Prevent a sync request from removing a user's busy presence status (#12213)

In trying to use the MSC3026 busy presence status, the user's status
would be set back to 'online' next time they synced. This change makes
it so that syncing does not affect a user's presence status if it
is currently set to 'busy': it must be removed through the presence
API.

The MSC defers to implementations on the behaviour of busy presence,
so this ought to remain compatible with the MSC.

tests/handlers/test_presence.py

@@ -657,6 +657,85 @@ class PresenceHandlerTestCase(unittest.HomeserverTestCase):
         # Mark user as online and `status_msg = None`
         self._set_presencestate_with_status_msg(user_id, PresenceState.ONLINE, None)
 
+    def test_set_presence_from_syncing_not_set(self):
+        """Test that presence is not set by syncing if affect_presence is false"""
+        user_id = "@test:server"
+        status_msg = "I'm here!"
+
+        self._set_presencestate_with_status_msg(
+            user_id, PresenceState.UNAVAILABLE, status_msg
+        )
+
+        self.get_success(
+            self.presence_handler.user_syncing(user_id, False, PresenceState.ONLINE)
+        )
+
+        state = self.get_success(
+            self.presence_handler.get_state(UserID.from_string(user_id))
+        )
+        # we should still be unavailable
+        self.assertEqual(state.state, PresenceState.UNAVAILABLE)
+        # and status message should still be the same
+        self.assertEqual(state.status_msg, status_msg)
+
+    def test_set_presence_from_syncing_is_set(self):
+        """Test that presence is set by syncing if affect_presence is true"""
+        user_id = "@test:server"
+        status_msg = "I'm here!"
+
+        self._set_presencestate_with_status_msg(
+            user_id, PresenceState.UNAVAILABLE, status_msg
+        )
+
+        self.get_success(
+            self.presence_handler.user_syncing(user_id, True, PresenceState.ONLINE)
+        )
+
+        state = self.get_success(
+            self.presence_handler.get_state(UserID.from_string(user_id))
+        )
+        # we should now be online
+        self.assertEqual(state.state, PresenceState.ONLINE)
+
+    def test_set_presence_from_syncing_keeps_status(self):
+        """Test that presence set by syncing retains status message"""
+        user_id = "@test:server"
+        status_msg = "I'm here!"
+
+        self._set_presencestate_with_status_msg(
+            user_id, PresenceState.UNAVAILABLE, status_msg
+        )
+
+        self.get_success(
+            self.presence_handler.user_syncing(user_id, True, PresenceState.ONLINE)
+        )
+
+        state = self.get_success(
+            self.presence_handler.get_state(UserID.from_string(user_id))
+        )
+        # our status message should be the same as it was before
+        self.assertEqual(state.status_msg, status_msg)
+
+    def test_set_presence_from_syncing_keeps_busy(self):
+        """Test that presence set by syncing doesn't affect busy status"""
+        # while this isn't the default
+        self.presence_handler._busy_presence_enabled = True
+
+        user_id = "@test:server"
+        status_msg = "I'm busy!"
+
+        self._set_presencestate_with_status_msg(user_id, PresenceState.BUSY, status_msg)
+
+        self.get_success(
+            self.presence_handler.user_syncing(user_id, True, PresenceState.ONLINE)
+        )
+
+        state = self.get_success(
+            self.presence_handler.get_state(UserID.from_string(user_id))
+        )
+        # we should still be busy
+        self.assertEqual(state.state, PresenceState.BUSY)
+
     def _set_presencestate_with_status_msg(
         self, user_id: str, state: str, status_msg: Optional[str]
     ):