CSI: don't overwrite context with empty value from request

[tgross]

When a volume is updated, we merge the new definition to the old. But the volume's context comes from the plugin and is likely not present in the user's volume specification. Which means that if the user re-submits the volume specification to make an adjustment to the volume, we wipe out the context field which might be required for subsequent operations by the CSI plugin. This was discovered to be a problem with the Terraform provider and fixed there, but it's also a problem for users of the volume create and volume register commands.

Update the merge so that we only overwrite the value of the context if it's been explictly set by the user. We still need to support user-driven updates to context for the volume register workflow.

Ref: hashicorp/terraform-provider-nomad#503
Fixes: democratic-csi/democratic-csi#438

Testing & Reproduction steps

Using the Democratic CSI plugin.

Before

$ nomad volume create ./volume.hcl
Created external volume csi-volume-nfs with ID csi-volume-nfs

$ nomad operator api "/v1/volume/csi/csi-volume-nfs?namespace=prod" | jq .Context
{
  "provisioner_driver": "nfs-client",
  "node_attach_driver": "nfs",
  "server": "192.168.1.170",
  "share": "/srv/nfs_data/v/csi-volume-nfs"
}

$ nomad volume create ./volume.hcl
Created external volume csi-volume-nfs with ID csi-volume-nfs

$ nomad operator api "/v1/volume/csi/csi-volume-nfs?namespace=prod" | jq .Context
{}

After

$ nomad volume create ./volume.hcl
Created external volume csi-volume-nfs with ID csi-volume-nfs

$ nomad operator api "/v1/volume/csi/csi-volume-nfs?namespace=prod" | jq .Context
{
  "server": "192.168.1.170",
  "share": "/srv/nfs_data/v/csi-volume-nfs",
  "provisioner_driver": "nfs-client",
  "node_attach_driver": "nfs"
}

$ nomad volume create ./volume.hcl
Created external volume csi-volume-nfs with ID csi-volume-nfs

$ nomad operator api "/v1/volume/csi/csi-volume-nfs?namespace=prod" | jq .Context
{
  "provisioner_driver": "nfs-client",
  "node_attach_driver": "nfs",
  "server": "192.168.1.170",
  "share": "/srv/nfs_data/v/csi-volume-nfs"
}

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[gulducat]

lgtm! I have one imaginary edge case to consider.

[gulducat]

this makes good sense for create, where the controller plugin generates the context dynamically, but I wonder about this edge case:

what if a user registers a volume with some context, but later wants to remove the context entirely? is that even a plausible concern? I suppose one could deregister the volume then re-register it with no context, since the actual volume out in the world would be unaffected.

[tgross]

The only time that would happen is if they updated the plugin and the new version of the plugin requires no context. In that case, you're almost certainly better off deregistering and re-registering