No worries, good question :>

The problem with bidirectional filesync is that it's an absolutely massive can of worms, very easy to mess up, and the consequences of messing up are usually the worst kind (loss of data). There's an insane amount of edgecases to keep in mind, and you need to get every edgecase right every single time, otherwise you might wipe someone's vacation photos, or suddenly downgrade someone's keepass database to an older version... And stuff like syncing multiple devices to the same server makes it balloon further.

I've started becoming more confident in copyparty's filesystem-index database, but it's still just a hint/guideline, with the filesystem being the only source of truth -- it's still not something I'd trust with tracking sync-state against one or more clients.

The bigger guys who offer bidirectional sync (nextcloud, syncthing, etc.) have spent years perfecting their logic, so I'd like to leave this in their capable hands.