Do not fail on non RFC3339 labels but log warning

[hiddeco]

Some image vendors have not read the spec properly and pass along
a timestamp in a different format than RFC3339. Before this change
the unmarshal of the JSON would fail for those labels and the affected
images would be excluded from our cache.

We now include those images in our cache and collect all labels that
failed parsing in a custom error, which we detect and log (instead
of bailing out).

Addresses #2075, but in a stricter way.

[2opremio]

Minor comment, otherwise LGTM

[2opremio]

Some image vendors have not read the spec properly and pass along a timestamp in a different format than RFC3339

Do we know what wrong formats they typically use? As @squaremo mentioned in Slack, we could maybe try to accommodate those other formats as well, by trying to parse them as a fallback.

[hiddeco]

@2opremio CentOS images use a custom yyyymmdd format (a datestamp basically, if that would be a thing). I do not know what other images use the labels with a wrong format.

I am personally opposed to support anything different than what the spec defines, as it would stimulate people not following rules in a way, and the specs we support clearly state:

This label contains the Date/Time the image was built. The value SHOULD be formatted according to RFC 3339.

I also do not think this will be an issue for any user in case they do not have (direct) control over what is defined in the labels, as we safely fallback to the timestamp the registry gives us.

[2opremio]

@2opremio CentOS images use a custom yyyymmdd format (a datestamp basically, if that would be a thing). I do not know what other images use the labels with a wrong format.

I am personally opposed to support anything different than what the spec defines, as it would stimulate people not following rules in a way, and the specs we support clearly state:

Makes sense. It may be worth reporting it to (or even creating a PR against) https://github.com/CentOS/CentOS-Dockerfiles

[2opremio]

Actually, this is the repo https://github.com/CentOS/sig-cloud-instance-images

[hiddeco]

It has been mentioned in an issue there about a year ago, just did not find the time yet to submit a PR.

Some other reasons that came to mind about not supporting different timestamps:

• we prioritize labels over the date from the registry, we can only do this if the given timestamp is more reliable, which is something we can not assume when someone does not follow the spec
• it adds complexity to our codebase to support something that’s essentially wrong

[2opremio]

Here's the culprit https://github.com/CentOS/sig-cloud-instance-build/blob/master/docker/containerbuild.sh#L74

[2opremio]

Upstream PR CentOS/sig-cloud-instance-build#151

[2opremio]

LGTM