The current process for upgrading FreeBSD is roughly, assuming that you're just using packages and not ports is:
* freebsd-update upgrade
* freebsd-update install
* reboot
* freebsd-update install
* pkg upgrade
* reboot
* freebsd-update install
* (optional) reboot
In comparison, the process for upgrading OpenSuse Tumbleweed, assuming that you're just using packaged software, is:
* zypper dup
* reboot
If the base system were managed by pkg, the FreeBSD version of the process should probably look a lot like the OpenSuse Tumbleweed version. Condensing upgrades down to a two-step process seems like it will be a significant quality-of-life improvement to me.
At least, I have heard this explanation in the context of OpenBSD, and assume it may also apply to FreeBSD.
To solve this, the updater could create a new boot environment[0], install everything in that boot environment using as many passes as required, then reboot into that environment. OpenSuse already does a similar thing[1], though I don't know how well it works.
[0] https://www.freebsd.org/cgi/man.cgi?beadm
[1] https://news.opensuse.org/2018/05/15/transactional-updates-i...
Any time a new syscall is wrapped the userland maintainers need to account for what happens when run on an old kernel and typically have some fallback logic.
At this point, I've only got two services (postgresql and mosquitto) which run on bare metal, everything else is in VMs. Once I move those off of FreeBSD the upgrade process will be greatly simplified (well, as simplified as FreeBSD gets anyway).
For base updates, we'd usually shut down services, install kernel and userland, and reboot --- I know this isn't the proper order, but most of the time the new userland is compatible enough to reboot (and you can usually power cycle if that doesn't work out). For updates where rebooting would be hard this way, we'd do it the right way, but then you need to add an extra reboot after installing userland, or restart all the services from base. If needed, we'd install the compat packages after the reboot. We would not update ports/pkg/etc at the same time; old binaries are expected to continue to work on a new system (and generally do). In house binaries are either compiled on the system they run, or centrally on a machine (or chroot/jail) running the oldest release on the fleet.
Generally,
When you need to upgrade software not from base, because of security updates or new features or ennui, you do that isolated from base changes. The only complication here is if you haven't updated base recently enough, pkg may provide binaries you can't run, and ports may provide software you can't compile (this one takes longer).
We would generally only update base for exceptional releases, or when it became hard to update non-base software on older releases. However, new machine installs would get the most recent release we had tested.