Fixes #10224: Clear our registered providers when validating configs with no provider

Prior to this commit, if you went to validate your Vagrantfile and
wanted to ignore the provider, Vagrant would still fail as it checks if
there are any registered providers that are installed and usable. This
commit mocks out all registered providers to bypass that for the
validate command so that Vagrant can just validate the config and ignore
any provider config blocks.

plugins/commands/validate/command.rb

@@ -30,7 +30,9 @@ module VagrantPlugins
         action_env = {}
         if options[:ignore_provider]
           action_env[:ignore_provider] = true
+          mockup_providers!
         end
+
         # Validate the configuration of all machines
         with_target_vms() do |machine|
           machine.action_raw(:config_validate, Vagrant::Action::Builtin::ConfigValidate, action_env)
@@ -41,6 +43,58 @@ module VagrantPlugins
         # Success, exit status 0
         0
       end
+
+      protected
+
+      # This method is required to bypass some of the provider checks that would
+      # otherwise raise exceptions before Vagrant could load and validate a config.
+      # It essentially ignores that there are no installed or usable prodivers so
+      # that Vagrant can go along and validate the rest of the Vagrantfile and ignore
+      # any provider blocks.
+      def mockup_providers!
+        require 'log4r'
+        logger = Log4r::Logger.new("vagrant::validate")
+        logger.debug("Overriding all registered provider classes for validate")
+
+        # Without setting up a tmp Environment, Vagrant will completely
+        # erase the local data dotfile and you can lose state after the
+        # validate command completes.
+        tmp_data_dir = Dir.mktmpdir("vagrant-validate-")
+        @env = Vagrant::Environment.new(
+          cwd: @env.cwd,
+          home_path: @env.home_path,
+          ui_class: @env.ui_class,
+          vagrantfile_name: @env.vagrantfile_name,
+          local_data_path: tmp_data_dir,
+          data_dir: tmp_data_dir
+        )
+
+        Vagrant.plugin("2").manager.providers.each do |key, data|
+          data[0].class_eval do
+            def initialize(machine)
+              @machine = machine
+            end
+
+            def machine_id_changed
+            end
+
+            def self.installed?
+              true
+            end
+
+            def self.usable?(raise_error=false)
+              true
+            end
+
+            def state
+              state_id = Vagrant::MachineState::NOT_CREATED_ID
+              short = :not_created
+              long = :not_created
+              Vagrant::MachineState.new(state_id, short, long)
+            end
+          end
+        end
+      end
     end
   end
 end

test/unit/plugins/commands/validate/command_test.rb

@@ -138,6 +138,7 @@ describe VagrantPlugins::CommandValidate::Command do
         VF
       end
       it "ignores provider specific configurations with the flag" do
+        allow(subject).to receive(:mockup_providers!).and_return(true)
         expect(iso_env.ui).to receive(:info).with(any_args) { |message, _|
           expect(message).to include("Vagrantfile validated successfully.")
         }