Implement fork-based daemon for scheduling jobs

Extracted from

https://github.com/discourse/discourse/blob/master/lib/demon/sidekiq.rb

Gemfile.lock

@@ -21,7 +21,7 @@ GEM
     minitest-around (0.4.1)
       minitest (~> 5.0)
     mustermann (1.0.3)
-    oj (3.6.10)
+    oj (3.6.11)
     ox (2.10.0)
     parallel (1.12.1)
     parser (2.5.1.2)
@@ -46,7 +46,7 @@ GEM
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
     redcarpet (3.4.0)
-    rubocop (0.59.1)
+    rubocop (0.59.2)
       jaro_winkler (~> 1.5.1)
       parallel (~> 1.10)
       parser (>= 2.5, != 2.5.1.1)

bin/schedule

@@ -7,10 +7,6 @@ require 'rufus-scheduler'
 
 scheduler = Rufus::Scheduler.new
 
-trap 'INT' do
-  scheduler.shutdown
-end
-
 scheduler.cron '*/15 15,16,17 * * 1-5' do
   `rake rates:current`
 end

config/unicorn.rb

@@ -6,6 +6,12 @@ preload_app true
 worker_processes worker_process_count
 timeout 10
 
-before_fork do |_, _|
+initialized = false
+before_fork do |_server, _worker|
   Sequel::DATABASES.each(&:disconnect)
+  unless initialized
+    require 'scheduler'
+    Scheduler.start
+    initialized = true
+  end
 end

docker-compose.override.yml

@@ -7,7 +7,3 @@ services:
       VIRTUAL_HOST: localhost
     ports:
       - "8080:8080"
-  scheduler:
-    build: .
-    environment:
-      APP_ENV: development

docker-compose.prod.yml

@@ -10,14 +10,6 @@ services:
         max-size: "50m"
         max-file: "10"
     restart: unless-stopped
-  scheduler:
-    env_file: .env
-    image: hakanensari/frankfurter
-    logging:
-      options:
-        max-size: "50m"
-        max-file: "10"
-    restart: unless-stopped
   nginx-proxy:
     image: jwilder/nginx-proxy
     labels:

docker-compose.yml

@@ -13,12 +13,5 @@ services:
       - '8080'
     links:
       - db
-  scheduler:
-    command: bin/schedule
-    entrypoint: bin/wait-for-it.sh db:5432 -s --
-    environment:
-      DATABASE_URL: postgres://postgres@db/postgres
-    links:
-      - db
 volumes:
   data:

lib/scheduler.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+class Scheduler
+  attr_reader :pid
+
+  def self.start
+    new.start
+  end
+
+  def initialize
+    @parent_pid = Process.pid
+  end
+
+  def start
+    return if pid
+
+    @pid = fork do
+      monitor_parent
+      run
+    end
+    monitor_child
+  end
+
+  private
+
+  def run
+    load 'bin/schedule'
+  end
+
+  def monitor_child
+    return if @child_monitor
+
+    @child_monitor = Thread.new do
+      loop do
+        sleep 5
+        unless alive?(pid)
+          @pid = nil
+          start
+        end
+      end
+    end
+  end
+
+  def monitor_parent
+    Thread.new do
+      loop do
+        exit unless alive?(@parent_pid)
+        sleep 1
+      end
+    end
+  end
+
+  def alive?(pid)
+    Process.getpgid(pid)
+    true
+  rescue Errno::ESRCH
+    false
+  end
+end

spec/scheduler_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require_relative 'helper'
+require 'scheduler'
+
+describe Scheduler do
+  let :scheduler do
+    Scheduler.new
+  end
+
+  after do
+    Process.kill('TERM', scheduler.pid) if scheduler.pid
+  end
+
+  it 'forks a scheduler' do
+    scheduler.start
+    Process.getpgid(scheduler.pid) # won't raise Errno::ESRCH
+  end
+end