From 64912a774199e40a68cab3020d3e98ad9bb8c6e5 Mon Sep 17 00:00:00 2001
From: Calen Pennington <cale@edx.org>
Date: Fri, 14 Jun 2013 21:17:08 -0400
Subject: [PATCH] Make assets watchers run as singletons

Previously, multiple copies of the watchers started from the different
shells would run simultaneously, which left the possiblity of zombie
watchers, increased resource consumption, and incorrect results. This
fixes that problem by only starting a watcher if that same command isn't
already in the process list.

Fixes LMS-499
---
 CHANGELOG.rst       |  3 +++
 Gemfile             |  1 +
 rakelib/assets.rake |  4 ++--
 rakelib/helpers.rb  | 12 ++++++++++++
 4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 9c7af1520b..18855e82ae 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -5,6 +5,9 @@ These are notable changes in edx-platform.  This is a rolling list of changes,
 in roughly chronological order, most recent first.  Add your entries at or near
 the top.  Include a label indicating the component affected.
 
+Common: Make asset watchers run as singletons (so they won't start if the
+watcher is already running in another shell).
+
 Common: Make rake provide better error messages if packages are missing.
 
 Common: Repairs development documentation generation by sphinx.
diff --git a/Gemfile b/Gemfile
index 7f7b146978..1ad685c34d 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,3 +4,4 @@ gem 'sass', '3.1.15'
 gem 'bourbon', '~> 1.3.6'
 gem 'colorize', '~> 0.5.8'
 gem 'launchy', '~> 2.1.2'
+gem 'sys-proctable', '~> 0.9.3'
diff --git a/rakelib/assets.rake b/rakelib/assets.rake
index 0c58047bc2..c935b0d53b 100644
--- a/rakelib/assets.rake
+++ b/rakelib/assets.rake
@@ -114,9 +114,9 @@ namespace :assets do
             task :_watch => (prereq_tasks + ["assets:#{asset_type}:debug"]) do
                 cmd = send(asset_type.to_s + "_cmd", watch=true, debug=true)
                 if cmd.kind_of?(Array)
-                    cmd.each {|c| background_process(c)}
+                    cmd.each {|c| singleton_process(c)}
                 else
-                    background_process(cmd)
+                    singleton_process(cmd)
                 end
             end
         end
diff --git a/rakelib/helpers.rb b/rakelib/helpers.rb
index 4b10bef709..3373214a19 100644
--- a/rakelib/helpers.rb
+++ b/rakelib/helpers.rb
@@ -1,4 +1,6 @@
 require 'digest/md5'
+require 'sys/proctable'
+require 'colorize'
 
 def find_executable(exec)
     path = %x(which #{exec}).strip
@@ -84,6 +86,16 @@ def background_process(*command)
     end
 end
 
+# Runs a command as a background process, as long as no other processes
+# tagged with the same tag are running
+def singleton_process(*command)
+    if Sys::ProcTable.ps.select {|proc| proc.cmdline.include?(command.join(' '))}.empty?
+        background_process(*command)
+    else
+        puts "Process '#{command.join(' ')} already running, skipping".blue
+    end
+end
+
 def environments(system)
     Dir["#{system}/envs/**/*.py"].select{|file| ! (/__init__.py$/ =~ file)}.map do |env_file|
         env_file.gsub("#{system}/envs/", '').gsub(/\.py/, '').gsub('/', '.')