Is fast-forward correct?

[aboodman]

It doesn't look like it to me.

It's only checking whether the incoming commit is taller than the existing commit. It doesn't check the ancestry at all. So committing x<-y<-z on top of a<-b would succeed. Worse, the ref height takes into account the entire chunk hierarchy including values, not just the commits. So committing something like x on top of a<-b<-c would also work if x contains a chunk hierarchy that is bigger than c's.

https://github.com/attic-labs/noms/blob/master/go/datas/database_common.go#L121

	if ok && newHeadRef.Height() <= currentHeadRef.Height() {
		return ErrMergeNeeded
	}

In order to implement fast-forward correctly, we must know all commits refs that are reachable from a given commit.