core: verify explicit sf types are usable

lib/vagrant/action/builtin/synced_folders.rb

@@ -124,10 +124,19 @@ module Vagrant
             impl = ""
             impl = data[:type].to_sym if data[:type]
 
-            if impl != "" && !plugins[impl]
-              # This should never happen because configuration validation
-              # should catch this case. But we put this here as an assert
-              raise "Internal error. Report this as a bug. Invalid: #{data[:type]}"
+            if impl != ""
+              impl_class = plugins[impl]
+              if !impl_class
+                # This should never happen because configuration validation
+                # should catch this case. But we put this here as an assert
+                raise "Internal error. Report this as a bug. Invalid: #{data[:type]}"
+              end
+
+              if !impl_class[0].new.usable?(machine)
+                # Verify that explicitly defined shared folder types are
+                # actually usable.
+                raise Errors::SyncedFolderUnusable, type: data[:type].to_s
+              end
             end
 
             # Keep track of this shared folder by the implementation.

lib/vagrant/errors.rb

@@ -512,6 +512,10 @@ module Vagrant
       error_key(:ssh_unavailable_windows)
     end
 
+    class SyncedFolderUnusable < VagrantError
+      error_key(:synced_folder_unusable)
+    end
+
     class UIExpectsTTY < VagrantError
       error_key(:ui_expects_tty)
     end

templates/locales/en.yml

@@ -580,6 +580,10 @@ en:
         Port: %{port}
         Username: %{username}
         Private key: %{key_path}
+      synced_folder_unusable: |=
+        The synced folder type '%{type}' is reporting as unusable for
+        your current setup. Please verify you have all the proper
+        prerequisites for using this shared folder type and try again.
       test_key: "test value"
       ui_expects_tty: |-
         Vagrant is attempting to interface with the UI in a way that requires

test/unit/vagrant/action/builtin/synced_folders_test.rb

@@ -160,6 +160,14 @@ describe Vagrant::Action::Builtin::SyncedFolders do
       result[:nfs].should == { "nfs" => folders["nfs"] }
     end
 
+    it "should error if an explicit type is unusable" do
+      plugins[:unusable] = [impl(false, "bad"), 15]
+      folders["root"] = { type: "unusable" }
+
+      expect { subject.synced_folders(machine) }.
+        to raise_error
+    end
+
     it "should ignore disabled folders" do
       folders["root"] = {}
       folders["foo"] = { disabled: true }