CI Flaky Timing Luck

Steps that passed by timing luck

Two failures with the same shape, both from a release pipeline:

• A loop wrapped one disk image per architecture. Each iteration mounted a volume under the same name. Releasing a volume is asynchronous, so the next mount either failed as busy or — worse — quietly mounted under a suffixed name while the script kept writing to the hardcoded path.
• A build step downloaded a native archive through the standard library's URL stream. The download answered with a redirect to a mirror; on a cross-protocol redirect the stream API saved the error page body as the target file and returned normally.

Neither is a race you introduced. Both are steps that were always wrong and were being covered by how fast the machine happened to be. A runner image update changes the timing, and they start failing — which is why the commit that "breaks" them is usually unrelated.

The expensive shape is not "fails intermittently", it is "succeeds while producing the wrong artifact." A hard failure costs one re-run. A step that reports success and hands the next job a zero-byte file, an error page, or an empty mount point costs an afternoon, and the traceback points somewhere it did not come from.

Traps

Never hardcode a mount point — capture the one the tool reports. The tool tells you where it actually mounted; a name collision changes that answer and nothing else warns you:

# adapted — condensed from the wrapping script, names genericised
MOUNT_POINT="$(hdiutil attach -nobrowse -noautoopen "$RW_DMG" 2>/dev/null \
  | grep -Eo '/Volumes/.*$' | tail -1 || true)"
[[ -d "$MOUNT_POINT" ]] || { echo "Could not mount $RW_DMG" >&2; exit 1; }
VOL_NAME="$(basename "$MOUNT_POINT")"

Everything downstream then derives from $MOUNT_POINT and $VOL_NAME. A script that writes to /Volumes/<Name> after a silent remount to /Volumes/<Name> 1 writes into the previous iteration's volume and produces an output missing whatever it thought it added.

Clean up before you start, not only after you finish. The previous iteration's cleanup may not have completed, and a cleanup you run only at the end never runs at all if the body failed:

# adapted — condensed from the wrapping script, names genericised
detach_stale_volumes() {
  for v in /Volumes/<Name> /Volumes/<Name>\ *; do
    [[ -e "$v" ]] && hdiutil detach -force "$v" >/dev/null 2>&1 || true
  done
}
detach_stale_volumes
for attempt in 1 2 3 4 5; do
  MOUNT_POINT="$(hdiutil attach … | grep -Eo '/Volumes/.*$' | tail -1 || true)"
  [[ -d "$MOUNT_POINT" ]] && break
  detach_stale_volumes; sleep 3
done

Note the glob covers the suffixed names too — those are exactly the leftovers a previous collision created.

Force and retry the release as well. The next iteration starts from whatever this one left behind, so a best-effort detach at the end of the body is worth a short retry loop.

Prefer a downloader that fails loudly on HTTP errors. The stdlib stream API does not follow a redirect that switches protocol — it hands back the redirect response itself, and copying that stream saves the redirect's own HTML page to disk under the artifact's filename. The download looks like it worked; the checksum is the only thing left to catch it. An external downloader with explicit flags fails instead:

// adapted
ProcessBuilder(
    "curl", "-fsSL",          // --fail: non-zero on HTTP errors instead of saving the error body
                              // -L: follows redirects across protocols and mirrors
    "--retry", "5", "--retry-delay", "5",
    "--retry-all-errors",     // plain --retry does NOT cover a mid-transfer receive failure;
                              // it retries only HTTP 5xx/408/429 and connection errors
    "-o", target.absolutePath, url,
).inheritIO().start().waitFor()

Check the result even after a zero exit, and delete the partial file. Otherwise an empty or truncated download is cached and every retry reproduces it:

check(exit == 0 && target.exists() && target.length() > 0) {
    if (target.exists()) target.delete()
    "download failed (exit $exit) for $url"
}

Mirrors are a lottery, so verify content rather than trusting the source. A release URL can redirect to a different mirror on every request. Pair the download with a digest check against a value pinned in your source — that is the only assertion that survives a mirror serving something else.

A cached artifact is a cached failure. Any caching layer in front of a download will happily preserve a bad file forever. Make the failure path delete, and make the cache key include whatever identifies the publication, not just the version.

Verifying it

Timing-luck steps have tells you can grep for. In shell steps and build scripts, look for:

grep -rn 'Volumes/\|/mnt/\|/tmp/[a-z-]*/' --include='*.sh' --include='*.yml' scripts .github
grep -rn 'openStream\|URL(\|readBytes()' --include='*.kts' --include='*.gradle' .
grep -rn 'sleep ' --include='*.sh' --include='*.yml' scripts .github

• a fixed path where a tool reports its own,
• a stdlib fetch where an HTTP error is possible,
• a bare sleep standing in for a condition — it is a timing assumption with a comment missing.

Then ask of each: if this step did the wrong thing, would the job still be green? Every yes is a place to add an assertion on the artifact — its existence, its size, its type, its digest — right where it is produced, rather than letting a later job discover it.