Two halves of one failure, either of which is enough to produce an
offline item that never starts.
The worker marked a transfer `completed` without checking it produced
any bytes, so a server that answered 200 with no body — an error page, a
transcode that yielded nothing — renamed a zero-byte `.part` into place
and published it as available offline. That is worse than failing: the
retry budget never applies and the UI shows the item as ready.
The media server then answered a request for that file with a span of
`{ start: 0, end: 0 }`. `end` is inclusive, so `Span::len()` reported
**one** byte: the response declared `Content-Length: 1` and streamed
nothing, which Chromium's media loader waits on forever. The user sees a
downloaded item that just never plays, with nothing explaining why.
A zero-length file has no satisfiable range, so `span_for` now returns
`None` and the server answers 416. An empty transfer is rejected as a
network error, which keeps the `.part` for a resume and lets the existing
retry budget do its job.
dtourolle
747ec0161c