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Upgrade build-log endpoint returns empty 200 for the in-flight job (2026-06-07)

Motivation

The Self-upgrade page renders three BuildLogViewers (api / frontend / terminal) and each polls GET /api/upgrade/jobs/{job_id}/build-log/{component} every 3 s while the upgrade is active. The per-component Append Blob is only created when that component's az acr build actually starts, so during the api build the frontend and terminal logs do not exist yet and the endpoint returned 404 build log not found on every poll.

Live App Insights showed this path as the single largest source of failed requests: 2,068 × 404 in 24 h for GET /api/upgrade/jobs/{job_id}/build-log/{component}, plus a browser console error on every poll. The SPA already treats that 404 as a benign "No log yet for this component", so the 404 carried no information — it was pure telemetry and console noise.

User-facing change

While an upgrade is mid-flight, a not-yet-built component's log now returns an empty 200 (the viewer keeps showing (empty) exactly as before) instead of a 404. No more per-poll browser console errors and no more 404 flood in App Insights. A genuinely unknown job_id (e.g. a job whose logs were pruned) still returns 404.

API / IaC diff summary

  • api/routes/upgrade.py — upgrade_build_log: on KeyError (blob missing) it now reads state.get_state().job_id and, when the requested job_id equals the current upgrade job, returns an empty 200 text/plain response. Any other job_id still raises 404. The state read happens only on the blob-missing path, so the happy path (blob present) is unchanged. 400 for an invalid job_id / component is unchanged, and the route's require_upgrade_admin gate is unchanged.
  • No IaC change.

Validation evidence

  • uv run pytest -q api/tests/test_upgrade_routes.py api/tests/test_persona_matrix.py — 85 passed, including the new test_build_log_endpoint_empty_200_for_current_job_not_yet_built (current job → empty 200; unknown job → 404) and the unchanged test_build_log_endpoint_404_for_missing / _requires_admin / _rejects_invalid_component.
  • uv run ruff check api/routes/upgrade.py api/tests/test_upgrade_routes.py — clean.
  • Live diagnosis: App Insights 24 h 4xx-by-path showed GET /api/upgrade/jobs/{job_id}/build-log/{component} 404 × 2068 (the symptom this fix removes).