nouuid mount option for XFS volumes lost when mountOptions are defined in StorageClass

[tongpu]

Describe the bug
In our environment we've run into the issue reported in #514, but we're already running v22.01.1, which should contain the fix.

After further investigation we believe that this is caused, because we've configured the discard mount option in our StorageClass, which seems to cause the nouuid option to not be added anymore.

Environment
Provide accurate information about the environment to help us reproduce the issue.

• Trident version: 22.01.1
• Trident installation flags used: -
• Container runtime: CRI-O 1.23.3
• Kubernetes version: 1.23.5
• Kubernetes orchestrator: OpenShift 4.10.35
• Kubernetes enabled feature gates: -
• OS: Red Hat Enterprise Linux CoreOS 410.84
• NetApp backend types: ONTAP

To Reproduce

• Create a StorageClass using ontap-san (or ontap-san-economy)
• Configure mountOptions: ["discard"] in the StorageClass
• Create a PV and attach it to a pod

Expected behavior

• Mount options for the PV should contain nouuid and discard

Observed behavior

• Mount options only contains discard

[clintonk]

Hello, @tongpu. This is expected, as the set of mount options specified in the storage class (if present) fully replace the set of mount options configured in a Trident backend. Mount options specified in both places could easily conflict, and given the wide array of available mount options, attempting to cleanly resolve such conflicts doesn't sound feasible.

https://github.com/NetApp-openstack-dev/trident/blob/master/frontend/csi/controller_server.go#L365

[tongpu]

@clintonk Quick follow-up to this. I've now added both nouuid and discard to the mountOptions in the StorageClass, but when I create a PVC from a VolumeSnapshot the discard option somehow gets dropped. Is this expected as well?