test(querier): reproduce the lost first runningDiff on a full cache hit

The metrics statement reads one step before the window for runningDiff,
and the cache does not keep that step.

Assisted-by: Claude Fable 5.1

tests/integration/tests/queriercommon/13_cache_running_diff.py

@@ -0,0 +1,69 @@
+from collections.abc import Callable
+from datetime import UTC, datetime, timedelta
+from http import HTTPStatus
+from uuid import uuid4
+
+from fixtures import types
+from fixtures.auth import USER_ADMIN_EMAIL, USER_ADMIN_PASSWORD
+from fixtures.metrics import Metrics
+from fixtures.querier import (
+    build_builder_query,
+    build_function,
+    get_all_series,
+    index_series_by_label,
+    make_query_request,
+)
+
+MINUTE = timedelta(minutes=1)
+
+
+def test_running_diff_full_hit_matches_no_cache(
+    signoz: types.SigNoz,
+    create_user_admin: None,  # pylint: disable=unused-argument
+    get_token: Callable[[str, str], str],
+    insert_metrics: Callable[[list[Metrics]], None],
+) -> None:
+    """
+    Setup:
+    A gauge that grows by 10 every minute over four minutes.
+
+    Tests:
+    runningDiff over the last three of those minutes, requested twice. The
+    metrics statement reads one step before the window so the first
+    interval has a difference. The second request, a full cache hit, must
+    return the same three differences as the request with noCache.
+    """
+    now = datetime.now(tz=UTC).replace(second=0, microsecond=0)
+    # the cache outlives the run, so a fixed name would serve the previous
+    # run's points back to this one
+    metric_name = f"cache_running_diff_{uuid4().hex[:8]}"
+    insert_metrics(
+        [
+            Metrics(
+                metric_name=metric_name,
+                labels={"service": "svc-a"},
+                timestamp=now - (14 - i) * MINUTE,
+                value=100.0 + 10 * i,
+                type_="Gauge",
+                is_monotonic=False,
+            )
+            for i in range(4)
+        ]
+    )
+    token = get_token(USER_ADMIN_EMAIL, USER_ADMIN_PASSWORD)
+    query = build_builder_query("A", metric_name, "avg", "avg", group_by=["service"], functions=[build_function("runningDiff")])
+    start_ms = int((now - 13 * MINUTE).timestamp() * 1000)
+    end_ms = int((now - 10 * MINUTE).timestamp() * 1000)
+
+    warm = make_query_request(signoz, token, start_ms, end_ms, [query], no_cache=False)
+    assert warm.status_code == HTTPStatus.OK, warm.text
+    cached = make_query_request(signoz, token, start_ms, end_ms, [query], no_cache=False)
+    assert cached.status_code == HTTPStatus.OK, cached.text
+    fresh = make_query_request(signoz, token, start_ms, end_ms, [query], no_cache=True)
+    assert fresh.status_code == HTTPStatus.OK, fresh.text
+
+    fresh_points = {point["timestamp"]: point["value"] for point in index_series_by_label(get_all_series(fresh.json(), "A"), "service")["svc-a"]["values"]}
+    assert sorted(fresh_points.values()) == [10, 10, 10], fresh_points
+
+    cached_points = {point["timestamp"]: point["value"] for point in index_series_by_label(get_all_series(cached.json(), "A"), "service")["svc-a"]["values"]}
+    assert cached_points == fresh_points