Revert "home: Don't use pointer to compute furthest_read_time."

This reverts commit 8f32db81a1.

This change unfortunately requires an index that we don't have, and
thus is incredibly expensive.  We'll need to do a thoughtful reworking
before we can integrate it again.

zerver/models.py

@@ -1962,10 +1962,6 @@ def get_usermessage_by_message_id(user_profile: UserProfile, message_id: int) ->
     except UserMessage.DoesNotExist:
         return None
 
-def get_latest_read_usermessage(user_profile: UserProfile) -> Optional[UserMessage]:
-    return UserMessage.objects.select_related().filter(user_profile=user_profile,
-                                                       flags=UserMessage.flags.read).last()
-
 class ArchivedUserMessage(AbstractUserMessage):
     """Used as a temporary holding place for deleted UserMessages objects
     before they are permanently deleted.  This is an important part of

zerver/tests/test_home.py

@@ -22,7 +22,7 @@ from zerver.lib.users import compute_show_invites_and_add_streams
 from zerver.models import (
     get_realm, get_stream, get_user, UserProfile,
     flush_per_request_caches, DefaultStream, Realm,
-    get_system_bot, UserMessage
+    get_system_bot,
 )
 from zerver.views.home import sent_time_in_epoch_seconds, compute_navbar_logo_url
 from corporate.models import Customer, CustomerPlan
@@ -458,19 +458,9 @@ class HomeTest(ZulipTestCase):
         user_profile.save()
 
         self.login_user(user_profile)
+        with patch('logging.warning') as mock:
             result = self._get_home_page()
-        self._sanity_check(result)
-
-    def test_no_read_user_messages(self) -> None:
-        user_profile = self.example_user('hamlet')
-        user_profile.pointer = 1
-        user_profile.save()
-
-        # Mark all messages as unread
-        UserMessage.objects.filter(user_profile=user_profile).update(flags=~UserMessage.flags.read)
-
-        self.login_user(user_profile)
-        result = self._get_home_page()
+        mock.assert_called_once_with('User %s has invalid pointer 999999' % (user_profile.id,))
         self._sanity_check(result)
 
     def test_topic_narrow(self) -> None:

zerver/views/home.py

@@ -13,7 +13,7 @@ from zerver.forms import ToSForm
 from zerver.models import Message, Stream, UserProfile, \
     Realm, UserMessage, \
     PreregistrationUser, \
-    get_latest_read_usermessage
+    get_usermessage_by_message_id
 from zerver.lib.events import do_events_register
 from zerver.lib.actions import do_change_tos_version, \
     realm_user_count
@@ -223,7 +223,10 @@ def home_real(request: HttpRequest) -> HttpResponse:
             register_ret['pointer'] = register_ret['max_message_id']
         furthest_read_time = None
     else:
-        latest_read = get_latest_read_usermessage(user_profile)
+        latest_read = get_usermessage_by_message_id(user_profile, user_profile.pointer)
+        if latest_read is None:
+            # Don't completely fail if your saved pointer ID is invalid
+            logging.warning("User %s has invalid pointer %s" % (user_profile.id, user_profile.pointer))
         furthest_read_time = sent_time_in_epoch_seconds(latest_read)
 
     # We pick a language for the user as follows: