Revert "Remove support for MSC3575 (sliding sync)"

This reverts commit d87848b9a6.

src/service/rooms/state_accessor.rs

@@ -7,6 +7,7 @@ use lru_cache::LruCache;
 use ruma::{
     events::{
         room::{
+            avatar::RoomAvatarEventContent,
             history_visibility::{
                 HistoryVisibility, RoomHistoryVisibilityEventContent,
             },
@@ -17,8 +18,8 @@ use ruma::{
         StateEventType,
     },
     state_res::Event,
-    EventId, OwnedRoomId, OwnedServerName, OwnedUserId, RoomId, ServerName,
-    UserId,
+    EventId, JsOption, OwnedRoomId, OwnedServerName, OwnedUserId, RoomId,
+    ServerName, UserId,
 };
 use serde_json::value::to_raw_value;
 use tracing::{error, warn};
@@ -507,6 +508,23 @@ impl Service {
         )
     }
 
+    #[tracing::instrument(skip(self))]
+    pub(crate) fn get_avatar(
+        &self,
+        room_id: &RoomId,
+    ) -> Result<JsOption<RoomAvatarEventContent>> {
+        self.room_state_get(room_id, &StateEventType::RoomAvatar, "")?.map_or(
+            Ok(JsOption::Undefined),
+            |s| {
+                serde_json::from_str(s.content.get()).map_err(|_| {
+                    Error::bad_database(
+                        "Invalid room avatar event in database.",
+                    )
+                })
+            },
+        )
+    }
+
     // Allowed because this function uses `services()`
     #[allow(clippy::unused_self)]
     #[tracing::instrument(skip(self), ret(level = "trace"))]
@@ -535,6 +553,24 @@ impl Service {
             .is_ok()
     }
 
+    #[tracing::instrument(skip(self))]
+    pub(crate) fn get_member(
+        &self,
+        room_id: &RoomId,
+        user_id: &UserId,
+    ) -> Result<Option<RoomMemberEventContent>> {
+        self.room_state_get(
+            room_id,
+            &StateEventType::RoomMember,
+            user_id.as_str(),
+        )?
+        .map_or(Ok(None), |s| {
+            serde_json::from_str(s.content.get()).map_err(|_| {
+                Error::bad_database("Invalid room member event in database.")
+            })
+        })
+    }
+
     /// Checks if a given user can redact a given event
     ///
     /// If `federation` is `true`, it allows redaction events from any user of