streams: Add notifications for message retention policy updates.

This is a part of #20289.

zerver/lib/actions.py

@@ -5016,9 +5016,49 @@ def do_change_stream_description(stream: Stream, new_description: str) -> None:
     send_event(stream.realm, event, can_access_stream_user_ids(stream))
 
 
-def do_change_stream_message_retention_days(
-    stream: Stream, message_retention_days: Optional[int] = None
+def send_change_stream_message_retention_days_notification(
+    user_profile: UserProfile, stream: Stream, old_value: Optional[int], new_value: Optional[int]
 ) -> None:
+    sender = get_system_bot(settings.NOTIFICATION_BOT, user_profile.realm_id)
+    user_mention = f"@_**{user_profile.full_name}|{user_profile.id}**"
+
+    # If switching from or to the organization's default retention policy,
+    # we want to take the realm's default into account.
+    if old_value is None:
+        old_value = stream.realm.message_retention_days
+    if new_value is None:
+        new_value = stream.realm.message_retention_days
+
+    with override_language(stream.realm.default_language):
+        if old_value == Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP["unlimited"]:
+            notification_string = _(
+                "{user} has changed the [message retention period](/help/message-retention-policy) for this stream from "
+                "**Forever** to **{new_value} days**. Messages will be automatically deleted after {new_value} days."
+            )
+            notification_string = notification_string.format(user=user_mention, new_value=new_value)
+        elif new_value == Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP["unlimited"]:
+            notification_string = _(
+                "{user} has changed the [message retention period](/help/message-retention-policy) for this stream from "
+                "**{old_value} days** to **Forever**."
+            )
+            notification_string = notification_string.format(user=user_mention, old_value=old_value)
+        else:
+            notification_string = _(
+                "{user} has changed the [message retention period](/help/message-retention-policy) for this stream from "
+                "**{old_value} days** to **{new_value} days**. Messages will be automatically deleted after {new_value} days."
+            )
+            notification_string = notification_string.format(
+                user=user_mention, old_value=old_value, new_value=new_value
+            )
+        internal_send_stream_message(
+            sender, stream, Realm.STREAM_EVENTS_NOTIFICATION_TOPIC, notification_string
+        )
+
+
+def do_change_stream_message_retention_days(
+    stream: Stream, acting_user: UserProfile, message_retention_days: Optional[int] = None
+) -> None:
+    old_message_retention_days_value = stream.message_retention_days
     stream.message_retention_days = message_retention_days
     stream.save(update_fields=["message_retention_days"])
 
@@ -5031,6 +5071,12 @@ def do_change_stream_message_retention_days(
         name=stream.name,
     )
     send_event(stream.realm, event, can_access_stream_user_ids(stream))
+    send_change_stream_message_retention_days_notification(
+        user_profile=acting_user,
+        stream=stream,
+        old_value=old_message_retention_days_value,
+        new_value=message_retention_days,
+    )
 
 
 def set_realm_permissions_based_on_org_type(realm: Realm) -> None:

zerver/tests/test_events.py

@@ -2492,8 +2492,10 @@ class SubscribeActionTest(BaseAction):
         events = self.verify_action(action, include_subscribers=include_subscribers, num_events=2)
         check_stream_update("events[0]", events[0])
 
-        action = lambda: do_change_stream_message_retention_days(stream, -1)
-        events = self.verify_action(action, include_subscribers=include_subscribers, num_events=1)
+        action = lambda: do_change_stream_message_retention_days(
+            stream, self.example_user("hamlet"), -1
+        )
+        events = self.verify_action(action, include_subscribers=include_subscribers, num_events=2)
         check_stream_update("events[0]", events[0])
 
         # Subscribe to a totally new invite-only stream, so it's just Hamlet on it

zerver/tests/test_subs.py

@@ -36,6 +36,7 @@ from zerver.lib.actions import (
     gather_subscriptions_helper,
     get_average_weekly_stream_traffic,
     get_default_streams_for_realm,
+    get_topic_messages,
     lookup_default_stream_groups,
     round_to_2_significant_digits,
     validate_user_access_to_subscribers_helper,
@@ -1272,6 +1273,55 @@ class StreamAdminTest(ZulipTestCase):
             stream = get_stream("stream_name1", user_profile.realm)
             self.assertEqual(stream.stream_post_policy, policy)
 
+    def test_change_stream_message_retention_days_notifications(self) -> None:
+        user_profile = self.example_user("desdemona")
+        self.login_user(user_profile)
+        realm = user_profile.realm
+        do_change_realm_plan_type(realm, Realm.PLAN_TYPE_SELF_HOSTED, acting_user=None)
+        stream = self.subscribe(user_profile, "stream_name1")
+
+        # Go from realm default (forever) to 2 days
+        result = self.client_patch(
+            f"/json/streams/{stream.id}", {"message_retention_days": orjson.dumps(2).decode()}
+        )
+        self.assert_json_success(result)
+        messages = get_topic_messages(user_profile, stream, "stream events")
+        self.assert_length(messages, 1)
+        expected_notification = (
+            f"@_**Desdemona|{user_profile.id}** has changed the [message retention period](/help/message-retention-policy) "
+            "for this stream from **Forever** to **2 days**. Messages will be automatically "
+            "deleted after 2 days."
+        )
+        self.assertEqual(messages[0].content, expected_notification)
+
+        # Go from 2 days to 8 days
+        result = self.client_patch(
+            f"/json/streams/{stream.id}", {"message_retention_days": orjson.dumps(8).decode()}
+        )
+        self.assert_json_success(result)
+        messages = get_topic_messages(user_profile, stream, "stream events")
+        self.assert_length(messages, 2)
+        expected_notification = (
+            f"@_**Desdemona|{user_profile.id}** has changed the [message retention period](/help/message-retention-policy) "
+            "for this stream from **2 days** to **8 days**. Messages will be automatically "
+            "deleted after 8 days."
+        )
+        self.assertEqual(messages[1].content, expected_notification)
+
+        # Go from 2 days to realm default (None on stream, forever/-1 on realm)
+        result = self.client_patch(
+            f"/json/streams/{stream.id}",
+            {"message_retention_days": orjson.dumps("realm_default").decode()},
+        )
+        self.assert_json_success(result)
+        messages = get_topic_messages(user_profile, stream, "stream events")
+        self.assert_length(messages, 3)
+        expected_notification = (
+            f"@_**Desdemona|{user_profile.id}** has changed the [message retention period](/help/message-retention-policy) "
+            "for this stream from **8 days** to **Forever**."
+        )
+        self.assertEqual(messages[2].content, expected_notification)
+
     def test_change_stream_message_retention_days(self) -> None:
         user_profile = self.example_user("desdemona")
         self.login_user(user_profile)
@@ -1286,7 +1336,7 @@ class StreamAdminTest(ZulipTestCase):
 
         do_change_realm_plan_type(realm, Realm.PLAN_TYPE_SELF_HOSTED, acting_user=None)
         events: List[Mapping[str, Any]] = []
-        with self.tornado_redirected_to_list(events, expected_num_events=1):
+        with self.tornado_redirected_to_list(events, expected_num_events=2):
             result = self.client_patch(
                 f"/json/streams/{stream.id}", {"message_retention_days": orjson.dumps(2).decode()}
             )
@@ -1313,7 +1363,7 @@ class StreamAdminTest(ZulipTestCase):
         self.assertNotIn(self.example_user("polonius").id, notified_user_ids)
         self.assertEqual(stream.message_retention_days, 2)
 
-        with self.tornado_redirected_to_list(events, expected_num_events=1):
+        with self.tornado_redirected_to_list(events, expected_num_events=2):
             result = self.client_patch(
                 f"/json/streams/{stream.id}",
                 {"message_retention_days": orjson.dumps("unlimited").decode()},
@@ -1335,7 +1385,7 @@ class StreamAdminTest(ZulipTestCase):
         stream = get_stream("stream_name1", realm)
         self.assertEqual(stream.message_retention_days, -1)
 
-        with self.tornado_redirected_to_list(events, expected_num_events=1):
+        with self.tornado_redirected_to_list(events, expected_num_events=2):
             result = self.client_patch(
                 f"/json/streams/{stream.id}",
                 {"message_retention_days": orjson.dumps("realm_default").decode()},

zerver/views/streams.py

@@ -272,10 +272,12 @@ def update_stream_backend(
         if not user_profile.is_realm_owner:
             raise OrganizationOwnerRequired()
         user_profile.realm.ensure_not_on_limited_plan()
-        message_retention_days_value = parse_message_retention_days(
+        new_message_retention_days_value = parse_message_retention_days(
             message_retention_days, Stream.MESSAGE_RETENTION_SPECIAL_VALUES_MAP
         )
-        do_change_stream_message_retention_days(stream, message_retention_days_value)
+        do_change_stream_message_retention_days(
+            stream, user_profile, new_message_retention_days_value
+        )
 
     if description is not None:
         if "\n" in description: