fix(playlists): one YouTube sync of a list at a time, across tabs and the scheduler

The client's in-flight flag covers the menus of one page only. A push reads YouTube, then writes
both sides for seconds and commits step by step, so a row lock ends at its first commit: two tabs,
or a push and the scheduled mirror refresh, could interleave into a doubled YouTube item or a
baseline recorded over the other's writes. `sync_claim` takes a session-level advisory lock on a
connection of its own (it survives those commits; a crashed worker's lock dies with its
connection, so no claim column is left stuck). Push and revert answer 409 `sync_busy`; the
scheduled refresh skips a list being synced and picks it up next run.

backend/tests/test_playlist_sync_claim.py

@@ -0,0 +1,111 @@
+"""E28 review C1 — one YouTube sync of a playlist at a time, whoever starts it.
+
+A push reads YouTube and then writes both sides for seconds, committing step by step, so the
+client's in-flight flag (one page's menus) and a row lock (gone at the first commit) both leave two
+tabs, or a push and the scheduled mirror refresh, free to interleave. The claim is an advisory lock
+on a connection of its own; the test holds it from outside, the way a second request would.
+"""
+
+from sqlalchemy import text
+
+from app.auth import require_write_scope
+from app.models import Playlist
+from app.routes import playlists as plroutes
+from app.sync import playlists as plsync
+
+
+def _linked(db, user, *, source="local"):
+    pl = Playlist(user_id=user.id, name="L", source=source, yt_playlist_id="PL123")
+    db.add(pl)
+    db.commit()
+    return pl
+
+
+class FakeYT:
+    """YouTube as far as these routes read it: two playlists, none with items."""
+
+    def __init__(self, *_):
+        pass
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *_):
+        return False
+
+    def iter_my_playlists(self):
+        return [{"id": "PL123", "title": "Renamed"}, {"id": "PL999", "title": "New"}]
+
+    def iter_my_playlist_video_ids(self, _ytid):
+        return []
+
+    def iter_playlist_items_with_ids(self, _ytid):
+        return []
+
+    def get_playlist_snippet(self, _ytid):
+        return {}
+
+
+def test_push_and_revert_refuse_while_the_list_is_claimed(
+    db, as_user, make_user, override_dep, monkeypatch
+):
+    user = make_user()
+    pl = _linked(db, user)
+    override_dep(require_write_scope, lambda: user)
+    monkeypatch.setattr(plroutes, "has_read_scope", lambda u: True)
+    ran: list[str] = []
+    monkeypatch.setattr(plroutes, "YouTubeClient", FakeYT)
+    monkeypatch.setattr(plroutes, "plan_merge", lambda d, u, p, live: {"units_estimate": 0})
+    monkeypatch.setattr(plroutes, "merge_playlist", lambda d, u, p, live: ran.append("push") or {})
+    monkeypatch.setattr(plroutes, "repull_playlist", lambda d, u, p: ran.append("revert") or {})
+    c = as_user(user)
+
+    with plsync.sync_claim(db, pl.id) as free:
+        assert free
+        for path in ("push", "revert-youtube"):
+            r = c.post(f"/api/playlists/{pl.id}/{path}")
+            assert r.status_code == 409
+            assert r.json()["detail"] == {"code": "playlist", "reason": "sync_busy"}
+    assert ran == []
+
+    # Released on the way out: the same list syncs again, and a claim is per list.
+    assert c.post(f"/api/playlists/{pl.id}/push").status_code == 200
+    assert c.post(f"/api/playlists/{pl.id}/revert-youtube").status_code == 200
+    assert ran == ["push", "revert"]
+
+
+def test_a_claim_is_released_when_its_block_raises(db, make_user):
+    pl = _linked(db, make_user())
+    try:
+        with plsync.sync_claim(db, pl.id) as free:
+            assert free
+            raise RuntimeError("boom")
+    except RuntimeError:
+        pass
+    # Asked of pg_locks, not by claiming again: a lock leaked on a pooled connection would be granted
+    # again to that same connection (advisory locks are re-entrant per session).
+    held = db.scalar(
+        text("SELECT count(*) FROM pg_locks WHERE locktype = 'advisory' AND objid = :id"),
+        {"id": pl.id},
+    )
+    assert held == 0
+
+
+def test_the_scheduled_refresh_skips_a_mirror_being_synced(db, make_user, monkeypatch):
+    user = make_user()
+    busy = _linked(db, user, source="youtube")
+    replaced: list[int] = []
+
+    monkeypatch.setattr(plsync, "has_read_scope", lambda u: True)
+    monkeypatch.setattr(plsync, "YouTubeClient", FakeYT)
+    monkeypatch.setattr(
+        plsync, "_replace_items_from_youtube", lambda d, yt, p, ids: replaced.append(p.id)
+    )
+
+    with plsync.sync_claim(db, busy.id):
+        out = plsync.sync_user_playlists(db, user)
+
+    assert out == {"synced": 1}
+    assert busy.id not in replaced and len(replaced) == 1
+    db.refresh(busy)
+    assert busy.name == "L"