ansible generated inventory: deal with orphan VMs

env.active_machines can potentiall return 'invalid' machines:
- Ignore machines that are not declared in current Vagrantfile
- Warn when machines are missing (it usually occurs when the VM is
  removed without `vagrant destroy` and some orphan metadata remains
  in .vagrant/machines/...)

plugins/provisioners/ansible/provisioner.rb

@@ -2,6 +2,7 @@ module VagrantPlugins
   module Ansible
     class Provisioner < Vagrant.plugin("2", :provisioner)
       def provision
+        @logger = Log4r::Logger.new("vagrant::provisioners::ansible")
         ssh = @machine.ssh_info
 
         # Connect with Vagrant user (unless --user or --private-key are
@@ -71,6 +72,9 @@ module VagrantPlugins
 
         ssh = @machine.ssh_info
 
+        # Managed machines
+        inventory_machines = []
+
         generated_inventory_file =
           @machine.env.root_path.join("vagrant_ansible_inventory")
 
@@ -78,8 +82,19 @@ module VagrantPlugins
           file.write("# Generated by Vagrant\n\n")
 
           @machine.env.active_machines.each do |am|
-            m = @machine.env.machine(*am)
-            file.write("#{m.name} ansible_ssh_host=#{m.ssh_info[:host]} ansible_ssh_port=#{m.ssh_info[:port]}\n")
+            begin
+              m = @machine.env.machine(*am)
+              if !m.ssh_info.nil?
+                file.write("#{m.name} ansible_ssh_host=#{m.ssh_info[:host]} ansible_ssh_port=#{m.ssh_info[:port]}\n")
+                inventory_machines << m
+              else
+                @logger.error("Auto-generated inventory: Impossible to get SSH information for machine '#{m.name} (#{m.provider_name})'. This machine should be recreated.")
+                # Let a note about this missing machine
+                file.write("# MISSING: '#{m.name}' machine was probably removed without using Vagrant. This machine should be recreated.\n")
+              end
+            rescue Vagrant::Errors::MachineNotFound => e
+              @logger.info("Auto-generated inventory: Skip machine '#{am[0]} (#{am[1]})', which is not configured for this Vagrant environment.")
+            end
           end
 
           # Write out groups information.  Only includes groups and
@@ -95,7 +110,7 @@ module VagrantPlugins
               included_groups << gname
               file.write("\n[#{gname}]\n")
               gmembers.each do |gm|
-                file.write("#{gm}\n") if @machine.env.active_machines.map { |m| m[0] }.include?(gm.to_sym)
+                file.write("#{gm}\n") if inventory_machines.map { |m| m.name }.include?(gm.to_sym)
               end
             end
           end