Prevent OS auto updates to deprecated versions

[marwinski]

How to categorize this issue?

/area os
/kind bug

What happened:

An automated OS update was triggered from a version 1.3 classified as supported to a 2.1 version classified as deprecated with a very short expiration time.

What you expected to happen:

OS image versions shall never be updated from a version classified as "supported" to a version classified as "deprecated".

How to reproduce it (as minimally and precisely as possible):

Assume you have the following setup:

2.1 preview
2.0 preview
1.3 supported

Now add a 2.2 version and deprecate the 2.0 and 2.1 versions.

2.2 preview
2.1 deprecated (with expiration date in 2 weeks time)
2.0 deprecated (with expiration date in 2 weeks time)
1.3 supported

As a result, 1.3 (supported) will be updated to 2.1 (deprecated) when automatic OS updated are enabled for the shoot cluster.

Anything else we need to know?:

While this behaviour appears awkward there is room for interpretation. According to https://github.com/gardener/gardener/blob/master/docs/usage/shoot_versions.md this may well be the desired behaviour. In this case I would consider it a design issue. There is a reason why an image is set to deprecated and (automatic updates) shall never update to a deprecated version.

This said, there is one exception: if the current version is about to expire it would be ok to update it to a newer deprecated version according to the update strategy.

Environment:

• Gardener version: any up-to-date version
• Kubernetes version (use kubectl version): n/a
• Cloud provider or hardware configuration: n/a
• Others: