Improve debootstrap time a bit, without local mirror

I've introduced two features to improve debootstrap time, auto proxy detection via squid-deb-proxy-client (by Michael Vogt) and cache directory support. It reduces time to create chroot environment without huge local mirror.

Let's create chroot environment without any new features.

$ time sudo debootstrap sid sid-chroot
I: Target architecture can be executed     
I: Retrieving InRelease                     
I: Checking Release signature           
I: Valid Release signature (key id 126C0D24BD8A2942CC7DF8AC7638D0442B90D010)
I: Retrieving Packages             
I: Validating Packages             
(snip)
I: Base system installed successfully. 

real    8m27.624s
user    1m52.732s
sys     0m10.786s

Then, use --cache-dir option.

$ time sudo debootstrap --cache-dir=/home/henrich/tmp/cache sid sid-chroot
E: /home/henrich/tmp/cache: No such directory

Yes, we should cache directory first.

$ mkdir ~/tmp/cache

Let's go.

$ time sudo debootstrap --cache-dir=/home/henrich/tmp/cache sid sid-chroot
I: Target architecture can be executed
I: Retrieving InRelease             
I: Checking Release signature         
I: Valid Release signature (key id 126C0D24BD8A2942CC7DF8AC7638D0442B90D010)
I: Retrieving Packages                 
I: Validating Packages                   
(snip)
I: Base system installed successfully. 

real    2m10.180s
user    1m47.428s
sys     0m8.196s

It cuts about 6 minutes! (of course, it depends on the mirror you choose). Then, try to use proxy feature.

$ sudo apt install squid-deb-proxy-client
(snip)
$ time sudo debootstrap sid sid-chroot
Using auto-detected proxy: http://192.168.10.13:8000/
I: Target architecture can be executed     
I: Retrieving InRelease                     
I: Checking Release signature           
I: Valid Release signature (key id 126C0D24BD8A2942CC7DF8AC7638D0442B90D010)
I: Retrieving Packages             
I: Validating Packages             
(snip)
I: Configuring systemd...
I: Base system installed successfully.

Can you see the words "Using auto-detected proxy: http://192.168.10.13:8000/"? It detects package proxy and use it. And its result is

real    2m15.995s
user    1m49.737s
sys     0m8.778s

Conclusion: If you already run squid-deb-proxy on some machine in local network, then install squid-deb-proxy-client and debootstrap automatically use it, or you can use --cache-dir option for speed up creating chroot environment via debootstrap. Especiall if you don't have good network conectivity, both features will help without effort.

Oh, and one more thing... Thomas Lange has proposed patches to improve debootstrap and it makes debootstrap much faster. If you're interested, please look into it.