Meta `Channel` doesn't conform the spec's CRD spec.subscribers requirement

[aliok]

Describe the bug
As noted in #3050 (comment), Channel doesn't doesn't allow subscribers in the spec.

Spec says this:

Spec Requirements
v1alpha1 Spec: each channel CRD MUST contain an array of subscribers: spec.subscribable.subscribers
v1beta1 Spec: each channel CRD MUST contain an array of subscribers: spec.subscribers

When a channel.messaging.knative.dev is created, by default Knative creates an InMemoryChannel. When we create a subscription against that Channel, InMemoryChannel will have the subscribers in the status, but it is not propagated to channel.messaging.knative.dev. Other fields like status.address are propagated fine.

Expected behavior
I should be able to create a channel.messaging.knative.dev with spec.subscribable.subscribers and #3050 should pass.

To Reproduce

cat <<EOS |kubectl apply -f -
---
apiVersion: messaging.knative.dev/v1alpha1
kind: Channel
metadata:
  name: foo
spec:
  subscribable:
    subscribers:
    - UID: "1234"
      ReplyUri: "foo.bar"
EOS

ends up in

Error from server (BadRequest): error when creating "channel_v1alpha1.yaml": admission webhook "validation.webhook.eventing.knative.dev" denied the request: validation failed: must not set the field(s): spec.subscribable.subscribers

Knative release version
0.14

Additional context
#3050

[vaikas]

So, not allowing the setting of subscribers is definitely intentional. Without this it's impossible to have a source of truth that's reliable. If we allow modifications to both channel as well as the backing channel, hilarity will ensue in trying to understand what the actual intent is.

Not propagating the status seems like a bug as they should be propagated to the Channel (from the backing channel, one way sync only so there's no problem like we have with two sources of modifiable truth).

I'd suggest we tackle the documentation for the Channel either as "non-conformant" Channel or talk about more in the context of a convenience factory method.
@grantr
@lionelvillard

[aliok]

So, for this issue, is it enough to change the spec statement

each channel CRD MUST contain an array of subscribers: spec.subscribers

to

other than channels.messaging.knative.dev, each channel CRD MUST contain an array of subscribers: spec.subscribers

?

[matzew]

Right, setting the subscribers is not for the spec, I added a comment here: #3050 (comment)

[matzew]

I'd suggest we tackle the documentation for the Channel either as "non-conformant" Channel or talk about more in the context of a convenience factory method.

@vaikas I think I would lean towards the later ?

[vaikas]

That was my knee jerk reaction as well.

[lberk]

ok I'll try and tackle this as part of reviewing's @aliok's #3016
/unassign @aliok
/assign @lberk