Bug#515160: debian-installer: Absolutely a bug
This is definitely a bug, though not one to put on the partitioner.
The installer needs to check the total size of packages that are going
to be installed before it tries to start downloading and unpacking
them, and this will (unfortunately, in terms of complexity) need to be
done on a per-mount-point basis. To be really stringent about it, it
should make sure there's enough free space left after installation
that the system doesn't immediately fall flat on its face. For
example, if /var is on the root partition, and it's so close to full
at install time that there's no free space a few days later, and
services can't start (can't write PID file), the installer should be
able to warn about that.
Given that all the above would be quite invasive (and I don't know if
individual installed-file size data is even available presently), the
conservative answer is to have debian-installer respond correctly when
it (or whatever underlying processes are doing the work) encounters
ENOSPC: report that there wasn't enough space on whichever partition
is full to proceed with installation. This is disappointing from a UI
design standpoint, but it at least moves the burden of diagnosing an
installation failure from the user to the software, where it belongs.