Box adding is done in a helper that ensures temporary files are cleaned

lib/vagrant/box_collection.rb

@@ -96,8 +96,7 @@ module Vagrant
 
       # Create a temporary directory since we're not sure at this point if
       # the box we're unpackaging already exists (if no provider was given)
-      temp_dir = Pathname.new(Dir.mktmpdir(TEMP_PREFIX))
+      with_temp_dir do |temp_dir|
-
         # Extract the box into a temporary directory.
         @logger.debug("Unpacking box into temporary directory: #{temp_dir}")
         result = Util::Subprocess.execute(
@@ -110,9 +109,13 @@ module Vagrant
           temp_dir = v1_upgrade(temp_dir)
         end
 
+        # We re-wrap ourselves in the safety net in case we upgraded.
+        # If we didn't upgrade, then this is still safe because the
+        # helper will only delete the directory if it exists
+        with_temp_dir(temp_dir) do |final_temp_dir|
           # Get an instance of the box we just added before it is finalized
           # in the system so we can inspect and use its metadata.
-      box = Box.new(name, provider, temp_dir)
+          box = Box.new(name, provider, final_temp_dir)
 
           # Get the provider, since we'll need that to at the least add it
           # to the system or check that it matches what is given to us.
@@ -150,14 +153,13 @@ module Vagrant
           # Go through each child and copy them one-by-one. This avoids
           # an issue where on Windows cross-device directory copies are
           # failing for some reason. [GH-1424]
-      temp_dir.children(true).each do |f|
+          final_temp_dir.children(true).each do |f|
             destination = final_dir.join(f.basename)
             @logger.debug("Moving: #{f} => #{destination}")
             FileUtils.mv(f, destination)
           end
-
+        end
-      # Remove the temporary directory
+      end
-      temp_dir.rmtree
 
       # Return the box
       find(name, provider)
@@ -324,5 +326,19 @@ module Vagrant
       # Return the temporary directory
       temp_dir
     end
+
+    # This is a helper that makes sure that our temporary directories
+    # are cleaned up no matter what.
+    #
+    # @param [String] dir Path to a temporary directory
+    # @return [Object] The result of whatever the yield is
+    def with_temp_dir(dir=nil)
+      dir ||= Dir.mktmpdir("vagrant-")
+      dir = Pathname.new(dir)
+
+      yield dir
+    ensure
+      dir.rmtree if dir.exist?
+    end
   end
 end