chore: limit Vec preallocation to follow serde convention

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• limit Vec preallocation to follow serde convention
• optimized one CID sweep, it could potentially reduce re-allocations
• see https://docs.serde.rs/src/serde/private/size_hint.rs.html

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

  • Reduced excessive memory preallocation during deserialization by applying a more cautious sizing strategy, improving memory stability.

• Tests

  • Added unit tests validating the cautious sizing behavior across representative sizes.

• Performance

  • Improved vector pre-allocation during bulk reads to reduce reallocations and improve efficiency.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

Adds a private helper size_hint_cautious_cid(...) that clamps preallocation for CID collections (1 MiB cap) and uses it in CBOR CID deserialization to compute reserved capacity; also preallocates the CID Vec in a chain store read path. No public API changes.

Changes

Cohort / File(s)	Summary
Encoding helper & tests src/utils/encoding/mod.rs	Adds private size_hint_cautious_cid(size_hint: usize) -> usize with MAX_PREALLOC_BYTES = 1 MiB and unit tests covering 0 and 1 MiB hints.
CID deserialization src/utils/encoding/cid_de_cbor.rs	Replace direct reserve from size hints with capacity computed via size_hint_cautious_cid(...) on both MapAccess and SeqAccess deserialization paths (affects allocation sizing only).
Chain store prealloc src/chain/store/chain_store.rs	Initialize CID result vector with Vec::with_capacity(amt.count() as usize) and populate via Amt::for_each_cacheless instead of index-based get loop.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• fix: reduce allocations in chain traversal #6009: Related changes touching src/utils/encoding/cid_de_cbor.rs allocation strategy (switch to SmallVec and adjust deserialization).

Suggested labels

rust

Suggested reviewers

• akaladarshi
• sudo-shashank

🚥 Pre-merge checks | ✅ 4

✅ 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 accurately summarizes the main change: limiting Vec preallocation during deserialization to follow serde convention, which is the primary objective of the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into main

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✨ 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 limit-allocs-deserialization

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/utils/encoding/mod.rs`:
- Around line 12-18: The helper size_hint_cautious<T> can divide by zero for
zero-sized types (ZSTs); update it to first check std::mem::size_of::<T>() and
if it is 0 return size_hint unchanged (or otherwise avoid dividing) before
computing MAX_PREALLOC_BYTES / size; modify the body of size_hint_cautious to
guard on the size (using a local let size = std::mem::size_of::<T>() and an
early return when size == 0) and then compute size_hint.min(MAX_PREALLOC_BYTES /
size).

🧹 Nitpick comments (1)

src/utils/encoding/mod.rs (1)

270-279: Add a ZST case to cover the new guard.

After guarding size_of::<T>() == 0, add a () test case so regressions are caught.

🧪 Suggested test tweak

     fn size_hint_cautious_test() {
         assert_eq!(size_hint_cautious::<u8>(0), 0);
         assert_eq!(size_hint_cautious::<u8>(1024 * 1024), 1024 * 1024);
         assert_eq!(size_hint_cautious::<u8>(2 * 1024 * 1024), 1024 * 1024);
         assert_eq!(
             size_hint_cautious::<u64>(2 * 1024 * 1024),
             (1024 * 1024) / std::mem::size_of::<u64>()
         );
+        assert_eq!(size_hint_cautious::<()>(2 * 1024 * 1024), 1024 * 1024);
     }

[akaladarshi]

@LesnyRumcajs CI is failing

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 63.21%. Comparing base (63a6089) to head (99c495a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/chain/store/chain_store.rs	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/utils/encoding/cid_de_cbor.rs	85.82% <100.00%> (+0.21%)	⬆️
src/utils/encoding/mod.rs	100.00% <100.00%> (ø)
src/chain/store/chain_store.rs	68.67% <80.00%> (ø)

... and 12 files with indirect coverage changes

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