Add daemonize option to CLI

README.md

@@ -152,13 +152,16 @@ Options:
   [--max-threads=COUNT]      # Maximum number of threads to use for working jobs. (env var: GOOD_JOB_MAX_THREADS, default: 5)
   [--queues=QUEUE_LIST]      # Queues to work from. (env var: GOOD_JOB_QUEUES, default: *)
   [--poll-interval=SECONDS]  # Interval between polls for available jobs in seconds (env var: GOOD_JOB_POLL_INTERVAL, default: 1)
+  [--daemonize]              # Run as a background daemon (default: false)
+  [--pidfile=PIDFILE]        # Path to write daemonized Process ID (env var: GOOD_JOB_PIDFILE, default: tmp/pids/good_job.pid)
 
 Executes queued jobs.
 
 All options can be configured with environment variables.
 See option descriptions for the matching environment variable name.
 
 == Configuring queues
+
 Separate multiple queues with commas; exclude queues with a leading minus;
 separate isolated execution pools with semicolons and threads with colons.
 ```

lib/good_job/cli.rb

@@ -15,6 +15,11 @@ class CLI < Thor
     # Requiring this loads the application's configuration and classes.
     RAILS_ENVIRONMENT_RB = File.expand_path("config/environment.rb")
 
+    # @!visibility private
+    def self.exit_on_failure?
+      true
+    end
+
     # @!macro thor.desc
     #   @!method $1
     #   @return [void]
@@ -27,7 +32,8 @@ class CLI < Thor
       See option descriptions for the matching environment variable name.
 
       == Configuring queues
-      \x5Separate multiple queues with commas; exclude queues with a leading minus;
+
+      Separate multiple queues with commas; exclude queues with a leading minus;
       separate isolated execution pools with semicolons and threads with colons.
 
     DESCRIPTION
@@ -43,10 +49,18 @@ class CLI < Thor
                   type: :numeric,
                   banner: 'SECONDS',
                   desc: "Interval between polls for available jobs in seconds (env var: GOOD_JOB_POLL_INTERVAL, default: 5)"
+    method_option :daemonize,
+                  type: :boolean,
+                  desc: "Run as a background daemon (default: false)"
+    method_option :pidfile,
+                  type: :string,
+                  desc: "Path to write daemonized Process ID (env var: GOOD_JOB_PIDFILE, default: tmp/pids/good_job.pid)"
     def start
       set_up_application!
       configuration = GoodJob::Configuration.new(options)
 
+      Daemon.new(pidfile: configuration.pidfile).daemonize if configuration.daemonize?
+
       notifier = GoodJob::Notifier.new
       poller = GoodJob::Poller.new(poll_interval: configuration.poll_interval)
       scheduler = GoodJob::Scheduler.from_configuration(configuration)

lib/good_job/configuration.rb

@@ -108,7 +108,7 @@ def poll_interval
 
     # Number of seconds to preserve jobs when using the +good_job cleanup_preserved_jobs+ CLI command.
     # This configuration is only used when {GoodJob.preserve_job_records} is +true+.
-    # @return [Boolean]
+    # @return [Integer]
     def cleanup_preserved_jobs_before_seconds_ago
       (
         options[:before_seconds_ago] ||
@@ -118,6 +118,20 @@ def cleanup_preserved_jobs_before_seconds_ago
       ).to_i
     end
 
+    # Tests whether to daemonize the process.
+    # @return [Boolean]
+    def daemonize?
+      options[:daemonize] || false
+    end
+
+    # Path of the pidfile to create when running as a daemon.
+    # @return [Pathname,String]
+    def pidfile
+      options[:pidfile] ||
+        env['GOOD_JOB_PIDFILE'] ||
+        Rails.application.root.join('tmp', 'pids', 'good_job.pid')
+    end
+
     private
 
     def rails_config

lib/good_job/daemon.rb

@@ -0,0 +1,59 @@
+module GoodJob
+  #
+  # Manages daemonization of the current process.
+  #
+  class Daemon
+    # The path of the generated pidfile.
+    # @return [Pathname,String]
+    attr_reader :pidfile
+
+    # @param pidfile [Pathname,String] Pidfile path
+    def initialize(pidfile:)
+      @pidfile = pidfile
+    end
+
+    # Daemonizes the current process and writes out a pidfile.
+    def daemonize
+      check_pid
+      Process.daemon
+      write_pid
+    end
+
+    private
+
+    def write_pid
+      File.open(pidfile, ::File::CREAT | ::File::EXCL | ::File::WRONLY) { |f| f.write(Process.pid.to_s) }
+      at_exit { File.delete(pidfile) if File.exist?(pidfile) }
+    rescue Errno::EEXIST
+      check_pid
+      retry
+    end
+
+    def delete_pid
+      File.delete(pidfile) if File.exist?(pidfile)
+    end
+
+    def check_pid
+      case pid_status(pidfile)
+      when :running, :not_owned
+        abort "A server is already running. Check #{pidfile}"
+      when :dead
+        File.delete(pidfile)
+      end
+    end
+
+    def pid_status(pidfile)
+      return :exited unless File.exist?(pidfile)
+
+      pid = ::File.read(pidfile).to_i
+      return :dead if pid.zero?
+
+      Process.kill(0, pid) # check process status
+      :running
+    rescue Errno::ESRCH
+      :dead
+    rescue Errno::EPERM
+      :not_owned
+    end
+  end
+end

spec/lib/good_job/daemon_spec.rb

@@ -0,0 +1,39 @@
+require 'rails_helper'
+
+RSpec.describe GoodJob::Daemon, skip_if_java: true do
+  let(:pidfile) { Rails.application.root.join('tmp/pidfile.pid') }
+  let(:daemon) { described_class.new(pidfile: pidfile) }
+
+  before do
+    FileUtils.mkdir_p Rails.application.root.join('tmp')
+    allow(Process).to receive(:daemon)
+    allow(daemon).to receive(:at_exit)
+  end
+
+  after do
+    File.delete(pidfile)
+  end
+
+  describe '#daemonize' do
+    it 'calls Process.daemon' do
+      daemon.daemonize
+      expect(Process).to have_received :daemon
+    end
+
+    it 'writes a pidfile' do
+      expect do
+        daemon.daemonize
+      end.to change { Pathname.new(pidfile).exist? }.from(false).to(true)
+    end
+
+    context 'when a pidfile already exists' do
+      before do
+        File.open(pidfile, "w") { |f| f.write(Process.pid) }
+      end
+
+      it 'aborts with a message' do
+        expect { daemon.daemonize }.to output("A server is already running. Check #{pidfile}\n").to_stderr.and raise_error SystemExit
+      end
+    end
+  end
+end