Happened to me a few months ago (whenever that big SSL 0-day was announced).

I use a 3rd party repo for ZFS drivers, which gets checked for compatibility with each new kernel release by the maintainers, so ZFS frequently stops me from upgrading, and crucially it stops me after I've already fetched the new package databases.

Running pacman -Sy and then installing an individual package isn't supported and it's understandable why.

Running pacman -Syu and having it break sticks you into this limbo where if you install anything before finishing the upgrade, you risk shooting yourself in the foot.

Interesting! How does the ZFS upgrade "stop" the package upgrade process, I wonder? Might be worth reporting a bug, maybe pacman can handle that kind of failure better, or maybe the ZFS package could be changed to fail more gracefully. I think the pacman devs would agree leaving the system in a partial-upgrade state is a bad thing to do.

It's not a bug, it's intended behavior.

The official release of ZFS on Linux I believe only supports kernel 5.x. Since arch is always on the cutting edge, the repo maintainers need to manually test the driver with each new kernel release before pushing it out to the world. They stop you from borking your system in the interim by having a strict version requirement for Linux on the zfs package. Pacman only does that check, though, once the -Sy operation has finished

Ohhh, I see what you're saying. Yeah, that is a tricky corner case :-/