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, 1 insertions, 0 deletions

diff --git a/changelog.d/16973.bugfix b/changelog.d/16973.bugfix
new file mode 100644
index 0000000000..a0e3a12304
--- /dev/null
+++ b/changelog.d/16973.bugfix
@@ -0,0 +1 @@
+Fix joining remote rooms when a module uses the `on_new_event` callback. This callback may now pass partial state events instead of the full state for remote rooms.