Give the date code one clock a test can freeze

zeeguu/core/test/frozen_clock.py

@@ -0,0 +1,40 @@
+"""
+Freeze the clock that every date computation in zeeguu.core.util.time reads.
+
+Streak code answers questions like "did they practice *today*?", and the answer
+depends on which timezone "today" is asked in. Tests that build their fixtures
+from the wall clock of the machine running them only exercise whatever
+UTC-vs-local relationship happens to hold at that moment -- which is why a suite
+can be green all afternoon and fail after local midnight. Freezing the clock
+lets a test name the instant it is testing, so both sides of a midnight are
+covered on every run.
+"""
+
+from contextlib import contextmanager
+from datetime import datetime, timezone
+from unittest.mock import patch
+
+
+@contextmanager
+def clock_at(utc_moment: str):
+    """
+    Pretend "now" is `utc_moment`, given as "YYYY-MM-DD HH:MM" in UTC.
+
+    Patches the one function the module derives server_now() and
+    user_local_today() from, so both move together.
+    """
+    frozen = datetime.fromisoformat(utc_moment).replace(tzinfo=timezone.utc)
+    with patch("zeeguu.core.util.time._now_utc", return_value=frozen):
+        yield frozen
+
+
+def server_time_at(utc_moment: str) -> datetime:
+    """The naive SERVER_TZ value the DB would hold for `utc_moment` (UTC)."""
+    from zeeguu.core.util.time import SERVER_TZ
+
+    return (
+        datetime.fromisoformat(utc_moment)
+        .replace(tzinfo=timezone.utc)
+        .astimezone(SERVER_TZ)
+        .replace(tzinfo=None)
+    )

zeeguu/core/test/test_friends.py

@@ -7,6 +7,7 @@
 from zeeguu.core.friends.friend_streak import update_streak
 from zeeguu.core.model.friendship import Friendship
 from zeeguu.core.model.user_language import UserLanguage
+from zeeguu.core.test.frozen_clock import clock_at
 from zeeguu.core.test.model_test_mixin import ModelTestMixIn
 from zeeguu.core.test.rules.user_rule import UserRule
 from zeeguu.core.util.time import SERVER_TZ, server_now, user_local_today, user_zone
@@ -53,6 +54,33 @@ def _set_last_practiced(self, user, practiced_at):
       session.add(user_language)
       session.commit()
 
+   def test_streak_counts_the_same_on_both_sides_of_a_users_midnight(self):
+      """
+      One scenario, two pinned instants: at 06:00 UTC it is still the 1st in
+      Auckland, at 12:00 UTC it is already the 2nd, while the server is on the
+      1st either way. The streak must come out the same.
+
+      Without a frozen clock this test can only ever exercise whichever of the
+      two relationships happens to hold when the suite runs -- the reason a
+      green afternoon run could fail after midnight.
+      """
+      self.user.timezone = "Pacific/Auckland"  # UTC+13 on this date
+      session.add(self.user)
+      session.commit()
+
+      for utc_moment in ["2026-03-01 06:00", "2026-03-01 12:00"]:
+         with clock_at(utc_moment):
+            self.friendship.friend_streak = 0
+            self.friendship.friend_streak_last_updated = None
+            self._set_last_practiced(self.user, practiced_days_ago(self.user))
+            self._set_last_practiced(
+               self.friend_user, practiced_days_ago(self.friend_user)
+            )
+
+            update_streak(self.friendship, session=session)
+
+            assert self.friendship.friend_streak == 1, f"at {utc_moment} UTC"
+
    def test_update_friend_streak_multiple_friends(self):
       from zeeguu.core.model.language import Language
       from zeeguu.core.model.user_language import UserLanguage

zeeguu/core/util/time.py

@@ -58,8 +58,21 @@ def user_zone(user):
     return SERVER_TZ
 
 
+def _now_utc():
+    """
+    The single clock reading every date computation in this module derives from.
+
+    Everything else here (server_now, user_local_today) is a projection of this
+    one instant into a different timezone, so a test that patches this function
+    moves *all* of them together and can pin what happens on either side of a
+    midnight -- without depending on the wall clock of the machine running it.
+    See zeeguu/core/test/frozen_clock.py.
+    """
+    return datetime.now(timezone.utc)
+
+
 def user_local_today(user):
-    return datetime.now(user_zone(user)).date()
+    return _now_utc().astimezone(user_zone(user)).date()
 
 
 def to_user_local_date(user, naive_server_dt):
@@ -78,7 +91,7 @@ def server_now():
     but it says *which* clock it means, so it stays correct if the constants
     at the top of this file ever change.
     """
-    return datetime.now(SERVER_TZ).replace(tzinfo=None)
+    return _now_utc().astimezone(SERVER_TZ).replace(tzinfo=None)
 
 
 """