Comment 30 for bug 1878225

I've managed to reproduce this issue by reproducing the network setup of autopkgtest.

All you have to do is setup your network to allow access only through an http proxy and drop all other traffic. Do that and you'll get:

stgraber@castiana:~$ autopkgtest-build-lxd ubuntu-daily:groovy
Creating autopkgtest-prepare-yzE
Starting autopkgtest-prepare-yzE
Created symlink /<email address hidden> → /dev/null.
Timed out waiting for container to boot
stgraber@castiana:~$

Note that the autopkgtest network drops traffic, it doesn't reject it, so you're not hitting normal connection failures that can be nicely handled. As a result snapd gets stuck in seeding.

Manually fixing the autopkgtest-build-lxd script to configure snapd's proxy in the container, gets you.

stgraber@castiana:~$ autopkgtest-build-lxd ubuntu-daily:groovy
Creating autopkgtest-prepare-ybH
Starting autopkgtest-prepare-ybH
Created symlink /<email address hidden> → /dev/null.
Container finished booting. Distribution Ubuntu, release groovy, architecture amd64
Running setup script /usr/share/autopkgtest/setup-commands/setup-testbed...
sh: Attempting to set up Debian/Ubuntu apt sources automatically
sh: Distribution appears to be Ubuntu
Get:1 http://us.archive.ubuntu.com/ubuntu groovy InRelease [267 kB]
Hit:2 http://us.archive.ubuntu.com/ubuntu groovy-updates InRelease
Hit:3 http://us.archive.ubuntu.com/ubuntu groovy-security InRelease
Get:4 http://us.archive.ubuntu.com/ubuntu groovy/main Sources [841 kB]
Get:5 http://us.archive.ubuntu.com/ubuntu groovy/multiverse Sources [177 kB]
Get:6 http://us.archive.ubuntu.com/ubuntu groovy/universe Sources [9906 kB]
Get:7 http://us.archive.ubuntu.com/ubuntu groovy/restricted Sources [6476 B]
Get:8 http://us.archive.ubuntu.com/ubuntu groovy/main amd64 Packages [975 kB]
Get:9 http://us.archive.ubuntu.com/ubuntu groovy/main amd64 c-n-f Metadata [29.5 kB]
Get:10 http://us.archive.ubuntu.com/ubuntu groovy/universe amd64 Packages [8734 kB]
Get:11 http://us.archive.ubuntu.com/ubuntu groovy/universe amd64 c-n-f Metadata [267 kB]
Get:12 http://us.archive.ubuntu.com/ubuntu groovy/multiverse amd64 Packages [154 kB]
Get:13 http://us.archive.ubuntu.com/ubuntu groovy/multiverse amd64 c-n-f Metadata [9320 B]
Get:14 http://us.archive.ubuntu.com/ubuntu groovy-updates/universe amd64 c-n-f Metadata [112 B]
Get:15 http://us.archive.ubuntu.com/ubuntu groovy-updates/multiverse amd64 c-n-f Metadata [116 B]
Get:16 http://us.archive.ubuntu.com/ubuntu groovy-security/universe amd64 c-n-f Metadata [116 B]
Get:17 http://us.archive.ubuntu.com/ubuntu groovy-security/multiverse amd64 c-n-f Metadata [116 B]
Fetched 21.4 MB in 4s (4957 kB/s)

Marking all tasks invalid and re-opening autopkgtest task as that's what's broken here.