Don't process PVCs for in-tree volumes

[jsafrane]

If I understand code in ProvisionExt, PVCs that require migrated in-tree provisioner in their StorageClass have their volume.beta.kubernetes.io/storage-provisioner annotation set to CSI driver name.

Therefore we do not need the provisioner library to listen for PVCs with in-tree provisioner at all and save some (little) CPU in unrelated PVC processing. I keep the check in ProvisionExt, just in case.

@ddebroy, PTAL carefuly.

/kind cleanup

Does this PR introduce a user-facing change?:

NONE

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[msau42]

/assign @ddebroy @davidz627
Will this work or does lib still need the option to check storageclass.provisioner?

[ddebroy]

The additionalProvisionerNames is still necessary to keep this check at https://github.com/kubernetes-sigs/sig-storage-lib-external-provisioner/blob/master/controller/controller.go#L1234 happy for migration scenarios. Note that check in lib-external-provisioner depends on the provisioner set in storage class rather than annotations.

Without populating additionalProvisionerNames, we end up with the following error message for migration scenarios.

controller.go:1235] provision "default/podpvc" class "standard": unknown provisioner 
"kubernetes.io/gce-pd" requested in claim's StorageClass

We discussed this a bit at #341 (comment)

I agree with the spirit of the PR. Maybe we can refactor https://github.com/kubernetes-sigs/sig-storage-lib-external-provisioner/blob/master/controller/controller.go#L1234 to allow for this?

[jsafrane]

I see. I reworked the PR to still pass additional provisioners to the lib, but rejecting PVCs in ShouldProvision. PTAL.

[ddebroy]

Only one nit: how about

var _ controller.Qualifier = &csiProvisioner{}

here. Otherwise lgtm.

[jsafrane]

var _ controller.Qualifier = &csiProvisioner{}

Good idea, added.

[ddebroy]

/lgtm