feat(mix): add coverRateFraction config to cover traffic

[chaitanyaprem]

Summary

Adds the cover_rate_fraction parameter (f ∈ (0.0, 1.0], default 0.7) to ConstantRateCoverTraffic, per Mix Cover Traffic spec §7. f scales cover emission rate relative to the max safe rate R / ((1 + L) * P), reserving budget headroom for forwarding spikes.

• Emission interval: ((1 + L) * P) / (f * R)
• Precompute target: (f * R) / (1 + L) (stored as field, not recomputed)

This helps reduce amount of cover traffic to plan and inturn gives more room for other traffic and also reduces proof generation overhead on the node.

Affected Areas

• Gossipsub
• Transports
• Peer Management / Discovery
• Protocol Logic — mix cover traffic
• Build / Tooling
• Other

Compatibility & Downstream Validation

API is backward-compatible; new param has a default. Cover traffic is not yet integrated in Nimbus/Waku/Codex.

Impact on Library Users

• New optional coverRateFraction: float = 0.7 on ConstantRateCoverTraffic.new().

Risk Assessment

Backward-compatible API; default behavior change is intentional per spec.

References

• Mix Cover Traffic spec update: feat: Mix Cover Traffic specification logos-co/logos-lips#311
• Prior cover traffic PR: feat(mix): cover traffic with constant rate #2243

[Copilot]

Pull request overview

Adds a configurable coverRateFraction to Mix ConstantRateCoverTraffic to scale cover emission/precompute rates below the maximum safe rate defined by the spec, with accompanying unit tests.

Changes:

• Introduces coverRateFraction: float = 0.7 (validated to (0.0, 1.0]) and exposes it via an accessor.
• Updates emission interval and precomputation sizing logic to incorporate the fraction and precomputes precomputeTarget.
• Adds unit tests covering the new formula, batch sizing impact, range validation, and a small-f edge case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
libp2p/protocols/mix/cover_traffic.nim	Adds coverRateFraction, adjusts emission/precompute computations, stores precomputeTarget, and exposes an accessor.
tests/libp2p/mix/test_cover_traffic.nim	Adds a new suite validating interval math, batch sizing behavior, parameter validation, and a small-fraction edge case.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.46%. Comparing base (adb2743) to head (4242c5c).

Files with missing lines	Patch %	Lines
libp2p/protocols/mix/cover_traffic.nim	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2322      +/-   ##
==========================================
+ Coverage   73.42%   73.46%   +0.03%     
==========================================
  Files         168      168              
  Lines       22221    22229       +8     
  Branches       20       20              
==========================================
+ Hits        16316    16330      +14     
+ Misses       5905     5899       -6     

Files with missing lines	Coverage Δ
libp2p/protocols/mix/cover_traffic.nim	73.38% <90.00%> (+1.72%)	⬆️

... and 5 files with indirect coverage changes

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[jm-clius]

As far as I can see, this matches the new spec and design intention. :)

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

scaledSlots is derived via (totalSlots.float * coverRateFraction).int, which makes the effective slot count sensitive to floating-point representation/truncation (as the test comment notes). Common decimal fractions (e.g. 0.7) can produce values like 69.999… for R=100 and truncate to 69, leading to an off-by-one emission rate and platform-dependent behavior. Consider making the conversion deterministic (e.g., apply an explicit floor with a small epsilon, or use a fixed-point/rational conversion) so coverRateFraction=0.7 reliably maps to 70% of R.