allow loading an archived room

src/matrix/Session.js

@@ -648,6 +648,25 @@ export class Session {
         }
         return observable;
     }
+
+    loadArchivedRoom(roomId, log = null) {
+        return this._platform.logger.wrapOrRun(log, "loadArchivedRoom", async log => {
+            log.set("id", roomId);
+            const txn = await this._storage.readTxn([
+                this._storage.storeNames.archivedRoomSummary,
+                this._storage.storeNames.roomMembers,
+            ]);
+            const summary = await txn.archivedRoomSummary.get(roomId);
+            if (summary) {
+                // TODO: should we really be using a Room here?
+                // Or rather an ArchivedRoom that shares a common base class with Room?
+                // That will make the Room code harder to read though ...
+                const room = this.createRoom(roomId);
+                await room.load(summary, txn, log);
+                return room;
+            }
+        });
+    }
 }
 
 export function tests() {

src/matrix/room/Room.js

@@ -442,7 +442,10 @@ export class Room extends EventEmitter {
                 const changes = await this._heroes.calculateChanges(this._summary.data.heroes, [], txn);
                 this._heroes.applyChanges(changes, this._summary.data);
             }
-            return this._syncWriter.load(txn, log);
+            // don't load sync writer for archived room
+            if (this.membership !== "leave") {
+                return this._syncWriter.load(txn, log);
+            }
         } catch (err) {
             throw new WrappedError(`Could not load room ${this._roomId}`, err);
         }

src/matrix/storage/idb/stores/RoomSummaryStore.js

@@ -42,6 +42,10 @@ export class RoomSummaryStore {
 		return this._summaryStore.put(summary);
 	}
 
+	get(roomId) {
+		return this._summaryStore.get(roomId);
+	}
+
 	async has(roomId) {
         const fetchedKey = await this._summaryStore.getKey(roomId);
         return roomId === fetchedKey;