Lock around chdir in Subprocess for thread safety

lib/vagrant/util/subprocess.rb

@@ -1,3 +1,5 @@
+require 'thread'
+
 require 'childprocess'
 require 'log4r'
 
@@ -12,6 +14,8 @@ module Vagrant
     # from the subprocess in real time, by simply passing a block to
     # the execute method.
     class Subprocess
+      @@chdir_lock = Mutex.new
+
       # The chunk size for reading from subprocess IO.
       READ_CHUNK_SIZE = 4096
 
@@ -72,8 +76,10 @@ module Vagrant
 
         # Start the process
         begin
-          Dir.chdir(workdir) do
-            process.start
+          @@chdir_lock.synchronize do
+            Dir.chdir(workdir) do
+              process.start
+            end
           end
         rescue ChildProcess::LaunchError => ex
           # Raise our own version of the error so that users of the class

plugins/providers/virtualbox/action/import.rb

@@ -7,6 +7,7 @@ module VagrantPlugins
         end
 
         def call(env)
+          raise "foo"
           env[:ui].info I18n.t("vagrant.actions.vm.import.importing",
                                :name => env[:machine].box.name)