core: statefile can track sources, not sure if we'll use it though

lib/vagrant/plugin/state_file.rb

@@ -38,6 +38,15 @@ module Vagrant
         save!
       end
 
+      # Adds a RubyGems index source to look up gems.
+      #
+      # @param [String] url URL of the source.
+      def add_source(url)
+        @data["sources"] ||= []
+        @data["sources"] << url if !@data["sources"].include?(url)
+        save!
+      end
+
       # This returns a hash of installed plugins according to the state
       # file. Note that this may _not_ directly match over to actually
       # installed gems.
@@ -55,6 +64,23 @@ module Vagrant
         save!
       end
 
+      # Remove a source for RubyGems.
+      #
+      # @param [String] url URL of the source
+      def remove_source(url)
+        @data["sources"] ||= []
+        @data["sources"].delete(url)
+        save!
+      end
+
+      # Returns the list of RubyGems sources that will be searched for
+      # plugins.
+      #
+      # @return [Array<String>]
+      def sources
+        @data["sources"] || []
+      end
+
       # This saves the state back into the state file.
       def save!
         @path.open("w+") do |f|

test/unit/vagrant/plugin/state_file_test.rb

@@ -57,6 +57,29 @@ describe Vagrant::Plugin::StateFile do
       subject.add_plugin("foo", version: "1.2.3")
       expect(subject.installed_plugins["foo"]["gem_version"]).to eql("1.2.3")
     end
+
+    describe "sources" do
+      it "should have no sources" do
+        expect(subject.sources).to be_empty
+      end
+
+      it "should add sources" do
+        subject.add_source("foo")
+        expect(subject.sources).to eql(["foo"])
+      end
+
+      it "should de-dup sources" do
+        subject.add_source("foo")
+        subject.add_source("foo")
+        expect(subject.sources).to eql(["foo"])
+      end
+
+      it "can remove sources" do
+        subject.add_source("foo")
+        subject.remove_source("foo")
+        expect(subject.sources).to be_empty
+      end
+    end
   end
 
   context "with an old-style file" do