Add experimental flag to guard development features

This commit introduces a special flag for enabling features that are not
ready for release. It can either be enabled by setting the
`VAGRANT_EXPERIMENTAL` flag to "1", or by setting it to a string of one
or more comma seperated values for specific features. It also adds a
couple of Vagrant developer focused methods for making it easier to
determine if the flag has been enabled, and if so, what features.

bin/vagrant

@@ -159,6 +159,31 @@ begin
     env.ui.warn(I18n.t("vagrant.general.not_in_installer") + "\n", prefix: false)
   end
 
+  # Acceptable experimental flag values include:
+  #
+  # Unset  - Disables experimental features
+  # 0      - Disables experimental features
+  # 1      - Enables all features
+  # String - Enables one or more features, separated by commas
+  if ENV["VAGRANT_EXPERIMENTAL"]
+    experimental = ENV["VAGRANT_EXPERIMENTAL"].to_s
+    if experimental == "0"
+      logger.debug("Experimental flag is set but disabled")
+    else
+      ui = Vagrant::UI::Prefixed.new(env.ui, "vagrant")
+      if experimental != "1"
+        logger.debug("Experimental flag is enabled")
+        features = experimental.split(',')
+        ui.warn(I18n.t("vagrant.general.experimental.features", features: features.join(", ")), bold: true, prefix: true, channel: :error)
+      elsif experimental == "1"
+        logger.debug("Experimental flag is enabled")
+        ui.warn(I18n.t("vagrant.general.experimental.all"), bold: true, prefix: true, channel: :error)
+      else
+        logger.warn("Experimental flag is set to an unsupported value")
+      end
+    end
+  end
+
   begin
     # Execute the CLI interface, and exit with the proper error code
     exit_status = env.cli(argv)

lib/vagrant/util/experimental.rb

@@ -0,0 +1,48 @@
+module Vagrant
+  module Util
+    class Experimental
+      VALID_FEATURES = [].freeze
+      class << self
+
+        # A method for determining if the experimental flag has been enabled
+        #
+        # @return [Boolean]
+        def enabled?
+          if !defined?(@_experimental)
+            experimental = ENV["VAGRANT_EXPERIMENTAL"].to_s
+            if experimental != "0" && !experimental.empty?
+              @_experimental = true
+            else
+              @_experimental = false
+            end
+          end
+          @_experimental
+        end
+
+        # A method for Vagrant internals to determine if a given feature
+        # has been abled and can be used.
+        #
+        # @param [String] - An array of strings of features to check against
+        # @return [Boolean] - A hash containing the original array and if it is valid
+        def feature_enabled?(feature)
+          experimental = ENV["VAGRANT_EXPERIMENTAL"].to_s.downcase
+          if experimental == "1"
+            return true
+          elsif VALID_FEATURES.include?(feature) &&
+                experimental.split(',').include?(feature)
+            return true
+          else
+            return false
+          end
+        end
+
+        # @private
+        # Reset the cached values for platform. This is not considered a public
+        # API and should only be used for testing.
+        def reset!
+          instance_variables.each(&method(:remove_instance_variable))
+        end
+      end
+    end
+  end
+end

templates/locales/en.yml

@@ -393,6 +393,18 @@ en:
         shown below.
 
         %{output}
+      experimental:
+        all: |-
+          You have enabled the experimental flag with all features enabled.
+          Please use with caution, as some of the features may not be fully
+          functional yet.
+        features: |-
+          You have requested to enabled the experimental flag with the following features:
+
+          Features:  %{features}
+
+          Please use with caution, as some of the features may not be fully
+          functional yet.
       not_in_installer: |-
         You appear to be running Vagrant outside of the official installers.
         Note that the installers are what ensure that Vagrant has all required

test/unit/vagrant/util/experimental_test.rb

@@ -0,0 +1,67 @@
+require File.expand_path("../../../base", __FILE__)
+
+require "vagrant/util/experimental"
+
+describe Vagrant::Util::Experimental do
+  include_context "unit"
+  before(:each) { described_class.reset! }
+  subject { described_class }
+
+  describe "#enabled?" do
+    it "returns true if enabled with '1'" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("1")
+      expect(subject.enabled?).to eq(true)
+    end
+
+    it "returns true if enabled with a list of features" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("list,of,features")
+      expect(subject.enabled?).to eq(true)
+    end
+
+    it "returns false if disabled" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("0")
+      expect(subject.enabled?).to eq(false)
+    end
+
+    it "returns false if not set" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return(nil)
+      expect(subject.enabled?).to eq(false)
+    end
+  end
+
+  describe "#feature_enabled?" do
+    before(:each) do
+      stub_const("Vagrant::Util::Experimental::VALID_FEATURES", ["secret_feature"])
+    end
+
+    it "returns true if flag set to 1" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("1")
+      expect(subject.feature_enabled?("anything")).to eq(true)
+    end
+
+    it "returns true if flag contains feature requested" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("secret_feature")
+      expect(subject.feature_enabled?("secret_feature")).to eq(true)
+    end
+
+    it "returns true if flag contains feature requested with other features 'enabled'" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("secret_feature,other_secret")
+      expect(subject.feature_enabled?("secret_feature")).to eq(true)
+    end
+
+    it "returns false if flag does not contain feature requested" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("secret_feature")
+      expect(subject.feature_enabled?("anything")).to eq(false)
+    end
+
+    it "returns false if flag set to 0" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return("0")
+      expect(subject.feature_enabled?("anything")).to eq(false)
+    end
+
+    it "returns false if flag is not set" do
+      allow(ENV).to receive(:[]).with("VAGRANT_EXPERIMENTAL").and_return(nil)
+      expect(subject.feature_enabled?("anything")).to eq(false)
+    end
+  end
+end