Gate librarian cog on a full ping round-trip, not a bare GET

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>

bot.py

@@ -30,17 +30,23 @@ import discord
 import requests
 from discord.ext import commands
 
-from communication_subroutine import comm_subroutine
+from communication_subroutine import comm_subroutine, librarian_ping
 from constants import (
     ENCODING,
     FILE_SERVICE_ADDRESS,
     GET_MP3,
+    LIBRARIAN_PING,
     LIBRARIAN_SERVICE_ADDRESS,
     LOGFILE,
     RADIO_SERVICE_ADDRESS,
     TOKEN,
+    service_headers,
 )
 
+# Round-trip health check budget for the librarian ping (bot -> librarian
+# internal queue -> pong back). Deliberately short so startup never stalls.
+LIBRARIAN_PING_TIMEOUT = 3.0
+
 logger = logging.getLogger("discord")
 logger.setLevel(logging.DEBUG)
 formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
@@ -110,9 +116,11 @@ SERVICE_EXTENSION_GROUPS = {
         "health_url": f"{RADIO_SERVICE_ADDRESS}/ping",
         "extensions": ["radio_commands"],
     },
-    # librarian: DOI / Crossref search
+    # librarian: DOI / Crossref search. NOTE: this one is NOT a plain GET - it
+    # is gated on a full ping round-trip (see _load_service_groups); the URL
+    # here is only the label used in the "unreachable" log line.
     "librarian": {
-        "health_url": f"{LIBRARIAN_SERVICE_ADDRESS}/",
+        "health_url": f"{LIBRARIAN_SERVICE_ADDRESS}{LIBRARIAN_PING}",
         "extensions": ["librarian_commands"],
     },
 }
@@ -154,17 +162,37 @@ async def _load_service_groups() -> bool:
         missing = [e for e in group["extensions"] if e not in client.extensions]
         if not missing:
             continue
-        err = await asyncio.to_thread(_service_health, group["health_url"])
-        if err is not None:
-            logger.warning(
-                "Service '%s' unreachable (%s) [%s: %s] - cogs stay disabled: %s",
-                service,
-                group["health_url"],
-                type(err).__name__,
-                err,
-                ", ".join(missing),
+        if service == "librarian":
+            # A plain GET only proves Flask is up. The librarian is only useful
+            # once its internal queue + worker are flowing, so prove exactly that
+            # with a ping that must complete the full round-trip (see
+            # communication_subroutine.librarian_ping).
+            alive = await asyncio.to_thread(
+                librarian_ping,
+                LIBRARIAN_SERVICE_ADDRESS,
+                LIBRARIAN_PING,
+                service_headers(),
+                LIBRARIAN_PING_TIMEOUT,
             )
-            continue
+            if not alive:
+                logger.warning(
+                    "Service 'librarian' ping round-trip failed (%s) - cogs stay disabled: %s",
+                    group["health_url"],
+                    ", ".join(missing),
+                )
+                continue
+        else:
+            err = await asyncio.to_thread(_service_health, group["health_url"])
+            if err is not None:
+                logger.warning(
+                    "Service '%s' unreachable (%s) [%s: %s] - cogs stay disabled: %s",
+                    service,
+                    group["health_url"],
+                    type(err).__name__,
+                    err,
+                    ", ".join(missing),
+                )
+                continue
         logger.info("Service '%s' is alive - enabling: %s", service, ", ".join(missing))
         for extension in missing:
             if await _load_extension_safe(extension):

communication_subroutine.py

@@ -4,10 +4,12 @@ import os
 import re
 import threading
 import time
+import uuid as uuidlib
 from queue import Empty, Queue
 from typing import Optional
 from urllib import request as urequest
 
+import requests
 from flask import Flask, abort, jsonify, request
 from waitress import serve
 
@@ -26,6 +28,11 @@ SRCHTITLE = re.compile(rb"StreamTitle=\\*(?P<title>[^;]*);").search
 
 awaiting_q = []
 incoming_q = Queue()
+# A health-check ping whose pong never comes back (dead/dropped librarian) would
+# otherwise leave its record in awaiting_q forever. scan_incoming sweeps ping
+# records older than this. Kept well above the ping timeout so a slow-but-alive
+# round-trip is never swept out from under a waiter.
+PING_TTL_SECONDS = 30
 app = Flask(__name__)
 
 
@@ -232,9 +239,36 @@ def scan_incoming(stop_event: Optional[threading.Event] = None):
         if stop_event and stop_event.is_set():
             logger.info("scan_incoming: stop requested")
             break
+        # Sweep stale health-check pings first: if a librarian is dead the pong
+        # never arrives, so drop ping records past their TTL. scan_incoming is
+        # the sole remover of awaiting_q, so this needs no lock (scan_queue only
+        # appends). Snapshot with list() so removal during iteration is safe.
+        now = time.monotonic()
+        for record in list(awaiting_q):
+            if (
+                getattr(record, "is_ping", False)
+                and getattr(record, "answered", None) is not None
+                and not record.answered.is_set()
+                and now - getattr(record, "created", now) > PING_TTL_SECONDS
+            ):
+                awaiting_q.remove(record)
         try:
             answer = incoming_q.get(block=False)
             logger.info("DATA FOUND")
+            # Health-check pong (shape: {"__pong__": uuid}). Close the waiter's
+            # event and drop its record. It must NEVER fall through to the
+            # result/orphan path below, or the librarian cog would later pull it
+            # off IN_COMM_Q and post a bogus "no results" message to Discord.
+            if isinstance(answer, dict) and "__pong__" in answer:
+                pong_uuid = answer["__pong__"]
+                for record in list(awaiting_q):
+                    if getattr(record, "uuid", None) == pong_uuid:
+                        event = getattr(record, "answered", None)
+                        if event is not None:
+                            event.set()
+                        awaiting_q.remove(record)
+                logger.info("PONG matched for %s", pong_uuid)
+                continue
             # Collect matched records and drop them from awaiting_q afterwards -
             # they used to stay forever (awaiting_q only ever grew), leaking
             # memory over the bot's uptime and letting a reused UUID re-match a
@@ -258,6 +292,55 @@ def scan_incoming(stop_event: Optional[threading.Event] = None):
             time.sleep(1)
 
 
+def librarian_ping(address: str, endpoint: str, headers: Optional[dict] = None,
+                   timeout: float = 3.0) -> bool:
+    """Health-check the librarian by round-tripping a ping through the FULL path.
+
+    The ping is a pseudo-query that exercises exactly the same machinery a real
+    search does, on BOTH sides:
+
+    * bot out: a ``QueryControl`` rides ``OUT_COMM_Q`` -> ``scan_queue`` ->
+      ``awaiting_q`` just like a real query,
+    * librarian: it is POSTed to the librarian, which must pull it off its OWN
+      internal queue and answer WITHOUT running a search,
+    * bot in: the pong comes back over ``/conjurer`` -> ``incoming_q`` ->
+      ``scan_incoming``, which matches it by uuid and sets our event.
+
+    Returns True only when that whole loop closes within ``timeout``. Never
+    blocks longer than roughly ``timeout`` and cannot deadlock: the POST is
+    bounded, the wait is bounded, and a ping whose pong never arrives is swept
+    out of ``awaiting_q`` by ``scan_incoming`` (PING_TTL_SECONDS).
+    """
+    logger = logging.getLogger("discord")
+    ping_uuid = str(uuidlib.uuid4())
+    answered = threading.Event()
+    query = QueryControl("healthcheck", ping_uuid, "__ping__", None)
+    query.is_ping = True
+    query.answered = answered
+    query.created = time.monotonic()
+    # Enter the bot-side comm queue BEFORE the POST, so the record is already in
+    # awaiting_q by the time the pong can come back (no lost-wakeup race).
+    OUT_COMM_Q.put(query)
+    try:
+        response = requests.post(
+            f"{address}{endpoint}",
+            json={"UUID": ping_uuid},
+            headers=headers or {},
+            timeout=timeout,
+        )
+    except requests.exceptions.RequestException as exc:
+        logger.info("Librarian ping POST failed (%s): %s", ping_uuid, exc)
+        return False  # stale record is swept by scan_incoming
+    if response.status_code != 200:
+        logger.info("Librarian ping rejected (%s): HTTP %s", ping_uuid, response.status_code)
+        return False
+    if answered.wait(timeout):
+        logger.info("Librarian ping round-trip OK (%s)", ping_uuid)
+        return True
+    logger.info("Librarian ping timed out after %ss (%s)", timeout, ping_uuid)
+    return False
+
+
 def get_stream_title(tag: bytes) -> str:
     title = ""
     if m := SRCHTITLE(tag):

conjurer_librarian/conjurer_librarian.py

@@ -411,7 +411,30 @@ class BackgroundTaskSearch(threading.Thread):
         while True:
             database = None
             ndb_database = None
-            librarian = librarian_queue.get()
+            item = librarian_queue.get()
+            # Health-check ping: it has flowed through the internal queue and is
+            # now pulled off it - that is the whole point. Pong it straight back
+            # with the same uuid and DO NOT run a search.
+            if isinstance(item, dict) and "__ping__" in item:
+                ping_uuid = item["__ping__"]
+                self.app.logger.info(
+                    "PING %s pulled off internal queue - ponging back (no search)",
+                    ping_uuid,
+                )
+                try:
+                    await asyncio.to_thread(
+                        requests.post,
+                        f"{MAIN_BOT_ADDRESS}{SEND_RESULTS}",
+                        json={"__pong__": ping_uuid},
+                        headers=_service_headers(),
+                        timeout=5,
+                    )
+                except requests.exceptions.RequestException as exc:
+                    self.app.logger.warning(
+                        "PING pong send failed for %s: %s", ping_uuid, exc
+                    )
+                continue
+            librarian = item
             self.app.logger.info("STARTED")
             result = await librarian.answer_query(librarian.deep_search)
             result = {librarian.uuid: result}
@@ -530,6 +553,29 @@ async def query_database():
     return return_data
 
 
+@app.route("/ping", methods=["POST"])
+async def ping_roundtrip():
+    _authorize_request()
+    """
+    Health-check round-trip.
+
+    Puts a lightweight ping marker onto the SAME internal ``librarian_queue``
+    that real searches go through and returns 200 immediately. The background
+    worker pulls it off the queue and pongs it back to the bot with the same
+    uuid, WITHOUT running any Crossref/DOI search. A successful pong therefore
+    proves the whole pipeline (HTTP in -> internal queue -> worker -> HTTP out)
+    is flowing, not just that Flask is up.
+    """
+    record = json.loads(request.data)
+    ping_uuid = record["UUID"]
+    app.logger.info("PING received %s - queued for round-trip", ping_uuid)
+    librarian_queue.put({"__ping__": ping_uuid})
+    return (
+        jsonify(isError=False, message="ping-queued", statusCode=200, data=ping_uuid),
+        200,
+    )
+
+
 @app.route("/get_partial_result", methods=["POST"])
 async def get_partial():
     _authorize_request()

constants.py

@@ -90,6 +90,9 @@ CREATE_PRIO_PLAYLIST = "/create_priority_playlist"
 REQUEST_MUSIC = "/request_radio_file"
 CLEAR_PRIO = "/clear_pr_pls"
 SEND_QUERY = "/query"
+# Health-check round-trip: a pseudo-query that the librarian must pull off its
+# own internal queue and answer (same uuid) WITHOUT running a real search.
+LIBRARIAN_PING = "/ping"
 TIME_BETWEEN_CALLS = 100000
 LAST_SPONTANEOUS_CALL = datetime.now()
 

tests/integration/test_librarian_ping.py

@@ -0,0 +1,121 @@
+"""Integration: the librarian health check is a full comm round-trip, not a GET.
+
+``librarian_ping`` injects a pseudo-query into the SAME machinery a real search
+uses - it rides ``OUT_COMM_Q`` -> ``scan_queue`` -> ``awaiting_q``, the librarian
+is expected to pull it off its own queue and pong it back over ``/conjurer`` ->
+``incoming_q`` -> ``scan_incoming``, which matches it by uuid and wakes the
+waiter. These tests stand in for the librarian with a stubbed ``requests.post``
+and assert the loop closes (and, crucially, that a pong never leaks into
+``IN_COMM_Q`` where the librarian cog would mistake it for a real result).
+"""
+import threading
+import time
+
+import pytest
+
+import communication_subroutine as cs
+
+
+def _drain(queue):
+    while not queue.empty():
+        queue.get()
+
+
+@pytest.fixture
+def comm_threads():
+    """Run scan_queue + scan_incoming (the two workers librarian_ping relies on)
+    for the duration of a test, on cleared shared state."""
+    cs.awaiting_q.clear()
+    _drain(cs.incoming_q)
+    _drain(cs.OUT_COMM_Q)
+    _drain(cs.IN_COMM_Q)
+    stop = threading.Event()
+    workers = [
+        threading.Thread(target=cs.scan_queue, kwargs={"stop_event": stop}, daemon=True),
+        threading.Thread(target=cs.scan_incoming, kwargs={"stop_event": stop}, daemon=True),
+    ]
+    for worker in workers:
+        worker.start()
+    yield
+    stop.set()
+    for worker in workers:
+        worker.join(timeout=3)
+
+
+def _await_in_awaiting(ping_uuid, timeout=2):
+    """Block until scan_queue has moved the ping into awaiting_q."""
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        if any(getattr(r, "uuid", None) == ping_uuid for r in list(cs.awaiting_q)):
+            return True
+        time.sleep(0.01)
+    return False
+
+
+class _Resp:
+    def __init__(self, status_code=200):
+        self.status_code = status_code
+
+
+def test_ping_round_trip_ok(comm_threads, monkeypatch):
+    # Stub librarian: only pong AFTER the record is in awaiting_q, mirroring the
+    # real network latency that always lets scan_queue win.
+    def fake_post(url, json=None, headers=None, timeout=None):
+        ping_uuid = json["UUID"]
+        _await_in_awaiting(ping_uuid)
+        cs.incoming_q.put({"__pong__": ping_uuid})
+        return _Resp(200)
+
+    monkeypatch.setattr(cs.requests, "post", fake_post)
+
+    assert cs.librarian_ping("http://lib", "/ping", {}, timeout=3.0) is True
+    # A pong must NEVER reach the cog's inbound queue...
+    assert cs.IN_COMM_Q.empty()
+    # ...and the ping record must be cleaned out of awaiting_q.
+    assert not any(getattr(r, "is_ping", False) for r in list(cs.awaiting_q))
+
+
+def test_ping_times_out_when_librarian_accepts_but_never_pongs(comm_threads, monkeypatch):
+    monkeypatch.setattr(cs.requests, "post", lambda *a, **k: _Resp(200))
+    start = time.monotonic()
+    assert cs.librarian_ping("http://lib", "/ping", {}, timeout=0.3) is False
+    # Bounded: it must not block much beyond the timeout.
+    assert time.monotonic() - start < 2.0
+
+
+def test_ping_false_when_librarian_unreachable(comm_threads, monkeypatch):
+    def boom(*a, **k):
+        raise cs.requests.exceptions.RequestException("no route to host")
+
+    monkeypatch.setattr(cs.requests, "post", boom)
+    assert cs.librarian_ping("http://lib", "/ping", {}, timeout=0.3) is False
+
+
+def test_ping_false_on_non_200(comm_threads, monkeypatch):
+    monkeypatch.setattr(cs.requests, "post", lambda *a, **k: _Resp(503))
+    assert cs.librarian_ping("http://lib", "/ping", {}, timeout=0.3) is False
+
+
+def test_orphan_pong_is_dropped_not_enqueued(comm_threads):
+    # A pong with no matching waiter (e.g. after the ping already timed out) must
+    # be silently dropped - never turned into an "Orphaned" IN_COMM_Q record that
+    # makes the librarian cog post a bogus "no results" message.
+    cs.incoming_q.put({"__pong__": "no-such-uuid"})
+    deadline = time.time() + 3
+    while time.time() < deadline and not cs.incoming_q.empty():
+        time.sleep(0.02)
+    time.sleep(0.2)  # give scan_incoming a beat to (not) enqueue anything
+    assert cs.IN_COMM_Q.empty()
+
+
+def test_real_result_still_reaches_in_comm_q(comm_threads):
+    # Guard the existing path: a normal {uuid: {...}} result must still match its
+    # QueryControl and land in IN_COMM_Q for the cog to render.
+    query = cs.QueryControl("user", "real-uuid", "jakieś zapytanie", None)
+    cs.OUT_COMM_Q.put(query)
+    assert _await_in_awaiting("real-uuid")
+    cs.incoming_q.put({"real-uuid": {"10.1/x": {"Title": ["Tytuł"], "type": "article"}}})
+    got = cs.IN_COMM_Q.get(timeout=3)
+    assert got.uuid == "real-uuid"
+    assert got.stop is True
+    assert "10.1/x" in got.entries