fix(vector-sync): sweep placeholder orphans at Pod startup (#101)

When the per-tenant nextcloud-mcp-server Pod OOMKills mid-batch, the
in-memory anyio processor queue is lost but the placeholder Qdrant
points (is_placeholder=true, status=pending) survive. The next Pod's
scanner re-runs, sees the existing placeholders, applies the
5 × VECTOR_SYNC_SCAN_INTERVAL staleness gate (~5h with the deployed
1h scan interval), and skips them. Result: 0 documents indexed for
the duration of the gate after every restart.

Stamps a process-level instance_id (UUID per Pod-process) onto every
placeholder write. A new sweep_orphan_placeholders helper, called
once from starlette_lifespan after the Qdrant client is initialised
and before the scanner / user-manager spawns, scrolls the collection
and deletes any placeholder whose instance_id doesn't match the
current Pod's (including placeholders with no instance_id field —
back-compat for pre-fix Pod versions). The scanner's next cycle
naturally re-creates fresh placeholders and queues work normally;
no DocumentTask reconstruction needed.

Sweep is one-shot at startup, not periodic — the existing staleness
gate still covers same-Pod recovery, and the cross-Pod-restart gap
was the only failure mode. Failure is non-fatal (logged via
vector_sync.orphan_sweep_failed) so a transient Qdrant hiccup at
boot doesn't prevent the scanner from running.

Both lifespan branches (single-user BasicAuth, OAuth / multi-user
BasicAuth) call the sweep via a module-local helper. A new
VECTOR_SYNC_ORPHAN_SWEEP_ENABLED setting (default True) provides
an escape hatch.

Closes Deck #101.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_placeholder.py

@@ -0,0 +1,196 @@
+"""Unit tests for placeholder orphan-sweep (card #101).
+
+When the per-tenant Pod OOMKills mid-batch the in-memory processor
+queue is lost. Placeholder points written to Qdrant survive, and the
+next Pod's scanner would skip them under the
+``5 × VECTOR_SYNC_SCAN_INTERVAL`` staleness gate (~5h with the deployed
+1h scan interval). ``sweep_orphan_placeholders`` runs once at Pod
+startup and deletes any placeholder whose ``instance_id`` payload
+doesn't match the current Pod-process, restoring throughput within
+one scan cycle.
+
+The sweep contract this file pins:
+
+* placeholder with ``instance_id != _INSTANCE_ID`` → deleted
+* placeholder with **absent** ``instance_id`` → deleted
+  (back-compat for placeholders written by pre-fix Pod versions)
+* placeholder with ``instance_id == _INSTANCE_ID`` → kept
+* empty / no-results scroll → no-op (no spurious delete call)
+* multi-page scroll → all pages visited, deletes batched per page
+"""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+from unittest.mock import AsyncMock
+
+import pytest
+
+from nextcloud_mcp_server.vector import placeholder as placeholder_module
+from nextcloud_mcp_server.vector.placeholder import sweep_orphan_placeholders
+
+
+def _scrolled_point(point_id, instance_id=...):
+    """Stand-in for a qdrant_client Record. Sweep reads only
+    ``id`` and ``payload``; passing ``instance_id=...`` (Ellipsis)
+    omits the field entirely (the back-compat case)."""
+    payload: dict = {"is_placeholder": True}
+    if instance_id is not ...:
+        payload["instance_id"] = instance_id
+    return SimpleNamespace(id=point_id, payload=payload)
+
+
+def _make_client(scroll_pages):
+    """Build an AsyncMock qdrant client whose ``scroll`` returns the
+    given ``(points, next_offset)`` pages in order, then mimics the
+    real client's exhausted-cursor return of ``([], None)``."""
+    client = AsyncMock()
+    pages = list(scroll_pages) + [([], None)]
+    client.scroll.side_effect = pages
+    return client
+
+
+@pytest.mark.unit
+async def test_sweeps_orphan_with_different_instance_id(monkeypatch):
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-new")
+    client = _make_client(
+        [
+            ([_scrolled_point("p1", instance_id="pod-old")], None),
+        ]
+    )
+
+    swept, kept = await sweep_orphan_placeholders(client, "nextcloud_content")
+
+    assert (swept, kept) == (1, 0)
+    client.delete.assert_awaited_once()
+    # Selector is the orphan's point id — own-Pod placeholders never
+    # reach the delete call.
+    assert client.delete.await_args.kwargs["points_selector"] == ["p1"]
+
+
+@pytest.mark.unit
+async def test_sweeps_placeholder_with_absent_instance_id(monkeypatch):
+    """Back-compat: placeholders written by pre-fix Pod versions have
+    no ``instance_id`` field. They MUST be treated as orphans so the
+    first deploy of this fix doesn't leave old placeholders behind."""
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-new")
+    client = _make_client(
+        [
+            ([_scrolled_point("legacy", instance_id=...)], None),
+        ]
+    )
+
+    swept, kept = await sweep_orphan_placeholders(client, "nextcloud_content")
+
+    assert (swept, kept) == (1, 0)
+    assert client.delete.await_args.kwargs["points_selector"] == ["legacy"]
+
+
+@pytest.mark.unit
+async def test_keeps_own_pod_placeholders(monkeypatch):
+    """A surviving Pod's own placeholders must NOT be deleted —
+    that's what the staleness gate is for, and the processor may
+    still be working through them."""
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-current")
+    client = _make_client(
+        [
+            (
+                [
+                    _scrolled_point("mine-1", instance_id="pod-current"),
+                    _scrolled_point("mine-2", instance_id="pod-current"),
+                ],
+                None,
+            ),
+        ]
+    )
+
+    swept, kept = await sweep_orphan_placeholders(client, "nextcloud_content")
+
+    assert (swept, kept) == (0, 2)
+    client.delete.assert_not_awaited()
+
+
+@pytest.mark.unit
+async def test_noop_when_no_placeholders_exist(monkeypatch):
+    """Cold-boot tenant with an empty collection — sweep does NOT
+    issue a delete request, so a trivially-empty batch can't trip
+    Qdrant validation on an empty selector list."""
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-fresh")
+    client = _make_client([([], None)])
+
+    swept, kept = await sweep_orphan_placeholders(client, "nextcloud_content")
+
+    assert (swept, kept) == (0, 0)
+    client.delete.assert_not_awaited()
+
+
+@pytest.mark.unit
+async def test_walks_multiple_scroll_pages(monkeypatch):
+    """Scroll cursor exhaustion is signalled by ``offset is None`` per
+    Qdrant's contract. Sweep must keep paging until the cursor is
+    exhausted, batching the delete per page."""
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-current")
+    client = _make_client(
+        [
+            (
+                [
+                    _scrolled_point("orphan-1", instance_id="pod-prev"),
+                    _scrolled_point("mine", instance_id="pod-current"),
+                ],
+                "next-cursor",
+            ),
+            (
+                [_scrolled_point("orphan-2", instance_id=...)],
+                None,
+            ),
+        ]
+    )
+
+    swept, kept = await sweep_orphan_placeholders(
+        client, "nextcloud_content", batch_size=2
+    )
+
+    assert (swept, kept) == (2, 1)
+    # One delete per page (each page had at least one orphan).
+    assert client.delete.await_count == 2
+    delete_selectors = [
+        call.kwargs["points_selector"] for call in client.delete.await_args_list
+    ]
+    assert delete_selectors == [["orphan-1"], ["orphan-2"]]
+
+
+@pytest.mark.unit
+async def test_write_placeholder_payload_includes_instance_id(monkeypatch):
+    """Pin the new payload contract: ``write_placeholder_point`` MUST
+    stamp the current Pod's ``_INSTANCE_ID`` onto the payload it
+    upserts, so the next Pod's sweep can identify these placeholders
+    as belonging to a different process."""
+    monkeypatch.setattr(placeholder_module, "_INSTANCE_ID", "pod-pinned")
+
+    fake_qdrant = AsyncMock()
+    fake_settings = SimpleNamespace(get_collection_name=lambda: "nextcloud_content")
+    fake_embedding = SimpleNamespace(get_dimension=lambda: 4)
+
+    async def fake_get_qdrant_client():
+        return fake_qdrant
+
+    monkeypatch.setattr(placeholder_module, "get_qdrant_client", fake_get_qdrant_client)
+    monkeypatch.setattr(placeholder_module, "get_settings", lambda: fake_settings)
+    monkeypatch.setattr(
+        placeholder_module, "get_embedding_service", lambda: fake_embedding
+    )
+
+    await placeholder_module.write_placeholder_point(
+        doc_id="d-42",
+        doc_type="note",
+        user_id="alice",
+        modified_at=1700000000,
+        etag="abc",
+    )
+
+    fake_qdrant.upsert.assert_awaited_once()
+    upserted_point = fake_qdrant.upsert.await_args.kwargs["points"][0]
+    assert upserted_point.payload["instance_id"] == "pod-pinned"
+    # Sanity: the other contract-pinning fields are still emitted.
+    assert upserted_point.payload["is_placeholder"] is True
+    assert upserted_point.payload["status"] == "pending"