Overview

We recently introduced OPFS VFS support for the PowerSync web SDK. Unlike IndexedDB-based VFS, which allows a single shared SQLite connection within a shared web worker across multiple tabs, OPFS requires each tab to have its own dedicated SQLite worker.

To manage concurrency, we use Navigator locks, ensuring exclusive access to SQLite connections. Table change notifications are propagated using a BroadcastChannel.

In multi-tab scenarios, a shared sync worker is responsible for syncing operations. This worker requires access to the SQLite database.

• IndexedDB VFS: A MessagePort to the shared SQLite worker is passed to the sync worker, allowing direct DB access.
• OPFS VFS: A MessagePort to the last connected tab’s dedicated SQLite worker is passed to the sync worker. This connection is updated as tabs are opened and closed.

The shared sync worker's DB connection is linked to a specific tab. If that tab is frozen or put to sleep (e.g., in Chrome’s Tab Freezing or Edge’s Sleeping Tabs), the connection becomes inaccessible, blocking SQLite queries in the sync worker.

Luckily browsers don't freeze or sleep tabs which are actively holding a web lock. This PR adds a hold on a web lock as soon as the connection is shared to the sync worker. In local testing this prevents tabs from being put to sleep.