fix: skip tipset if it's part of current chain in chain follower

[hanabi1224]

Summary of changes

Changes introduced in this pull request:

• to not rewind the head when receiving old heads from laggy peers

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
  • Optimized chain synchronization to eliminate redundant tipset processing when equivalent blocks already exist at the same height, improving sync efficiency.

[coderabbitai]

Walkthrough

This PR optimizes chain synchronization by adding an early-exit check to SyncStateMachine::add_full_tipset that skips redundant insertion/merging when the chain index already contains an equivalent tipset at the same epoch height, queried via chain_index().tipset_by_height() with null-tipset resolution strategy.

Changes

Chain Follower Duplicate Tipset Avoidance

Layer / File(s)	Summary
Imports src/chain_sync/chain_follower.rs	ResolveNullTipset type imported from crate::chain::index to enable height-based tipset resolution with strategy selection.
Core Deduplication Logic src/chain_sync/chain_follower.rs	Early-exit check added in SyncStateMachine::add_full_tipset that queries the chain index for a tipset at the incoming tipset's height and returns early if the resolved tipset key matches the incoming tipset key, preventing redundant insertion/merging.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• ChainSafe/forest#6780: Early-return logic calls ChainIndex::tipset_by_height to avoid inserting duplicate tipsets, directly relating to that PR's modifications to tipset_by_height cache and semantics.
• ChainSafe/forest#6880: Both PRs modify behavior around tipset_by_height; this PR calls it with ResolveNullTipset::TakeOlder to prevent redundant inserts while that PR changes the function's underlying cache and skip-interval logic.

Suggested reviewers

• LesnyRumcajs
• akaladarshi

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: skip tipset if it's part of current chain in chain follower' directly and clearly describes the main change: preventing insertion/merging of tipsets that are already in the current chain.
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.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ 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/chain-follower-fix

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch hm/chain-follower-fix

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

🧹 Nitpick comments (1)

src/chain_sync/chain_follower.rs (1)

701-709: ⚡ Quick win

Reuse the existing heaviest snapshot in this guard.

Line 704 fetches head again even though heaviest was already read at Line 688. Reusing it avoids mixed-head decisions and an extra lookup.

Suggested diff

-        if let Ok(Some(ts)) = self.cs.chain_index().tipset_by_height(
+        if let Ok(Some(ts)) = self.cs.chain_index().tipset_by_height(
             tipset.epoch(),
-            self.cs.heaviest_tipset(),
+            heaviest.clone(),
             ResolveNullTipset::TakeOlder,
         ) && ts.key() == tipset.key()
         {
             return;
         }

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/chain_sync/chain_follower.rs` around lines 701 - 709, The guard is
re-fetching the current head instead of reusing the previously read `heaviest`
snapshot, which can cause mixed-head decisions and an extra lookup; change the
call to `self.cs.chain_index().tipset_by_height(..., self.cs.heaviest_tipset(),
...)` to pass the existing `heaviest` variable (the one read at line 688) into
`self.cs.chain_index().tipset_by_height(tipset.epoch(), heaviest,
ResolveNullTipset::TakeOlder)` so the comparison `ts.key() == tipset.key()` uses
a consistent snapshot.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/chain_sync/chain_follower.rs`:
- Around line 701-709: The guard is re-fetching the current head instead of
reusing the previously read `heaviest` snapshot, which can cause mixed-head
decisions and an extra lookup; change the call to
`self.cs.chain_index().tipset_by_height(..., self.cs.heaviest_tipset(), ...)` to
pass the existing `heaviest` variable (the one read at line 688) into
`self.cs.chain_index().tipset_by_height(tipset.epoch(), heaviest,
ResolveNullTipset::TakeOlder)` so the comparison `ts.key() == tipset.key()` uses
a consistent snapshot.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 8f23e39d-42bb-4aff-a9d8-f01124f67f54

📥 Commits

Reviewing files that changed from the base of the PR and between 4cea4c5 and f92e280.

📒 Files selected for processing (1)

• src/chain_sync/chain_follower.rs

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.22%. Comparing base (4cea4c5) to head (f92e280).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/chain_sync/chain_follower.rs	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/chain_sync/chain_follower.rs	38.16% <57.14%> (+0.18%)	⬆️

... and 8 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4cea4c5...f92e280. Read the comment docs.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.