No space left results in process hanging

I've been using OBuilder with the runc and zfs backends and have bumped into this a few times. If ZFS runs out of space on the device whilst running some runc build step it seems to hang for me. If this happens during other parts of the build then as you would expect the error bubbles up to the top. For me, this reproduces the error:

truncate --size 2G zfs.img
sudo zpool create obuilder-test $PWD/zfs.img
dune exec -- ./main.exe build -f example.spec . --store=zfs:obuilder-test --verbosity=debug

It happens during the opam update and seems to just hang. If you ctrl+c the process and try again, it will tell you there's no space left on the device. I could also be doing something completely wrong too!

Start by reproducing the command in the issue with the runc and zfs backends, using the 2G zfs image and debug verbosity. Trace how the opam update build step handles a full ZFS device and compare it with the error path observed after Ctrl+C; done means the process reports the no-space condition instead of hanging.