logs: fix logs.disabled on Windows

[tgross]

On Windows the executor returns an error when trying to open the NUL device when we pass it os.DevNull for the stdout/stderr paths. Instead of opening the device, use the discard pipe so that we have platform-specific behavior from the executor itself.

Fixes: #17148

---

Because our E2E tests are a much more realistic exercise of the behavior, I ran the following Windows batch job on our E2E cluster with Windows 2016. The job has log collection enabled:

jobspec

job "example" {

  type = "batch"

  constraint {
    attribute = "${attr.kernel.name}"
    value     = "windows"
  }

  group "group" {

    task "task" {
      driver = "raw_exec"

      template {
        data = <<EOH
$Number = 1..30
$Number | ForEach-Object {
    if ($_ % 5 -eq 0 -and $_ % 3 -eq 0)
    {
        Write-Host "FizzBuzz"
    }
    elseif ($_ % 3 -eq 0)
    {
        Write-Host "Fizz"
    }
    elseif ($_ % 5 -eq 0)
    {
        Write-Host "Buzz"
    }
    else {
        $_
    }
}
  EOH

        destination = "local/factorial.ps1"
      }

      # logs {
      #   disabled = true
      # }

      config {
        command = "powershell"
        args    = ["local/factorial.ps1"]
      }
    }
  }
}

And got the following expected results:

successful run with logs

$ nomad job run example.nomad
==> 2023-05-17T13:52:07-04:00: Monitoring evaluation "bba99715"
    2023-05-17T13:52:07-04:00: Evaluation triggered by job "example"
    2023-05-17T13:52:08-04:00: Allocation "9e724e91" created: node "2d7b2be9", group "group"
    2023-05-17T13:52:08-04:00: Evaluation status changed: "pending" -> "complete"
==> 2023-05-17T13:52:08-04:00: Evaluation "bba99715" finished with status "complete"
$ nomad alloc status 9e7
ID                  = 9e724e91-ad2e-9538-c4f0-587068874fd4
Eval ID             = bba99715
Name                = example.group[0]
Node ID             = 2d7b2be9
Node Name           = EC2AMAZ-3RIPI07
Job ID              = example
Job Version         = 0
Client Status       = complete
Client Description  = All tasks have completed
Desired Status      = run
Desired Description = <none>
Created             = 9s ago
Modified            = 8s ago

Task "task" is "dead"
Task Resources:
CPU        Memory          Disk     Addresses
0/100 MHz  45 MiB/300 MiB  300 MiB

Task Events:
Started At     = 2023-05-17T17:52:08Z
Finished At    = 2023-05-17T17:52:08Z
Total Restarts = 0
Last Restart   = N/A

Recent Events:
Time                       Type        Description
2023-05-17T13:52:08-04:00  Terminated  Exit Code: 0
2023-05-17T13:52:08-04:00  Started     Task started by client
2023-05-17T13:52:07-04:00  Task Setup  Building Task Directory
2023-05-17T13:52:07-04:00  Received    Task received by client
$ nomad alloc logs 9e7
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz
Fizz
22
23
Fizz
Buzz
26
Fizz
28
29
FizzBuzz

And then I uncommented the logs { disabled = true } block and ran it again, and got the expected results.

successful run without logs

$ nomad alloc status d78
ID                  = d7822543-8548-249f-b522-2e828b43809a
Eval ID             = 6c6c696d
Name                = example.group[0]
Node ID             = 2d7b2be9
Node Name           = EC2AMAZ-3RIPI07
Job ID              = example
Job Version         = 1
Client Status       = complete
Client Description  = All tasks have completed
Desired Status      = run
Desired Description = <none>
Created             = 7s ago
Modified            = 6s ago

Task "task" is "dead"
Task Resources:
CPU        Memory          Disk     Addresses
0/100 MHz  50 MiB/300 MiB  300 MiB

Task Events:
Started At     = 2023-05-17T17:53:43Z
Finished At    = 2023-05-17T17:53:43Z
Total Restarts = 0
Last Restart   = N/A

Recent Events:
Time                       Type        Description
2023-05-17T13:53:43-04:00  Terminated  Exit Code: 0
2023-05-17T13:53:43-04:00  Started     Task started by client
2023-05-17T13:53:42-04:00  Task Setup  Building Task Directory
2023-05-17T13:53:42-04:00  Received    Task received by client

$ nomad alloc logs d78
Failed to read stdout file: error reading file: Unexpected response code: 404 (log entry for task "task" and log type "stdout" not found)

[jrasell]

LGTM! Just noting there is no changelog entry or backport labels, if we wanted those before merging.

[tgross]

Changelog fixed. Will merge and backport once that's ✔️

[tgross]

Only backporting to 1.5.x because 1.4.x and 1.3.x didn't get the feature.