Comment 2 for bug 73752

I think we could also have the client enforce this in a general sense.

It wouldn't be an absolute enforcement, since with plain filesystem commands, you don't really have the ability to force this sort of thing. But it wouldn't be hard to have the remote .bzr/branch/branch.conf contain "preserve_mainline=True"

And have the standard client refuse to push revisions which would change the existing revision history of the target branch.

We would need a branch format bump, to ensure that older clients won't try to update a branch with that flag set. But I don't see it requiring a smart server.

The smart server is just another layer that can be even more strict about enforcing it.