demo script for demonizing using fork exec

minor refinements to demon

lib/demon/base.rb

@@ -7,10 +7,10 @@ class Demon::Base
     @demons
   end
 
-  def self.start(count = 1)
+  def self.start(count = 1, verbose: false)
     @demons ||= {}
     count.times do |i|
-      (@demons["#{prefix}_#{i}"] ||= new(i)).start
+      (@demons["#{prefix}_#{i}"] ||= new(i, verbose: verbose)).start
     end
   end
 
@@ -38,16 +38,18 @@ class Demon::Base
   attr_reader :pid, :parent_pid, :started, :index
   attr_accessor :stop_timeout
 
-  def initialize(index)
+  def initialize(index, rails_root: nil, parent_pid: nil, verbose: false)
     @index = index
     @pid = nil
-    @parent_pid = Process.pid
+    @parent_pid = parent_pid || Process.pid
     @started = false
     @stop_timeout = 10
+    @rails_root = rails_root || Rails.root
+    @verbose = verbose
   end
 
   def pid_file
-    "#{Rails.root}/tmp/pids/#{self.class.prefix}_#{@index}.pid"
+    "#{@rails_root}/tmp/pids/#{self.class.prefix}_#{@index}.pid"
   end
 
   def alive?(pid = nil)
@@ -153,8 +155,15 @@ class Demon::Base
 
   private
 
+  def verbose(msg)
+    if @verbose
+      puts msg
+    end
+  end
+
   def write_pid_file
-    FileUtils.mkdir_p(Rails.root + "tmp/pids")
+    verbose("writing pid file #{pid_file} for #{@pid}")
+    FileUtils.mkdir_p(@rails_root + "tmp/pids")
     File.open(pid_file, 'w') do |f|
       f.write(@pid)
     end
@@ -190,7 +199,7 @@ class Demon::Base
   end
 
   def establish_app
-    Discourse.after_fork
+    Discourse.after_fork if defined?(Discourse)
 
     Signal.trap("HUP") do
       begin

script/demon_test/child

@@ -0,0 +1,31 @@
+#!/usr/bin/env ruby
+
+$parent_pid = ARGV[0].to_i
+
+puts "Hello from #{Process.pid} my parent is #{$parent_pid}"
+
+Thread.new do
+
+  def alive?(pid)
+    Process.kill(0, pid)
+    true
+  rescue
+    false
+  end
+
+  while true
+    begin
+      unless alive?($parent_pid)
+        STDERR.puts "Parent was terminated!"
+        Process.kill "TERM", Process.pid
+        sleep 10
+        Process.kill "KILL", Process.pid
+      end
+    rescue => e
+      STDERR.puts "URGENT monitoring thread had an exception #{e}"
+    end
+    sleep 1
+  end
+end
+
+sleep

script/demon_test/parent.rb

@@ -0,0 +1,25 @@
+require File.expand_path("../../../config/environment", __FILE__)
+
+puts "Parent is now loaded"
+
+class ForkExecDemon < Demon::Base
+  def self.prefix
+    "fork-exec-child"
+  end
+
+  def run
+    if @pid = fork
+      write_pid_file
+      return
+    end
+
+    exec "./child #{parent_pid}"
+  end
+end
+
+ForkExecDemon.start(1, verbose: true)
+
+while true
+  ForkExecDemon.ensure_running
+  sleep 0.1
+end