ci(reborn): share one Rust cache across the closure (fix per-crate cache LRU eviction)

[serrrfirat]

Problem (root cause of the closure's re-downloads + flakes)

The closure's crate-tests matrix used a per-crate cache key (reborn-tests-<crate>), so it produced ~60 caches (registry + target each, ~0.4–0.7 GB) plus 4 per-partition root caches (~1 GB each) → ~30+ GB competing for GitHub's ~10 GB per-repo cache LRU.

They evict each other almost as fast as they're written. Live snapshot when we investigated:

24 total repo caches, 18.0 GB  (limit ~10 GB → actively evicting)
only 10 of 64 closure crates had a cache present

So most jobs hit "No cache found" → re-download the crates.io registry from cold every run → and ~60 parallel cold downloads amplify transient registry flakes (the SSL_ERROR_SYSCALL / HTTP2 reds). It was LRU eviction, not code changes — the keys were valid; the entries were just evicted.

Fix

Switch both matrices to rust-cache shared-key:

• crate-tests → shared-key: reborn-tests-crates — one entry for all crate jobs; the registry + shared-dep build is downloaded/compiled once and stays resident, not ~60×.
• root partitions → shared-key: reborn-tests-root — all 4 partitions compile the identical root build, so one shared cache is strictly better than 4 copies.

Drops the reborn-tests cache footprint from ~30+ GB / 68 entries → ~2 entries that comfortably fit the LRU → the registry stays resident → no more cold re-downloads.

Trade-off

Crate-test target caching is now shared rather than per-crate, so a given crate may recompile its own (small) crate-specific code more often — but the shared dependency build (the bulk) and the registry (the download-relevant part) are always cached. The download/flake fix is the priority; if per-crate compile time regresses, we can add a separate per-crate target cache later.

Relationship to #5115

Complementary: this removes the cause of cold downloads (cache eviction); CARGO_NET_RETRY (#5115) remains the safety net for the now-rare cold download.

Automated agent-authored.

[gemini-code-assist]

Note

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between b1924ab and 171c73e.

📒 Files selected for processing (1)

• .github/workflows/reborn-tests.yml

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📝 Walkthrough

Summary by CodeRabbit

Release Notes

• Chores
  • Optimized CI/CD cache configuration to improve test job efficiency.

Walkthrough

The reborn-tests workflow updates Rust cache keying in two jobs. The ~60 crate-test matrix jobs move from per-package keys to a single reborn-tests-crates key, and the four root parity partitions move from per-partition keys to a single reborn-tests-root key.

Changes

CI Cache Key Consolidation

Layer / File(s)	Summary
Shared cache keys for crate-test and root parity jobs .github/workflows/reborn-tests.yml	crate-test jobs replace reborn-tests-${{ matrix.package }} with constant reborn-tests-crates; root parity partitions replace reborn-tests-root-${{ matrix.partition }} with constant reborn-tests-root. Both changes collapse per-variant cache entries into a single shared entry.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

Two cache keys, once sixty-four,
Now one each—no more, no more.
Matrix variants share the pile,
Registry reused in style.
🦀 One key to rule them all.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	Description includes root cause analysis, solution details, trade-offs, and relationship to prior work, but omits most template sections (Change Type, Validation, Security Impact, Blast Radius, Review track).	Complete required sections: mark CI/Infrastructure as Change Type; confirm validation steps (cargo fmt/clippy if applicable); confirm no security/trust-boundary impact; specify blast radius and review track.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title follows Conventional Commits (ci scope), clearly describes the fix (shared cache strategy), and accurately reflects the main change in the workflow file.
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.

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