The librarian health check was a plain GET to '/', which only proved
Flask was listening - not that the service could actually take a query,
run it through its internal queue+worker, and answer back. So the cog
could load against a librarian whose worker was wedged or that couldn't
reach the bot on the return leg.
Replace it with a ping that travels the SAME path a real search does, on
both sides:
bot: QueryControl -> OUT_COMM_Q -> scan_queue -> awaiting_q
librarian: POST /ping -> librarian_queue -> worker pulls it off
(no Crossref/DOI search) -> pongs back with the same uuid
bot: /conjurer -> incoming_q -> scan_incoming matches uuid, wakes waiter
The cog enables only when that whole loop closes within 3s. This also
proves the librarian->bot return path, which a GET never did.
Safety: uuid is random per ping; the wait and POST are both bounded so
startup can't stall; a pong that finds no waiter is dropped (never
orphaned into IN_COMM_Q, which would make the cog post a bogus 'no
results' message); and a ping whose pong never returns is swept out of
awaiting_q after PING_TTL_SECONDS so nothing leaks. All awaiting_q writes
stay within scan_queue (append) and scan_incoming (remove) - no locks,
no cross-thread mutation.
Integration tests cover: OK round-trip, timeout when accepted-but-no-pong,
unreachable, non-200, orphan-pong-dropped, and that real results still
reach IN_COMM_Q. Suite: 24 integration + 41 unit green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
