Add Delayed::Plugins::Pidfile

The new plugin creates a pidfile at location
`#{Rails.root}/tmp/delayed_job.pid` when a worker starts and then
removes it when a worker stops. It uses `lifecycle.around(:execute)` to
achieve this.

The file is created in "write exclusive" mode. This means if the file
already exists, a Errno::EEXIST exception is raised. This ensures that a
worker doesn't overwrite a pidfile in use.

This plugin is useful to allow an outside observer (e.g. a healthcheck)
to check if the worker started successfully.

The plugin is not installed by default for backwards compatibility.
Users can use it by adding to their initializer:

Delayed::Worker.plugins << Delayed::Plugin::Pidfile

refs collectiveidea#875

README.md

@@ -483,6 +483,19 @@ Cleaning up
 ===========
 You can invoke `rake jobs:clear` to delete all jobs in the queue.
 
+Plugins
+=======
+
+`Delayed::Plugin::Pidfile` creates a pidfile at location
+`#{Rails.root}/tmp/delayed_job.pid` when starting a worker (e.g. with `rails
+jobs:work`). If the file already exists, a `Errno::EEXIST` error is raised.
+
+To use, add to `config/initializers/delayed_job_config.rb`:
+
+```rb
+Delayed::Worker.plugins << Delayed::Plugin::Pidfile
+```
+
 Having problems?
 ================
 Good places to get help are:

lib/delayed/plugins/pidfile.rb

@@ -0,0 +1,25 @@
+require 'fileutils'
+
+module Delayed
+  module Plugins
+    class Pidfile < Delayed::Plugin
+      callbacks do |lifecycle|
+        lifecycle.around(:execute) do |worker, &block|
+          dir = File.dirname(pidfile)
+          FileUtils.mkdir_p(dir)
+
+          File.write(pidfile, "#{Process.pid}\n", mode => 'wx')
+          begin
+            block.call(worker)
+          ensure
+            File.unlink(pidfile)
+          end
+        end
+      end
+
+      def self.pidfile
+        "#{Rails.root}/tmp/delayed_job.pid"
+      end
+    end
+  end
+end

spec/delayed/plugins/pidfile_spec.rb

@@ -0,0 +1,49 @@
+require 'helper'
+require 'delayed/plugins/pidfile'
+require 'fileutils'
+
+describe Delayed::Plugins::Pidfile do
+  around do |example|
+    original_plugins = Delayed::Worker.plugins
+    begin
+      example.run
+    ensure
+      Delayed::Worker.plugins = original_plugins
+    end
+  end
+
+  it 'creates a pidfile and then removes it' do
+    Delayed::Worker.plugins << Delayed::Plugins::Pidfile
+
+    pidfile_contents = nil
+    Delayed::Worker.plugins << Class.new(Delayed::Plugin) do
+      callbacks do |lifecycle|
+        lifecycle.around(:execute) do
+          pidfile_contents = File.read(Delayed::Plugins::Pidfile.pidfile)
+        end
+      end
+    end
+
+    expect(File.exist?(Delayed::Plugins::Pidfile.pidfile)).to be(false)
+
+    worker = Delayed::Worker.new
+    Delayed::Worker.lifecycle.run_callbacks(:execute, worker) {}
+
+    expect(pidfile_contents).to eq("#{Process.pid}\n")
+    expect(File.exist?(Delayed::Plugins::Pidfile.pidfile)).to be(false)
+  end
+
+  it 'raises an exception if pidfile already exists' do
+    Delayed::Worker.plugins << Delayed::Plugins::Pidfile
+
+    FileUtils.touch(Delayed::Plugins::Pidfile.pidfile)
+    begin
+      worker = Delayed::Worker.new
+      expect { Delayed::Worker.lifecycle.run_callbacks(:execute, worker) {} }.to raise_error(Errno::EEXIST)
+      # Doesn't remove the file.
+      expect(File.exist?(Delayed::Plugins::Pidfile.pidfile)).to be(true)
+    ensure
+      File.unlink(Delayed::Plugins::Pidfile.pidfile)
+    end
+  end
+end