fix: await processFn to prevent buffer pool race condition

[lodekeeper]

Motivation

Fixes a race condition that can cause state corruption and First offset must equal to fixedEnd errors on restart.

See discussion: https://discord.com/channels/593655374469660673/1469368525180113078

Description

The using keyword in serializeState.ts releases the buffer back to the pool when the block exits. Since processFn is async (returns a Promise), the buffer was being released before the DB write completed.

If another serialization (checkpoint state or archive state) happened before the write finished, it would:

1. Get the same buffer from the pool
2. Call fill(0) on it (per BufferPool.alloc behavior)
3. Corrupt the data being written by the first serialization

This could cause First offset must equal to fixedEnd 0 != <large number> errors on restart when the corrupted state is read.

Fix

Add await before processFn(stateBytes) to ensure the buffer is not released until the async operation completes.

---

AI Disclosure: This PR was authored with AI assistance (Lodekeeper/Claude).

[gemini-code-assist]

Summary of Changes

Hello @lodekeeper, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a significant data integrity issue by correcting a race condition during state serialization. By ensuring that buffers are not returned to the pool until all asynchronous write operations are complete, it prevents potential state corruption and system errors, enhancing the stability and reliability of the application.

Highlights

• Race Condition Fix: Addressed a critical race condition in state serialization that could lead to state corruption and 'First offset must equal to fixedEnd' errors upon system restart.
• Root Cause Identified: The issue stemmed from a buffer being prematurely released back to the buffer pool by the using keyword in serializeState.ts because the processFn was an async operation that was not awaited, allowing another serialization to reuse and corrupt the buffer before the initial write completed.
• Solution Implemented: The fix involves adding an await keyword before processFn(stateBytes) to ensure that the buffer is not released until the asynchronous database write operation has fully completed, thereby preventing data corruption.

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Changelog

• packages/beacon-node/src/chain/serializeState.ts
  • Modified the serializeState function to await the processFn call.
  • Added a comment explaining the necessity of await to prevent premature buffer release.

Activity

• The pull request was authored with AI assistance (Lodekeeper/Claude).
• A discussion related to this issue can be found on Discord: https://discord.com/channels/593655374469660673/1469368525180113078

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[gemini-code-assist]

Code Review

This pull request effectively resolves a critical race condition within the serializeState function. By adding await before processFn(stateBytes), the buffer is now guaranteed to not be released back to the pool until the asynchronous operation completes. This directly prevents the state corruption and First offset must equal to fixedEnd errors described in the motivation. The added comment provides excellent clarity on the purpose of this crucial change, significantly improving code maintainability and understanding for future developers.

[nflaig]

going to merge this so that we can cut another rc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.34%. Comparing base (ba49cac) to head (215ad70).
⚠️ Report is 1 commits behind head on unstable.

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable    #8877   +/-   ##
=========================================
  Coverage     52.34%   52.34%           
=========================================
  Files           848      848           
  Lines         63488    63487    -1     
  Branches       4704     4704           
=========================================
  Hits          33230    33230           
+ Misses        30189    30188    -1     
  Partials         69       69           

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