networkconfig: single-source the aggregator-selection formula (#2968 item 5)

[momosh-ssv]

Item 5 of #2968. Pure refactor — zero behavioral change.

beacon/goclient (pre-fork path) and protocol/v2/ssv/runner (post-fork AggregatorCommittee path) each hand-rolled the same selection computation — sha256(slotSig), first 8 bytes little-endian, mod max(1, committeeCount/targetAggregatorsPerCommittee), == 0 — which must stay bit-identical or pre/post-Boole aggregator selection diverges (a consensus-level split on who aggregates).

Extract networkconfig.IsAggregatorSelected and delegate both sites. The runner keeps its mockable IsAggregator field (default now wraps the shared helper); the pooled hasher is dropped (no remaining users; selection runs once per aggregator duty, not per message).

Deep review notes (passed): byte-for-byte equivalence verified in both directions against the pre-change implementations (hash input, digest slicing, endianness, clamp-before-mod order, truncating division); with AggregatorCommitteeRunner already routing through goclient.IsAggregator, every selection path now converges on the single helper, making drift structurally impossible; test seam (protocol/v2/ssv/testing/runner.go overrides) intact; parity test covers the clamp-to-1 case.

[greptile-apps]

Greptile Summary

Centralizes beacon-chain aggregator selection without changing its formula.

• Adds networkconfig.IsAggregatorSelected for the SHA-256, little-endian, modulo-based selection check.
• Delegates both the beacon client and runner implementations to the shared helper.
• Removes the runner’s pooled hasher and adds parity, edge-case, and concurrency coverage.

Confidence Score: 5/5

The PR appears safe to merge with no actionable behavioral, build, or security regressions identified.

Both aggregator-selection paths preserve the existing parameter order and converge on a helper with the same SHA-256 input, little-endian digest interpretation, division, minimum-modulo clamp, and selection comparison.

Important Files Changed

Filename	Overview
networkconfig/aggregator.go	Introduces a shared implementation that preserves the previous hash, byte-order, division, clamp, and modulo behavior.
beacon/goclient/aggregator.go	Replaces the local selection formula with an argument-equivalent call to the shared helper.
protocol/v2/ssv/runner/aggregator.go	Preserves the mockable function field while replacing the pooled hashing closure with the shared stateless helper.
networkconfig/aggregator_test.go	Covers representative committee sizes and independently recomputes the documented selection formula.
protocol/v2/ssv/runner/aggregator_test.go	Verifies runner/helper parity while retaining concurrent determinism coverage.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A[Pre-fork beacon/goclient] --> H[networkconfig.IsAggregatorSelected]
    B[Post-fork AggregatorRunner] --> H
    C[AggregatorCommitteeRunner] --> A
    H --> D[SHA-256 slot signature]
    D --> E[First 8 bytes, little-endian]
    E --> F[Modulo max 1, committee count / target]
    F --> G[Selected when remainder is zero]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.1%. Comparing base (09a74d7) to head (32b3904).

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