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

@@ -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.

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'

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

rakelib/helpers.rb

 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('/', '.')