fix(base): Create the room::display_name response processor.

This patch creates the `room::display_name::update_for_rooms` response
processor. It also creates the `changes::save_only` response processor.
Finally, this patch uses both to compute the new display name for all
rooms and save them, for sliding sync only.

Author: Hywan

crates/matrix-sdk-base/src/response_processors/changes.rs

@@ -25,6 +25,13 @@ use crate::{
     Result, StateChanges,
 };
 
+/// Save the [`StateChanges`] from the [`Context`] inside the [`BaseStateStore`]
+/// only! The changes aren't applied on the in-memory rooms.
+#[instrument(skip_all)]
+pub async fn save_only(context: Context, state_store: &BaseStateStore) -> Result<()> {
+    save_changes(&context.state_changes, state_store, None).await
+}
+
 /// Save the [`StateChanges`] from the [`Context`] inside the
 /// [`BaseStateStore`], and apply them on the in-memory rooms.
 #[instrument(skip_all)]

crates/matrix-sdk-base/src/response_processors/room/display_name.rs

@@ -0,0 +1,37 @@
+// Copyright 2025 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use super::super::Context;
+use crate::{rooms::UpdatedRoomDisplayName, store::BaseStateStore, sync::RoomUpdates};
+
+pub async fn update_for_rooms(
+    context: &mut Context,
+    room_updates: &RoomUpdates,
+    state_store: &BaseStateStore,
+) {
+    for room in room_updates
+        .left
+        .keys()
+        .chain(room_updates.joined.keys())
+        .chain(room_updates.invited.keys())
+        .chain(room_updates.knocked.keys())
+        .filter_map(|room_id| state_store.room(room_id))
+    {
+        // Compute the display name. If it's different, let's register the `RoomInfo` in
+        // the `StateChanges`.
+        if let Ok(UpdatedRoomDisplayName::New(_)) = room.compute_display_name().await {
+            context.state_changes.room_infos.insert(room.room_id().to_owned(), room.clone_info());
+        }
+    }
+}

crates/matrix-sdk-base/src/response_processors/room/mod.rs

@@ -17,6 +17,7 @@ use tokio::sync::broadcast::Sender;
 
 use crate::{store::ambiguity_map::AmbiguityCache, RequestedRequiredStates, RoomInfoNotableUpdate};
 
+pub mod display_name;
 pub mod msc4186;
 pub mod sync_v2;
 

crates/matrix-sdk-base/src/sliding_sync.rs

@@ -254,6 +254,7 @@ impl BaseClient {
 
         context.state_changes.ambiguity_maps = ambiguity_cache.cache;
 
+        // Save the changes and apply them.
         processors::changes::save_and_apply(
             context,
             &self.state_store,
@@ -262,12 +263,19 @@ impl BaseClient {
         )
         .await?;
 
+        let mut context = processors::Context::default();
+
         // Now that all the rooms information have been saved, update the display name
-        // cache (which relies on information stored in the database). This will
-        // live in memory, until the next sync which will saves the room info to
-        // disk; we do this to avoid saving that would be redundant with the
-        // above. Oh well.
-        room_updates.update_in_memory_caches(&self.state_store).await;
+        // of the updated rooms (which relies on information stored in the database).
+        processors::room::display_name::update_for_rooms(
+            &mut context,
+            &room_updates,
+            &self.state_store,
+        )
+        .await;
+
+        // Save the new display name updates if any.
+        processors::changes::save_only(context, &self.state_store).await?;
 
         Ok(SyncResponse {
             rooms: room_updates,