MAAS "Images" page seems to imply that i386/20.04 is available, but it's not

Up until 20.04 every version of Ubuntu was supported on every architecture. The UI was built upon this assumption. As such the websocket currently only sends the list of available Ubuntu versions. Architectures are only sent once the image has been downloaded.

One of the things I noticed when I reproduced this is while the API gives an error when you try to download 20.04 i386 the websocket silently ignores i386 missing. This is a UI bug but will require some backend work as well.

@design - What do you need from the backend to fix this? Should MAAS include the architectures for each image a user can select to download? Should MAAS send an error when an architecture is unavailable? Both?

@ltrager I think both would be the safest option, but definitely the former.

I've split the backend work into LP:1879827 and reassigning this to Caleb for design to complete the frontend work. I was only able to add an "unsupported_arches" field to the Ubuntu releases list. The websocket always creates a BootSourceSelection even if the BootResource doesn't exit. Raising any error is viewed by logging and the front end as a larger failure.

The boot images page has received very few updates since it was first written. Both the front end and back end assume every Ubuntu release supports every architecture. On the back end there are a lot of inefficiencies that causes a lot more database queries then are really needed. I think both the front and back end need a rewrite at this point.

Also, as reported in bug #1880697 (which is marked as a duplicate of this one), MAAS seems to be putting error messages on its web UI related to its inability to download the 32-bit images. I'm attaching the screen shot from that bug report to this one. (Note the "Failed to import images..." notice at the top of the page.)

@ltrager My plan was to implement radio buttons for each selected release where you would then select architectures on a per-release basis. At the moment the bootresource.save_ubuntu websocket handler doesn't allow this - you can only pass it a list of releases and a list of architectures. How much work would it be to update it? If it's significant I can just implement what was suggested in the original bug report (a message saying "Combination unavailable") but we would also need to make sure that error messages don't show up too.

@caleb - See the attached branch. Does that work for you?

@ltrager Seems to work fine on my end. I've got a PR ready that just needs to be reviewed which allows you to select architectures on a per-release basis. It turned out to be a lot trickier than I originally thought - hoping we don't have to touch the images page again until we start rewriting the whole thing.

https://github.com/canonical-web-and-design/maas-ui/pull/1231