write and remove archived summary when leaving/rejoining

src/matrix/room/Room.js

@@ -250,6 +250,9 @@ export class Room extends EventEmitter {
     async writeSync(roomResponse, isInitialSync, {summaryChanges, decryptChanges, roomEncryption, retryEntries}, txn, log) {
         log.set("id", this.id);
         const isRejoin = summaryChanges.membership === "join" && this._summary.data.membership === "leave";
+        if (isRejoin) {
+            this._summary.tryRemoveArchive(txn);
+        }
         const {entries: newEntries, newLiveKey, memberChanges} =
             await log.wrap("syncWriter", log => this._syncWriter.writeSync(roomResponse, isRejoin, txn, log), log.level.Detail);
         let allEntries = newEntries;
@@ -276,8 +279,13 @@ export class Room extends EventEmitter {
         // also apply (decrypted) timeline entries to the summary changes
         summaryChanges = summaryChanges.applyTimelineEntries(
             allEntries, isInitialSync, !this._isTimelineOpen, this._user.id);
-        // write summary changes, and unset if nothing was actually changed
+        // only archive a room if we had previously joined it
-        summaryChanges = this._summary.writeData(summaryChanges, txn);
+        if (summaryChanges.membership === "leave" && this.membership === "join") {
+            summaryChanges = this._summary.removeAndWriteArchive(summaryChanges, txn);
+        } else {
+            // write summary changes, and unset if nothing was actually changed
+            summaryChanges = this._summary.writeData(summaryChanges, txn);
+        }
         if (summaryChanges) {
             log.set("summaryChanges", summaryChanges.diff(this._summary.data));
         }

src/matrix/room/RoomSummary.js

@@ -302,6 +302,20 @@ export class RoomSummary {
 		}
 	}
 
+    /** move summary to archived store when leaving the room */
+    removeAndWriteArchive(data, txn) {
+        txn.roomSummary.remove(data.roomId);
+        if (data !== this._data) {
+            txn.archivedRoomSummary.set(data.serialize());
+            return data;
+        }
+    }
+
+    /** delete archived summary when rejoining the room */
+    tryRemoveArchive(txn) {
+        txn.archivedRoomSummary.remove(this._data.roomId);
+    }
+
     async writeAndApplyData(data, storage) {
         if (data === this._data) {
             return false;