[Experimental] Spec non-compliance: `spec.subscriber` MUST be set

[evankanderson]

Describe the bug
As part of knative/specs#30 to simplify the user experience of Subscriptions, the spec was changed from requiring one of spec.subscriber or spec.reply to be set to requiring that spec.subscriber is set.

Expected behavior
An admission validation error if a Subscription is created without a subscriber.

To Reproduce
Create a Subscription like so:

apiVersion: messaging.knative.dev/v1
kind: Subscription
metadata:
  name: i-am-a-bad-bad-subscription
spec:
  channel:
    kind: Channel
    apiVersion: messaging.knative.dev/v1
    name: dont-care
  reply:
    uri: http://dev/null

Knative release version

0.26

Additional context
Should probably get this in for 1.0

[salaboy]

@evankanderson is this a blocker for GA?
Can you assign it to me? I can probably work on this one

[evankanderson]

I think it should be a blocker for GA (possibly with a flag to disable the validation until Oct 2022 because this is a v1 resource).

You can assign an issue to yourself with /assign (fyi)

/assign @salaboy

[lionelvillard]

I agree, this is a blocker for GA. @salaboy Beware this is potentially a breaking change, as @evankanderson hinted at it with adding a flag.

[salaboy]

@evankanderson @lionelvillard from the change in the spec I am assuming that spec.reply shouldn't be validated anymore. In other words, a missing reply will not cause validation to fail. Is this a correct assumption?

[lionelvillard]

Reopening since we decided to implement this as an experimental feature and there are followup tasks:

• Alpha: feature disabled by default. At this stage, Knative eventing is conformant when the flag is enabled. (0.27)
• Beta: feature is enabled by default, can be disabled. At this stage, Knative eventing is conformant by default (0.28)
• GA. feature cannot be disabled. (0.29)

[lionelvillard]

@salaboy do you have some cycle to promote this feature to Beta?