fix(plex): an HLS artifact names the session it belongs to, and a paused viewer says so

Deep-review round 5, over round 4's own fixes.

- Sessions are keyed by rating_key alone while a session's DIRECTORY is per (key, offset, tag), and
  the artifact URL carried no session identity — so once a seek-restart or a second viewer of the
  same item took the registry slot, the old viewer's request for `seg_12.ts` was answered 200 with
  the new session's twelfth segment: bytes from a different `-ss` offset under `-copyts`, on a
  timeline the client is not on. Wrong content with a success status is the one failure a player
  cannot notice, and round 4's reclaim-on-404 could never fire on it. The id is in the path now (so
  the playlist's relative names resolve back into the same session), a mismatch is a 404, and the
  player reclaims at the playhead. The entry playlist's auto-start is gone with it: it could only
  ever begin at 0, silently discarding the viewer's position.

- Round 4 exempted paused viewers from the reaper by keeping EVERY finished session for the full
  ten-minute idle window — one whole segmented copy of a film per item, on the volume the download
  center shares. The signal it needed already existed: an open player checkpoints its position every
  10 s whether or not it is playing, so that route now touches the session and the periodic pass is
  back on the short rule.

- POST /session's opportunistic reap was never scoped to the key being started: it walked the whole
  registry with the 30 s finished rule, so pressing play on ANY item — even a direct-play one, the
  call ran before that early return — killed another viewer's paused session. It is gone; the
  periodic pass owns reaping.

- A segment that can never arrive no longer polls out the full 20 s: the producer's handle is passed
  to `wait_for`, so a finished ffmpeg answers at once. This is a sync route, so each such wait parked
  a threadpool worker AND a pooled DB connection, and an ordinary seek past the produced region was
  enough to do it.

- The DataError handler's re-raise branch — the half that keeps a genuine DataError a 500 — is
  pinned, so simplifying it to an unconditional 422 cannot pass in silence.

Six regression tests (two route-level, in the DB lane), each mutation-checked red on the assertion
it is named after. pytest 1124, vitest 999.

backend/app/main.py

@@ -112,11 +112,14 @@ async def _reap_hls_sessions() -> None:
     ffmpeg still working through the film, its directory a whole segmented copy of it — lived until
     somebody pressed play again or the process restarted.
 
-    `reap_abandoned`, not `reap_idle`: a finished session is also a paused viewer's."""
+    A FINISHED session is reaped on the short rule here, not kept for the full idle window: an open
+    player checkpoints every 10 s and that route touches the session, so a paused viewer is no longer
+    invisible to this pass (see `reap_idle`). Keeping every finished session for ten minutes instead
+    held one whole segmented copy of a film per item on the download volume (round 5)."""
     while True:
         await asyncio.sleep(_HLS_REAP_EVERY_S)
         try:
-            await asyncio.to_thread(plex_stream.reap_abandoned)
+            await asyncio.to_thread(plex_stream.reap_idle)
         except Exception:
             log.exception("HLS session reap failed")
 

backend/app/plex/stream.py

@@ -21,6 +21,7 @@ CPU-only prod host; only P3's full transcode is CPU-heavy.
 import logging
 import os
 import re
+import secrets
 import shutil
 import subprocess
 import threading
@@ -99,6 +100,14 @@ class HlsSession:
         hw: bool = False,
         tonemap: bool = False,
     ):
+        # Unguessable per-session id, carried in the URLs the client is handed. Sessions are keyed
+        # by rating_key alone, so a seek-restart or a SECOND viewer of the same item replaces the
+        # registered one — and the old viewer's next request for `seg_12.ts`, resolved by rating_key,
+        # was answered 200 with the NEW session's twelfth segment, produced from a different offset
+        # under `-copyts`. Wrong bytes, not an error, so the client's reclaim never fired on it
+        # (deep-review round 5). A request naming a session that is no longer registered is a 404,
+        # which the player recovers from by restarting at the playhead.
+        self.id = secrets.token_urlsafe(9)
         self.key = key
         self.dir = directory
         self.proc = proc
@@ -826,16 +835,19 @@ _FINISHED_IDLE_S = 30
 
 
 def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
-    """Kill sessions nobody has fetched from for `_SESSION_IDLE_S` — or, once their ffmpeg has
-    finished, for `finished_idle_s`.
+    """Kill sessions nobody has said they still want for `_SESSION_IDLE_S` — or, once their ffmpeg
+    has finished, for `finished_idle_s`.
 
-    The short rule is only safe where a NEW session is being requested (the default, POST /session):
-    a finished session is also what a PAUSED viewer has. A remux runs far faster than realtime, so
+    A finished session is also what a PAUSED viewer has: a remux runs far faster than realtime, so
     ffmpeg is done a minute or two in and writes ENDLIST; hls.js then stops asking for the playlist,
-    and with its forward buffer full a paused player fetches nothing at all. The periodic reaper
-    therefore uses `reap_abandoned` — with the 30 s rule, a one-minute pause killed the session and
-    playback died ~30 s after resuming (deep-review round 4). A viewer paused for longer than
-    `_SESSION_IDLE_S` still loses it; the player restarts one at the playhead when a fetch 404s."""
+    and with its forward buffer full a paused player fetches nothing at all. With the 30 s rule that
+    killed a one-minute pause, and playback died ~30 s after resuming (deep-review round 4).
+
+    What makes the short rule safe again is that fetching is no longer the only signal: an open
+    player checkpoints its position every 10 s, and that route calls `touch_session`, so a viewer who
+    is merely paused says so far more often than this rule asks (round 5). Round 4's blanket
+    `reap_abandoned` bought the same safety by keeping every finished session for ten minutes — each
+    one a whole segmented copy of the film on a volume shared with the download center."""
     now = time.time()
     dropped = 0
     with _lock:
@@ -849,7 +861,11 @@ def reap_idle(finished_idle_s: float = _FINISHED_IDLE_S) -> int:
     return dropped
 
 
-def reap_abandoned() -> int:
-    """The periodic reaper's pass: only what nobody has fetched from for `_SESSION_IDLE_S`, finished
-    or not (see reap_idle for why a finished session is not yet an abandoned one)."""
-    return reap_idle(finished_idle_s=_SESSION_IDLE_S)
+def touch_session(key: str) -> None:
+    """Say a viewer is still there for `key`, without fetching anything. `last_access` used to move
+    only on an HLS file fetch, which a paused player does not do — so the only way to keep a paused
+    viewer's session alive was to keep EVERY abandoned one just as long."""
+    with _lock:
+        s = _sessions.get(key)
+        if s is not None:
+            s.last_access = time.time()

backend/app/routes/plex/playback.py

@@ -323,6 +323,10 @@ def item_progress(
     `_should_push_resume`). `final` (sent on pause / seek / pagehide / unmount) pushes immediately and
     resets the throttle window, so Plex sees a settled position the instant the user pauses or seeks."""
     it = item_or_404(db, rating_key)
+    # An open player checkpoints every 10 s whether or not it is playing, so this is the signal that a
+    # PAUSED viewer still wants their HLS session — the one thing `last_access` could not see, since
+    # it moved only on a file fetch and a paused player fetches nothing (see stream.reap_idle).
+    plex_stream.touch_session(str(rating_key))
     position = max(0, int(body.position_seconds))
     duration = int(body.duration_seconds)
     final = body.final
@@ -561,7 +565,6 @@ def stream_session(
     HEVC/VP9/AV1/…) take the same HLS path but with the video re-encoded to H.264 (VAAPI or libx264).
     Selecting a non-default audio track (`audio` = stream ordinal) forces the HLS path so the chosen audio
     can be muxed in. Subtitles are independent WebVTT tracks (GET /subtitle) — no session restart."""
-    plex_stream.reap_idle()  # opportunistic cleanup of idle sessions on each new playback
     it = item_or_404(db, rating_key)
     transcode = it.playable == "transcode"
     # `multi` (item has ≥2 audio tracks) forces the HLS path so ALL audio tracks ship as renditions and
@@ -585,7 +588,9 @@ def stream_session(
     return {
         "mode": "hls",
         "start_s": s.media_start_s,
-        "url": f"/api/plex/stream/{it.rating_key}/hls/{s.entry}",
+        # The session id is part of the path (not a query param) so the playlist's RELATIVE segment
+        # names resolve back into the same session — see `stream_hls_file` for why it has to.
+        "url": f"/api/plex/stream/{it.rating_key}/hls/{s.id}/{s.entry}",
         "transcode": s.transcode,
         "hw": s.hw,
         "tonemap": s.tonemap,
@@ -624,35 +629,39 @@ _HLS_CT = {
 }
 
 
-@router.get("/stream/{rating_key}/hls/{filename}")
+@router.get("/stream/{rating_key}/hls/{session_id}/{filename}")
 def stream_hls_file(
     rating_key: str,
+    session_id: str,
     filename: str,
     user: User = Depends(current_user),
-    db: Session = Depends(get_db),
 ) -> Response:
-    """Serve a file (playlist / segment / subtitle) from the current HLS session directory. The
-    playlist references its segments/subtitles by relative name, all resolving back here."""
+    """Serve a file (playlist / segment / subtitle) from ONE HLS session's directory. The playlist
+    references its segments/subtitles by relative name, all resolving back here.
+
+    The session id in the path is what makes "back here" mean the same session. Sessions are keyed by
+    rating_key alone and a seek-restart or a second viewer of the same item replaces the registered
+    one, while the DIRECTORY is per (key, offset, tag) — so resolving by rating_key handed the old
+    viewer the new session's `seg_12.ts`, produced from a different offset under `-copyts`: wrong
+    bytes with a 200, which the player has no way to notice (deep-review round 5). A request for a
+    session that is no longer the registered one is a 404, and the player reclaims at the playhead.
+
+    That reclaim is also why a request with no live session is no longer auto-started here: the
+    auto-start could only ever begin at 0, silently throwing away the viewer's position."""
     # Filename must be a plain hls artifact — no path separators, only the expected extensions.
     ext = "." + filename.rsplit(".", 1)[-1] if "." in filename else ""
     if ext not in _HLS_CT or "/" in filename or "\\" in filename or ".." in filename:
         raise HTTPException(status_code=404, detail="Not found")
     s = plex_stream.current_session(str(rating_key))
-    if s is None:
-        # No session yet (e.g. reaped): the ENTRY playlist may auto-start one at 0 — master.m3u8 for a
-        # multi-audio item, index.m3u8 for a single-audio remux item.
-        it = item_or_404(db, rating_key)
-        if filename == "master.m3u8":
-            s = plex_stream.start_session(db, it, 0.0, multi=True)
-        elif filename == "index.m3u8" and it.playable == "remux":
-            s = plex_stream.start_session(db, it, 0.0)
-        else:
+    if s is None or s.id != session_id:
         raise HTTPException(status_code=404, detail="No active playback session")
-        if s is None:
-            raise HTTPException(status_code=404, detail="Media file not found on disk")
     path = s.dir / filename
     timeout = 30 if ext == ".m3u8" else 20
-    if not plex_stream.wait_for(path, timeout):
+    # `proc` so a session whose ffmpeg has already exited answers at once instead of polling out the
+    # whole timeout: this is a SYNC route on the ~40-slot threadpool, holding a pooled DB connection
+    # with it, so every such wait is a worker (and a connection) parked for 20 s — an ordinary seek
+    # past the produced region was enough to do it (round 5).
+    if not plex_stream.wait_for(path, timeout, proc=s.proc):
         raise HTTPException(status_code=404, detail="Not available yet")
     return Response(
         content=path.read_bytes(),

backend/tests/test_hls_reaper.py

@@ -1,21 +1,35 @@
-"""Idle HLS sessions are reaped on a timer, not only when the next session is requested.
+"""Idle HLS sessions are reaped on a timer, and a viewer who is merely PAUSED keeps theirs.
 
 plex/stream.py promises "a periodic reaper kills idle ones", but `reap_idle` ran only inside
 POST /session: after the last playback, an abandoned session (its ffmpeg still working through the
-film, its directory a whole segmented copy of it) lived until somebody pressed play again.
+film, its directory a whole segmented copy of it) lived until somebody pressed play again
+(deep-review round 4).
 
 And it must not take a PAUSED viewer's session: a remux finishes long before the film does, and a
-paused player with a full buffer fetches nothing — so "finished and unfetched for 30 s" (the rule a
-new session request applies) is not "abandoned".
+paused player with a full buffer fetches nothing, so "finished and unfetched for 30 s" does not mean
+"abandoned". Round 4 bought that safety by keeping EVERY finished session for the full ten-minute
+idle window — one segmented copy of a film per item, on the volume the download center shares. Round
+5 replaced the blanket retention with the signal that already existed: an open player checkpoints its
+position every 10 s whether or not it is playing, and that route says "still here".
+
+Both call sites are pinned here, because the bug each time was that only one of them moved: the
+periodic pass, and the fact that starting a session no longer sweeps OTHER viewers' sessions at all.
 """
 
 import asyncio
 import time
+from types import SimpleNamespace
 
 import pytest
+from fastapi.testclient import TestClient
 
 import app.main as app_main
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
 from app.main import app
+from app.models import PlexItem, PlexLibrary, User
+from app.plex import paths as plex_paths
 from app.plex import stream
 
 
@@ -27,7 +41,7 @@ class _Configured:
         return False
 
 
-def test_idle_sessions_are_reaped_while_nobody_starts_a_new_one(monkeypatch):
+def test_the_periodic_pass_is_the_one_that_runs_and_it_uses_the_short_rule(monkeypatch):
     async def _no_sweep():
         return None
 
@@ -39,7 +53,9 @@ def test_idle_sessions_are_reaped_while_nobody_starts_a_new_one(monkeypatch):
     monkeypatch.setattr(app_main, "shutdown_scheduler", lambda: None)
     monkeypatch.setattr(app_main.plex_subtitles, "shutdown", lambda: None)
     monkeypatch.setattr(app_main, "_HLS_REAP_EVERY_S", 0.01)
-    monkeypatch.setattr(app_main.plex_stream, "reap_abandoned", lambda: reaped.append(1) or 0)
+    # `reap_idle`, i.e. the SHORT finished rule: a finished session nobody has said they still want
+    # is a whole copy of a film sitting on the download volume.
+    monkeypatch.setattr(app_main.plex_stream, "reap_idle", lambda: reaped.append(1) or 0)
 
     async def _cycle():
         async with app_main.lifespan(app):
@@ -79,12 +95,10 @@ def sessions(monkeypatch):
     return live
 
 
-def test_the_periodic_pass_keeps_a_paused_viewers_finished_session(tmp_path, sessions):
-    # A remux that finished a minute ago, its viewer paused with a full buffer since.
-    s = _Session(tmp_path, "paused", finished=True, idle_s=90)
-    sessions["12345"] = s
-    assert stream.reap_abandoned() == 0
-    assert "12345" in sessions and s.dir.exists() and not s.proc.killed
+def test_the_periodic_pass_reaps_a_finished_session_nobody_has_claimed(tmp_path, sessions):
+    sessions["12345"] = _Session(tmp_path, "done", finished=True, idle_s=45)
+    assert stream.reap_idle() == 1
+    assert sessions == {}
 
 
 def test_the_periodic_pass_still_reaps_what_nobody_has_touched_for_the_idle_limit(
@@ -96,11 +110,95 @@ def test_the_periodic_pass_still_reaps_what_nobody_has_touched_for_the_idle_limi
     sessions["2"] = _Session(
         tmp_path, "gone_live", finished=False, idle_s=stream._SESSION_IDLE_S + 5
     )
-    assert stream.reap_abandoned() == 2
+    assert stream.reap_idle() == 2
     assert sessions == {}
 
 
-def test_a_new_session_request_still_reaps_finished_sessions_quickly(tmp_path, sessions):
-    sessions["12345"] = _Session(tmp_path, "done", finished=True, idle_s=45)
-    assert stream.reap_idle() == 1
-    assert sessions == {}
+def test_touching_a_session_keeps_it_through_the_short_rule(tmp_path, sessions):
+    s = _Session(tmp_path, "paused", finished=True, idle_s=90)
+    sessions["12345"] = s
+    stream.touch_session("12345")
+    assert stream.reap_idle() == 0
+    assert "12345" in sessions and s.dir.exists() and not s.proc.killed
+
+
+# --- the two call sites, driven through the real routes (DB lane) -------------------------------
+
+
+def _library_item(db, rating_key: str, *, playable: str = "remux"):
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    it = PlexItem(
+        rating_key=rating_key,
+        library_id=lib.id,
+        kind="movie",
+        title="A Movie",
+        file_path="/media/a.mkv",
+        playable=playable,
+    )
+    db.add(it)
+    db.commit()
+    return it
+
+
+def _client(db, user):
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: user
+    return TestClient(app)
+
+
+def test_a_paused_viewers_checkpoint_keeps_their_session_alive(db, monkeypatch, tmp_path, sessions):
+    """The whole reason the short rule is safe again: a paused player fetches nothing, so
+    `last_access` never moved on its own — but it does keep checkpointing its position."""
+    monkeypatch.setattr(state, "_configured_cache", True)
+    it = _library_item(db, "paused1")
+    user = User(email="paused@example.com")
+    db.add(user)
+    db.commit()
+    s = _Session(tmp_path, "paused", finished=True, idle_s=90)
+    sessions["paused1"] = s
+    try:
+        resp = _client(db, user).post(
+            f"/api/plex/item/{it.rating_key}/progress",
+            json={"position_seconds": 600, "duration_seconds": 3000, "final": False},
+        )
+    finally:
+        app.dependency_overrides.clear()
+    assert resp.status_code == 200, resp.text
+    assert stream.reap_idle() == 0, "the checkpoint did not say the viewer is still there"
+    assert "paused1" in sessions and not s.proc.killed
+
+
+def test_starting_a_session_leaves_another_viewers_paused_one_alone(
+    db, monkeypatch, tmp_path, sessions
+):
+    """The opportunistic sweep this route used to run was not scoped to the key being started: it
+    walked the WHOLE registry with the 30 s finished rule, so pressing play on any item killed a
+    paused viewer's session — the very case round 4 set out to protect (found in round 5)."""
+    monkeypatch.setattr(state, "_configured_cache", True)
+    monkeypatch.setattr(plex_paths, "local_media_path", lambda db_, fp: "/local/a.mkv")
+    started = _library_item(db, "starting")
+    user = User(email="starter@example.com")
+    db.add(user)
+    db.commit()
+    paused = _Session(tmp_path, "paused", finished=True, idle_s=90)
+    sessions["someone-else"] = paused
+
+    def fake_start(db_, item, start, audio=None, aoff=0.0, multi=False, transcode=False):
+        return SimpleNamespace(
+            id="sid",
+            media_start_s=start,
+            entry="index.m3u8",
+            transcode=transcode,
+            hw=False,
+            tonemap=False,
+        )
+
+    monkeypatch.setattr(stream, "start_session", fake_start)
+    try:
+        resp = _client(db, user).post(f"/api/plex/stream/{started.rating_key}/session")
+    finally:
+        app.dependency_overrides.clear()
+    assert resp.status_code == 200, resp.text
+    assert "someone-else" in sessions and not paused.proc.killed

backend/tests/test_not_found_detail.py

@@ -51,3 +51,24 @@ def test_a_nul_in_a_request_string_is_a_422_not_a_500(db, make_user, as_user):
     assert r.status_code == 422, r.text
     assert r.json()["detail"] == {"code": "invalid_input", "reason": "nul"}
     assert client.get("/api/plex/playlists?contains_group=1,%00").status_code == 422
+
+
+def test_any_other_data_error_is_still_a_500():
+    """The other half of the app-wide handler, and the half a refactor can delete in silence: it is
+    registered for EVERY DataError on every route and decides on a substring of the driver's message,
+    so only a NUL may become a 422. Without this, simplifying it to an unconditional 422 — it reads
+    like dead defensive code — would answer a numeric overflow or an over-long ingested title with
+    "your input was bad", losing the traceback that names the real fault (deep-review round 5)."""
+    import asyncio
+
+    import pytest
+    from sqlalchemy.exc import DataError
+
+    from app.main import _nul_in_text
+
+    overflow = DataError("SELECT 1", {}, Exception("numeric field overflow"))
+    with pytest.raises(DataError):
+        asyncio.run(_nul_in_text(None, overflow))  # re-raised: it stays the 500 it always was
+
+    nul = DataError("SELECT 1", {}, Exception("invalid byte sequence: NUL (0x00)"))
+    assert asyncio.run(_nul_in_text(None, nul)).status_code == 422

backend/tests/test_plex_hls_file_route.py

@@ -0,0 +1,145 @@
+"""An HLS artifact request names the session it belongs to, and gets nothing else.
+
+Sessions are keyed by rating_key ALONE, while a session's directory is per (key, offset, tag). So a
+seek-restart — or a second viewer of the same item — replaces the registered session, and the old
+viewer's next request for `seg_12.ts`, resolved by rating_key, was answered 200 with the NEW
+session's twelfth segment: bytes produced from a different `-ss` offset under `-copyts`, i.e. a
+timeline the client is not on. Wrong content with a success status is the one failure a player cannot
+notice, and the reclaim-on-404 added in round 4 could never fire on it (deep-review round 5).
+
+DB lane (`db` fixture) — skipped when no Postgres is reachable.
+"""
+
+import time
+
+import pytest
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import PlexItem, PlexLibrary, User
+from app.plex import stream as plex_stream
+
+
+class _Proc:
+    def poll(self):
+        return 0  # finished: `wait_for` may not poll out its whole timeout on a missing file
+
+    def terminate(self):
+        pass
+
+    def wait(self, timeout=None):
+        return 0
+
+
+class _Session:
+    def __init__(self, session_id: str, directory):
+        self.id = session_id
+        self.dir = directory
+        self.proc = _Proc()
+        self.last_access = time.time()
+
+
+@pytest.fixture
+def sessions(monkeypatch):
+    live: dict = {}
+    monkeypatch.setattr(plex_stream, "_sessions", live)
+    return live
+
+
+def _client(db, monkeypatch):
+    monkeypatch.setattr(state, "_configured_cache", True)
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    db.add(
+        PlexItem(
+            rating_key="m1",
+            library_id=lib.id,
+            kind="movie",
+            title="A Movie",
+            file_path="/media/a.mkv",
+            playable="remux",
+        )
+    )
+    user = User(email="hls@example.com")
+    db.add(user)
+    db.commit()
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: user
+    return TestClient(app)
+
+
+def test_a_segment_is_served_from_the_session_the_url_names(db, monkeypatch, tmp_path, sessions):
+    d = tmp_path / "m1_0_0_+0.00"
+    d.mkdir()
+    (d / "seg_12.ts").write_bytes(b"mine")
+    sessions["m1"] = _Session("sid-a", d)
+    client = _client(db, monkeypatch)
+    try:
+        r = client.get("/api/plex/stream/m1/hls/sid-a/seg_12.ts")
+    finally:
+        app.dependency_overrides.clear()
+    assert r.status_code == 200, r.text
+    assert r.content == b"mine"
+
+
+def test_a_segment_of_a_replaced_session_is_a_404_not_the_new_sessions_bytes(
+    db, monkeypatch, tmp_path, sessions
+):
+    """Viewer A is at 0:00; viewer B seeks to 25:00 and their session takes the registry slot. A's
+    next segment must fail honestly — the player reclaims at the playhead — not arrive as B's."""
+    theirs = tmp_path / "m1_1500_0_+0.00"
+    theirs.mkdir()
+    (theirs / "seg_12.ts").write_bytes(b"theirs")
+    sessions["m1"] = _Session("sid-b", theirs)
+    client = _client(db, monkeypatch)
+    try:
+        r = client.get("/api/plex/stream/m1/hls/sid-a/seg_12.ts")
+    finally:
+        app.dependency_overrides.clear()
+    assert r.status_code == 404, r.text
+    assert b"theirs" not in r.content
+
+
+def test_a_playlist_with_no_live_session_is_a_404_not_a_silent_restart_at_zero(
+    db, monkeypatch, sessions
+):
+    """The entry playlist used to auto-start a session when none was registered — always at 0.0,
+    silently throwing away the viewer's position. A 404 sends the player through its own reclaim,
+    which restarts at the playhead."""
+    started: list[float] = []
+    monkeypatch.setattr(
+        plex_stream,
+        "start_session",
+        lambda *a, **k: started.append(a[2] if len(a) > 2 else 0.0),
+    )
+    client = _client(db, monkeypatch)
+    try:
+        r = client.get("/api/plex/stream/m1/hls/sid-gone/index.m3u8")
+    finally:
+        app.dependency_overrides.clear()
+    assert r.status_code == 404, r.text
+    assert started == []
+
+
+def test_a_segment_that_will_never_arrive_fails_at_once(db, monkeypatch, tmp_path, sessions):
+    """`stream_hls_file` is a SYNC route: it runs on the ~40-slot threadpool and holds a pooled DB
+    connection while it waits. Waiting without the producer's handle polled out the whole 20 s even
+    when that ffmpeg had already exited and no segment could ever appear — one ordinary seek past the
+    produced region parked a worker and a connection for 20 s (deep-review round 5)."""
+    d = tmp_path / "m1_0_0_+0.00"
+    d.mkdir()
+    sessions["m1"] = _Session("sid-a", d)  # its _Proc.poll() reports a finished ffmpeg
+    client = _client(db, monkeypatch)
+    started = time.monotonic()
+    try:
+        r = client.get("/api/plex/stream/m1/hls/sid-a/seg_99999.ts")
+    finally:
+        app.dependency_overrides.clear()
+    elapsed = time.monotonic() - started
+    assert r.status_code == 404, r.text
+    # The wait is 20 s; anything in that neighbourhood means the producer's exit was not consulted.
+    assert elapsed < 2.0, f"waited {elapsed:.1f}s for a segment that can never arrive"

backend/tests/test_plex_playback_session.py

@@ -58,7 +58,12 @@ def _post(db, user, monkeypatch, rating_key, *, captured):
     def fake_start(db_, it, start, audio, aoff, multi, transcode=False):
         captured["transcode"] = transcode
         return SimpleNamespace(
-            media_start_s=start, entry="index.m3u8", transcode=transcode, hw=False, tonemap=False
+            id="sid",
+            media_start_s=start,
+            entry="index.m3u8",
+            transcode=transcode,
+            hw=False,
+            tonemap=False,
         )
 
     monkeypatch.setattr(plex_stream, "start_session", fake_start)