Both gateway SSE read loops used urlopen(timeout=600) and re-checked cancel only
BETWEEN lines, so a half-open gateway (TCP open, zero bytes) pinned the worker
for the full 10 minutes and ignored Stop.
Route raw-line iteration through _iter_sse_lines_cancellable and bound the socket
to a byte-silence budget (_gateway_read_timeout_secs, default 120s, env
HERMES_WEBUI_GATEWAY_READ_TIMEOUT). A read timeout is TERMINAL — CPython's
socket.makefile latches _timeout_occurred on the first socket.timeout, after
which every read raises a bare OSError, so the connection can't be resumed. On a
read timeout (or that poisoned-socket OSError) the generator surfaces the user's
Stop (yield b"" -> caller emits cancel) or re-raises so the caller reports the
stall. A stream that keeps emitting bytes within the window never trips it, so a
genuinely alive (if slow) turn is untouched; only >budget of total byte-silence
is treated as a dead gateway. Both loops (runs-API + legacy) changed identically.
The test fake now models the timeout-poison latch (a bare OSError after the first
timeout) so it reflects real socket semantics; the previous stateless re-raise
had made a multi-window "recovery" path look green that cannot occur in
production.
Refs #2476
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
