Environment#batch and forcing disable through env var

lib/vagrant/batch_action.rb

@@ -6,23 +6,46 @@ module Vagrant
   # This class executes multiple actions as a single batch, parallelizing
   # the action calls if possible.
   class BatchAction
-    def initialize
+    def initialize(disable_parallel=false)
       @actions          = []
+      @disable_parallel = disable_parallel
       @logger           = Log4r::Logger.new("vagrant::batch_action")
     end
 
+    # Add an action to the batch of actions that will be run.
+    #
+    # This will **not** run the action now. The action will be run
+    # when {#run} is called.
+    #
+    # @param [Machine] machine The machine to run the action on
+    # @param [Symbol] action The action to run
+    # @param [Hash] options Any additional options to send in.
     def action(machine, action, options=nil)
       @actions << [machine, action, options]
     end
 
+    # Run all the queued up actions, parallelizing if possible.
+    #
+    # This will parallelize if and only if the provider of every machine
+    # supports parallelization. Parallelizing can additionally be disabled
+    # by passing the option into the initializer of this class.
     def run
       par = true
+
+      if @disable_parallel
+        par = false
+        @logger.info("Disabled parallelization by force.")
+      end
+
+      if par
         @actions.each do |machine, _, _|
           if !machine.provider_options[:parallel]
+            @logger.info("Disabling parallelization because provider doesn't support it: #{machine.provider_name}")
             par = false
             break
           end
         end
+      end
 
       @logger.info("Batch action will parallelize: #{par.inspect}")
 
@@ -35,6 +58,9 @@ module Vagrant
         threads << thread
       end
 
+      # Join the threads, which will return immediately if parallelization
+      # if disabled, because we already joined on them. Otherwise, this
+      # will wait for completion of all threads.
       threads.map(&:join)
     end
   end

lib/vagrant/environment.rb

@@ -188,6 +188,21 @@ module Vagrant
       result
     end
 
+    # This creates a new batch action, yielding it, and then running it
+    # once the block is called.
+    #
+    # This handles the case where batch actions are disabled by the
+    # VAGRANT_NO_PARALLEL environmental variable.
+    def batch
+      BatchAction.new(!!ENV["VAGRANT_NO_PARALLEL"]).tap do |b|
+        # Yield it so that the caller can setup actions
+        yield b
+
+        # And run it!
+        b.run
+      end
+    end
+
     # This returns the provider name for the default provider for this
     # environment. The provider returned is currently hardcoded to "virtualbox"
     # but one day should be a detected valid, best-case provider for this

test/unit/vagrant/batch_action_test.rb

@@ -5,10 +5,12 @@ require File.expand_path("../../base", __FILE__)
 describe Vagrant::BatchAction do
   let(:called_actions) { [] }
   let!(:lock) { Mutex.new }
+  let(:provider_name) { "test" }
   let(:provider_options) { {} }
 
   def new_machine(options)
     double("machine").tap do |m|
+      m.stub(:provider_name => provider_name)
       m.stub(:provider_options => options)
       m.stub(:action) do |action, opts|
         lock.synchronize do

test/unit/vagrant/environment_test.rb

@@ -46,6 +46,36 @@ describe Vagrant::Environment do
     end
   end
 
+  describe "batching" do
+    let(:batch) do
+      double("batch") do |b|
+        b.stub(:run)
+      end
+    end
+
+    context "without the disabling env var" do
+      it "should run without disabling parallelization" do
+        with_temp_env("VAGRANT_NO_PARALLEL" => nil) do
+          Vagrant::BatchAction.should_receive(:new).with(false).and_return(batch)
+          batch.should_receive(:run)
+
+          instance.batch {}
+        end
+      end
+    end
+
+    context "with the disabling env var" do
+      it "should run with disabling parallelization" do
+        with_temp_env("VAGRANT_NO_PARALLEL" => "yes") do
+          Vagrant::BatchAction.should_receive(:new).with(true).and_return(batch)
+          batch.should_receive(:run)
+
+          instance.batch {}
+        end
+      end
+    end
+  end
+
   describe "current working directory" do
     it "is the cwd by default" do
       Dir.mktmpdir do |temp_dir|