Nomad template reload with raw_exec

issue.md

This issue is described in hashicorp/nomad#5459 but this is a really simple to reproduce version of the same.

nomad_sighup is a really simple Go program, which listens for SIGHUP signal to be sent and prints "Received SIGHUP signal". It listens for this signal in an infinite for-loop.

The nomad deployment specs, deploys the same binary with raw_exec driver. There's a template block defined, which is a dummy configuration template. It watches for a Nomad service object doggo-web and updates whenever the Address/IP of this service updates. When that happens, Nomad is configured to send a SIGHUP signal to the underlying process.

sighup.go

package main

import (
	"fmt"
	"os"
	"os/signal"
	"syscall"
)

func main() {
	sigChan := make(chan os.Signal, 1)
	signal.Notify(sigChan, syscall.SIGHUP)

	for {
		sig := <-sigChan
		switch sig {
		case syscall.SIGHUP:
			fmt.Println("Received SIGHUP signal")
		}
	}

}

sighup.nomad

job "sighup" {
  datacenters = ["dc1"]
  type        = "service"
  namespace   = "default"

  group "sighup" {
    count = 1

    restart {
      attempts = 3
      delay    = "15s"
      interval = "3m"
      mode     = "delay"
    }

    task "app" {
      driver = "raw_exec"

      # Dummy config which queries nomad services and changes everytime the service registration changes.
      template {
        data = <<EOF
{{- with nomadService "doggo-web" }}
backend doggo.internal
  {{- range nomadService "doggo-web" }}
  server "doggo-web-{{ .Address }}-{{ .Port }}" {{ .Address }}:{{ .Port }} check inter 5s weight 1
  {{- end }}
{{ end }}
EOF

        destination   = "$${NOMAD_TASK_DIR}/dummy.conf"
        change_mode   = "signal"
        change_signal = "SIGHUP"
      }

      # This is the Go program available in `PATH` to the raw_exec driver.
      config {
        command = "nomad_sighup"
      }
    }

  }
}