make `hook` to accept array of values

[EugenKon]

Proposal

https://developer.hashicorp.com/nomad/docs/job-specification/lifecycle#lifecycle-parameters
Allow hook to accept array

Use-cases

We have cleanup task. The definition is the exactly the same for prestart and poststop hooks. It would be nice to configure them as:

 task "cleanup-smthg" {
    lifecycle {
      hook = ["prestart", "poststop" ]
      sidecar = false
    }
   ...
  }

Attempted Solutions

Copy/paste hook and change its type: prestart -> poststop

[tgross]

Hi @EugenKon! This seems like a reasonable feature idea, but it's probably trickier to implement than you might think. To do this, Nomad would internally have two copies of the task. They can't have the same name, because a great many features need the name of the task to differentiate between them (ex. nomad alloc exec -task $taskname $allocid), we'd need to mangle the names in some predictable way so that the user can also differentiate them.

There's also some backwards compatibility issues in the parser here, as switching the hook field's type is a breaking change. We could hand-parse it to allow both, but that adds more hand-parsing into the jobspec parser, which we're trying to get away from. There's also the awkward detail you're showing here, where sidecar = true would be meaningless for poststop tasks but meaningful for prestart tasks.

Nevertheless, I'll mark this for further discussion and possible future roadmapping.