Root cause of "segment_end never written, response just runs until
Netlify kills it": the Fetch spec only honors AbortSignal for the
request/header phase. Once you have `res.body` and start reading
from it, aborting that same signal does NOT close the body stream —
any in-flight `reader.read()` hangs indefinitely.
So when the segment timer fired and called segmentAbort.abort(), the
running `for await (const event of parseSse(upstream))` stayed
blocked on a `reader.read()` that never resolved. The while loop
never exited, segment_end was never written, controller.close() was
never called, and Netlify eventually pulled the rug somewhere
between ~49 s and ~60 s in.
Fix: parseSse now takes an optional AbortSignal and, when it fires,
calls `reader.cancel()` directly. The WHATWG ReadableStream spec
guarantees that any pending read() resolves with `{done: true}`
after cancel, which lets the for-await exit promptly and the body
loop re-evaluate `while (!done && !segmenting)` → break →
writeJson(segment_end) → controller.close(). Confirmed in the curl
trace: heartbeats now stop at the budget mark and segment_end fires
before Netlify's cap.
Co-Authored-By: alex <alex@semipublic.co>
Devin AI
ed0069ed89