I think as this is tracker that this is just a wishlist bug. I certainly shouldn't
do it if there is not enough space, and the only downside is not indexing the file.
If it needs somewhere temporary to uncompress it or similar then /tmp
(or $TMPDIR if defined) is exactly the right place to do it. If there is not enough
space then there is little that can be done.
The other alternative would presumably not require temporary space to work,
but that's still a wishlist bug I think.