refactor: remove build() and async shuffling calculation

[guha-rahul]

Motivation

• When lodestar-z happens, BeaconStateAllForks will be a blocker and The build() method in ShufflingCache depends on it.
• Post-Fulu proposer lookahead is stored in BeaconState, requiring shufflings synchronously during epoch transitions—making the async build() pattern no longer viable.

Description

• Remove build() method from IShufflingCache interface and ShufflingCache class
• Add set() to IShufflingCache interface to add shufflings
• Remove asyncShufflingCalculation

Closes #8653

AI Assistance Disclosure

• External Contributors: I have read the contributor guidelines and disclosed my usage of AI below.
  use claude to understand how ShufflingCache avoids recomputation

[gemini-code-assist]

Summary of Changes

Hello @guha-rahul, 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 refactors the shuffling calculation mechanism by removing the asynchronous build() method and the asyncShufflingCalculation flag. This change is motivated by upcoming lodestar-z compatibility issues with BeaconStateAllForks and the need for synchronous shuffling calculations for proposer lookahead in the post-Fulu era. The IShufflingCache interface has been updated to reflect these changes, now providing a set() method for directly adding shufflings.

Highlights

• Removal of Asynchronous Shuffling: The build() method and the asyncShufflingCalculation flag have been removed, shifting shuffling computations to a synchronous model to align with new requirements.
• IShufflingCache Interface Update: The build() method in the IShufflingCache interface has been replaced with a set() method, allowing direct addition of EpochShuffling objects to the cache.
• BeaconStateAllForks Compatibility: The changes address a future blocker related to BeaconStateAllForks and lodestar-z by removing the dependency of the build() method on it.
• Post-Fulu Requirements: This refactor ensures that proposer lookahead, which requires synchronous shuffling post-Fulu, is correctly handled within the system.

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[gemini-code-assist]

Code Review

This pull request effectively refactors the shuffling calculation by removing the asynchronous build() method and the asyncShufflingCalculation option. This change simplifies the codebase by enforcing synchronous shuffling calculation, which is a necessary step for upcoming features like post-Fulu proposer lookahead. The changes are consistent across all modified files, including interfaces, implementations, and tests. The IShufflingCache interface is updated to use a set() method, and the implementation in ShufflingCache correctly makes this method public. The epoch processing logic in EpochCache and EpochTransitionCache is also cleaned up by removing the now-obsolete async path. Overall, this is a solid refactoring. I have a couple of minor suggestions to improve code clarity and documentation.

[gemini-code-assist]

This comment is misleading as the asynchronous shuffling build logic has been removed in this pull request. It should be removed to avoid confusion for future developers.

[gemini-code-assist]

The comment for the set method in the IShufflingCache interface could be more descriptive. The implementation in ShufflingCache also handles resolving promises if they exist. Reflecting this behavior in the interface documentation would improve clarity for developers using this interface.

Suggested change

	/**
	* Queue asynchronous build for an EpochShuffling
	* Add an EpochShuffling to the ShufflingCache.
	*/
	/**
	* Add an EpochShuffling to the ShufflingCache. If a promise for the shuffling is present it will
	* resolve the promise with the built shuffling.
	*/

[wemeetagain]

generally lgtm

[twoeths]

this PR misses the context of #6938:

• state-transition should not have ShufflingCache dependency, we will not have it in lodestar-z also
• beacon-node should populate the ShufflingCache instead, see https://github.com/ChainSafe/lodestar/pull/6938/changes#diff-c21a87c055e2b34c02de2fa28ecc52c53837159f567eef5fb22e80ed55827044L341 . That's also the direction for integrating lodestar-z state-transition, need to think how to pull shuffling there
• a lot of metrics in ShuffllingCache introduced by #6938 becomes useless
• if we regen state for attestation verification, we should also populate ShufflingCache https://github.com/ChainSafe/lodestar/pull/6938/changes#diff-29e6daed96db2b885b1c4f516eb55bf7e6916e298d50f78bd2f8c57811481d2fL902
• add more comments on why we have to compute shuffling for epoch n + 2 postfulu (with proposer lookahead spec change)

in the end it'd be more about reverting #6938, and we should deploy on a feature group to make sure there is no regression
also would like @matthewkeil to have a close look to make sure we don't miss anything

[matthewkeil]

this PR misses the context of #6938:

• state-transition should not have ShufflingCache dependency, we will not have it in lodestar-z also
• beacon-node should populate the ShufflingCache instead, see https://github.com/ChainSafe/lodestar/pull/6938/changes#diff-c21a87c055e2b34c02de2fa28ecc52c53837159f567eef5fb22e80ed55827044L341 . That's also the direction for integrating lodestar-z state-transition, need to think how to pull shuffling there
• a lot of metrics in ShuffllingCache introduced by feat: async shuffling refactor #6938 becomes useless
• if we regen state for attestation verification, we should also populate ShufflingCache https://github.com/ChainSafe/lodestar/pull/6938/changes#diff-29e6daed96db2b885b1c4f516eb55bf7e6916e298d50f78bd2f8c57811481d2fL902
• add more comments on why we have to compute shuffling for epoch n + 2 postfulu (with proposer lookahead spec change)

in the end it'd be more about reverting #6938, and we should deploy on a feature group to make sure there is no regression also would like @matthewkeil to have a close look to make sure we don't miss anything

@twoeths It looks like you covered changes that will get reverted from #6938. The gist Rahul will be that there should be no shuffling cache on the epochCtx anymore. I think if you remove that from the EpochCache and follow all the codepaths that break from the change you will find most of the other hanging references not mentioned above.