pub version: u32,           // always 2
    pub program_id: String,
    pub recipient_count: u32,
    pub total_amount: i128,
    pub remaining_balance: i128,
    pub idempotency_key: Option<String>, // the key that triggered this payout (if any)
}

BatchPayoutReplayedEvent (replay detected)

Emitted instead of BatchPayoutEvent when a duplicate key is detected. Auditors can use this event to confirm that no double-payment occurred.

pub struct BatchPayoutReplayedEvent {
    pub version: u32,           // always 2
    pub program_id: String,
    pub idempotency_key: String, // the key that was replayed
}

Event topic symbol: BatPayRp

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Storage

Key	Type	Location	Description
PayIdem	Vec<String>	Persistent	Set of consumed idempotency keys

Keys are never expired or pruned. This is intentional: the storage cost is bounded by the number of distinct batch payouts ever executed, which is small relative to the escrow lifetime.

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Choosing an Idempotency Key

The key must be unique per logical payout batch. Recommended strategies:

• UUID v4 generated by the backend before the first attempt.
• Content hash of (program_id, sorted_recipients, amounts, timestamp).

The key must be ≤ 64 bytes (Soroban String limit for this contract).

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Security Assumptions

1. Replay detection is pre-transfer. The key lookup happens before any token transfer, so a replay can never partially execute.
2. Write-after-success. The key is persisted only after all transfers complete. A mid-batch failure leaves the key unconsumed, allowing a safe retry.
3. No key expiry. Keys are permanent. Callers must not reuse keys across logically distinct payouts.
4. Authorization unchanged. Idempotency wraps the existing batch_payout_internal which enforces the same authorization, pause, and circuit-breaker checks.

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Test Coverage

All tests live in contracts/program-escrow/src/test_batch_operations.rs.

Test	What it verifies
test_idempotent_batch_payout_first_call_succeeds	Fresh key executes payout and reduces balance
test_idempotent_batch_payout_replay_no_double_payment	Replay returns same balance; no extra transfer
test_idempotent_batch_payout_replay_emits_audit_event	Replay emits BatchPayoutReplayedEvent with correct key
test_idempotent_batch_payout_audit_trail_integrity	Verifies full audit trail: BATCH_PAYOUT only on first call, BatPayRp on replay
test_idempotent_batch_payout_distinct_keys_all_execute	Three distinct keys each execute independently
test_idempotent_batch_payout_partial_overlap	Mix of new and duplicate keys; only new keys transfer
test_idempotent_batch_payout_complex_retry_interleaving	Complex mix of unique and replayed keys maintains state integrity
test_idempotent_replay_does_not_grow_payout_history	Replays never append to payout_history
test_idempotent_batch_payout_by_replay_no_double_payment	Delegate variant respects idempotency

Run with:

cargo test -p program-escrow test_idempotent