* Start the webhook before informers sync.
Some webhooks (e.g. conversion) are required to list resources, so by delaying those until after informers have synced, we create a deadlock when they run in the same process. This change has two key parts:
1. Start the webhook immediately when our process starts, and issue a callback from sharedmain when the informers have synced.
2. Block `Admit` calls until informers have synced (all conversions are exempt), unless they have been designated by implementing `webhook.StatelessAdmissionController`.
Our built-in admission controllers (defaulting, validation, configmap validation) have all been marked as stateless, the main case where we want to block `Admit` calls is when we require the informer to have synchronized to populate indices for Bindings.
* Add missing err declaration
By combining our validation logic into our mutating webhook we were previously allowing for mutating webhooks evaluated after our own to modify our resources into invalid shapes. There are no guarantees around ordering of mutating webhooks (that I could find), so the only way to remedy this properly is to split apart the two into separate webhook configurations:
- `defaulting`: which runs during the mutating admission webhook phase
- `validation`: which runs during the validating admission webhook phase.
The diagram in [this post](https://kubernetes.io/blog/2019/03/21/a-guide-to-kubernetes-admission-controllers/) is very helpful in illustrating the flow of webhooks.
Fixes: https://github.com/knative/pkg/issues/847
Matt Moore
Matt Moore