Add MSC3030 experimental client and federation API endpoints to get the closest event to a given timestamp (#9445)

MSC3030: https://github.com/matrix-org/matrix-doc/pull/3030

Client API endpoint. This will also go and fetch from the federation API endpoint if unable to find an event locally or we found an extremity with possibly a closer event we don't know about.
```
GET /_matrix/client/unstable/org.matrix.msc3030/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>
{
    "event_id": ...
    "origin_server_ts": ...
}
```

Federation API endpoint:
```
GET /_matrix/federation/unstable/org.matrix.msc3030/timestamp_to_event/<roomID>?ts=<timestamp>&dir=<direction>
{
    "event_id": ...
    "origin_server_ts": ...
}
```

Co-authored-by: Erik Johnston <erik@matrix.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 8fbc75aa65..cce85526e7 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -110,6 +110,7 @@ class FederationServer(FederationBase):
         super().__init__(hs)
 
         self.handler = hs.get_federation_handler()
+        self.storage = hs.get_storage()
         self._federation_event_handler = hs.get_federation_event_handler()
         self.state = hs.get_state_handler()
         self._event_auth_handler = hs.get_event_auth_handler()
@@ -200,6 +201,48 @@ class FederationServer(FederationBase):
 
         return 200, res
 
+    async def on_timestamp_to_event_request(
+        self, origin: str, room_id: str, timestamp: int, direction: str
+    ) -> Tuple[int, Dict[str, Any]]:
+        """When we receive a federated `/timestamp_to_event` request,
+        handle all of the logic for validating and fetching the event.
+
+        Args:
+            origin: The server we received the event from
+            room_id: Room to fetch the event from
+            timestamp: The point in time (inclusive) we should navigate from in
+                the given direction to find the closest event.
+            direction: ["f"|"b"] to indicate whether we should navigate forward
+                or backward from the given timestamp to find the closest event.
+
+        Returns:
+            Tuple indicating the response status code and dictionary response
+            body including `event_id`.
+        """
+        with (await self._server_linearizer.queue((origin, room_id))):
+            origin_host, _ = parse_server_name(origin)
+            await self.check_server_matches_acl(origin_host, room_id)
+
+            # We only try to fetch data from the local database
+            event_id = await self.store.get_event_id_for_timestamp(
+                room_id, timestamp, direction
+            )
+            if event_id:
+                event = await self.store.get_event(
+                    event_id, allow_none=False, allow_rejected=False
+                )
+
+                return 200, {
+                    "event_id": event_id,
+                    "origin_server_ts": event.origin_server_ts,
+                }
+
+        raise SynapseError(
+            404,
+            "Unable to find event from %s in direction %s" % (timestamp, direction),
+            errcode=Codes.NOT_FOUND,
+        )
+
     async def on_incoming_transaction(
         self,
         origin: str,