Strip tz in _parse_dt dateutil fallback (naive-datetime contract) (#2557)

_parse_dt documents that it returns naive datetimes (CalendarEvent.dtstart is
naive) and every return path strips tz — except the last-resort dateutil
fallback, which returned dateutil's value verbatim. An offset-bearing non-ISO
input (e.g. RFC-2822 'Mon, 05 Jan 2026 14:00:00 +0900', which fromisoformat
rejects but dateutil parses) leaked a tz-aware datetime into the naive dtstart
column via create_event/update_event -> _parse_dt_pair. On read-back,
_expand_rrule compares ev.dtstart against naive window bounds and raised
'can't compare offset-naive and offset-aware datetimes' (500 / no events).

Normalize the fallback to UTC-naive, mirroring the fromisoformat branch. Naive
inputs are unchanged.

(cherry picked from commit b03b6b91df21c1a3ad3c447f23f35b8b19e6d1b1)

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

routes/calendar_routes.py

@@ -399,7 +399,17 @@ def _parse_dt(s: str) -> datetime:
     # Last resort: dateutil's fuzzy parser
     try:
         from dateutil import parser as _du
-        return _du.parse(s)
+        parsed = _du.parse(s)
+        # Strip tz like every other return path above — this function's
+        # contract is naive datetimes (CalendarEvent.dtstart is naive). An
+        # offset-bearing non-ISO input (e.g. RFC-2822 "Mon, 05 Jan 2026
+        # 14:00:00 +0900") otherwise leaked tz-aware into the naive column and
+        # crashed read-back comparisons in _expand_rrule with "can't compare
+        # offset-naive and offset-aware datetimes".
+        if parsed.tzinfo is not None:
+            from datetime import timezone as _tz
+            return parsed.astimezone(_tz.utc).replace(tzinfo=None)
+        return parsed
     except Exception:
         raise ValueError(f"could not parse datetime: {s!r}")
 

tests/test_calendar_parse_dt_naive.py

@@ -0,0 +1,46 @@
+"""Regression: _parse_dt's dateutil fallback must return naive datetimes.
+
+_parse_dt documents that it returns local-naive datetimes to match the DB
+schema (CalendarEvent.dtstart is naive), and every return path strips tz —
+except the last-resort dateutil branch, which returned dateutil's value
+verbatim. An offset-bearing non-ISO input (e.g. RFC-2822
+"Mon, 05 Jan 2026 14:00:00 +0900", which datetime.fromisoformat rejects but
+dateutil parses) therefore leaked a tz-aware datetime into the naive dtstart
+column. On read-back, _expand_rrule compares ev.dtstart against naive window
+bounds and raises "can't compare offset-naive and offset-aware datetimes".
+
+The fallback now normalizes to UTC and strips tz, exactly like the ISO path.
+"""
+import pytest
+
+from tests.test_null_owner_gates import _import_calendar_helpers
+
+# Inputs datetime.fromisoformat() rejects (so they hit the dateutil fallback)
+# but that carry a numeric UTC offset dateutil resolves to tz-aware.
+_OFFSET_NONISO = [
+    "Mon, 05 Jan 2026 14:00:00 +0900",
+    "January 5, 2026 14:00 +0900",
+]
+
+
+@pytest.mark.parametrize("s", _OFFSET_NONISO)
+def test_parse_dt_dateutil_fallback_returns_naive(s):
+    cal = _import_calendar_helpers()
+    d = cal._parse_dt(s)
+    assert d.tzinfo is None, f"{s!r} leaked tz-aware: {d!r}"
+    # +0900 14:00 -> 05:00 UTC, naive.
+    assert (d.hour, d.minute) == (5, 0)
+
+
+@pytest.mark.parametrize("s", _OFFSET_NONISO)
+def test_parse_dt_pair_fallback_returns_naive(s):
+    cal = _import_calendar_helpers()
+    dt, _is_utc = cal._parse_dt_pair(s)
+    assert dt.tzinfo is None, f"{s!r} leaked tz-aware via _parse_dt_pair: {dt!r}"
+
+
+def test_parse_dt_naive_input_unchanged():
+    cal = _import_calendar_helpers()
+    d = cal._parse_dt("January 5, 2026 14:00")  # no offset -> stays as parsed
+    assert d.tzinfo is None
+    assert (d.hour, d.minute) == (14, 0)