fix(chunk-post): key sorted-peer memo by set, not length

[vilenarios]

What

ArweaveCompositeClient.getSortedChunkPostPeers memoizes its weighted peer ordering with a normalizer of eligiblePeers.length. Two different eligible-peer sets of equal size therefore share one cached ordering for the full CHUNK_POST_SORTED_PEERS_CACHE_DURATION_MS window (10s default).

This keys the memo by the set of peer ids instead (order-independent slice().sort().join()).

Why it matters

The length-only key quietly narrows chunk-POST seeding diversity: a churning-but-same-size peer set keeps reusing the first set's ordering, so the same peers get prioritized for up to 10s even as the eligible set changes. For a gateway whose job is to be a good seeding target, fanout diversity is exactly what we don't want to silently pin.

Surfaced during an architecture review of chunk fanout (the broader durability work concluded the gateway should optimize fanout quality, not duplicate the bundler's durability engine — this is one of those quality fixes).

Cost

Still cheap: one sort+join over the bounded postChunk peer set (tens–low hundreds of entries), far less than the weighted selectPeers + filtering the memo guards. The 10s maxAge still absorbs gradual weight drift. Does not mutate the caller's array.

Tests

Extracted the keying as the pure, exported chunkPostPeersCacheKey and unit-tested it directly:

• distinct same-length sets ⇒ distinct keys (the bug)
• order-independent for the same set
• no input mutation
• empty set

yarn test:file src/arweave/composite-client.test.ts → 15/15 pass; eslint clean.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 0fe8dbe4-b95b-40d3-8970-df6889e570ae

📥 Commits

Reviewing files that changed from the base of the PR and between 55ef923 and d43b86b.

📒 Files selected for processing (2)

• src/arweave/composite-client.test.ts
• src/arweave/composite-client.ts

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📝 Walkthrough

Walkthrough

Adds an exported chunkPostPeersCacheKey helper that generates a stable, order-independent memoization key by sorting a copy of the peer list and joining it. Updates getSortedChunkPostPeers to use this key instead of eligiblePeers.length, and adds tests covering collision-avoidance, order-independence, non-mutation, and empty input.

Peer memoization key fix

Layer / File(s)	Summary
chunkPostPeersCacheKey helper and memoizer update src/arweave/composite-client.ts	Exports chunkPostPeersCacheKey that sorts a copy of the peer array and joins it; updates the getSortedChunkPostPeers normalizer to use this set-based key instead of eligiblePeers.length.
Tests for chunkPostPeersCacheKey src/arweave/composite-client.test.ts	Adds import and a describe block verifying no same-length collisions, order-independence, no input mutation, and empty-string result for an empty peer array.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Possibly related PRs

• ar-io/ar-io-node#445: Introduced the memoized peer sorting architecture in composite-client.ts that this PR's memoization key fix directly targets.
• ar-io/ar-io-node#468: Modifies the preferred-vs-non-preferred peer ordering logic inside the same getSortedChunkPostPeers function updated here.

Suggested reviewers

• djwhitt

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: switching chunk-post peer memoization from length-based to set-based keying.
Description check	✅ Passed	The description accurately matches the changeset and explains the bug, fix, and tests.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/chunk-post-peer-memo-key

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.76%. Comparing base (96ede32) to head (d43b86b).
⚠️ Report is 17 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #804      +/-   ##
===========================================
+ Coverage    78.65%   78.76%   +0.11%     
===========================================
  Files          132      132              
  Lines        49980    50109     +129     
  Branches      3763     3781      +18     
===========================================
+ Hits         39312    39470     +158     
+ Misses       10620    10591      -29     
  Partials        48       48              

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[vilenarios]

Superseded by #812 — consolidated into the feature/chunk-fanout integration branch (F + B1 + B2 + dashboard as one coherent PR). All commits from this branch are included there; closing to review as a unit.