fix: a first watch state and a first Watch later survive two requests at once

The first checkpoint of an item and the final flush beside it both read no
state and both inserted; the loser failed on uq_plex_user_item. Two first saves
each made a Watch later list. Both are created by an ON CONFLICT DO NOTHING
insert and then read; a partial unique index holds one Watch later per user, and
the migration folds an existing second list into the first. Plex keeps one
per-item state map (rollup.item_state_map).

backend/tests/test_migration_0112.py

@@ -0,0 +1,68 @@
+"""Migration 0112's data step, DB-lane: a user who already has two Watch later lists keeps the oldest,
+with the other's videos moved in after its own (one it already held is not added twice), and the
+one-per-user index then refuses a second list.
+
+The schema half is checked by test_migrations.py; a video lost on the way is invisible there.
+"""
+
+from datetime import datetime, timezone
+
+import pytest
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import text
+from sqlalchemy.exc import IntegrityError
+
+from app.catalog.models import Channel, Video
+from app.db import engine
+
+
+def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
+    user, other = make_user(), make_user()
+    db.add(Channel(id="UCm", title="M"))
+    now = datetime.now(timezone.utc)
+    for v in ("v1", "v2", "v3"):
+        db.add(Video(id=v, channel_id="UCm", published_at=now, duration_seconds=60))
+    db.commit()
+    cfg = Config("alembic.ini")
+    command.downgrade(cfg, "0111_audit_request_id")
+    try:
+        with engine.begin() as conn:
+            ids = conn.execute(
+                text(
+                    "INSERT INTO playlists (user_id, name, kind, position) VALUES"
+                    " (:u, 'Watch later', 'watch_later', 0), (:u, 'Watch later', 'watch_later', 1),"
+                    " (:o, 'Watch later', 'watch_later', 0) RETURNING id"
+                ),
+                {"u": user.id, "o": other.id},
+            ).scalars()
+            first, second, theirs = list(ids)
+            conn.execute(
+                text(
+                    "INSERT INTO playlist_items (playlist_id, video_id, position) VALUES"
+                    " (:a, 'v1', 0), (:b, 'v2', 0), (:b, 'v1', 1), (:c, 'v3', 0)"
+                ),
+                {"a": first, "b": second, "c": theirs},
+            )
+    finally:
+        command.upgrade(cfg, "head")
+
+    with engine.connect() as conn:
+        lists = conn.execute(
+            text("SELECT id, user_id FROM playlists WHERE kind = 'watch_later' ORDER BY id")
+        ).all()
+        assert [tuple(r) for r in lists] == [(first, user.id), (theirs, other.id)]
+        items = conn.execute(
+            text(
+                "SELECT playlist_id, video_id, position FROM playlist_items ORDER BY playlist_id, position"
+            )
+        ).all()
+        assert [tuple(r) for r in items] == [(first, "v1", 0), (first, "v2", 1), (theirs, "v3", 0)]
+    with pytest.raises(IntegrityError), engine.begin() as conn:
+        conn.execute(
+            text(
+                "INSERT INTO playlists (user_id, name, kind, position)"
+                " VALUES (:u, 'Watch later', 'watch_later', 2)"
+            ),
+            {"u": user.id},
+        )

backend/tests/test_plex_service.py

@@ -12,7 +12,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 
-from app.plex import library, queries, service
+from app.plex import library, queries, rollup, service
 from app.plex.models import PlexCollection, PlexItem, PlexLibrary, PlexShow, PlexState
 from app.plex.query import LibraryFilters
 
@@ -71,9 +71,9 @@ def test_a_users_watch_state_is_theirs_alone(db, make_user):
     _state(db, owner, it, status="watched")
     assert queries.state(db, owner, it.id).status == "watched"
     assert queries.state(db, stranger, it.id) is None
-    assert queries.states(db, stranger, [it.id]) == {}
-    assert set(queries.states(db, owner, [it.id])) == {it.id}
-    assert queries.states(db, owner, []) == {}
+    assert rollup.item_state_map(db, stranger, [it.id]) == {}
+    assert set(rollup.item_state_map(db, owner, [it.id])) == {it.id}
+    assert rollup.item_state_map(db, owner, []) == {}
 
 
 def test_the_grid_shows_the_callers_state_and_hides_only_what_they_hid(db, make_user):
@@ -421,3 +421,18 @@ def test_only_the_owners_own_link_pushes_their_watch_state(db, make_user):
     db.commit()
     assert watch_sync.link_for_push(db, owner.id).id == link.id
     assert watch_sync.link_for_push(db, other.id) is None
+
+
+def test_a_first_write_that_lost_the_race_updates_the_winners_row(db, make_user):
+    """The first checkpoint of an item and the `final` flush beside it both read "no state"; the one
+    that writes second must land on the row the first made, not fail on `uq_plex_user_item`."""
+    user = make_user()
+    it = _movie(db, _lib(db), "m-race")
+    _state(db, user, it, position_seconds=10)  # the request that won, committed
+    st = service.checkpoint(db, user, it.id, None, 42, NOW, dirty=True)  # read "none" before it
+    assert (st.position_seconds, st.synced_to_plex) == (42, False)
+    st = service.set_status(db, user, it.id, None, "hidden")
+    assert st.status == "hidden"
+    db.commit()
+    rows = db.scalars(select(PlexState).where(PlexState.item_id == it.id)).all()
+    assert [(r.position_seconds, r.status) for r in rows] == [(42, "hidden")]

backend/tests/test_reader_service.py

@@ -308,3 +308,21 @@ def test_a_pin_is_one_per_thing_and_only_its_owners(db, make_user):
     pins.remove(db, pins.own_pin(db, user, pin.id))
     db.commit()
     assert db.execute(select(ComfortPin)).first() is None
+
+
+def test_a_watch_later_save_that_lost_the_race_uses_the_winners_list(db, make_user, monkeypatch):
+    """The first two saves at once both read "no Watch later"; the second must land in the list the
+    first made, not make one of its own."""
+    user = make_user()
+    won = service.watch_later(db, user)
+    db.commit()
+    real = queries.watch_later
+    stale = [True]  # this request's first read happened before the winner committed
+
+    def read(d, u):
+        return None if stale and stale.pop() else real(d, u)
+
+    monkeypatch.setattr(queries, "watch_later", read)
+    assert service.watch_later(db, user).id == won.id
+    db.commit()
+    assert [p.id for p in queries.playlists_for(db, user) if p.kind == "watch_later"] == [won.id]