> The simple way to fork a sqlite db is just to copy it

SQLite recommends against doing that [1] while a transaction is active, because the copy might have some pre/post data in it which could potentially cause corruption. Presumably this isn't an issue with copy-on-write filesystems assuming they copy atomically, but I probably still wouldn't do it, instead use the backup API or VACUUM INTO (if nothing else, because VACUUM INTO will also obviously VACUUM the copy).

If you're not attached/writing to it, then of course it's fine.

[1]: https://www.sqlite.org/howtocorrupt.html#_backup_or_restore_...