prevent overlapping sync accumulator persists

add a flag to stop the sync worker trying to persist to indexeddb
if there are already persists in flight. accumulates user presence
updates in RAM to stop them being lost if the persist is skipped.

hopefully fixes https://github.com/vector-im/element-web/issues/21541

src/store/indexeddb-local-backend.ts

@@ -127,6 +127,8 @@ export class LocalIndexedDBStoreBackend implements IIndexedDBBackend {
     private db: IDBDatabase = null;
     private disconnected = true;
     private _isNewlyCreated = false;
+    private _isPersisting = false;
+    private _pendingUserPresenceData: UserTuple[] = [];
 
     /**
      * Does the actual reading from and writing to the indexeddb
@@ -401,11 +403,22 @@ export class LocalIndexedDBStoreBackend implements IIndexedDBBackend {
     public async syncToDatabase(userTuples: UserTuple[]): Promise<void> {
         const syncData = this.syncAccumulator.getJSON(true);
 
+        if (this._isPersisting) {
+            logger.warn("Skipping syncToDatabase() as persist already in flight");
+            this._pendingUserPresenceData.push(...userTuples);
+            return;
+        } else {
+            userTuples.unshift(...this._pendingUserPresenceData);
+            this._isPersisting = true;
+        }
+
         await Promise.all([
             this.persistUserPresenceEvents(userTuples),
             this.persistAccountData(syncData.accountData),
             this.persistSyncData(syncData.nextBatch, syncData.roomsData),
-        ]);
+        ]).finally(() => {
+            this._isPersisting = false;
+        });
     }
 
     /**
@@ -427,7 +440,9 @@ export class LocalIndexedDBStoreBackend implements IIndexedDBBackend {
                 nextBatch,
                 roomsData,
             }); // put == UPSERT
-            return txnAsPromise(txn).then();
+            return txnAsPromise(txn).then(() => {
+                logger.log("Persisted sync data up to", nextBatch);
+            });
         });
     }