Better locking within handle_box_url

This improves locking in the face of parallel providers and
handling box_url parameters. This avoids downloading a box multiple
times.

lib/vagrant/action/builtin/handle_box_url.rb

@@ -1,3 +1,5 @@
+require "thread"
+
 module Vagrant
   module Action
     module Builtin
@@ -6,18 +8,42 @@ module Vagrant
       # sequence for a provider after configuration validation but before
       # you attempt to use any box.
       class HandleBoxUrl
+        @@big_lock = Mutex.new
+        @@handle_box_url_locks = Hash.new { |h,k| h[k] = Mutex.new }
+
         def initialize(app, env)
           @app = app
         end
 
         def call(env)
           if !env[:machine].box
+            # Get a "big lock" to make sure that our more fine grained
+            # lock access is thread safe.
+            lock = nil
+            @@big_lock.synchronize do
+              lock = @@handle_box_url_locks[env[:machine].config.vm.box]
+            end
+
             # We can assume a box URL is set because the Vagrantfile
             # validation should do this for us. If not, though, we do
             # raise a terrible runtime error.
             box_name = env[:machine].config.vm.box
             box_url  = env[:machine].config.vm.box_url
 
+            lock.synchronize do
+              # First see if we actually have the box now.
+              has_box = false
+
+              formats = env[:machine].provider_options[:box_format] ||
+                env[:machine].provider_name
+              [formats].flatten.each do |format|
+                if env[:box_collection].find(box_name, format)
+                  has_box = true
+                  break
+                end
+              end
+
+              if !has_box
                 # Add the box then reload the box collection so that it becomes
                 # aware of it.
                 env[:ui].info I18n.t(
@@ -35,6 +61,8 @@ module Vagrant
                   # Just ignore this, since it means the next part will succeed!
                   # This can happen in a multi-threaded environment.
                 end
+              end
+            end
 
             # Reload the environment and set the VM to be the new loaded VM.
             env[:machine] = env[:machine].env.machine(