Add basic loop detection for distro_dispatch

lib/vagrant/machine.rb

@@ -132,14 +132,26 @@ module Vagrant
       raise Errors::MachineGuestNotReady if !communicate.ready?
 
       # Load the initial guest.
-      guest = load_guest(config.vm.guest)
+      last_guest = config.vm.guest
+      guest      = load_guest(last_guest)
 
       # Loop and distro dispatch while there are distros.
       while true
         distro = guest.distro_dispatch
         break if !distro
 
-        guest = load_guest(distro)
+        # This is just some really basic loop detection and avoiding for
+        # guest classes. This is just here to help implementers a bit
+        # avoid a situation that is fairly easy, since if you subclass
+        # a parent which does `distro_dispatch`, you'll end up dispatching
+        # forever.
+        if distro == last_guest
+          @logger.warn("Distro dispatch loop in '#{distro}'. Exiting loop.")
+          break
+        end
+
+        last_guest = distro
+        guest      = load_guest(distro)
       end
 
       # Return the result

test/unit/vagrant/machine_test.rb

@@ -249,6 +249,24 @@ describe Vagrant::Machine do
       config.vm.guest = :parent
       instance.guest.should be_kind_of(guest_child)
     end
+
+    it "should protect against loops in the distro dispatch" do
+      # Create the classes and dispatch the parent into the child
+      guest_parent = Class.new(Vagrant.plugin("1", :guest)) do
+        def distro_dispatch
+          :parent
+        end
+      end
+
+      # Register the classes
+      register_plugin do |p|
+        p.guest(:parent) { guest_parent }
+      end
+
+      # Test that the result is the child
+      config.vm.guest = :parent
+      instance.guest.should be_kind_of(guest_parent)
+     end
   end
 
   describe "setting the ID" do