Add version capability to darwin host plugin

lib/vagrant/errors.rb

@@ -356,6 +356,10 @@ module Vagrant
       error_key(:darwin_mount_failed)
     end
 
+    class DarwinVersionFailed < VagrantError
+      error_key(:darwin_version_failed)
+    end
+
     class DestroyRequiresForce < VagrantError
       error_key(:destroy_requires_force)
     end

plugins/hosts/darwin/cap/version.rb

@@ -0,0 +1,23 @@
+module VagrantPlugins
+  module HostDarwin
+    module Cap
+      class Version
+        def self.version(env)
+          r = Vagrant::Util::Subprocess.execute("sw_vers", "-productVersion")
+          if r.exit_code != 0
+            raise Vagrant::Errors::DarwinVersionFailed,
+              version: r.stdout,
+              error: r.stderr
+          end
+          begin
+            Gem::Version.new(r.stdout)
+          rescue => err
+            raise Vagrant::Errors::DarwinVersionFailed,
+              version: r.stdout,
+              error: err.message
+          end
+        end
+      end
+    end
+  end
+end

plugins/hosts/darwin/plugin.rb

@@ -70,6 +70,11 @@ module VagrantPlugins
         require_relative "cap/nfs"
         Cap::NFS
       end
+
+      host_capability("darwin", "version") do
+        require_relative "cap/version"
+        Cap::Version
+      end
     end
   end
 end

test/unit/plugins/hosts/darwin/cap/version_test.rb

@@ -0,0 +1,47 @@
+require_relative "../../../../base"
+require_relative "../../../../../../plugins/hosts/darwin/cap/version"
+
+describe VagrantPlugins::HostDarwin::Cap::Version do
+  describe ".version" do
+    let(:product_version) { "10.5.1" }
+    let(:env) { double(:env) }
+    let(:exit_code) { 0 }
+    let(:stderr) { "" }
+    let(:stdout) { product_version }
+    let(:result) {
+      Vagrant::Util::Subprocess::Result.new(exit_code, stdout, stderr)
+    }
+
+    before do
+      allow(Vagrant::Util::Subprocess).to receive(:execute).
+        with("sw_vers", "-productVersion").
+        and_return(result)
+    end
+
+    it "should return a Gem::Version" do
+      expect(described_class.version(env)).to be_a(Gem::Version)
+    end
+
+    it "should equal the defined version" do
+      expect(described_class.version(env)).to eq(Gem::Version.new(product_version))
+    end
+
+    context "when version cannot be parsed" do
+      let(:product_version) { "invalid"  }
+
+      it "should raise a failure error" do
+        expect { described_class.version(env) }.
+          to raise_error(Vagrant::Errors::DarwinVersionFailed)
+      end
+    end
+
+    context "when command execution fails" do
+      let(:exit_code) { 1 }
+
+      it "should raise a failure error" do
+        expect { described_class.version(env) }.
+          to raise_error(Vagrant::Errors::DarwinVersionFailed)
+      end
+    end
+  end
+end