Comment 8 for bug 1657440

[Impact]
An apt update of an apt repository that does not use InRelease during the time it is being updated can cause the gpg file to not be downloaded and updated. This makes the packages from the repository be unable to be authenticated.

The Ubuntu Cloud Archive is one of the archives that meets this criteria.

The impact to downstream automation deployment code is that if they are adding the UCA repo to a system and calling apt update during the time the UCA is being updated by Canonical, the repo can get into a state where the Release.gpg file is not there and all package installs will fail due to "unauthenticated packages" error.

[Test Case]
A detailed python script was attached.

To reproduce this outside that script you would want to:
1. Add the UCA repo
2. Do the following in a loop starting at 43 minutes after the hour and run it until 55 minutes after the hour:
2.1 Remove these files to simulate the UCA repo being added the first time.
/var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release
/var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_Release.gpg
/var/lib/apt/lists/ubuntu-cloud.archive.canonical.com_ubuntu_dists_xenial-updates_newton_main_binary*Packages

2.2 apt-get update
3. Check the state of the 3 files you deleted. If you have the _Release file but not the _Release.gpg you have recreated the issue.
4. If you have not recreated the issue, continue GOTO 2 and continue to loop.

[Regression Potential]
Unknown