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## Summary - add a SHA-pinned sccache action to reuse unchanged Rust compilation units when the exact relay artifact cache misses - keep pull requests read-only while preserving cache writes for trusted `main` and `release` pushes - stop saving isolated exact relay-artifact caches from PRs, reducing cache churn - preserve the exact artifact cache as the zero-build fast path ## Why this is an experiment The relay artifact job currently misses its exact cache whenever any file under `crates/**` changes, forcing a full workspace rebuild. PR #4975 spent roughly 21 minutes in that job for a one-file `buzz-sdk` change. sccache targets the relevant reuse boundary—individual compiler inputs—but the repository cache pool is already under heavy eviction pressure, so this PR does **not** claim a proven timing win yet. ## Safety - `Mozilla-Actions/sccache-action` is pinned to commit `fc920bf0ec8de6ee65d409111f7ec508035751ba` - `RUSTC_WRAPPER` is scoped only to `Build relay artifacts` - PRs use `READ_ONLY`; trusted `push` runs (`main` and `release`) use `READ_WRITE` - the existing exact finished-artifact cache remains the first/fast path - finished artifacts are saved only by trusted pushes, preserving the former trust boundary - workflow permissions remain `contents: read`; no `pull_request_target` path is introduced - the pinned action automatically emits sccache hit/miss/error/write/duration statistics in its post-job hook ## Validation - `actionlint .github/workflows/ci.yml` - `git diff --check` - desktop release-cache contract test - release-ref contract test - independent code-shape reviews from Princess Donut and Mongo: 9/10, no remaining findings ## Measurement plan 1. purge obsolete PR-scoped `relay-artifacts-*` cache entries before measurement 2. merge/push a trusted writer to populate sccache 3. run a representative one-crate PR 4. compare relay job duration and automatic sccache statistics against the 21–22 minute baseline 5. retain this only if the warm run demonstrates material improvement --------- Signed-off-by: Wes <wesbillman@users.noreply.github.com> Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>