perf: reuse unchanged live snapshot values (Closes #519)

tests/test_live_snapshot.py

@@ -137,6 +137,48 @@ async def test_live_snapshot_omits_section_bodies_at_known_revisions(monkeypatch
     assert "freshness" in second
 
 
+@pytest.mark.anyio
+async def test_warm_unchanged_polls_only_load_shared_metadata(monkeypatch):
+    async def user():
+        return {"id": 1, "login": "timmy"}
+
+    async def empty():
+        return []
+
+    monkeypatch.setattr(main, "current_user", user)
+    monkeypatch.setattr(main, "repos", empty)
+    monkeypatch.setattr(main, "issues", empty)
+    monkeypatch.setattr(main, "pull_requests", empty)
+    monkeypatch.setattr(main, "activity_events", lambda _user: empty())
+    monkeypatch.setattr(main, "notifications", empty)
+    first = payload(await main.live_snapshot())
+    full_load = main._live_snapshot_store.load
+    metadata_load = main._live_snapshot_store.load_metadata
+    calls = {"full": 0, "metadata": 0}
+
+    def counted_full_load():
+        calls["full"] += 1
+        return full_load()
+
+    def counted_metadata_load():
+        calls["metadata"] += 1
+        return metadata_load()
+
+    monkeypatch.setattr(main._live_snapshot_store, "load", counted_full_load)
+    monkeypatch.setattr(main._live_snapshot_store, "load_metadata", counted_metadata_load)
+
+    for _ in range(10):
+        response = await main.live_snapshot(
+            context_revision=first["revisions"]["context"],
+            events_revision=first["revisions"]["events"],
+            notifications_revision=first["revisions"]["notifications"],
+        )
+        assert response.status_code == 200
+        assert "context" not in payload(response)
+
+    assert calls == {"full": 0, "metadata": 10}
+
+
 @pytest.mark.anyio
 async def test_live_snapshot_token_from_another_worker_never_suppresses_content(monkeypatch, tmp_path):
     async def user():

tests/test_live_snapshot_store.py

@@ -1,5 +1,6 @@
 import threading
 import os
+import sqlite3
 
 import pytest
 
@@ -7,6 +8,26 @@ from src import live_snapshot_store
 from src.live_snapshot_store import LiveSnapshotStore, RefreshLeaseLost
 
 
+def test_metadata_load_does_not_retrieve_or_decode_snapshot_value(tmp_path):
+    store = LiveSnapshotStore(tmp_path / "live.sqlite3", clock=lambda: 100.0)
+    with sqlite3.connect(store.path) as connection:
+        connection.execute(
+            "UPDATE live_snapshot SET value_json = ? WHERE singleton = 1",
+            ("not-json",),
+        )
+
+    metadata = store.load_metadata()
+
+    assert metadata.created_at == {
+        section: None for section in ("context", "events", "notifications")
+    }
+    assert metadata.revisions == {
+        section: 0 for section in ("context", "events", "notifications")
+    }
+    with pytest.raises(ValueError):
+        store.load()
+
+
 def test_independent_stores_racing_for_refresh_have_one_lease_winner(tmp_path):
     path = tmp_path / "live.sqlite3"
     first = LiveSnapshotStore(path, clock=lambda: 100.0)