messages: Create function to calculate first visible message id.

zerver/lib/cache.py

@@ -410,6 +410,11 @@ def flush_realm(sender: Any, **kwargs: Any) -> None:
     users = realm.get_active_users()
     delete_user_profile_caches(users)
 
+    # Deleting realm or updating message_visibility_limit
+    # attribute should clear the last_visible_message_id cache.
+    if kwargs.get('update_fields') is None or "message_visibility_limit" in kwargs['update_fields']:
+        cache_delete(realm_last_visible_message_id_cache_key(realm))
+
     if realm.deactivated:
         cache_delete(realm_user_dicts_cache_key(realm.id))
         cache_delete(active_user_ids_cache_key(realm.id))
@@ -420,6 +425,10 @@ def realm_alert_words_cache_key(realm):
     # type: (Realm) -> Text
     return u"realm_alert_words:%s" % (realm.string_id,)
 
+def realm_last_visible_message_id_cache_key(realm):
+    # type: (Realm) -> Text
+    return u"realm_last_visible_message_id:%s" % (realm.string_id,)
+
 # Called by models.py to flush the stream cache whenever we save a stream
 # object.
 def flush_stream(sender: Any, **kwargs: Any) -> None:

zerver/lib/message.py

@@ -13,6 +13,7 @@ from zerver.lib.cache import (
     generic_bulk_cached_fetch,
     to_dict_cache_key,
     to_dict_cache_key_id,
+    realm_last_visible_message_id_cache_key,
 )
 from zerver.lib.request import JsonableError
 from zerver.lib.stream_subscription import (
@@ -846,7 +847,18 @@ def apply_unread_message_event(user_profile: UserProfile,
     if 'mentioned' in flags:
         state['mentions'].add(message_id)
 
-# This function will be used when we restrict search history for certain realms,
-# but now it just allows all realms to see all messages.
+# 4 hours is probably frequent enough to encourage upgrade
+# and largely mitigates the cache-miss problem. We also
+# don't need to restrict messages to exactly
+# message_visibility_limit. The query is also reasonably fast
+# so cache misses can be managed without cron job.
+@cache_with_key(realm_last_visible_message_id_cache_key, timeout=3600*4)
 def get_first_visible_message_id(realm: Realm) -> int:
-    return 0
+    if realm.message_visibility_limit is None:
+        return 0
+
+    try:
+        return Message.objects.filter(sender__realm=realm).values('id').\
+            order_by('-id')[realm.message_visibility_limit - 1]["id"]
+    except IndexError:
+        return 0

zerver/tests/test_messages.py

@@ -22,6 +22,8 @@ from zerver.lib.message import (
     MessageDict,
     messages_for_ids,
     sew_messages_and_reactions,
+    get_first_visible_message_id,
+    realm_last_visible_message_id_cache_key,
 )
 
 from zerver.lib.test_helpers import (
@@ -2731,3 +2733,29 @@ class MessageHydrationTest(ZulipTestCase):
 
         self.assertIn('class="user-mention"', new_message['content'])
         self.assertEqual(new_message['flags'], ['mentioned'])
+
+class MessageVisibilityTest(ZulipTestCase):
+    def test_get_first_visible_message_id_with_no_limit(self) -> None:
+        realm = get_realm("zulip")
+        realm.message_visibility_limit = None
+        realm.save()
+
+        self.assertEqual(get_first_visible_message_id(realm), 0)
+
+    def test_get_first_visible_message_id_with_limit(self) -> None:
+        Message.objects.all().delete()
+        message_ids = [self.send_stream_message(self.example_email("othello"), "Scotland") for i in range(15)]
+
+        realm = get_realm("zulip")
+        realm.message_visibility_limit = 10
+        realm.save()
+
+        expected_message_id = message_ids[5]
+        self.assertEqual(get_first_visible_message_id(realm), expected_message_id)
+        # If the message_visibility_limit is greater than number of messages
+        # get_first_visible_message_id should return 0
+        message_visibility_limit = 50
+        realm.message_visibility_limit = message_visibility_limit
+        realm.save()
+
+        self.assertEqual(get_first_visible_message_id(realm), 0)