That is conflict resolution, over the set of files. Which is why Unison provides the "reconciling changes" UI.
> Which implies you need to choose one node from {A,B,C} that is the most likely to be available to sync the other two to.
Yes, that is a way to avoid update conflicts.
> IMO a topological sort would be an even stricter requirement than spanning tree, in that if one node becomes available, you can't sync anything "below" it!
Yes, it is also a way to avoid update conflicts.
> (eg I use unison for maildir).
Why? IMAP already takes care of synchronization, without any potential problems.
> Why? IMAP already takes care of synchronization, without any potential problems.
Less configuration, less attack surface. Sharing mail over NFS is a common thing, and I view unison as a type of distributed filesystem.
Create file F on A. Sync A-B, A-C. Delete F on A. Sync A-B. ?????. Sync B-C. Sync A-B. File F now re-exists on A (and B and C).
??? is some event where you cannot sync A-C to directly save A's changes on C, yet you still want to save any changes from B on C. Say a remote machine crashes and becomes unavailable, you didn't have the time over 3G, or some other type of ad-hoc craziness which is why you're using distributed syncing in the first place.
Which implies you need to choose one node from {A,B,C} that is the most likely to be available to sync the other two to. That node can also provide connectivity to a larger network, which generalizes to a spanning tree.
IMO a topological sort would be an even stricter requirement than spanning tree, in that if one node becomes available, you can't sync anything "below" it! Also what does syncing disks "in order" mean when changes can happen at any time? (eg I use unison for maildir).