Configuring temp directory for virt-builder

I was playing around with virt-builder to speed up configuration for some VMs. I was building images on my laptop, which has a /var partition that's only about 9 GiB, with only a couple gigabytes free. By default on my machine, virt-builder uses a directory under /var/tmp to extract VM images. Maybe you can see where this is going--the extracted image wouldn't fit in my /var partition and virt-builder failed.

I couldn't tell that a full partition was the problem at first. I enabled an environment variable to see debug messages: export LIBGUESTFS_DEBUG=1. This told me a command called "supermin" was failing. It was writing its output in /var/tmp, and I saw that it was running out of disk space.

I wanted to change the cache directory, but I couldn't find a command-line flag or environment variable in the manpage for configuring it. I took a look at the code instead, and I found my way to the build_supermin_appliance function by searching for a log statement "run supermin".

From there I found a variable int_cachedir. Thankfully a docstring in the code says I can change it by setting the environment variable LIBGUESTFS_CACHEDIR. I set it to $HOME/.cache/virt-builder and I was able to build the image successfully!