client-lib: fee support in RedeemNote

[louisinger]

related to arkade-os/go-sdk#98

@altafan please review

Summary by CodeRabbit

• Bug Fixes
  • Improved redemption fee handling: fees are now assessed per note and validated so they never exceed note value; individual note contributions and the final redeemable amount are adjusted accordingly.
  • Added off-chain address validation and output-fee calculation to ensure accurate, fee-aware final transfers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 9ffbe74 and 575d784.

📒 Files selected for processing (1)

• pkg/client-lib/batch_session.go

🚧 Files skipped from review as they are similar to previous changes (1)

• pkg/client-lib/batch_session.go

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Walkthrough

RedeemNotes now initializes a fee estimator from account info, computes and validates per-note input fees (rejecting notes where fees >= note value), aggregates adjusted note amounts, resolves offchain addresses, computes output fees for the transfer, and subtracts output fees from the receiver amount before forming final receivers.

Changes

Cohort / File(s)	Summary
Fee Estimation & Redemption Logic pkg/client-lib/batch_session.go	RedeemNotes updated to fetch account info, initialize a fee estimator, compute per-note input fees with validation and per-note amount adjustments, aggregate adjusted amounts, fetch offchain addresses, compute per-transfer output fees, subtract output fees from the receiver amount, and construct final receivers.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant RedeemNotes
    participant FeeEstimator
    participant OffchainResolver
    participant ReceiverCreator

    Caller->>RedeemNotes: request redeem(notes)
    RedeemNotes->>FeeEstimator: init from account info
    loop per note
        RedeemNotes->>FeeEstimator: estimate input fee(note)
        FeeEstimator-->>RedeemNotes: input fee
        RedeemNotes->>RedeemNotes: validate fee < note.value, adjust amount
    end
    RedeemNotes->>OffchainResolver: fetch offchain address(es)
    OffchainResolver-->>RedeemNotes: address(es)
    RedeemNotes->>FeeEstimator: estimate output fee(total amount)
    FeeEstimator-->>RedeemNotes: output fee
    RedeemNotes->>ReceiverCreator: create receiver(amount - output fee, address)
    ReceiverCreator-->>Caller: return receivers

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• RedeemNotes missing fee estimation go-sdk#98: Addresses missing fee estimation by adding per-note and output fee calculations and validations in RedeemNotes.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'client-lib: fee support in RedeemNote' directly and clearly summarizes the main change: adding fee support functionality to the RedeemNote method in the client library.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

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• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@pkg/client-lib/batch_session.go`:
- Around line 137-142: The check that validates fees vs amount should reject
cases where outputFeesSats is equal to amount as well as greater; update the
conditional in the function that contains the receiver/fees logic (the block
that currently returns "fees (%d) exceed amount (%d)") to use >= instead of > so
it returns an error for fee-equals-amount, preventing receiver.Amount from being
set to zero and avoiding creation of receiversOutput with a zero-valued
receiver.
- Line 116: Guard the uint64 accumulation to prevent silent wrap: in the loop
where amount is incremented using "amount += noteValue - feesSats", first ensure
noteValue >= feesSats (avoid underflow), compute delta := noteValue - feesSats,
then check overflow before adding (e.g., if amount > math.MaxUint64 - delta) and
return an error if it would overflow; update the code paths that use "amount",
"noteValue", and "feesSats" to handle and propagate this error instead of
allowing wrapping.

---

ℹ️ Review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 0ed1abd and 9ffbe74.

📒 Files selected for processing (1)

• pkg/client-lib/batch_session.go