Allow Ansible provisioner to run reliably in parallel

The Ansible Vagrant provisioner has a race where the inventory file is
updated every time the provisioner runs unless a file is provided.

Therefore if Ansible attempts to provision two nodes in parallel, you
may see the following race:
   * System A writes the inventory file and calls Ansible.
   * System B starts to provision and truncates the file before
     creating a new one.
   * Ansible on system A now attempts to read the inventory
     file, which is blank. Ansible bombs out with "ERROR: provided
     hosts list is empty".

To fix this, we only allow Vagrant to update the inventory file if
it needs to.

plugins/provisioners/ansible/provisioner.rb

@@ -1,9 +1,12 @@
 require "vagrant/util/platform"
+require "thread"
 
 module VagrantPlugins
   module Ansible
     class Provisioner < Vagrant.plugin("2", :provisioner)
 
+      @@lock = Mutex.new
+
       def initialize(machine, config)
         super
 
@@ -117,55 +120,63 @@ module VagrantPlugins
         FileUtils.mkdir_p(generated_inventory_dir) unless File.directory?(generated_inventory_dir)
         generated_inventory_file = generated_inventory_dir.join('vagrant_ansible_inventory')
 
-        generated_inventory_file.open('w') do |file|
-          file.write("# Generated by Vagrant\n\n")
+        inventory = "# Generated by Vagrant\n\n"
 
-          @machine.env.active_machines.each do |am|
-            begin
-              m = @machine.env.machine(*am)
-              m_ssh_info = m.ssh_info
-              if !m_ssh_info.nil?
-                file.write("#{m.name} ansible_ssh_host=#{m_ssh_info[:host]} ansible_ssh_port=#{m_ssh_info[:port]}\n")
-                inventory_machines[m.name] = m
-              else
-                @logger.error("Auto-generated inventory: Impossible to get SSH information for machine '#{m.name} (#{m.provider_name})'. This machine should be recreated.")
-                # Let a note about this missing machine
-                file.write("# MISSING: '#{m.name}' machine was probably removed without using Vagrant. This machine should be recreated.\n")
-              end
-            rescue Vagrant::Errors::MachineNotFound => e
-              @logger.info("Auto-generated inventory: Skip machine '#{am[0]} (#{am[1]})', which is not configured for this Vagrant environment.")
+        @machine.env.active_machines.each do |am|
+          begin
+            m = @machine.env.machine(*am)
+            m_ssh_info = m.ssh_info
+            if !m_ssh_info.nil?
+              inventory += "#{m.name} ansible_ssh_host=#{m_ssh_info[:host]} ansible_ssh_port=#{m_ssh_info[:port]}\n"
+              inventory_machines[m.name] = m
+            else
+              @logger.error("Auto-generated inventory: Impossible to get SSH information for machine '#{m.name} (#{m.provider_name})'. This machine should be recreated.")
+              # Let a note about this missing machine
+              inventory += "# MISSING: '#{m.name}' machine was probably removed without using Vagrant. This machine should be recreated.\n"
             end
+          rescue Vagrant::Errors::MachineNotFound => e
+            @logger.info("Auto-generated inventory: Skip machine '#{am[0]} (#{am[1]})', which is not configured for this Vagrant environment.")
           end
+        end
 
-          # Write out groups information.
-          # All defined groups will be included, but only supported
-          # machines and defined child groups will be included.
-          # Group variables are intentionally skipped.
-          groups_of_groups = {}
-          defined_groups = []
+        # Write out groups information.
+        # All defined groups will be included, but only supported
+        # machines and defined child groups will be included.
+        # Group variables are intentionally skipped.
+        groups_of_groups = {}
+        defined_groups = []
 
-          config.groups.each_pair do |gname, gmembers|
-            # Require that gmembers be an array
-            # (easier to be tolerant and avoid error management of few value)
-            gmembers = [gmembers] if !gmembers.is_a?(Array)
+        config.groups.each_pair do |gname, gmembers|
+          # Require that gmembers be an array
+          # (easier to be tolerant and avoid error management of few value)
+          gmembers = [gmembers] if !gmembers.is_a?(Array)
 
-            if gname.end_with?(":children")
-              groups_of_groups[gname] = gmembers
-              defined_groups << gname.sub(/:children$/, '')
-            elsif !gname.include?(':vars')
-              defined_groups << gname
-              file.write("\n[#{gname}]\n")
-              gmembers.each do |gm|
-                file.write("#{gm}\n") if inventory_machines.include?(gm.to_sym)
-              end
-            end
-          end
-
-          defined_groups.uniq!
-          groups_of_groups.each_pair do |gname, gmembers|
-            file.write("\n[#{gname}]\n")
+          if gname.end_with?(":children")
+            groups_of_groups[gname] = gmembers
+            defined_groups << gname.sub(/:children$/, '')
+          elsif !gname.include?(':vars')
+          defined_groups << gname
+          inventory += "\n[#{gname}]\n"
             gmembers.each do |gm|
-              file.write("#{gm}\n") if defined_groups.include?(gm)
+              inventory += "#{gm}\n" if inventory_machines.include?(gm.to_sym)
+            end
+          end
+        end
+
+        defined_groups.uniq!
+        groups_of_groups.each_pair do |gname, gmembers|
+          inventory += "\n[#{gname}]\n"
+          gmembers.each do |gm|
+            inventory += "#{gm}\n" if defined_groups.include?(gm)
+          end
+        end
+
+        @@lock.synchronize do
+          if ! File.exists?(generated_inventory_file) or
+             inventory != File.read(generated_inventory_file)
+
+            generated_inventory_file.open('w') do |file|
+              file.write(inventory)
             end
           end
         end