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A download froze part-way through, on Firefox, with an empty console and a node
that stayed perfectly healthy. Three separate measurements cleared the node
(615 MB pulled whole over MNP), the transport (three files interleaved on one
connection, 1.5 GB, all whole) and the service worker (three concurrent 150 MB
streams in real Firefox 154) — because none of them was wrong.
The empty console was the evidence. `_sendAndWait` logs every timeout, so no
chunk request had expired: the client was not waiting on the node. Of the three
awaits left on that path only one was unbounded.
**A service worker with no event for about thirty seconds is terminated**, and
`respondWith(new Response(stream))` does not extend its life while the response
is still being written. The reader vanished mid-file and `writable.write()` then
never resolved and never rejected — no error, no log, no failed transfer, just a
progress bar that stops. The first stress probe wrote 450 MB in two seconds and
passed: fast enough to hide it entirely.
Measured in Firefox 154, writing 1 MB every 2 s: without the ping it stalled at
17 MB after 59 s; with it, 40 MB in 80 s, complete.
- the page pings the worker every 10 s while it writes, and the worker
answers. Receiving a message is an event, and an event resets the timer;
- that interval stops itself after two minutes with no write. A target can be
opened and never written to — a transfer cancelled while it waits for a slot
never runs, so nothing calls close() or abort() — and a timer nobody clears
pings for the life of the page. It also kept the Node test process alive for
ever, which is the same defect wearing a louder symptom;
- `writable.write()` is bounded at 60 s and fails with a message naming the
chunk. That does not fix whatever stopped a sink; it turns an unexplainable
freeze into a failed transfer that says so, which is the difference between
a mystery and a bug report.
Also: `Content-Disposition` lost a filename to a single apostrophe.
`encodeURIComponent` leaves `'` alone and `'` is the delimiter in RFC 5987's
`filename*=<charset>'<lang>'<value>`, so the header became unparseable and
Firefox named the file after the URL — 449 MB of film arrived complete as
"mtsshk9w-ohqty535". `(`, `)` and `*` get the same treatment, and a plain ASCII
`filename=` rides alongside.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST
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On Firefox and Safari the service worker is the only unbounded way to write a
download to disk: neither has the File System Access API, and OPFS is not a
substitute — measured on Firefox 154, its quota is exactly 10% of the volume's
size (389,233,459 bytes of a 3,892,334,592-byte volume, refused to the byte),
which a film exceeds. So when this path declines, a large download has nowhere
left to go, which makes its reliability a correctness property.
Four ways it declined, all of them avoidable:
- it was registered inside the first click on Download, so that click paid
install, activate and claim while somebody watched a button do nothing;
- `_swReady` cached a null for the life of the page. One slow first click left
the tab unable to stream anything again, curable only by a reload nobody knew
to do. Only a successful controller is remembered now;
- control was waited for with a 3 s cap. It is 15 s, and a page that is active
but not controlled asks the worker to claim again (`mbdl-claim`) instead of
declaring the path unavailable;
- a missed navigation gave up at once. It gets a second attempt with a fresh id
and iframe, the failed one torn down completely first.
Also closes a MessagePort leaked per download, and gives the reason a name
(`lastStreamFailure`) so a refusal can say what happened. The timeouts became
parameters: the defaults are the production values, no caller passes any, and
the tests do not spend a minute waiting.
`openTarget` gets an unrelated but adjacent fix, in the same file: it creates
the destination with `getFileHandle({create: true})`, so an empty file exists
before the first byte, and `abort()` leaves the target untouched — every
cancelled download left a 0-byte file behind, and since `freeName` avoids
collisions, three cancels left film.mkv, film (2).mkv and film (3).mkv, all
empty. Its `abort()` now removes the entry. Safe here and only here, because
`freeName` guarantees the name was not taken: the `showSaveFilePicker` path
must not do the same, where the person may have picked an existing file whose
contents `abort()` correctly preserves. Verified by hand in Chrome.
test_streamed_download_reliability.py runs the real module under Node against a
stubbed browser — it fails if the null is cached again.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST
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