diff --git a/synapse/events/auto_accept_invites.py b/synapse/events/auto_accept_invites.py
new file mode 100644
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+++ b/synapse/events/auto_accept_invites.py
@@ -0,0 +1,196 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright 2021 The Matrix.org Foundation C.I.C
+# Copyright (C) 2024 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+# Originally licensed under the Apache License, Version 2.0:
+# <http://www.apache.org/licenses/LICENSE-2.0>.
+#
+# [This file includes modifications made by New Vector Limited]
+#
+#
+import logging
+from http import HTTPStatus
+from typing import Any, Dict, Tuple
+
+from synapse.api.constants import AccountDataTypes, EventTypes, Membership
+from synapse.api.errors import SynapseError
+from synapse.config.auto_accept_invites import AutoAcceptInvitesConfig
+from synapse.module_api import EventBase, ModuleApi, run_as_background_process
+
+logger = logging.getLogger(__name__)
+
+
+class InviteAutoAccepter:
+ def __init__(self, config: AutoAcceptInvitesConfig, api: ModuleApi):
+ # Keep a reference to the Module API.
+ self._api = api
+ self._config = config
+
+ if not self._config.enabled:
+ return
+
+ should_run_on_this_worker = config.worker_to_run_on == self._api.worker_name
+
+ if not should_run_on_this_worker:
+ logger.info(
+ "Not accepting invites on this worker (configured: %r, here: %r)",
+ config.worker_to_run_on,
+ self._api.worker_name,
+ )
+ return
+
+ logger.info(
+ "Accepting invites on this worker (here: %r)", self._api.worker_name
+ )
+
+ # Register the callback.
+ self._api.register_third_party_rules_callbacks(
+ on_new_event=self.on_new_event,
+ )
+
+ async def on_new_event(self, event: EventBase, *args: Any) -> None:
+ """Listens for new events, and if the event is an invite for a local user then
+ automatically accepts it.
+
+ Args:
+ event: The incoming event.
+ """
+ # Check if the event is an invite for a local user.
+ is_invite_for_local_user = (
+ event.type == EventTypes.Member
+ and event.is_state()
+ and event.membership == Membership.INVITE
+ and self._api.is_mine(event.state_key)
+ )
+
+ # Only accept invites for direct messages if the configuration mandates it.
+ is_direct_message = event.content.get("is_direct", False)
+ is_allowed_by_direct_message_rules = (
+ not self._config.accept_invites_only_for_direct_messages
+ or is_direct_message is True
+ )
+
+ # Only accept invites from remote users if the configuration mandates it.
+ is_from_local_user = self._api.is_mine(event.sender)
+ is_allowed_by_local_user_rules = (
+ not self._config.accept_invites_only_from_local_users
+ or is_from_local_user is True
+ )
+
+ if (
+ is_invite_for_local_user
+ and is_allowed_by_direct_message_rules
+ and is_allowed_by_local_user_rules
+ ):
+ # Make the user join the room. We run this as a background process to circumvent a race condition
+ # that occurs when responding to invites over federation (see https://github.com/matrix-org/synapse-auto-accept-invite/issues/12)
+ run_as_background_process(
+ "retry_make_join",
+ self._retry_make_join,
+ event.state_key,
+ event.state_key,
+ event.room_id,
+ "join",
+ bg_start_span=False,
+ )
+
+ if is_direct_message:
+ # Mark this room as a direct message!
+ await self._mark_room_as_direct_message(
+ event.state_key, event.sender, event.room_id
+ )
+
+ async def _mark_room_as_direct_message(
+ self, user_id: str, dm_user_id: str, room_id: str
+ ) -> None:
+ """
+ Marks a room (`room_id`) as a direct message with the counterparty `dm_user_id`
+ from the perspective of the user `user_id`.
+
+ Args:
+ user_id: the user for whom the membership is changing
+ dm_user_id: the user performing the membership change
+ room_id: room id of the room the user is invited to
+ """
+
+ # This is a dict of User IDs to tuples of Room IDs
+ # (get_global will return a frozendict of tuples as it freezes the data,
+ # but we should accept either frozen or unfrozen variants.)
+ # Be careful: we convert the outer frozendict into a dict here,
+ # but the contents of the dict are still frozen (tuples in lieu of lists,
+ # etc.)
+ dm_map: Dict[str, Tuple[str, ...]] = dict(
+ await self._api.account_data_manager.get_global(
+ user_id, AccountDataTypes.DIRECT
+ )
+ or {}
+ )
+
+ if dm_user_id not in dm_map:
+ dm_map[dm_user_id] = (room_id,)
+ else:
+ dm_rooms_for_user = dm_map[dm_user_id]
+ assert isinstance(dm_rooms_for_user, (tuple, list))
+
+ dm_map[dm_user_id] = tuple(dm_rooms_for_user) + (room_id,)
+
+ await self._api.account_data_manager.put_global(
+ user_id, AccountDataTypes.DIRECT, dm_map
+ )
+
+ async def _retry_make_join(
+ self, sender: str, target: str, room_id: str, new_membership: str
+ ) -> None:
+ """
+ A function to retry sending the `make_join` request with an increasing backoff. This is
+ implemented to work around a race condition when receiving invites over federation.
+
+ Args:
+ sender: the user performing the membership change
+ target: the user for whom the membership is changing
+ room_id: room id of the room to join to
+ new_membership: the type of membership event (in this case will be "join")
+ """
+
+ sleep = 0
+ retries = 0
+ join_event = None
+
+ while retries < 5:
+ try:
+ await self._api.sleep(sleep)
+ join_event = await self._api.update_room_membership(
+ sender=sender,
+ target=target,
+ room_id=room_id,
+ new_membership=new_membership,
+ )
+ except SynapseError as e:
+ if e.code == HTTPStatus.FORBIDDEN:
+ logger.debug(
+ f"Update_room_membership was forbidden. This can sometimes be expected for remote invites. Exception: {e}"
+ )
+ else:
+ logger.warn(
+ f"Update_room_membership raised the following unexpected (SynapseError) exception: {e}"
+ )
+ except Exception as e:
+ logger.warn(
+ f"Update_room_membership raised the following unexpected exception: {e}"
+ )
+
+ sleep = 2**retries
+ retries += 1
+
+ if join_event is not None:
+ break
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