Port `EventInternalMetadata` class to Rust (#16782)

There are a couple of things we need to be careful of here:

1. The current python code does no validation when loading from the DB,
so we need to be careful to ignore such errors (at least on jki.re there
are some old events with internal metadata fields of the wrong type).
2. We want to be memory efficient, as we often have many hundreds of
thousands of events in the cache at a time.

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

1 files changed, 106 insertions, 0 deletions

diff --git a/synapse/synapse_rust/events.pyi b/synapse/synapse_rust/events.pyi
new file mode 100644
index 0000000000..423ede5969
--- /dev/null
+++ b/synapse/synapse_rust/events.pyi
@@ -0,0 +1,106 @@
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# 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>.
+
+from typing import Optional
+
+from synapse.types import JsonDict
+
+class EventInternalMetadata:
+    def __init__(self, internal_metadata_dict: JsonDict): ...
+
+    stream_ordering: Optional[int]
+    """the stream ordering of this event. None, until it has been persisted."""
+
+    outlier: bool
+    """whether this event is an outlier (ie, whether we have the state at that
+    point in the DAG)"""
+
+    out_of_band_membership: bool
+    send_on_behalf_of: str
+    recheck_redaction: bool
+    soft_failed: bool
+    proactively_send: bool
+    redacted: bool
+
+    txn_id: str
+    """The transaction ID, if it was set when the event was created."""
+    token_id: int
+    """The access token ID of the user who sent this event, if any."""
+    device_id: str
+    """The device ID of the user who sent this event, if any."""
+
+    def get_dict(self) -> JsonDict: ...
+    def is_outlier(self) -> bool: ...
+    def copy(self) -> "EventInternalMetadata": ...
+    def is_out_of_band_membership(self) -> bool:
+        """Whether this event is an out-of-band membership.
+
+        OOB memberships are a special case of outlier events: they are membership events
+        for federated rooms that we aren't full members of. Examples include invites
+        received over federation, and rejections for such invites.
+
+        The concept of an OOB membership is needed because these events need to be
+        processed as if they're new regular events (e.g. updating membership state in
+        the database, relaying to clients via /sync, etc) despite being outliers.
+
+        See also https://element-hq.github.io/synapse/develop/development/room-dag-concepts.html#out-of-band-membership-events.
+
+        (Added in synapse 0.99.0, so may be unreliable for events received before that)
+        """
+        ...
+    def get_send_on_behalf_of(self) -> Optional[str]:
+        """Whether this server should send the event on behalf of another server.
+        This is used by the federation "send_join" API to forward the initial join
+        event for a server in the room.
+
+        returns a str with the name of the server this event is sent on behalf of.
+        """
+        ...
+    def need_to_check_redaction(self) -> bool:
+        """Whether the redaction event needs to be rechecked when fetching
+        from the database.
+
+        Starting in room v3 redaction events are accepted up front, and later
+        checked to see if the redacter and redactee's domains match.
+
+        If the sender of the redaction event is allowed to redact any event
+        due to auth rules, then this will always return false.
+        """
+        ...
+    def is_soft_failed(self) -> bool:
+        """Whether the event has been soft failed.
+
+        Soft failed events should be handled as usual, except:
+            1. They should not go down sync or event streams, or generally
+               sent to clients.
+            2. They should not be added to the forward extremities (and
+               therefore not to current state).
+        """
+        ...
+    def should_proactively_send(self) -> bool:
+        """Whether the event, if ours, should be sent to other clients and
+        servers.
+
+        This is used for sending dummy events internally. Servers and clients
+        can still explicitly fetch the event.
+        """
+        ...
+    def is_redacted(self) -> bool:
+        """Whether the event has been redacted.
+
+        This is used for efficiently checking whether an event has been
+        marked as redacted without needing to make another database call.
+        """
+        ...
+    def is_notifiable(self) -> bool:
+        """Whether this event can trigger a push notification"""
+        ...