synced_folders/nfs: can say functional is false explicitly

plugins/synced_folders/nfs/config.rb

@@ -3,17 +3,20 @@ require "vagrant"
 module VagrantPlugins
   module SyncedFolderNFS
     class Config < Vagrant.plugin("2", :config)
+      attr_accessor :functional
       attr_accessor :map_uid
       attr_accessor :map_gid
 
       def initialize
         super
 
+        @functional = UNSET_VALUE
         @map_uid    = UNSET_VALUE
         @map_gid    = UNSET_VALUE
       end
 
       def finalize!
+        @functional = true if @functional == UNSET_VALUE
         @map_uid = :auto if @map_uid == UNSET_VALUE
         @map_gid = :auto if @map_gid == UNSET_VALUE
       end

plugins/synced_folders/nfs/synced_folder.rb

@@ -25,6 +25,12 @@ module VagrantPlugins
       end
 
       def usable?(machine, raise_error=false)
+        # If the machine explicitly said NFS is not supported, then
+        # it isn't supported.
+        if !machine.config.nfs.functional
+          return false
+        end
+
         # We don't currently support NFS on Windows
         if Vagrant::Util::Platform.windows?
           return false

test/unit/plugins/synced_folders/nfs/config_test.rb

@@ -5,17 +5,13 @@ require Vagrant.source_root.join("plugins/synced_folders/nfs/config")
 describe VagrantPlugins::SyncedFolderNFS::Config do
   subject { described_class.new }
 
-  describe "#map_gid" do
-    it "defaults to :auto" do
+  context "defaults" do
+    before do
       subject.finalize!
-      expect(subject.map_gid).to eq(:auto)
-    end
     end
 
-  describe "#map_uid" do
-    it "defaults to nil" do
-      subject.finalize!
-      expect(subject.map_uid).to eq(:auto)
-    end
+    its(:functional) { should be_true }
+    its(:map_gid) { should eq(:auto) }
+    its(:map_uid) { should eq(:auto) }
   end
 end