The sync heavy lifting in Dropbox is handled by librsync, or at least was at one point. librsync is very mature open source software, and this bug pertains to a particular interaction between the Dropbox GUI and a feature (selective sync) which they have somewhat tacked on to the core library. Long story short, you don't necessarily have to be Dropbox or employ a couple hundred software engineers to get inotify + rsync working well.
> The sync heavy lifting in Dropbox is handled by librsync, or at least was at one point.
Nope, highly custom process that involves librsync very little. The sophistication of what has to be done to solve this problem well would probably surprise you.
He wasn't talking about rsync the binary but rather about librsync [0], the library that allows you to calculate a diff between two byte arrays without needing the cooperation of the other side -- ie you can calculate an efficient delta to send to remote without needing remote to be online at the moment you calculate, and without needing the complete old version, only a signature of it.
A good example use of it is the rdiff tool that allows you to do exactly what I said before. A better real-world example would be duplicity [1] or rdiff-backup [2] that use librsync to de-duplicate backups, without needing access to the whole previous value, only a small signature of it.
