diff --git a/backend/app/downloads/folders.py b/backend/app/downloads/folders.py
index 570d9598..4a3769ef 100644
--- a/backend/app/downloads/folders.py
+++ b/backend/app/downloads/folders.py
@@ -136,7 +136,14 @@ def done_expiry(db: Session, folder_id: int | None, kept: bool) -> datetime | No
     folder that keeps files)."""
     if kept or folder_id is None:
         return None
-    days = db.execute(select(FileFolder.retention_days).where(FileFolder.id == folder_id)).scalar()
+    # Read under a share lock, held to the caller's commit: a retention change (`set_retention` locks
+    # the folder) waits for this file and restamps it too, or this read waits for the change and
+    # takes the new value — never the old value stamped on a file the change did not see.
+    days = db.execute(
+        select(FileFolder.retention_days)
+        .where(FileFolder.id == folder_id)
+        .with_for_update(read=True)
+    ).scalar()
     return None if days is None else datetime.now(timezone.utc) + timedelta(days=days)
 
 
@@ -208,18 +215,30 @@ def set_retention(db: Session, folder: FileFolder, days: int | None) -> None:
     """Change how long `folder` keeps a file (None = until deleted). The done files already in it
     move by the difference — each keeps the age it has — but none ends up closer than the GC's
     warning window, so a shortened retention still warns before it takes anything."""
+    # Wait out a file being put in it or stamped from it right now (`own_folder`, `done_expiry`), so
+    # that file is among the ones restamped below. Re-read: another change may have landed first.
+    db.execute(
+        select(FileFolder)
+        .where(FileFolder.id == folder.id)
+        .with_for_update()
+        .execution_options(populate_existing=True)
+    )
     old = folder.retention_days
     if old == days:
         return
     folder.retention_days = days
     now = datetime.now(timezone.utc)
     floor = now + timedelta(days=sysconfig.get_int(db, "download_gc_grace_days"))
+    # Locked as they are read: one moved out or deleted meanwhile is no longer this folder's to time.
     jobs = db.execute(
-        select(DownloadJob).where(
+        select(DownloadJob)
+        .where(
             DownloadJob.folder_id == folder.id,
             DownloadJob.status == JobStatus.DONE,
             DownloadJob.kept.is_(False),
         )
+        .with_for_update()
+        .execution_options(populate_existing=True)
     ).scalars()
     for job in jobs:
         if days is None:
@@ -238,7 +257,18 @@ def delete_own(db: Session, folder: FileFolder) -> int:
         raise FolderError("file_folder_not_own")
     # Wait out a file being put in it right now (`own_folder`), then take that one home too.
     db.execute(select(FileFolder.id).where(FileFolder.id == folder.id).with_for_update())
-    jobs = db.execute(select(DownloadJob).where(DownloadJob.folder_id == folder.id)).scalars().all()
+    # Locked as they are read: a file moved out meanwhile stays where it went, and one deleted
+    # meanwhile is not written back (its gone row would fail the delete).
+    jobs = (
+        db.execute(
+            select(DownloadJob)
+            .where(DownloadJob.folder_id == folder.id)
+            .with_for_update()
+            .execution_options(populate_existing=True)
+        )
+        .scalars()
+        .all()
+    )
     for job in jobs:
         job.folder_id = source_folder(db, job.user_id, source_key_for(db, job)).id
         restamp(db, job)
diff --git a/backend/app/downloads/gc.py b/backend/app/downloads/gc.py
index 981c4b85..d0041c96 100644
--- a/backend/app/downloads/gc.py
+++ b/backend/app/downloads/gc.py
@@ -22,7 +22,7 @@ import time
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
 
-from sqlalchemy import func, or_, select
+from sqlalchemy import func, or_, select, update
 from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
@@ -82,6 +82,59 @@ def _relock_expired(
     return job if still else None
 
 
+def _mark_warned(db: Session, job_id: int, expires_at: datetime) -> DownloadJob | None:
+    """Mark one file warned if it still goes when the pass saw it go — not deleted, kept, moved or
+    re-timed since (each of those sets a fresh expiry, and its own warning to come) — and return it.
+    The UPDATE's row lock holds it so until the commit."""
+    hit = db.execute(
+        update(DownloadJob)
+        .where(
+            DownloadJob.id == job_id,
+            DownloadJob.status == JobStatus.DONE,
+            DownloadJob.kept.is_(False),
+            DownloadJob.expiry_warned.is_(False),
+            DownloadJob.expires_at == expires_at,
+        )
+        .values(expiry_warned=True)
+        .returning(DownloadJob.id)
+        .execution_options(synchronize_session=False)
+    ).scalar_one_or_none()
+    return db.get(DownloadJob, job_id) if hit is not None else None
+
+
+def _on_disk(db: Session) -> int:
+    """The bytes the cache holds now (every asset whose file is on disk)."""
+    return (
+        db.execute(
+            select(func.coalesce(func.sum(MediaAsset.size_bytes), 0)).where(quota.ON_DISK)
+        ).scalar()
+        or 0
+    )
+
+
+def _relock_evictable(db: Session, asset_id: int) -> tuple[MediaAsset | None, list[DownloadJob]]:
+    """Re-read an eviction candidate and its holders under their locks (the asset, then its jobs —
+    the order the user routes take): between the pass's SELECT and here a holder may have kept it,
+    or every holder deleted it."""
+    asset = db.execute(
+        select(MediaAsset)
+        .where(MediaAsset.id == asset_id)
+        .with_for_update()
+        .execution_options(populate_existing=True)
+    ).scalar_one_or_none()
+    holders = (
+        db.execute(
+            select(DownloadJob)
+            .where(DownloadJob.asset_id == asset_id)
+            .with_for_update()
+            .execution_options(populate_existing=True)
+        )
+        .scalars()
+        .all()
+    )
+    return asset, list(holders)
+
+
 def _drop_job(db: Session, job: DownloadJob, ntype: str) -> None:
     """Take a file out of its user's library: tell them, release the job's hold on the asset (the
     file goes with the last holder), delete the row (its shares and links cascade)."""
@@ -176,29 +229,28 @@ def run_download_gc(db: Session) -> dict:
     grace_days = sysconfig.get_int(db, "download_gc_grace_days")
     warned = expired = evicted = reaped = 0
 
-    # 1. Pre-expiry warnings — one per file, the first run inside the grace window.
+    # 1. Pre-expiry warnings — one per file, the first run inside the grace window. Each mark is
+    #    conditional on the expiry the SELECT saw and committed on its own: a file deleted since is
+    #    passed over (a write to its gone row would fail the whole run), and one kept, moved or re-timed
+    #    since keeps its fresh expiry unwarned.
     warn_before = now + timedelta(days=grace_days)
-    soon = (
-        db.execute(
-            select(DownloadJob).where(
-                DownloadJob.status == JobStatus.DONE,
-                DownloadJob.kept.is_(False),
-                DownloadJob.expiry_warned.is_(False),
-                DownloadJob.expires_at.is_not(None),
-                DownloadJob.expires_at > now,
-                DownloadJob.expires_at <= warn_before,
-            )
+    soon = db.execute(
+        select(DownloadJob.id, DownloadJob.expires_at).where(
+            DownloadJob.status == JobStatus.DONE,
+            DownloadJob.kept.is_(False),
+            DownloadJob.expiry_warned.is_(False),
+            DownloadJob.expires_at.is_not(None),
+            DownloadJob.expires_at > now,
+            DownloadJob.expires_at <= warn_before,
         )
-        .scalars()
-        .all()
-    )
-    for job in soon:
-        _notify(
-            db, job, db.get(MediaAsset, job.asset_id) if job.asset_id else None, "download_expiring"
-        )
-        job.expiry_warned = True
-        warned += 1
-    if soon:
+    ).all()
+    db.rollback()  # the pass-1 snapshot holds no locks; each mark takes its own below
+    for job_id, expires_at in soon:
+        job = _mark_warned(db, job_id, expires_at)
+        if job is not None:
+            asset = db.get(MediaAsset, job.asset_id) if job.asset_id else None
+            _notify(db, job, asset, "download_expiring")
+            warned += 1
         db.commit()
 
     # 2. Expiry — a file past its folder's retention leaves the library (warned in pass 1).
@@ -224,19 +276,14 @@ def run_download_gc(db: Session) -> dict:
     #    fetch again) and never a file anyone kept; an evicted file leaves every holder's library.
     cap = sysconfig.get_int(db, "download_total_max_bytes")
     if cap > 0:
-        total = (
-            db.execute(
-                select(func.coalesce(func.sum(MediaAsset.size_bytes), 0)).where(quota.ON_DISK)
-            ).scalar()
-            or 0
-        )
+        total = _on_disk(db)
         if total > cap:
             kept_assets = select(DownloadJob.asset_id).where(
                 DownloadJob.kept.is_(True), DownloadJob.asset_id.is_not(None)
             )
-            candidates = (
+            candidate_ids = (
                 db.execute(
-                    select(MediaAsset)
+                    select(MediaAsset.id)
                     .where(
                         MediaAsset.status == "ready",
                         MediaAsset.source_kind != "upload",
@@ -247,30 +294,20 @@ def run_download_gc(db: Session) -> dict:
                 .scalars()
                 .all()
             )
-            candidate_ids = [a.id for a in candidates]
             db.rollback()
             for asset_id in candidate_ids:
                 if total <= cap:
                     break
                 # Locked and re-read like an expiry: a holder may have kept it, or every holder
-                # deleted it (the file is then already gone and counted out) since the SELECT.
-                asset = db.execute(
-                    select(MediaAsset)
-                    .where(MediaAsset.id == asset_id)
-                    .with_for_update()
-                    .execution_options(populate_existing=True)
-                ).scalar_one_or_none()
-                holders = (
-                    db.execute(
-                        select(DownloadJob)
-                        .where(DownloadJob.asset_id == asset_id)
-                        .with_for_update()
-                        .execution_options(populate_existing=True)
-                    )
-                    .scalars()
-                    .all()
-                )
-                if asset is None or asset.status != "ready" or any(j.kept for j in holders):
+                # deleted it since the SELECT.
+                asset, holders = _relock_evictable(db, asset_id)
+                if asset is None or asset.status != "ready":
+                    # Its file went (or is being fetched afresh) since the sum: the disk is counted
+                    # again, or the pass would evict another user's file for space already free.
+                    db.commit()
+                    total = _on_disk(db)
+                    continue
+                if any(j.kept for j in holders):
                     db.commit()
                     continue
                 total -= asset.size_bytes or 0
diff --git a/backend/app/downloads/service.py b/backend/app/downloads/service.py
index 3da8fd6b..7fa7f2b2 100644
--- a/backend/app/downloads/service.py
+++ b/backend/app/downloads/service.py
@@ -65,6 +65,23 @@ def reference_url(job, asset) -> str | None:
     return None
 
 
+def library_left_behind(db: Session, files_root: str) -> bool:
+    """True when the library names files and none of the newest of them is under `files_root` — a
+    fresh, empty volume mounted in the library's place (a compose file changed under an install, its
+    old volume left behind). The mount check cannot see that: the new volume is a real mount."""
+    rels = (
+        db.execute(
+            select(MediaAsset.rel_path)
+            .where(MediaAsset.status == "ready", MediaAsset.rel_path.is_not(None))
+            .order_by(MediaAsset.id.desc())
+            .limit(20)
+        )
+        .scalars()
+        .all()
+    )
+    return bool(rels) and not any(storage.abs_path(files_root, r).exists() for r in rels if r)
+
+
 def get_or_create_asset(
     db: Session, source_kind: str, source_ref: str, format_sig: str
 ) -> MediaAsset:
@@ -77,7 +94,12 @@ def get_or_create_asset(
         MediaAsset.source_ref == source_ref,
         MediaAsset.format_sig == format_sig,
     )
-    asset = db.execute(stmt).scalar_one_or_none()
+    # Locked to the caller's commit: a release or the GC freeing this asset meanwhile (they lock it
+    # too) either waits for the new hold, or has deleted the row by the time this read returns — and
+    # the caller then gets a fresh one, never a job pointing at a row that is gone.
+    asset = db.execute(
+        stmt.with_for_update().execution_options(populate_existing=True)
+    ).scalar_one_or_none()
     if asset is not None:
         # A previously-FAILED shared asset must be re-attempted for the new job about to hold it —
         # reset it to pending (+clear the error) so the worker actually re-downloads instead of
diff --git a/backend/app/downloads/storage.py b/backend/app/downloads/storage.py
index 8a003117..c3de9bf8 100644
--- a/backend/app/downloads/storage.py
+++ b/backend/app/downloads/storage.py
@@ -68,20 +68,52 @@ _FOLD_MAP = str.maketrans(
 )
 
 
-def files_root_problem(root: str, system_root: str = "/") -> str | None:
-    """Why `root` cannot be the files library's volume, or None when it can. A root that sits on the
-    container's own filesystem is the image's empty directory, not a mounted volume: the library
-    would answer 404 for every file, and what a download writes would go when the container does —
-    the state an install is in when its compose file still mounts the library somewhere else."""
+def _mount_points(mountinfo: str) -> list[str]:
+    """The mount points a /proc/<pid>/mountinfo table lists (its fifth field, octal escapes undone)."""
+    points = []
+    for line in mountinfo.splitlines():
+        fields = line.split(" ")
+        if len(fields) > 4:
+            points.append(re.sub(r"\\([0-7]{3})", lambda m: chr(int(m.group(1), 8)), fields[4]))
+    return points
+
+
+def _in_container() -> bool:
+    return Path("/.dockerenv").exists() or Path("/run/.containerenv").exists()
+
+
+def _read_mountinfo() -> str | None:
+    try:
+        return Path("/proc/self/mountinfo").read_text()
+    # silent-ok: no mount table to read (not Linux) — the caller then cannot tell, and refuses nothing.
+    except OSError:
+        return None
+
+
+def files_root_problem(
+    root: str, mountinfo: str | None = None, in_container: bool | None = None
+) -> str | None:
+    """Why `root` cannot be the files library's volume, or None when it can. In a container, a root
+    that is no mount of its own (nor inside one) is the image's empty directory: the library would
+    answer 404 for every file, and what a download writes would go when the container does — the
+    state an install is in when its compose file still mounts the library somewhere else.
+
+    Asked of the kernel's mount table, not by comparing devices: a storage driver that keeps the
+    container's root on the host's own disk (vfs, the fallback under Docker-in-LXC on older ZFS) gives
+    a mounted volume the same device as `/`. Outside a container a plain directory is a fine root."""
     path = Path(root)
     if not path.is_dir():
         return f"{root} does not exist"
-    try:
-        if path.stat().st_dev == Path(system_root).stat().st_dev:
-            return f"{root} is not a mounted volume (it is the container's own empty directory)"
-    # silent-ok: the failure IS the answer — returned as the reason, which the caller logs.
-    except OSError as e:
-        return f"{root} cannot be read ({e})"
+    if in_container if in_container is not None else _in_container():
+        table = mountinfo if mountinfo is not None else _read_mountinfo()
+        if table is not None:
+            real = os.path.realpath(root)
+            mounted = any(
+                m != "/" and (real == m or real.startswith(m.rstrip("/") + "/"))
+                for m in _mount_points(table)
+            )
+            if not mounted:
+                return f"{root} is not a mounted volume (it is the container's own empty directory)"
     if not os.access(path, os.W_OK):
         return f"{root} is not writable"
     return None
diff --git a/backend/app/downloads/upload.py b/backend/app/downloads/upload.py
index 00668379..7e382a19 100644
--- a/backend/app/downloads/upload.py
+++ b/backend/app/downloads/upload.py
@@ -70,7 +70,9 @@ def ingest_upload(
     rejected upload leaves nothing behind and the caller just cleans up its temp file."""
     quota.check_upload(db, user_id, size)  # the file's size is known → charge it up front
     if folder_id is not None:
-        folders.upload_target(db, user_id, folder_id)
+        # Refused before any work; held only once the job is about to name it (below), not through
+        # the probe and the poster, which a rename or delete of the folder would wait out.
+        folders.upload_target(db, user_id, folder_id, hold=False)
 
     data = probe.probe_json(src_path)
     vcodec, acodec, width, height, duration = probe.facts(data)
@@ -138,7 +140,7 @@ def ingest_upload(
         db.add(job)
         folders.place(db, job)
         if folder_id is not None:
-            job.folder_id = folder_id
+            job.folder_id = folders.upload_target(db, user_id, folder_id).id
         if not convert:
             folders.stamp_done(db, job)
         service.incr_ref_count(db, asset.id)
diff --git a/backend/app/main.py b/backend/app/main.py
index cd7cbba2..795456b7 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -28,6 +28,7 @@ log = logs.get_logger(__name__)
 from app import bodylimit, collation, logsink, state, sysconfig  # noqa: E402 — logging is configured first
 from app.config import settings  # noqa: E402 — logging is configured first
 from app.db import SessionLocal, engine  # noqa: E402 — logging is configured first
+from app.downloads import service as download_service  # noqa: E402 — logging is configured first
 from app.downloads import storage  # noqa: E402 — logging is configured first
 from app.plex import stream as plex_stream  # noqa: E402 — logging is configured first
 from app.plex import subtitles as plex_subtitles  # noqa: E402 — logging is configured first
@@ -84,6 +85,18 @@ async def lifespan(app: FastAPI):
             # With the traceback: this catch spans a connection, a query and a coercion at boot,
             # and `%s` of the exception does not say which of them raised.
             log.exception("could not read the stored log level; staying at %s", current_level())
+        if not problem:
+            try:
+                left = download_service.library_left_behind(db, settings.files_root)
+            except Exception:  # a database not reachable yet: the check is a hint, never a gate
+                log.exception("could not check the files library against its database")
+            else:
+                if left:
+                    log.critical(
+                        "The files library is empty but the database names files: the volume "
+                        "mounted at %s is not the one that holds them. See docs/self-hosting.md.",
+                        settings.files_root,
+                    )
         if not state.is_configured(db):
             token = state.rotate_setup_token(db)
             url = f"{settings.app_base}/setup?token={token}"
diff --git a/backend/app/routes/files.py b/backend/app/routes/files.py
index 3c67f709..ca773f43 100644
--- a/backend/app/routes/files.py
+++ b/backend/app/routes/files.py
@@ -339,10 +339,6 @@ def _serialize_folder(folder: FileFolder, count: int, size: int) -> dict:
     }
 
 
-def _own_folder_or_404(db: Session, user: User, folder_id: int) -> FileFolder:
-    return owned_or_404(db, FileFolder, folder_id, user, what="folder")
-
-
 @router.get("/folders")
 def list_folders(user: User = Depends(current_user), db: Session = Depends(get_db)) -> list[dict]:
     """The caller's folders with how many files each holds and their size. A source folder its
@@ -407,7 +403,7 @@ def update_folder(
 ) -> dict:
     """Rename one of the caller's own folders (a source folder's name is its origin's), and/or set
     how long any of their folders keeps a file (`folders.set_retention`)."""
-    folder = _own_folder_or_404(db, user, folder_id)
+    folder = owned_or_404(db, FileFolder, folder_id, user, what="folder")
     sent = body.model_fields_set
     if "name" in sent:
         if folder.kind != folders.FolderKind.OWN:
@@ -457,7 +453,7 @@ def delete_folder(
     folder_id: int, user: User = Depends(current_user), db: Session = Depends(get_db)
 ) -> dict:
     """Delete one of the caller's own folders; its files go back to their source folders."""
-    folder = _own_folder_or_404(db, user, folder_id)
+    folder = owned_or_404(db, FileFolder, folder_id, user, what="folder")
     try:
         moved = folders.delete_own(db, folder)
     except folders.FolderError as e:
diff --git a/backend/tests/test_file_folders.py b/backend/tests/test_file_folders.py
index 4c1b068f..9bd40d3a 100644
--- a/backend/tests/test_file_folders.py
+++ b/backend/tests/test_file_folders.py
@@ -308,21 +308,22 @@ def test_one_users_expiry_leaves_another_users_copy(db, make_user, _root):
     assert (_root / "a.mp4").exists()
 
 
-def _meanwhile(monkeypatch, act):
-    """Run `act` in another session (another request) between the GC's expiry SELECT and its
-    handling of each file it found — the window a user's click can land in."""
-    real = gcmod._relock_expired
+def _meanwhile(monkeypatch, act, step="_relock_expired"):
+    """Run `act` in another session (another request) between a GC pass's SELECT and its handling of
+    each row it found — the window a user's click can land in. `step` is the pass's per-row re-read
+    (expiry, warning, eviction); `act` gets that row's id."""
+    real = getattr(gcmod, step)
 
-    def relock(db, job_id, asset_id, now):
+    def wrapped(db, key, *rest):
         other = SessionLocal()
         try:
-            act(other, job_id)
+            act(other, key)
             other.commit()
         finally:
             other.close()
-        return real(db, job_id, asset_id, now)
+        return real(db, key, *rest)
 
-    monkeypatch.setattr(gcmod, "_relock_expired", relock)
+    monkeypatch.setattr(gcmod, step, wrapped)
 
 
 def test_a_file_kept_while_the_gc_runs_stays(db, make_user, _root, monkeypatch):
@@ -376,6 +377,236 @@ def test_the_cache_cap_never_evicts_a_kept_file_and_an_evicted_one_leaves_every_
     assert (_root / "old.mp4").exists() and not (_root / "new.mp4").exists()
 
 
+def test_a_file_deleted_while_the_gc_warns_does_not_stop_the_run(db, make_user, _root, monkeypatch):
+    # Written back from a stale read, the warning of a deleted file failed the whole run: nothing
+    # expired or was evicted that cycle, nor the next while the race lasted.
+    (_root / "a.mp4").write_bytes(b"x")
+    (_root / "b.mp4").write_bytes(b"x")
+    uid = make_user().id
+    _job(db, uid, _asset(db), expires_at=NOW + timedelta(hours=1))
+    gone = _job(
+        db, uid, _asset(db, ref="bbbbbbbbbbb", rel="b.mp4"), expires_at=NOW - timedelta(minutes=1)
+    )
+
+    def user_deletes(other, jid):
+        service.delete(other, other.get(DownloadJob, jid))
+
+    _meanwhile(monkeypatch, user_deletes, step="_mark_warned")
+    result = _gc(db)
+    assert (result["warned"], result["expired"]) == (0, 1)
+    db.expire_all()
+    assert db.get(DownloadJob, gone.id) is None
+    assert _notices(db, uid, "download_expiring") == 0
+
+
+def test_a_file_re_timed_while_the_gc_warns_keeps_its_new_expiry_unwarned(
+    db, make_user, _root, monkeypatch
+):
+    # Moved, un-kept or its folder's retention changed: a fresh expiry, warned when it comes — not
+    # marked warned now for a date it no longer has.
+    uid = make_user().id
+    job = _job(db, uid, _asset(db), expires_at=NOW + timedelta(hours=1))
+    later = NOW + timedelta(days=30)
+
+    def re_timed(other, jid):
+        moved = other.get(DownloadJob, jid)
+        moved.expires_at = later
+        moved.expiry_warned = False
+
+    _meanwhile(monkeypatch, re_timed, step="_mark_warned")
+    assert _gc(db)["warned"] == 0
+    db.expire_all()
+    fresh = db.get(DownloadJob, job.id)
+    assert (fresh.expires_at, fresh.expiry_warned) == (later, False)
+    assert _notices(db, uid, "download_expiring") == 0
+
+
+def _two_over_the_cap(db, make_user, _root):
+    sysconfig.set_value(db, "download_total_max_bytes", 15)
+    db.commit()
+    a, b = make_user().id, make_user().id
+    for name in ("old.mp4", "new.mp4"):
+        (_root / name).write_bytes(b"x")
+    old = _asset(db, ref="oldoldoldol", rel="old.mp4", size=10)
+    new = _asset(db, ref="newnewnewne", rel="new.mp4", size=10)
+    old.last_access_at = NOW - timedelta(days=9)
+    new.last_access_at = NOW - timedelta(days=1)
+    return _job(db, a, old), _job(db, b, new)
+
+
+def test_a_file_deleted_while_the_cache_is_over_its_cap_frees_that_space_and_no_more(
+    db, make_user, _root, monkeypatch
+):
+    # Its bytes still counted, the pass went on to evict another user's file for space already free.
+    jold, jnew = _two_over_the_cap(db, make_user, _root)
+
+    def holder_deletes(other, asset_id):
+        if asset_id == jold.asset_id:
+            service.delete(other, other.get(DownloadJob, jold.id))
+
+    _meanwhile(monkeypatch, holder_deletes, step="_relock_evictable")
+    assert _gc(db)["evicted"] == 0
+    db.expire_all()
+    assert db.get(DownloadJob, jnew.id) is not None
+    assert (_root / "new.mp4").exists()
+
+
+def test_a_file_kept_while_the_cache_is_over_its_cap_stays(db, make_user, _root, monkeypatch):
+    jold, jnew = _two_over_the_cap(db, make_user, _root)
+
+    def holder_keeps(other, asset_id):
+        if asset_id == jold.asset_id:
+            other.get(DownloadJob, jold.id).kept = True
+
+    _meanwhile(monkeypatch, holder_keeps, step="_relock_evictable")
+    assert _gc(db)["evicted"] == 1  # the newer one goes in its place
+    db.expire_all()
+    assert db.get(DownloadJob, jold.id) is not None
+    assert (_root / "old.mp4").exists()
+    assert db.get(DownloadJob, jnew.id) is None
+
+
+def _in_thread(work):
+    """Run `work(other_session)` in another thread and session; returns (thread, outcome list)."""
+    import threading
+
+    outcome: list[object] = []
+
+    def run():
+        other = SessionLocal()
+        try:
+            outcome.append(work(other))
+            other.commit()
+        except Exception as e:  # noqa: BLE001 — the assertion below names what went wrong
+            outcome.append(e)
+        finally:
+            other.close()
+
+    t = threading.Thread(target=run)
+    t.start()
+    return t, outcome
+
+
+def test_a_retention_changed_while_a_file_goes_in_times_that_file_too(db, make_user):
+    # The change used to list the folder's files before the arriving one was in, so that one kept the
+    # old retention — deleted later from a folder now set to keep files forever.
+    uid = make_user().id
+    own = FileFolder(user_id=uid, kind=folders.FolderKind.OWN, name="Mine", retention_days=7)
+    db.add(own)
+    db.commit()
+    own_id = own.id
+    job = _job(db, uid, _asset(db))
+    target = folders.own_folder(db, uid, own_id)  # held from the check until this commits
+    t, outcome = _in_thread(
+        lambda other: folders.set_retention(other, other.get(FileFolder, own_id), None)
+    )
+    t.join(timeout=1)
+    folders.move(db, job, target)  # stamped with the 7 days it sees
+    db.commit()
+    t.join(timeout=10)
+    assert outcome == [None]
+    db.expire_all()
+    assert db.get(DownloadJob, job.id).expires_at is None
+
+
+def test_a_retention_changed_while_a_file_finishes_there_times_that_file_too(db, make_user):
+    # The worker's finish read the folder's old retention unlocked, while the change listed the done
+    # files before this one was among them.
+    uid = make_user().id
+    own = FileFolder(user_id=uid, kind=folders.FolderKind.OWN, name="Mine", retention_days=7)
+    db.add(own)
+    db.commit()
+    own_id = own.id
+    job = _job(db, uid, _asset(db), status=JobStatus.RUNNING)
+    folders.move(db, job, own)
+    db.commit()
+    job.status = JobStatus.DONE
+    folders.stamp_done(db, job)  # the finish: stamped with the 7 days it reads
+    db.flush()
+    t, outcome = _in_thread(
+        lambda other: folders.set_retention(other, other.get(FileFolder, own_id), None)
+    )
+    t.join(timeout=1)
+    db.commit()
+    t.join(timeout=10)
+    assert outcome == [None]
+    db.expire_all()
+    assert db.get(DownloadJob, job.id).expires_at is None
+
+
+def test_a_file_moved_out_while_its_folder_is_deleted_stays_where_it_went(db, make_user):
+    uid = make_user().id
+    f = FileFolder(user_id=uid, kind=folders.FolderKind.OWN, name="F")
+    g = FileFolder(user_id=uid, kind=folders.FolderKind.OWN, name="G")
+    db.add_all([f, g])
+    db.commit()
+    f_id, g_id = f.id, g.id
+    job = _job(db, uid, _asset(db))
+    folders.move(db, job, f)
+    db.commit()
+    folders.move(db, job, folders.own_folder(db, uid, g_id))
+    db.flush()  # the move is written, not yet committed
+    t, outcome = _in_thread(lambda other: folders.delete_own(other, other.get(FileFolder, f_id)))
+    t.join(timeout=1)
+    assert t.is_alive()  # the delete waits for the file it would take home
+    db.commit()
+    t.join(timeout=10)
+    assert outcome == [0]
+    db.expire_all()
+    assert db.get(DownloadJob, job.id).folder_id == g_id
+
+
+def test_a_file_deleted_while_its_folder_is_deleted_does_not_fail_the_delete(db, make_user, _root):
+    (_root / "a.mp4").write_bytes(b"x")
+    uid = make_user().id
+    f = FileFolder(user_id=uid, kind=folders.FolderKind.OWN, name="F")
+    db.add(f)
+    db.commit()
+    f_id = f.id
+    job = _job(db, uid, _asset(db))
+    folders.move(db, job, f)
+    db.commit()
+    service.delete(db, db.get(DownloadJob, job.id))
+    db.flush()  # deleted, not yet committed
+    t, outcome = _in_thread(lambda other: folders.delete_own(other, other.get(FileFolder, f_id)))
+    t.join(timeout=1)
+    db.commit()
+    t.join(timeout=10)
+    assert outcome == [0]
+    db.expire_all()
+    assert db.get(FileFolder, f_id) is None
+
+
+def test_a_file_asked_for_while_its_cached_copy_is_freed_gets_a_fresh_one(db, make_user):
+    # The enqueue read the cached copy unlocked: freed meanwhile, its job pointed at a row that was gone
+    # (a foreign-key failure, a 500).
+    asset = _asset(db)
+    asset_id, sig = asset.id, asset.format_sig
+    db.commit()
+    db.execute(select(MediaAsset).where(MediaAsset.id == asset_id).with_for_update())
+    db.delete(db.get(MediaAsset, asset_id))
+    db.flush()  # the release, not yet committed
+    t, outcome = _in_thread(
+        lambda other: service.get_or_create_asset(other, "youtube", "abcdefghijk", sig).id
+    )
+    t.join(timeout=1)
+    assert t.is_alive()  # it waits for the release
+    db.commit()
+    t.join(timeout=10)
+    [fresh] = outcome
+    assert isinstance(fresh, int) and fresh != asset_id
+
+
+def test_an_empty_volume_in_the_librarys_place_is_noticed(db, make_user, _root):
+    # A fresh volume is a real, writable mount: only the files the library names tell it is the wrong one.
+    assert service.library_left_behind(db, str(_root)) is False  # nothing named yet
+    _asset(db, rel="a.mp4")
+    db.commit()
+    assert service.library_left_behind(db, str(_root)) is True
+    (_root / "a.mp4").write_bytes(b"x")
+    assert service.library_left_behind(db, str(_root)) is False
+
+
 def test_deleting_a_user_takes_their_folders_and_files_with_them(db, make_user):
     # Both tables cascade from users; the files' folder FK refuses deleting a folder that still holds
     # files, which must not stop the account's own deletion.
diff --git a/backend/tests/test_files_api.py b/backend/tests/test_files_api.py
index 5a3ddf3e..1f4f4395 100644
--- a/backend/tests/test_files_api.py
+++ b/backend/tests/test_files_api.py
@@ -498,6 +498,50 @@ def test_an_upload_to_a_folder_it_may_not_use_is_refused_before_its_body_is_read
     assert db.execute(select(DownloadJob.id)).all() == before
 
 
+def test_an_upload_holds_its_folder_only_once_its_file_is_about_to_be_in_it(
+    db, make_user, as_user, _root, upload_headers, monkeypatch
+):
+    # Held from the start, the folder waited out the probe and the poster (up to a minute) before a
+    # rename or a delete of it could go on; not held at all, a delete could take it from under the file.
+    alice = make_user()
+    own = _own(db, alice.id)
+    steps: list[object] = []
+    real = folders.own_folder
+
+    def own_folder(db_, user_id, folder_id, *, hold=True):
+        steps.append(("folder", hold))
+        return real(db_, user_id, folder_id, hold=hold)
+
+    def probe(_p):
+        steps.append("probe")
+
+    monkeypatch.setattr(folders, "own_folder", own_folder)
+    monkeypatch.setattr(uploadmod.probe, "probe_json", probe)
+    r = as_user(alice).post(
+        "/api/files/upload", content=b"x", headers=upload_headers("a.txt", folder=own.id)
+    )
+    assert r.status_code == 200, r.text
+    # The route's own look before the body, the ingest's look, the probe, then the hold.
+    assert steps[-3:] == [("folder", False), "probe", ("folder", True)]
+
+
+def test_a_file_moved_holds_the_folder_it_goes_to(db, make_user, as_user, monkeypatch):
+    alice = make_user()
+    job = _file(db, alice.id)
+    own = _own(db, alice.id)
+    holds: list[bool] = []
+    real = folders.own_folder
+
+    def own_folder(db_, user_id, folder_id, *, hold=True):
+        holds.append(hold)
+        return real(db_, user_id, folder_id, hold=hold)
+
+    monkeypatch.setattr(folders, "own_folder", own_folder)
+    r = as_user(alice).patch(f"/api/files/{job.id}", json={"folder_id": own.id})
+    assert r.status_code == 200, r.text
+    assert holds == [True]
+
+
 def test_a_folder_deleted_while_its_upload_streams_refuses_the_upload(
     db, make_user, as_user, _root, _no_probe, upload_headers, monkeypatch
 ):
diff --git a/backend/tests/test_storage.py b/backend/tests/test_storage.py
index 7525e773..53d92b27 100644
--- a/backend/tests/test_storage.py
+++ b/backend/tests/test_storage.py
@@ -177,15 +177,29 @@ def test_sidecars_keep_the_full_basename_when_the_title_has_a_dot(tmp_path: Path
     assert "Chan_-_2026-08-12_-_Sonka_Hazilag.nfo" not in names  # the old truncated orphan
 
 
+def _mounts(*points):
+    """A /proc/self/mountinfo table with these mount points (the fields around them are filler)."""
+    return "\n".join(f"{i} 1 0:{i} / {p} rw - ext4 /dev/x rw" for i, p in enumerate(points, 22))
+
+
 def test_a_files_root_on_the_containers_own_disk_is_refused(tmp_path):
     # The image's empty /files, left unmounted by an old compose file: every file would 404 and
     # every download would go with the container.
-    assert "not a mounted volume" in files_root_problem(
-        str(tmp_path), system_root=str(tmp_path.parent)
-    )
+    root = str(tmp_path)
+    plain = _mounts("/", "/proc", "/etc/hosts")
+    assert "not a mounted volume" in files_root_problem(root, plain, in_container=True)
     assert "does not exist" in files_root_problem(str(tmp_path / "gone"))
-    # On another filesystem than the container's own, and writable: the volume.
-    assert files_root_problem(str(tmp_path), system_root="/proc") is None
+    # Its own mount, or inside one: the volume — whatever device it reports. On a storage driver that
+    # keeps the container's root on the host's disk (vfs) a volume has the same device as /.
+    assert files_root_problem(root, _mounts("/", root), in_container=True) is None
+    assert files_root_problem(root, _mounts("/", str(tmp_path.parent)), in_container=True) is None
+    # A mount point with a space in it, as the kernel escapes it.
+    spaced = tmp_path / "my files"
+    spaced.mkdir()
+    escaped = str(spaced).replace(" ", "\\040")
+    assert files_root_problem(str(spaced), _mounts("/", escaped), in_container=True) is None
+    # Outside a container a plain directory is a fine root.
+    assert files_root_problem(root, plain, in_container=False) is None
 
 
 def test_the_old_name_of_the_files_root_still_finds_the_files(monkeypatch):
diff --git a/docker-compose.home.yml.example b/docker-compose.home.yml.example
index ea55314f..7ac42b60 100644
--- a/docker-compose.home.yml.example
+++ b/docker-compose.home.yml.example
@@ -86,7 +86,7 @@ services:
     volumes:
       # Downloaded media. Defaults to a Docker-managed named volume; set FILES_HOST_PATH in .env
       # to a host directory (e.g. one your Plex server reads) to keep the Plex-style tree there.
-      - ${FILES_HOST_PATH:-siftlode_files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-siftlode_files}}:/files
       # Plex media, read-only. Only used when the optional Plex module is enabled: set
       # PLEX_MEDIA_HOST_PATH in .env to the host directory holding the Plex library (the local side
       # of plex_path_map, default /plex-media). Left unset → an empty named volume (harmless; the
@@ -137,7 +137,7 @@ services:
         condition: service_started
     networks: [internal]
     volumes:
-      - ${FILES_HOST_PATH:-siftlode_files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-siftlode_files}}:/files
       # Plex media (read-only) — same mount as the api; used by Plex clip jobs (a later phase).
       - ${PLEX_MEDIA_HOST_PATH:-siftlode_plex_media}:/plex-media:ro
     mem_limit: 2g
diff --git a/docker-compose.selfhost.yml b/docker-compose.selfhost.yml
index fecf70a1..596266b9 100644
--- a/docker-compose.selfhost.yml
+++ b/docker-compose.selfhost.yml
@@ -62,7 +62,7 @@ services:
     volumes:
       # Downloaded media. Defaults to a Docker-managed named volume; set FILES_HOST_PATH in .env
       # to a host directory (e.g. one your Plex server reads) to keep the Plex-style tree there.
-      - ${FILES_HOST_PATH:-siftlode_files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-siftlode_files}}:/files
       # Plex library, read-only. The Plex module streams files itself, so it needs to SEE them: set
       # PLEX_MEDIA_HOST_PATH in .env to the host directory holding the library (the local side of the
       # admin "path map", whose default is /data=/plex-media). Left unset it falls back to an empty
@@ -111,7 +111,7 @@ services:
         condition: service_started
     networks: [internal]
     volumes:
-      - ${FILES_HOST_PATH:-siftlode_files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-siftlode_files}}:/files
       # Same read-only Plex library as the API — the worker resolves library paths too.
       - ${PLEX_MEDIA_HOST_PATH:-siftlode_plex_media}:/plex-media:ro
     security_opt:
diff --git a/docker-compose.yml b/docker-compose.yml
index 673dfed1..af7a1e59 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -47,7 +47,7 @@ services:
     volumes:
       # Downloaded media. Defaults to a Docker-managed named volume; set FILES_HOST_PATH in .env
       # to a host directory (e.g. one your Plex server reads) to keep the Plex-style tree there.
-      - ${FILES_HOST_PATH:-files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-files}}:/files
       # Plex library, read-only. The Plex module streams files itself, so it needs to SEE them: set
       # PLEX_MEDIA_HOST_PATH in .env to the host directory holding the library (the local side of the
       # admin "path map", whose default is /data=/plex-media). Left unset it falls back to an empty
@@ -91,7 +91,7 @@ services:
       bgutil-pot:
         condition: service_started
     volumes:
-      - ${FILES_HOST_PATH:-files}:/files
+      - ${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-files}}:/files
       # Same read-only Plex library as the API — the worker resolves library paths too.
       - ${PLEX_MEDIA_HOST_PATH:-plex_media}:/plex-media:ro
     logging: *logging
diff --git a/docs/self-hosting.md b/docs/self-hosting.md
index 3180bcbc..83d71d9e 100644
--- a/docs/self-hosting.md
+++ b/docs/self-hosting.md
@@ -260,7 +260,8 @@ compose file from before still mounts `/downloads` and sets `DOWNLOAD_ROOT`, and
 that name. To move to the new layout, download the new
 [`docker-compose.selfhost.yml`](../docker-compose.selfhost.yml) over your old one, then:
 
-- **If you set a host directory**, rename `DOWNLOAD_HOST_PATH` to `FILES_HOST_PATH` in `.env`.
+- **If you set a host directory**, nothing to do: the new compose file still reads
+  `DOWNLOAD_HOST_PATH` when `FILES_HOST_PATH` is not set. Rename it in `.env` when it suits you.
 - **If you used the default volume**, copy it over once before you start the new version (Docker
   cannot rename a volume). Compose names a volume after the project, which is your folder's name
   unless you set one, so look the old one up rather than guessing it:
@@ -277,10 +278,15 @@ that name. To move to the new layout, download the new
 
   Open the Files library and check your files play; only then remove the old volume
   (`docker volume rm "$OLD"`).
+- **If you build from source**, a `git pull` brings the new `docker-compose.yml`, whose default
+  volume is `files` (it was `downloads`): copy it over the same way, with `downloads` and `files` in
+  place of `siftlode_downloads` and `siftlode_files`, before you start the new version.
 
 If the files library is not mounted where the app looks, the API says so in its log at start
 (`The files library is not reachable`), and the download worker refuses to start rather than write
-files into a container that throws them away.
+files into a container that throws them away. A fresh, empty volume in the library's place (the old
+one not copied over) is a real mount, so the worker runs; the API's log says that too, at start
+(`The files library is empty but the database names files`). Copy the old volume over, as above.
 
 **The registry keeps only the newest few releases, not every version the changelog lists.** A pin is
 "decide when you take the next hop", not "stay here forever": once you are several releases behind,
diff --git a/frontend/src/lib/compose.contract.test.ts b/frontend/src/lib/compose.contract.test.ts
new file mode 100644
index 00000000..a2ac0415
--- /dev/null
+++ b/frontend/src/lib/compose.contract.test.ts
@@ -0,0 +1,23 @@
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { describe, expect, it } from "vitest";
+
+// The compose files an install runs from. The library's host directory was renamed (DOWNLOAD_HOST_PATH
+// → FILES_HOST_PATH); a `.env` that still sets the old name must keep mounting that directory, not a
+// fresh empty volume the app would take for the library. Held here because this lane can read the
+// whole repository; the backend's test lane sees only backend/.
+const REPO = fileURLToPath(new URL("../../../", import.meta.url));
+const COMPOSE = [
+  "docker-compose.yml",
+  "docker-compose.selfhost.yml",
+  "docker-compose.home.yml.example",
+];
+
+describe("compose files · the library mount", () => {
+  it.each(COMPOSE)("%s falls back to the old host-path name on every service", (file) => {
+    const text = readFileSync(REPO + file, "utf8");
+    const mounts = text.match(/^\s*- .*:\/files\s*$/gm) ?? [];
+    expect(mounts.length).toBe(2); // the API and the worker
+    for (const m of mounts) expect(m).toContain("${FILES_HOST_PATH:-${DOWNLOAD_HOST_PATH:-");
+  });
+});