[sodre]

Summary

A Fargate job never hit the Rust cache an EC2 job saved. Swatinem/rust-cache hashes CARGO*, CC*, CFLAGS*, CXX*, CMAKE* and RUST* environment variables into its key (source), and Fargate tasks set CARGO_BUILD_JOBS. The cap on cargo's parallelism moves from the environment into a config file.

 Fargate entrypoint (ci-runner-idle-exit)
+  CI_RUNNER_VCPUS=<n> set → write /home/runner/.cargo/config.toml: [build] jobs = <n>
+                            (an existing file is kept; a bad value warns and writes nothing)
 launcher RunTask env (integration branch, #32)
-  CARGO_BUILD_JOBS=<n>, CMAKE_BUILD_PARALLEL_LEVEL=<n>, MAKEFLAGS=-j<n>
+  CI_RUNNER_VCPUS=<n>, MAKEFLAGS=-j<n>

Cargo reads .cargo/config.toml from every ancestor of the working directory, so the file covers any checkout under /home/runner/_work. rust-cache only hashes config files inside the workspace, so this one leaves the key alone.

Evidence

• Before (live): zae-zcm's Fargate run computed rust-cache key …-ef9743d9-…, against EC2's …-2fd77104-…. The first wave built cold.
• After: image/smoke.sh covers five cases (no variable, the file's contents, its owner runner, an existing file kept, a bad value). It passed on amd64 and arm64: https://github.com/zeroae/ci-runner/actions/runs/43560324849.
• Still to measure: a Fargate job hitting EC2's cache, which needs this image plus the launcher change deployed.

Merge Danger

Door: two-way.

Blast Radius: Fargate only.

EC2 doesn't use this entrypoint. Until the launcher change deploys, the old environment variables still apply, and the file is not written (no CI_RUNNER_VCPUS).

Part of #18 (#29).

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