scheduler: prevent -Inf in job anti affinity scoring

[nvanthao]

Similar to #17198, we are seeing

job-anti-affinity": -Inf,

I am yet able to replicate the issue. My understanding is that with desiredCount of 0, there should be no allocation placement, hence no ranking/feasibility code should run.

nomad/scheduler/generic_sched.go

Lines 454 to 469 in 13ee343

	return nil
	}
	// Compute the placements
	place := make([]placementResult, 0, len(results.place))
	for _, p := range results.place {
	s.queuedAllocs[p.taskGroup.Name] += 1
	place = append(place, p)
	}
	destructive := make([]placementResult, 0, len(results.destructiveUpdate))
	for _, p := range results.destructiveUpdate {
	s.queuedAllocs[p.placeTaskGroup.Name] += 1
	destructive = append(destructive, p)
	}
	return s.computePlacements(destructive, place)

That being said, it is happening and I believe the fix is straightforward.

I'll update the PR with more unit tests if possible once we understood the root cause.

[tgross]

@nvanthao can you confirm which version of Nomad you're using? You had [NOMAD-401] in the title here for some reason as well. If that's a Jira reference, well we don't use Jira in Nomad Eng so if you can do me (and our community members) a favor and put the details of whatever the problem is here in the PR or a new issue, that'd be swell.

[nvanthao]

Thanks @tgross,

Nomad version is 1.3.8 and this is the recent job history

Version     = 76
Stable      = false
Submit Date = 2023-07-21T13:40:00Z

Diff        =
+/- Job: "<job-id>"
+/- Task Group: "<tg>"
  +/- Count: "3" => "0"
      Task: "<task>"
 
Version     = 75
Stable      = true
Submit Date = 2023-07-21T05:40:00Z
Diff        =

+/- Job: "<job-id>"
+/- Task Group: "<tg>" 
  +/- Count: "0" => "3"
      Task: "<task"

Version     = 74
Stable      = false
Submit Date = 2023-07-20T19:16:53Z
Diff        =
+/- Job: "<task>"
+/- Task Group: "<tg>"
  +/- Count: "1" => "0"
      Task: "<task>"

I don't know how to replicate this issue yet.

[tgross]

@nvanthao the fix described in #17198 is not present in Nomad 1.3.8. It shipped in the (currently unsupported) 1.3.15. See the 1.3.15 changelog.

I'm going to close this for now. If you can reproduce on a currently supported version of Nomad with the fix, I'd be happy to revisit.