fetch-snaps --channel parameter does not support track/channel syntax for "latest/*"

[Kyler Hornor]

Expectation:

While declaring --channel=latest/stable is redundant, declaring the latest track should still fetch the content as expected. [potentially] If no revisions are downloaded, an error should indicate this to the user so the empty archive isn't pushed to the offline environment.

Behavior:

Declaring the latest track in a fetch-snaps run instead fetches a nearly empty archive, only containing the channel-map.json and the meta.json file. This can be confusing to end-users, as `snap info $name` prepends the latest track to the channel names.

Example:

Below we use jq as a test snap. Note that it doesn't grab any revisions in the test where the latest track is declared.

ubuntu@focal:~$ sudo snap-store-proxy fetch-snaps jq --channel=latest/stable --arch=amd64
Fetching channel map info for jq
Downloaded jq to /var/snap/snap-store-proxy/common/downloads/jq-20220614T144528.tar.gz

ubuntu@focal:~$ sudo snap-store-proxy fetch-snaps jq --channel=stable --arch=amd64
Fetching channel map info for jq
Fetching revision 6 for jq
Downloaded jq to /var/snap/snap-store-proxy/common/downloads/jq-20220614T144534.tar.gz

[Kyler Hornor (kylerhornor)]

Its worth noting that this is only an issue with the latest track. `snap-store-proxy fetch-snaps powershell --arch=amd64 --channel=lts/stable` works as expected. The issue isn't with using track/channel as a whole, but rather solely with directly declaring the latest track.

[Kyler Hornor (kylerhornor)]

I think I see the `issue`. get_fetch_urls() in fetch.py tries to build the fetch using the channel name passed down from cli.py. The channel-map.json does not prepend `latest/` to the names of the default tracks, so the raw match here fails. Could just add a simple check/trim if that is appropriate.

This seems to be accommodated for in snapd (or maybe it doesn't use the channel map at all?), but not in snap-store-proxy.

[Przemysław Suliga (suligap)]

Confirmed. Thanks for the report! We also have an internal bug for tracking this issue.

Note that for fetching snaps for import to an offline store the store-admin snap can be used as well (and it does not have this issue), eg.:

    store-admin export snaps jq bare --channel latest/stable --channel edge --arch amd64 --arch arm64