ci: add R2-backed shared sccache

[UnArbosFive]

Summary

• use Firecracker MMDSv2 reader credentials for the shared Cloudflare R2 compiler cache on Fireactions turbo runners
• retain GitHub Actions sccache as the legacy-runner fallback and ordinary Cargo as the fail-open path
• add a protected main-only writer warmer using the sccache-writer environment
• consolidate direct Rust compilation paths and retain Cargo registry/git caching without target tarballs where shared sccache applies

Depends on #2846 and intentionally targets its bittensor-core-exploration branch.

Security boundaries

• pull requests receive only the bucket-scoped read credential from MMDS
• writer credentials are available only through the protected sccache-writer environment on trusted main push/schedule/dispatch events
• credentials are masked and never printed
• R2 or MMDS failures disable the cache rather than failing compilation

Validation

• .github/scripts/test-sccache-configure.sh
• actionlint -config-file .github/actionlint.yaml
• bash -n .github/scripts/sccache-configure.sh .github/scripts/test-sccache-configure.sh
• git diff --check origin/pr-2846-head..HEAD

The R2 bucket, reader MMDS metadata, action allowlist, and protected writer environment are live. The BHS turbo canary proved reader GET succeeds and writer PUT is denied.

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Project	Deployment	Actions	Updated (UTC)
subtensor	Ready	Preview, Comment	Jul 12, 2026 12:13am

[github-actions]

🛡️ AI Review — Skeptic (security review)

VERDICT: SAFE

VERY HIGH account-age scrutiny, mitigated by admin repository permission; no Gittensor association or author/committer mismatch. Intentional stacked branch: codex/subtensor-r2-sccache → bittensor-core-exploration.

The writer workflow is restricted to trusted main events and its protected environment; ordinary PR jobs receive only the validated, bucket-scoped read credential. The trusted AI-review instruction files are unchanged.

Findings

No findings.

Conclusion

No malicious behavior or security vulnerability was identified. The shared-cache credential boundary remains fail-open without exposing a PR-accessible writer path.

---

🔍 AI Review — Auditor (domain review)

VERDICT: 👍

Gittensor association UNKNOWN; the author has limited public history but repository admin permission, so full correctness scrutiny was applied.

The PR is consistent with its description: R2 reader credentials are validated, writer credentials remain behind the protected main-only workflow, and cache failures degrade to GHA sccache or ordinary Cargo. No runtime code changed, and this base branch has no spec-version check.

The helper still permits writer mode on bittensor-core-exploration and codex/subtensor-r2-sccache, although no workflow supplies writer credentials there. This is a non-blocking description discrepancy and should be removed after rollout.

Overlapping PRs are stacked dependencies or separate CI experiments; none is a competing implementation.

Findings

No findings.

Conclusion

The shared-cache and multi-architecture build integration is coherent, fail-open, and adequately tested. No merge-blocking domain issues were found.

[github-actions]

🔄 AI review updated — Skeptic: SAFE Auditor: 👍

[UnArbosFive]

Live validation on fa-turbo is complete:

• protected test-config-boundary workflow passes on Ubuntu
• MMDS reader configuration validated
• pinned sccache v0.15.0 started with SCCACHE_BACKEND=r2
• endpoint, region=auto, TLS, and subtensor/v1 were exported
• a short compile recorded 798 cache misses and 798 expected write denials under the read-only credential; compilation continued normally
• no credentials appeared in the inspected logs

The runtime workflow was cancelled immediately after this proof, and all other active workflows from this PR were cancelled to free the self-hosted runners. Zero PR workflows are currently active.

[UnArbosFive]

Temporary benchmark writer access is now included:

• exact trusted branch: bittensor-core-exploration (PR bittensor-core: one Rust core (keys, keyfiles, timelock, codec, RFC-0078 digest, Ledger) under a thin Python SDK #2846 head)
• allowed writer events: branch push and manual workflow_dispatch
• pull_request remains read-only
• schedules remain main-only
• the branch is now protected, so it qualifies for the existing protected-branches-only sccache-writer Environment
• the trusted-branch boundary test passes on Ubuntu

Merging this draft into bittensor-core-exploration will itself trigger the writer warmer and populate R2.

[github-actions]

🔄 AI review updated — Skeptic: SAFE Auditor: 👍

[UnArbosFive]

Final review and rollout status:

• thermo-nuclear maintainability review: no structural or security blockers
• latest bittensor-core: one Rust core (keys, keyfiles, timelock, codec, RFC-0078 digest, Ledger) under a thin Python SDK #2846 head re-fetched after rollout; still dcc180a99 and fully contained in this branch
• writer warmer now asserts SCCACHE_BACKEND=r2 and the boundary test exercises writer activation
• shared-cache changes now trigger real Check Rust, eco-tests, and Runtime Checks path filters
• AX41 benchmarking now uses R2 sccache instead of target-directory tarballs
• all 10 Fireactions hosts are converged: active service, config mode 0600, valid schema, and one read-only MMDS block per pool
• final CI is running normally and has not been cancelled

PR is ready for review and merge. Merging into bittensor-core-exploration triggers the trusted writer warmer automatically.

[github-actions]

AI review — see the sticky summary comment for the verdict and the inline comments below for specific findings.

[github-actions]

[HIGH] PR head receives shared-cache writer execution

This enables the writer workflow directly on the branch whose code is still under review. Once the protected environment run is approved, this branch controls the checked-out local actions and all Cargo build scripts while writer credentials are present, so a hostile follow-up commit could exfiltrate the credentials or populate malicious artifacts in the cache consumed by other PRs. The hard-coded exception also remains reusable after merge. Remove the PR-head branch from this workflow and from prepare_writer’s allowlist; validate writer behavior only after merging into a protected trusted branch.

[github-actions]

🔄 AI review updated — Skeptic: VULNERABLE

[github-actions]

AI review — see the sticky summary comment for the verdict and the inline comments below for specific findings.

[github-actions]

[HIGH] PR head receives shared-cache writer execution

This still lets code under review execute with protected R2 writer credentials and populate the shared compiler cache. A hostile commit on this branch could exfiltrate the credentials or poison cached artifacts later consumed by ordinary PR jobs. Remove codex/subtensor-r2-sccache from this trigger, the job-level if, and prepare_writer in .github/scripts/sccache-configure.sh; only trusted post-review branches should receive writer access.

[github-actions]

🔄 AI review updated — Skeptic: VULNERABLE

[github-actions]

🔄 AI review updated — Skeptic: SAFE Auditor: 👍

[github-actions]

🔄 AI review updated — Skeptic: SAFE Auditor: 👍

[github-actions]

🔄 AI review updated — Skeptic: SAFE Auditor: 👍

[UnArbosFive]

Superseded by #2857. The runner-density branch contains this PR’s complete cache commit history unchanged, followed by the runner-pool and try-runtime placement work. Consolidating on #2857 preserves the latest combined CI run and avoids another force-push/retest cycle. This branch is being retained for history until the unified PR lands.