Add warning to plan when duplicate job names are detected

[kiwakiri]

Problem Description:

Running nomad-pack run for a single service with a single variables file, the command stopped all other jobs that were previously deployed under the same pack deployment name, without any warning during nomad-pack plan.

Expected behavior:

nomad-pack plan should detect that there are existing jobs with the same name and issue a warning that the jobs will be stopped as part of plan execution.

Actual behavior:

nomad-pack plan does not indicate that any existing jobs will be stopped.
All jobs previously deployed under the same pack deployment name are stopped during run with the warning:
warning: Job '<job name>' is no longer defined in pack deployment '<deployment name>' - stopping

Environment:

nomad-pack version: v0.4.2 (cd4d2a4)
Nomad version: 1.9.6+ent

[dmoss18]

@kiwakiri I'm a little confused how this happened. We use the same pack to deploy different microservices as nomad jobs, each with their own variables file. We've been running this way in prod for a year now with great success.

In this incident, the only thing we changed in the pack template was adding a couple identity blocks to a task

identity {
        name        = "vault_default"
        aud         = ["vault.io"]
        ttl         = "1h"
        change_mode = "restart"
      }

      identity {
        name        = "consul_default"
        aud         = ["consul.io"]
        ttl         = "1h"
        change_mode = "restart"
      }

Then running nomad-pack run ... killed every other job we had that was using that pack. Can you clarify why this change caused the cascading job failures? We've changed the pack templates before and haven't encountered this. Is there something about identity that changes the job spec materially enough that nomad-pack sees it as a different pack?