How do I implement file locking in a VFS?

[rhashimoto]

The example IndexedDB virtual filesystem implementation checks and sets an isLocked flag in IndexedDB and just returns a SQLITE_BUSY error when attempting to lock a file that is already locked (instead of waiting to obtain the lock). [this is no longer how it works] While this might work for some applications, it has some drawbacks:

• A retry-on-busy approach is prone to starvation.
• The lock can be orphaned if the lock holder exits without releasing it.
  • Ideally a lock expires automatically with the holder lifetime, which is not the case when the lock is stored in IndexedDB.
  • Orphaned locks typically require user-initiated clearing.

Here are some alternative mechanisms for synchronization on the web platform:

• Web Locks API
  • Looks great.
  • Doesn't directly support upgradeable locks, i.e. from shared to exclusive.
  • Not supported on Safari (but in the Technology Preview release as of 12/20/21).
  • Example use as a VFS mix-in is here.
• Atomics
  • Workers in same cluster only (i.e. not between browser tabs).
  • Allows synchronous blocking, so it can work with the synchronous SQLite build (which is smaller and faster).
  • Not supported on Safari (but in preview behind a flag). [Supported now]
  • Severe restrictions on including content from other origins.
• Service Worker
  • Do-It-Yourself synchronization.
  • Supported across the evergreen browsers.
  • See proof-of-concept discussion.