ifupdown does not render viable /etc/network/interfaces on bionic ec2 cloud-images

Filing this bug for clarification and tracking on Ubuntu Bionic and later.

Canonical cloudimages for Ec2 and other clouds leave an artifact that disables /etc/network/interface with the following content:

# ifupdown has been replaced by netplan(5) on this system. See
# /etc/netplan for current configuration.
# To re-enable ifupdown on this system, you can run:
# sudo apt install ifupdown

In following the suggested text, installing the ifupdown deb continues to leave a broken system because ifupdown.postinst script creates an /etc/network/interfaces.d parts directory but does not write out a working /etc/network/interfaces file if the file exists on the system.

This leaves a broken /etc/network/interfaces files which doesn't source /etc/network/interfaces.d/* and 3rd parties cannot use the interfaces.d parts directory to extend networking config.

Cloud-init is one of those users of /etc/network/interfaces.d/ for rendering system networking on cloud images. Per LP: #1867029

I have attached ifupdwown-test.sh which exercises this issue on lxc by mocking the CPC cloudimage artifact.