Ci on BXM.net

Memo in GitHub Actions Skipped a Rust Release Build and Caught Runners With Different Compilers

Thu, 22 Oct 2026 00:00:00 +0000

Most CI jobs build the same thing again and again. A docs fix or a typo in a README triggers the whole pipeline, and the release build runs as if the code had changed. This case study puts BXM Memo in front of one such step, a release build of the bxm-canon binary, and keeps Memo’s cache between jobs with actions/cache.

The workflow is in the repository as .github/workflows/case-study-memo.yml. Five jobs run one after another, each on a fresh GitHub-hosted runner. plain builds without Memo for the baseline. cold builds through Memo with an empty cache, warm changes nothing, docs edits the README, and source edits a Rust source file. The inputs are canon/src/**, Cargo.toml and Cargo.lock, and the salt is rustc --version, so a different compiler means a different result. We ran the whole workflow twice.

Replaying 13 Commits Through Memo Cut Test Time From 228 to 79 Seconds and Turned Up Two Flaky Tests

Thu, 22 Oct 2026 00:00:00 +0000

BXM’s repository has two packages with their own test suites: Canon, tested with cargo test, and Memo, tested with go test. Most commits touch one of them, or neither. A docs change or a new JavaScript build script doesn’t change either suite’s result, yet a plain CI job runs both every time.

So we replayed the repository’s own history. All 13 commits from 0.1.0 to the end of 0.3.0 were checked out one after another, and both suites ran at each one. That happened twice: once plain, and once with each suite behind Memo. Canon’s suite takes canon/** and the spec’s test vectors as inputs. Memo’s takes memo/**. Memo needs a declared output, so each suite writes its log to a file and that file is the output.

Why Did My Build Step Run Again, Six Cache Misses and What Memo Said About Each

Thu, 22 Oct 2026 00:00:00 +0000

A cache that misses without saying why is almost as annoying as no cache. Someone notices the step ran again, nobody knows why, and the cache key gets a random suffix “to be safe”. After that the cache never hits.

BXM Memo keeps the manifest of the last run of each named job. On a miss it compares the two and says what changed. We put that to work on a small report step: a Python script that totals sales per region from monthly CSV files into out/report.txt. The step runs through Memo with the script and data/*.csv as inputs, REPORT_CURRENCY as an environment variable that changes the result, and the Python version as the salt.