[eschultz]

Summary

The 2026-10-25 scheduled run failed on fedora-kde-44 with a 404, but the file exists. download.fedoraproject.org redirects each request to a mirror, and that run landed on one missing the ISO. fedora-workstation-44 uses the same URL and passed in the same run.

check-sources treated every 404 as final after one request. That is right for a URL served by the host we asked, and wrong when a redirector picks a different mirror per request.

• A 404/410 from a host other than the one requested now goes through the existing retry ladder, and the failure names the mirror.
• A 404 from the requested host still fails after one request.
• New self-test: a redirector sends the first request to a stale mirror and the second to a good one.
• nixos-26.05 releasesUrl pointed at releases.nixos.org/nixos/26.05/, an S3 prefix with no index page (404). It now uses the bucket's browsable listing, ?prefix=nixos/26.05/.

Test plan

• bun check-sources.ts --self-test: 6/6 pass, including the new stale-mirror case
• bun run typecheck, bun run validate: clean
• bun run releases -- --check: nixos-26.05 now returns 200