Fix joining remote rooms when a `on_new_event` callback is registered (#16973)

Since Synapse 1.76.0, any module which registers a `on_new_event`
callback would brick the ability to join remote rooms.
This is because this callback tried to get the full state of the room,
which would end up in a deadlock.

Related:
https://github.com/matrix-org/synapse-auto-accept-invite/issues/18

The following module would brick the ability to join remote rooms:

```python
from typing import Any, Dict, Literal, Union
import logging

from synapse.module_api import ModuleApi, EventBase

logger = logging.getLogger(__name__)

class MyModule:
    def __init__(self, config: None, api: ModuleApi):
        self._api = api
        self._config = config

        self._api.register_third_party_rules_callbacks(
            on_new_event=self.on_new_event,
        )

    async def on_new_event(self, event: EventBase, _state_map: Any) -> None:
        logger.info(f"Received new event: {event}")

    @staticmethod
    def parse_config(_config: Dict[str, Any]) -> None:
        return None
```

This is technically a breaking change, as we are now passing partial
state on the `on_new_event` callback.
However, this callback was broken for federated rooms since 1.76.0, and
local rooms have full state anyway, so it's unlikely that it would
change anything.

1 files changed, 9 insertions, 14 deletions

diff --git a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
index 7a3255aeef..9f7a04372d 100644
--- a/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
+++ b/synapse/module_api/callbacks/third_party_event_rules_callbacks.py
@@ -366,7 +366,7 @@ class ThirdPartyEventRulesModuleApiCallbacks:
         if len(self._check_threepid_can_be_invited_callbacks) == 0:
             return True
 
-        state_events = await self._get_state_map_for_room(room_id)
+        state_events = await self._storage_controllers.state.get_current_state(room_id)
 
         for callback in self._check_threepid_can_be_invited_callbacks:
             try:
@@ -399,7 +399,7 @@ class ThirdPartyEventRulesModuleApiCallbacks:
         if len(self._check_visibility_can_be_modified_callbacks) == 0:
             return True
 
-        state_events = await self._get_state_map_for_room(room_id)
+        state_events = await self._storage_controllers.state.get_current_state(room_id)
 
         for callback in self._check_visibility_can_be_modified_callbacks:
             try:
@@ -427,7 +427,13 @@ class ThirdPartyEventRulesModuleApiCallbacks:
             return
 
         event = await self.store.get_event(event_id)
-        state_events = await self._get_state_map_for_room(event.room_id)
+
+        # We *don't* want to wait for the full state here, because waiting for full
+        # state will persist event, which in turn will call this method.
+        # This would end up in a deadlock.
+        state_events = await self._storage_controllers.state.get_current_state(
+            event.room_id, await_full_state=False
+        )
 
         for callback in self._on_new_event_callbacks:
             try:
@@ -490,17 +496,6 @@ class ThirdPartyEventRulesModuleApiCallbacks:
                 )
         return True
 
-    async def _get_state_map_for_room(self, room_id: str) -> StateMap[EventBase]:
-        """Given a room ID, return the state events of that room.
-
-        Args:
-            room_id: The ID of the room.
-
-        Returns:
-            A dict mapping (event type, state key) to state event.
-        """
-        return await self._storage_controllers.state.get_current_state(room_id)
-
     async def on_profile_update(
         self, user_id: str, new_profile: ProfileInfo, by_admin: bool, deactivation: bool
     ) -> None: