volumes: return better error messages for unsupported task drivers

[tgross]

Fixes #7774 (partially #6664 as well)

When an allocation runs for a task driver that can't support volume mounts, the mounting will fail in a way that can be hard to understand. With host volumes this usually means failing silently, whereas with CSI the operator gets inscrutable internals exposed in the nomad alloc status.

This changeset adds a MountConfig field to the task driver Capabilities response. We validate this when the csi_hook or volume_hook fires and return a user-friendly error.

Three notes:

• We don't currently have a way to get driver capabilities up to the server, except through attributes. Validating this when the user initially submits the jobspec would be even better than what we're doing here (and could be useful for all our other capabilities), but that's out of scope for this PR.
• The MountConfig enum starts with "supports all" in order to support community plugins in a backwards compatible way, rather than cutting them off from volume mounting unexpectedly.
• I haven't included an update to the internals docs. I'm going to do that in a separate PR so I can include the missing networking fields as well without cluttering this PR.

[tgross]

Cherry-picked to 0.11.3 branch as 4b0fe71

[shantanugadgil]

hi, this error is still present in 0.12.0-beta1 with host volumes

job "host_volume_1" {
  datacenters = ["dc1"]

  type = "batch"

  group "mygroup" {
    volume "myworkspace" {
      type      = "host"
      source    = "workspace"
      read_only = true
    }

    task "mytask" {
      driver = "raw_exec"

      volume_mount {
        volume      = "myworkspace"
        destination = "/myworkspace"
      }

      template {
        data = <<EOH
#!/bin/bash

set -u

id -a

hostname

env | sort

sleep 30
EOH

        destination = "local/runme.bash"
      }

      config {
        command = "/bin/bash"
        args    = ["-x", "local/runme.bash"]
      }
    }
  }
}

[tgross]

hi, this error is still present in 0.12.0-beta1 with host volumes

Hi @shantanugadgil You should be getting an error because the raw_exec driver has MountConfigSupportNone. If you're not getting an error to that effect, can you open a new issue with the error message you're getting?

[shantanugadgil]

@tgross the issue is that plan and run do not complain about it, and silently accept it.

The error is seen in the status command, but I was hoping that the plan/run would not let this proceed.

[tgross]

Yes, that's a known limitation. See the top of the PR:

We don't currently have a way to get driver capabilities up to the server, except through attributes. Validating this when the user initially submits the jobspec would be even better than what we're doing here (and could be useful for all our other capabilities), but that's out of scope for this PR.

But if you want to open a new issue with that as a feature request, feel free!

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.