docs(sui-indexer): add coin management notes and confirm no race condition

[Rcc999]

Closes: #310

Summary by Sourcery

Document IKA coin management and clarify concurrency guarantees in the sui-indexer.

Enhancements:

• Clarify concurrency assumptions and usage of fetchAllIkaCoins in code comments for the IKA client.

Documentation:

• Add README documentation describing IKA coin usage, coin selection flow, and concurrency behavior in the indexer.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Documents how IKA coin selection and merging work in the Sui indexer and clarifies that concurrency characteristics of the Cloudflare worker prevent race conditions when selecting coins.

Sequence diagram for IKA coin selection and merging in SuiIndexer

sequenceDiagram
    actor CFWorkerCron
    participant SuiIndexer
    participant IkaClientImp
    participant CoinOps
    participant SuiRPC

    CFWorkerCron->>SuiIndexer: processSolvedRedeems()
    loop each_redeem_request
        SuiIndexer->>IkaClientImp: fetchAllIkaCoins(owner)
        IkaClientImp->>SuiRPC: query IKA coins for owner
        SuiRPC-->>IkaClientImp: CoinStruct[]
        IkaClientImp-->>SuiIndexer: CoinStruct[]

        SuiIndexer->>CoinOps: selectCoins(coins, targetAmount)
        CoinOps-->>SuiIndexer: selectedCoins

        alt multiple_coins_needed
            SuiIndexer->>CoinOps: prepareCoin(selectedCoins)
            CoinOps->>SuiRPC: merge coins on chain
            SuiRPC-->>CoinOps: mergedCoin
            CoinOps-->>SuiIndexer: mergedCoin
        else single_coin_sufficient
            SuiIndexer-->>SuiIndexer: use single coin directly
        end

        SuiIndexer->>SuiRPC: presign/sign transaction using coin
        SuiRPC-->>SuiIndexer: transaction result
    end

    %% Concurrency characteristic: only one CFWorkerCron instance runs
    %% and processSolvedRedeems loop is sequential, so no parallel
    %% coin selection occurs

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Class diagram for IkaClientImp IKA coin management method

classDiagram
    class IkaClientImp {
        +fetchAllIkaCoins(owner string) CoinStruct[]
    }

    class CoinStruct

    IkaClientImp ..> CoinStruct : returns

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File-Level Changes

Change	Details	Files
Document IKA coin management behavior, selection algorithm, and concurrency guarantees in the Sui indexer README.	Added an IKA Coin Management section describing the purpose of IKA coins for presign/sign requests and where the coin selection logic lives. Outlined the step-by-step flow: fetching all IKA coins, selecting coins to meet a target amount, and merging coins when multiple are required. Documented concurrency characteristics, noting that Cloudflare workers run one cron at a time and that processSolvedRedeems processes sequentially, eliminating parallel coin selection.	packages/sui-indexer/README.md
Clarify intended usage and concurrency assumptions of fetchAllIkaCoins in the Ika client implementation.	Extended the fetchAllIkaCoins method comment to describe its role in coin selection and merging. Documented the reliance on Cloudflare worker single-cron execution and sequential processSolvedRedeems processing to avoid parallel coin selection. Left a TODO note highlighting the potential need for a maxCoins limit parameter unchanged.	packages/sui-indexer/src/ika_client.ts

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#310	Add an appropriate note or comment in the Ika client explaining the coin selection/merge behavior and its concurrency characteristics.	✅
#310	Determine whether there is a real race condition problem in coin selection (i.e., whether there is any flow where two transactions can select the same coins in parallel without waiting for completion).	✅
#310	If a race condition exists in coin selection, implement a solution to prevent parallel transactions from trying to use the same coins.	✅

Possibly linked issues

• Workers: check the flows to make parallel transactions when selecting ika coin #310: The PR adds IKA coin docs and ika_client comments confirming no concurrent coin selection race condition, matching the issue.
• Workers: check the flows to make parallel transactions when selecting ika coin #310: Yes, the PR documents coin selection and explicitly explains the absence of race conditions in workers

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[sourcery-ai]

Hey - I've left some high level feedback:

• The concurrency explanation relies on Cloudflare cron behavior; consider clarifying or linking to the specific CF guarantee and explicitly scoping the assumption (e.g., what happens with manual triggers, multiple environments, or future parallel workers) so the "no race condition" claim remains accurate over time.
• Since fetchAllIkaCoins is documented as safe because processSolvedRedeems is sequential, it may be worth calling out in the comments that introducing any parallel caller or reusing this helper elsewhere would require revisiting the race-condition assumptions to avoid misuse.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The concurrency explanation relies on Cloudflare cron behavior; consider clarifying or linking to the specific CF guarantee and explicitly scoping the assumption (e.g., what happens with manual triggers, multiple environments, or future parallel workers) so the "no race condition" claim remains accurate over time.
- Since `fetchAllIkaCoins` is documented as safe because `processSolvedRedeems` is sequential, it may be worth calling out in the comments that introducing any parallel caller or reusing this helper elsewhere would require revisiting the race-condition assumptions to avoid misuse.

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