Cache the machine objects created in Environment#machine.

Each machine and provider backing should be represented by only one
machine.

lib/vagrant/environment.rb

@@ -138,6 +138,10 @@ module Vagrant
     #   backed by.
     # @return [Machine]
     def machine(name, provider)
+      cache_key = [name, provider]
+      @machines ||= {}
+      return @machines[cache_key] if @machines.has_key?(cache_key)
+
       vm_config = config.for_vm(name)
       if !vm_config
         raise Errors::MachineNotFound, :name => name, :provider => provider
@@ -149,7 +153,7 @@ module Vagrant
       end
 
       box = boxes.find(vm_config.vm.box, provider)
-      Machine.new(name, provider_cls, vm_config, box, self)
+      @machines[cache_key] = Machine.new(name, provider_cls, vm_config, box, self)
     end
 
     # This returns a list of the configured machines for this environment.

test/unit/vagrant/environment_test.rb

@@ -238,12 +238,20 @@ VF
   end
 
   describe "getting a machine" do
+    # A helper to register a provider for use in tests.
+    def register_provider(name)
+      provider_cls = Class.new(Vagrant.plugin("2", :provider))
+
+      register_plugin("2") do |p|
+        p.provider(name) { provider_cls }
+      end
+
+      provider_cls
+    end
+
     it "should return a machine object with the correct provider" do
       # Create a provider
-      foo_provider = Class.new(Vagrant.plugin("2", :provider))
-      register_plugin("2") do |p|
-        p.provider("foo") { foo_provider }
-      end
+      foo_provider = register_provider("foo")
 
       # Create the configuration
       isolated_env = isolated_environment do |e|
@@ -263,6 +271,33 @@ VF
       machine.provider.should be_kind_of(foo_provider)
     end
 
+    it "should cache the machine objects by name and provider" do
+      # Create a provider
+      foo_provider = register_provider("foo")
+      bar_provider = register_provider("bar")
+
+      # Create the configuration
+      isolated_env = isolated_environment do |e|
+        e.vagrantfile(<<-VF)
+Vagrant.configure("2") do |config|
+  config.vm.box = "base"
+  config.vm.define "vm1"
+  config.vm.define "vm2"
+end
+VF
+      end
+
+      env = isolated_env.create_vagrant_env
+      vm1_foo = env.machine(:vm1, :foo)
+      vm1_bar = env.machine(:vm1, :bar)
+      vm2_foo = env.machine(:vm2, :foo)
+
+      vm1_foo.should eql(env.machine(:vm1, :foo))
+      vm1_bar.should eql(env.machine(:vm1, :bar))
+      vm1_foo.should_not eql(vm1_bar)
+      vm2_foo.should eql(env.machine(:vm2, :foo))
+    end
+
     it "should raise an error if the VM is not found" do
       expect { instance.machine("i-definitely-dont-exist", :virtualbox) }.
         to raise_error(Vagrant::Errors::MachineNotFound)