Edit from @negz: I'd like to drop the resource.ClientApplicator. For *Unstructured resources we've mostly already replaced it with SSA. For concrete types, I've found a few places where it's been easy to just replace it with Get + Update. Using https://pkg.go.dev/sigs.k8s.io/[email protected]/pkg/controller/controllerutil#CreateOrUpdate would probably be fine too.

Original issue follows:

What happened?

Looking at the claim controller, updating the XR (cp in the code), the following race with other actors in the system (controllers, user with kubectl) exists:

1. reconciler gets latest XR state cp
2. reconciler updates in the cp whatever it wants to update
3. reconciler calls Apply with cp
   • applicator gets current state via client
   • applicator sets RV cp.RV = current.RV
   • applicator calls update with cp

Assuming between 1 and 3a some client updates the XR out of band, e.g. some other controller, or kubectl. 3c will wipe out that change, won't it?

Above is just one example for that issue. The apply func is widely used in Crossplane.

How can we reproduce it?

This will likely show up as subtle bugs as this is some kind of time travel, or controllers fighting for their desired state of the world and racing with their informers.

What environment did it happen in?

Crossplane version: main