Bring auto-accept invite logic into Synapse (#17147)

This PR ports the logic from the
[synapse_auto_accept_invite](https://github.com/matrix-org/synapse-auto-accept-invite)
module into synapse.

I went with the naive approach of injecting the "module" next to where
third party modules are currently loaded. If there is a better/preferred
way to handle this, I'm all ears. It wasn't obvious to me if there was a
better location to add this logic that would cleanly apply to all
incoming invite events.

Relies on https://github.com/element-hq/synapse/pull/17166 to fix linter
errors.

1 files changed, 196 insertions, 0 deletions

diff --git a/synapse/events/auto_accept_invites.py b/synapse/events/auto_accept_invites.py
new file mode 100644
index 0000000000..d88ec51d9d
--- /dev/null
+++ 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