v154 · abort reason

Cancelling mid-stream

A download read chunk by chunk, ended part-way for one of four different causes, and routed on the reason. One controller, four meanings — which is why a bare AbortError at the read site was not enough.

Read a body, then stop it

idle
Chunks read
0
Bytes
0
Ended by
—
Reason reached the reader?
—

How the handler routed it

Log

Start the download, then end it with one of the causes.

The request targets /public/slow-stream, a same-origin endpoint that emits 256 KB in 8 KB pieces with a delay between them — because a real file arrives in a single chunk over localhost and there would be no mid-stream to cancel in. Everything else is ordinary: the loop reads the response body, and aborting the fetch errors that body stream so the next read() rejects with whatever the abort carried. Every one of those buttons calls abort() on the same controller. The only thing distinguishing them is the value passed in — and the only reason the handler can see it is that Chrome 154 delivers it to the stream reader rather than replacing it with a generic error.

code path

const reader = response.body.getReader();
try {
  for (;;) {
    const { done, value } = await reader.read();
    if (done) break;
    await sink.write(value);
  }
} catch (error) {
  switch (error?.cause) {
    case "USER":     return dismissQuietly();
    case "QUOTA":    return offerToFreeSpace();
    case "NAVIGATE": return;                 // nobody is watching
    case "STALE":    return;                 // a newer request owns the UI
    default:         throw error;            // a real failure
  }
}

see also