fix: improve lookback tipset check error message

[hanabi1224]

Summary of changes

Ports the error message improvement from https://github.com/filecoin-project/lotus/pull/13680/changes#diff-b62548166087e88f8ca2c5613b67b4a44fc090cf65311824d3efbcd1e87182c9R185

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Outside contributions

• I have read and agree to the CONTRIBUTING document.
• I have read and agree to the AI Policy document. I understand that failure to comply with the guidelines will lead to rejection of the pull request.

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of lookback height calculations when the requested round would go past the current chain height.
  • For newer network versions, the system now returns a clear error instead of falling back to an unexpected result.
  • Preserved the previous fallback behavior for older network versions and genesis cases.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 60c619e5-8c44-4bc0-a3b1-0f6d3860a437

📥 Commits

Reviewing files that changed from the base of the PR and between cb9c72b and b940820.

📒 Files selected for processing (2)

• src/chain/store/chain_store.rs
• src/chain/store/errors.rs

🔗 Linked repositories identified

CodeRabbit considers these linked repositories for cross-repo context during reviews:

• filecoin-project/lotus (manual)

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Walkthrough

This PR modifies ChainStore::get_lookback_tipset_for_round_blocking to restrict the previous unconditional fallback (recomputing state root against the heaviest tipset) to network versions ≤ V3 or genesis epoch. A new Error::LookbackHeightOverflow variant is added and returned for other cases.

Changes

Lookback overflow error handling

Layer / File(s)	Summary
New error variant and type import src/chain/store/errors.rs	Adds crate::prelude::* import, switches NullRound to prelude ChainEpoch, and adds LookbackHeightOverflow { lookback_height, base_height } variant with formatted error message.
Guarded lookback fallback logic src/chain/store/chain_store.rs	Restricts the state-root recomputation fallback to network version ≤ V3 or genesis epoch; returns Error::LookbackHeightOverflow otherwise when lookback height exceeds the heaviest tipset epoch.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• ChainSafe/forest#7289: Both PRs modify ChainStore::get_lookback_tipset_for_round_blocking, changing its tipset lookup/error behavior.
• ChainSafe/forest#7290: Both PRs modify the same function, adjusting the fallback branch logic around genesis/overflow handling.

Suggested reviewers: LesnyRumcajs, akaladarshi

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: stricter lookback tipset handling with an improved error message.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch hm/strict-lookback-check

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch hm/strict-lookback-check

Warning

Review ran into problems

🔥 Problems

Linked repositories: Couldn't analyze filecoin-project/lotus - clone failed: Clone operation failed: 13 INTERNAL: Received RST_STREAM with code 2 (Internal server error)

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.31%. Comparing base (b2b29fa) to head (b940820).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/chain/store/chain_store.rs	0.00%	17 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/chain/store/errors.rs	12.50% <ø> (ø)
src/chain/store/chain_store.rs	69.92% <0.00%> (-0.71%)	⬇️

... and 10 files with indirect coverage changes

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[akaladarshi]

@hanabi1224 I think adding a small unit test for this change will be helpful

nit: You can use the guard-first shape here, but that just my personal preference, you can leave or take it.