core: check if provider is usable when requesting a machine

lib/vagrant/errors.rb

@@ -496,6 +496,10 @@ module Vagrant
       error_key(:provider_not_found)
     end
 
+    class ProviderNotUsable < VagrantError
+      error_key(:provider_not_usable)
+    end
+
     class ProvisionerFlagInvalid < VagrantError
       error_key(:provisioner_flag_invalid)
     end

lib/vagrant/vagrantfile.rb

@@ -124,6 +124,16 @@ module Vagrant
       provider_options = provider_plugin[1]
       box_formats      = provider_options[:box_format] || provider
 
+      # Test if the provider is usable or not
+      begin
+        provider_cls.usable?(true)
+      rescue Errors::VagrantError => e
+        raise Errors::ProviderNotUsable,
+          machine: name.to_s,
+          provider: provider.to_s,
+          message: e.to_s
+      end
+
       # Add the sub-machine configuration to the loader and keys
       vm_config_key = "#{object_id}_machine_#{name}"
       @loader.set(vm_config_key, sub_machine.config_procs)

templates/locales/en.yml

@@ -855,6 +855,12 @@ en:
       provider_not_found: |-
         The provider '%{provider}' could not be found, but was requested to
         back the machine '%{machine}'. Please use a provider that exists.
+      provider_not_usable: |-
+        The provider '%{provider}' that was requested to back the machine
+        '%{machine}' is reporting that it isn't usable on this system. The
+        reason is shown below:
+
+        %{message}
       provisioner_flag_invalid: |-
         '%{name}' is not a known provisioner. Please specify a valid
         provisioner.

test/unit/vagrant/vagrantfile_test.rb

@@ -26,7 +26,14 @@ describe Vagrant::Vagrantfile do
 
   # A helper to register a provider for use in tests.
   def register_provider(name, config_class=nil, options=nil)
-    provider_cls = Class.new(Vagrant.plugin("2", :provider))
+    provider_cls = Class.new(Vagrant.plugin("2", :provider)) do
+      if options && options[:unusable]
+        def self.usable?(raise_error=false)
+          raise Vagrant::Errors::VagrantError if raise_error
+          false
+        end
+      end
+    end
 
     register_plugin("2") do |p|
       p.provider(name, options) { provider_cls }
@@ -298,6 +305,13 @@ describe Vagrant::Vagrantfile do
       expect { subject.machine_config(:default, :foo, boxes) }.
         to raise_error(Vagrant::Errors::ProviderNotFound)
     end
+
+    it "raises an error if the provider is not usable" do
+      register_provider("foo", nil, unusable: true)
+
+      expect { subject.machine_config(:default, :foo, boxes) }.
+        to raise_error(Vagrant::Errors::ProviderNotUsable)
+    end
   end
 
   describe "#machine_names" do