fix(exit-certificate): AET-04: fail-fast when a token balanceOf batch errors in Step B

[joanestebanr]

🔄 Changes Summary

• Fix (High): fetchAllTokenBalances (Step B1) silently discarded per-token RPC errors (log.Warnf + return), leaving the failed token absent from the balance map. Step C then treated its entire LBT supply as SC-locked and Step D routed the whole supply to exitAddress, excluding the real EOA holders from the certificate.
• Made it fail-fast, consistent with its sibling helpers (classifyAddresses, fetchETHBalances): fetchAllTokenBalances now returns an error and uses errgroup.WithContext (limited to tokenConcurrency) to cancel the remaining scans on the first failure. RunStepB1 propagates it and aborts the pipeline.
• Added a doc comment explaining why the error must not be dropped.

⚠️ Breaking Changes

• None. No config, API, or CLI changes. Behaviour change only: a persistent per-token RPC failure now aborts the pipeline (after the existing RPC retries) instead of producing a misrouted certificate.

📋 Config Updates

• None.

✅ Testing

• 🤖 Automatic: Added TestFetchAllTokenBalancesFailFast (a failing balanceOf batch must return an error, never omit the token). Updated TestFetchAllTokenBalances to the new signature. Full package suite passes (go test ./tools/exit_certificate/) and golangci-lint reports 0 issues.

🐞 Issues

• Closes exit_tool: SP001: When any wrapped token's balanceOf batch fails inside step_b.fetchAllTokenBalances(), the error is silently discarded #1679

🔗 Related PRs

• N/A

📝 Notes

• Chose fail-fast as the default without adding an opt-out flag (e.g. ignoreTokenBalanceFetchError) to keep the config surface minimal; can add one if a knowingly-degraded test mode is desired.

🤖 Generated with Claude Code

[claude]

Claude finished @joanestebanr's task in 1m 11s —— View job

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Code Review Complete ✅

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🚀 What's New

This PR fixes a critical bug in Step B1 of the exit-certificate tool where token balance fetch errors were silently ignored (logged as warnings and dropped). The fix makes fetchAllTokenBalances fail-fast using errgroup.WithContext, ensuring RPC errors abort the pipeline instead of producing misrouted certificates that exclude real EOA holders.

🐛 Bug Fixes

• Critical Fix: fetchAllTokenBalances now returns errors instead of silently dropping failed tokens with log.Warnf. Previously, a persistent RPC error for a token's balanceOf batch would leave that token absent from the balance map, causing Step C to treat the entire LBT supply as SC-locked and misroute it to exitAddress, excluding real EOA holders from the certificate.
• Migrated from manual WaitGroup + semaphore pattern to errgroup.WithContext for proper error propagation and context cancellation.

📋 Config Updates

None.

⚠️ Breaking Changes

None. No config, API, or CLI changes. Behavior change only: A persistent per-token RPC failure now aborts the pipeline (after existing RPC retries) instead of producing an incorrect certificate.

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Review Feedback

🟢 Code Quality: Excellent

Strengths:

1. Perfect use of errgroup - The migration from manual WaitGroup + buffered channel semaphore to errgroup.WithContext is exactly the right pattern for this use case. The code is cleaner and more maintainable.

2. Excellent documentation - The doc comment on fetchAllTokenBalances (lines 261-267) clearly explains why failing fast is critical. This is exactly the kind of comment that prevents future regressions.

3. Proper error wrapping - Error messages include context ("fetch balances for token %s: %w") which will make debugging much easier.

4. Comprehensive test coverage - The new TestFetchAllTokenBalancesFailFast test specifically guards against the regression, and the existing test was properly updated.

5. Consistent with sibling functions - Making this consistent with classifyAddresses and fetchETHBalances improves code maintainability.

🟢 Correctness: Solid

Implementation details verified:

• ✅ tok := token loop variable capture is correct (prevents closure bug)
• ✅ g.SetLimit(tokenConcurrency) properly limits concurrency to 4
• ✅ Context propagation: goroutines use gctx (from errgroup) instead of ctx
• ✅ Mutex protection on tokenBalances map is preserved
• ✅ Error unwrapping chain is correct with %w format

🟢 Security: No Issues

No security concerns identified. The change actually improves security by preventing silent failures that could lead to incorrect certificate generation.

🟢 Performance: Equivalent or Better

• Concurrency limit remains at 4 tokens in parallel (tokenConcurrency)
• Errgroup provides early cancellation on first error, potentially faster failure detection
• No additional allocations or overhead introduced

🟢 Testing: Well Covered

Test coverage analysis:

• ✅ TestFetchAllTokenBalancesFailFast - New test ensures errors propagate
• ✅ TestFetchAllTokenBalances - Updated to check error return
• ✅ newErrorRPCServer - Reusable test helper for simulating RPC failures
• ✅ All existing Step B1 integration tests pass with the new signature

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Summary

This is a high-quality bug fix that addresses a critical correctness issue. The implementation is clean, well-documented, and properly tested. The choice to fail-fast without adding an opt-out flag is correct for this use case—a certificate with missing token balances is worse than no certificate.

Recommendation: Approve and merge. 🚀

No changes requested.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[joanestebanr]

AET-04 Silent Token Balance Fetch Failure In fetchAllTokenBalances