fix(nonce): Koios fallback for η_ph in TICKN computation

[wcatz]

Summary

• TICKN nonce computation failed because blocks table has gaps (epochs 284-612 missing)
• Added fetchLastBlockHashFromKoios() fallback when DB doesn't have blocks for the required epoch
• Enables epoch 614 leaderlog computation before epoch starts

Test plan

• All 20 tests pass
• go vet clean
• Deploy and verify epoch 614 leaderlog computes

🤖 Generated with Claude Code

Summary by CodeRabbit

Bug Fixes

• Improved nonce computation resilience with enhanced data retrieval: The system now implements a two-tier approach for obtaining epoch data—attempting local cache first, then automatically falling back to external data sources if needed—ensuring more reliable nonce calculations even during synchronization interruptions.

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

The changes modify GetNonceForEpoch in fullMode to implement a fallback strategy: when the previous epoch's last block hash is unavailable locally, the code queries Koios instead. A new helper method fetchLastBlockHashFromKoios handles this external API lookup, enabling epoch nonce computation even when local data is missing.

Changes

Cohort / File(s)	Summary
Nonce Fallback Logic nonce.go	Added fetchLastBlockHashFromKoios method to query Koios for a given epoch's last block hash. Modified GetNonceForEpoch (fullMode) to use this fallback when local DB lookup fails or returns empty, enabling graceful degradation to external API when internal data is unavailable.

Sequence Diagram

sequenceDiagram
    participant Client
    participant GetNonceForEpoch
    participant LocalDB
    participant Koios
    
    Client->>GetNonceForEpoch: GetNonceForEpoch(fullMode, candidateNonce exists)
    GetNonceForEpoch->>LocalDB: Query previous epoch's last block hash
    alt Local DB has data
        LocalDB-->>GetNonceForEpoch: Return block hash
        GetNonceForEpoch-->>Client: Compute & return nonce
    else Local DB empty or error
        LocalDB-->>GetNonceForEpoch: Empty/Error
        GetNonceForEpoch->>Koios: fetchLastBlockHashFromKoios(epoch)
        Koios-->>GetNonceForEpoch: Return hex hash
        GetNonceForEpoch-->>Client: Compute & return nonce
    end

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• fix(nonce): remove wrong hardcoded epoch nonces #64: Also modifies nonce.go to enable Koios-based fallback for computing epoch nonces when local data is unavailable.
• fix(nonce): prevent computing nonces from incomplete chain data #76: Updates GetNonceForEpoch full-mode behavior to implement Koios fallback retrieval when local chain data is insufficient.
• fix(nonce): add Koios verification fallback for backfill #61: Extends nonce.go with Koios-based fallback and verification for epoch nonces when local data is missing or inconsistent.

Poem

🐰 A nonce needs a path, both local and far,
When the database sleeps, we look to a star,
Koios retrieves what the cache cannot hold,
Fallback paths shimmer, resilience takes fold! ✨

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/tickn-koios-fallback

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