Acquire lock to prevent concurrent creation of master VM for the same box.

plugins/providers/virtualbox/action/import_master.rb

@@ -13,16 +13,16 @@ module VagrantPlugins
         def call(env)
           master_id_file = env[:machine].box.directory.join("master_id")
           
+          env[:machine].env.lock(env[:machine].box.name, retry: true) do
             env[:master_id] = master_id_file.read.chomp if master_id_file.file?          
             if env[:master_id] && env[:machine].provider.driver.vm_exists?(env[:master_id])
               # Master VM already exists -> nothing to do - continue.
-            @app.call(env) 
+              @logger.info("Master VM for '#{env[:machine].box.name}' already exists (id=#{env[:master_id]}) - skipping import step.")
+              return @app.call(env) 
             end          
             
             env[:ui].info I18n.t("vagrant.actions.vm.clone.importing", name: env[:machine].box.name)
             
-          #TODO - prevent concurrent creation of master vms for the same box.
-           
             # Import the virtual machine
             ovf_file = env[:machine].box.directory.join("box.ovf").to_s
             env[:master_id] = env[:machine].provider.driver.import(ovf_file) do |progress|
@@ -48,10 +48,14 @@ module VagrantPlugins
             master_id_file.open("w+") do |f|
               f.write(env[:master_id])
             end
+          end          
           
           # If we got interrupted, then the import could have been
           # interrupted and its not a big deal. Just return out.
-          return if env[:interrupted]
+          if env[:interrupted]
+            @logger.info("Import of master VM was interrupted -> exiting.")
+            return
+          end
 
           # Import completed successfully. Continue the chain
           @app.call(env)