use xz files for stage0 download if on system with xz

[stefson]

I noticed that if ./x.py is run from a git checkout, the build script downloads the rust-std and rustc, and cargo stage0 archives from static.rust-lang.org in the tar.gz format. Those are a lot bigger in size than the also delivered tar.xz files, usually adds some 30% in download time and consumed bandwith. Do you think you could possibly change the script to use tar.xz in the future?

here's a snipet from my log:

downloading https://static.rust-lang.org/dist/2019-09-26/rust-std-1.38.0-x86_64-unknown-linux-gnu.tar.gz.sha256
running: curl -# -y 30 -Y 10 --connect-timeout 30 --retry 3 -Sf -o /var/tmp/portage/dev-lang/rust-999/temp/tmprecglk92.sha256 https://static.rust-lang.org/dist/2019-09-26/rust-std-1.38.0-x86_64-unknown-linux-gnu.tar.gz.sha256
################################################################################### 100,0%
downloading https://static.rust-lang.org/dist/2019-09-26/rust-std-1.38.0-x86_64-unknown-linux-gnu.tar.gz
running: curl -# -y 30 -Y 10 --connect-timeout 30 --retry 3 -Sf -o /var/tmp/portage/dev-lang/rust-999/temp/tmp_2fjvg1r https://static.rust-lang.org/dist/2019-09-26/rust-std-1.38.0-x86_64-unknown-linux-gnu.tar.gz
################################################################################### 100,0%
verifying /var/tmp/portage/dev-lang/rust-999/temp/tmp_2fjvg1r

[guanqun]

I did a quick check and the boostrap script uses tarfile module to do the unpacking, and as our build system need to support at least python 2.7 which unfortunately doesn't contain the support of xz out of box. see the reference here: https://docs.python.org/2/library/tarfile.html
however, when we transition to python 3, then this tarfile supports xz: https://docs.python.org/3/library/tarfile.html

[guanqun]

I created a PR to use .tar.xz if x.py is called with python3. I tried locally with msvc and it works fine.

[stefson]

does this work depending on wether it's either

python2 ./x.py build for using tar.gz

or python3 ./x.py build for using tar.xz then?

[guanqun]

that's right. because python 2 doesn't have xz support in its tarball library, see the link above. we could import other libraries but that would require extra dependencies.