The 'CI timing report' job is pure observability — it collects per-job/step
durations from the GitHub API after the run and publishes an HTML gantt
report + PR-vs-main timing diff. It gates nothing (all-checks-pass does not
include it), yet it could redden a PR: the script makes dozens of paginated
API calls with the shared repo GITHUB_TOKEN and had zero retry handling, so
a single 403 (rate-limit burst when several PRs run CI concurrently) failed
the job. Observed twice in a row on PR #59805.
- api_get(): retry 403/429/5xx and connection errors with exponential
backoff, honoring Retry-After / X-RateLimit-Reset (max 5 attempts, 120s
cap). Non-transient statuses (404 etc.) still fail fast.
- main(): exhausted retries raise TimingsUnavailable, caught to emit a
degraded summary line + placeholder HTML artifact and exit 0 — a metrics
collector must never fail the PR's checks. No timings JSON is written on
the degraded path so an empty baseline can never be cached.
- ci.yml: baseline-save steps on main skip gracefully when no JSON exists.
Verified with a mocked urlopen harness: retry-then-success (3 attempts),
exhausted-retries -> TimingsUnavailable, 404 fails fast without retry,
degraded main() exits 0 with summary + placeholder and no JSON, and the
--from-json happy path is unchanged.
