The enhanced-tab "Sync" button's stale-removal phase deleted any track whose file
wasn't on disk, with NO guard — so if the music storage was momentarily
unavailable (sleeping NAS, dropped mount, unmounted Docker volume, WSL hiccup),
os.path.exists returned False for EVERY file and one click wiped the whole artist
(tracks + their now-"empty" albums) from the DB. The deep-scan path already had a
50%-stale safety net (#828); this endpoint never got one.
- New core/library/stale_guard.py: is_implausible_stale_removal(missing, total) —
a tested rule (skip removal when missing > 50% of a >=5-track set), centralised
so every stale-removal site can share it.
- sync_artist_library: if the guard trips, SKIP removal (delete nothing), return
removal_skipped + warn; the frontend shows "storage may be offline — skipped"
instead of silently deleting. Empty-album cleanup now also only runs on the
non-skipped path and uses `album_id IS NOT NULL` (fixes the NOT IN-with-NULL
no-op). Frontend also refreshes the view on additions, not just removals.
- @admin_only on the endpoint — it deletes tracks + albums but was ungated, while
the sibling delete_album endpoint is gated.
Deep scan was already safe (different mechanism: server-diff + its own 50% guard).
Tests: guard unit rules; endpoint skips removal when all files missing (keeps the
tracks), removes only the genuinely-gone few otherwise, and 403s for non-admins.
7 new tests pass.
4d1b9a5639