[bug] Nomad panics when attempting to scale a job by a negative value

[jrasell]

Nomad version

Version: Nomad v0.11.2-dev
Commit: f5c84fca8e128be8ff609f3b6bfc0230a25d3adf

Operating system and Environment details

Reproduced on Mac and Linux amd64.

Issue

When attempting to scale a job task group by a negative value, the Nomad server panics.

Reproduction steps

• run Nomad locally via nomad agent -dev
• register the job as defined below nomad run <jobfile>
• attempt to scale using the payload below curl --request POST --data @payload.json http://localhost:4646/v1/job/example/scale
• observe Nomad panic

Job file

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

  group "cache" {

    count = 1

    task "redis" {
      driver = "docker"

      config {
        image = "redis:3.2"
      }
    }
  }
}

API Payload

{
  "Count": -1,
  "Target": {
    "Group": "cache"
  }
}

Nomad Logs

panic: bitmap must be positive size

goroutine 122 [running]:
github.com/hashicorp/nomad/scheduler.bitmapFrom(0xc001b89d40, 0xffffffffffffffff, 0x2, 0x2, 0xc001b89d40)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/reconcile_util.go:428 +0x25c
github.com/hashicorp/nomad/scheduler.newAllocNameIndex(...)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/reconcile_util.go:387
github.com/hashicorp/nomad/scheduler.(*allocReconciler).computeGroup(0xc00072b040, 0xc001396740, 0x5, 0xc001b89c20, 0xc001b89b90)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/reconcile.go:362 +0x339
github.com/hashicorp/nomad/scheduler.(*allocReconciler).Compute(0xc00072b040, 0xc001b894a0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/reconcile.go:207 +0x170
github.com/hashicorp/nomad/scheduler.(*GenericScheduler).computeJobAllocs(0xc00072a820, 0xc00000d2c0, 0xc0002693b0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/generic_sched.go:347 +0x337
github.com/hashicorp/nomad/scheduler.(*GenericScheduler).process(0xc00072a820, 0x0, 0x0, 0x0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/generic_sched.go:254 +0x35e
github.com/hashicorp/nomad/scheduler.retryMax(0x5, 0xc000bfbe00, 0xc000bfbdf0, 0x6, 0x6f650a0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/util.go:280 +0x56
github.com/hashicorp/nomad/scheduler.(*GenericScheduler).Process(0xc00072a820, 0xc0013f1800, 0x6f650a0, 0xc00095eff0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/scheduler/generic_sched.go:153 +0x28c
github.com/hashicorp/nomad/nomad.(*Worker).invokeScheduler(0xc0005152d0, 0xc001b89320, 0xc0013f1800, 0xc0010cfc80, 0x24, 0x0, 0x0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/nomad/worker.go:268 +0x421
github.com/hashicorp/nomad/nomad.(*Worker).run(0xc0005152d0)
/Users/jrasell/go/src/github.com/hashicorp/nomad/nomad/worker.go:129 +0x294
created by github.com/hashicorp/nomad/nomad.NewWorker
/Users/jrasell/go/src/github.com/hashicorp/nomad/nomad/worker.go:81 +0x152

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