providers/virtualbox: wrap version set in mutex

plugins/providers/virtualbox/driver/meta.rb

@@ -1,4 +1,5 @@
 require "forwardable"
+require "thread"
 
 require "log4r"
 
@@ -20,6 +21,7 @@ module VagrantPlugins
         # We cache the read VirtualBox version here once we have one,
         # since during the execution of Vagrant, it likely doesn't change.
         @@version = nil
+        @@version_lock = Mutex.new
 
         # The UUID of the virtual machine we represent
         attr_reader :uuid
@@ -36,6 +38,7 @@ module VagrantPlugins
           @logger = Log4r::Logger.new("vagrant::provider::virtualbox::meta")
           @uuid = uuid
 
+          @@version_lock.synchronize do
             if !@@version
               # Read and assign the version of VirtualBox we know which
               # specific driver to instantiate.
@@ -48,6 +51,7 @@ module VagrantPlugins
                 raise Vagrant::Errors::VirtualBoxNotDetected
               end
             end
+          end
 
           # Instantiate the proper version driver for VirtualBox
           @logger.debug("Finding driver for VirtualBox version: #{@@version}")