providers/docker: synced folder usable? raises error, tests

/cc @fgrehm

plugins/providers/docker/errors.rb

@@ -1,12 +1,21 @@
 module VagrantPlugins
   module DockerProvider
     module Errors
-      class ImageNotConfiguredError < Vagrant::Errors::VagrantError
+      class DockerError < Vagrant::Errors::VagrantError
+        error_namespace("docker_provider.errors")
+      end
+
+      class ImageNotConfiguredError < DockerError
         error_key(:docker_provider_image_not_configured)
       end
-      class NfsWithoutPrivilegedError < Vagrant::Errors::VagrantError
+
+      class NfsWithoutPrivilegedError < DockerError
         error_key(:docker_provider_nfs_without_privileged)
       end
+
+      class SyncedFolderNonDocker < DockerError
+        error_key(:synced_folder_non_docker)
+      end
     end
   end
 end

plugins/providers/docker/synced_folder.rb

@@ -1,9 +1,18 @@
 module VagrantPlugins
   module DockerProvider
     class SyncedFolder < Vagrant.plugin("2", :synced_folder)
-      def usable?(machine)
+      def usable?(machine, raise_error=false)
         # These synced folders only work if the provider is Docker
-        machine.provider_name == :docker
+        if machine.provider_name != :docker
+          if raise_error
+            raise Errors::SyncedFolderNonDocker,
+              provider: machine.provider_name.to_s
+          end
+
+          return false
+        end
+
+        true
       end
 
       def prepare(machine, folders, _opts)

templates/locales/providers_docker.yml

@@ -18,14 +18,14 @@ en:
       config:
         cmd_not_set: |-
           The Docker command has not been set!
-
-  vagrant:
-    errors:
       docker_provider_nfs_without_privileged: |-
         You've configured a NFS synced folder but didn't enable privileged
         mode for the container. Please set the `privileged` option to true
         on the provider block from your Vagrantfile, recreate the container
         and try again.
-
       docker_provider_image_not_configured: |-
         The base Docker image has not been set for the '%{name}' VM!
+      synced_folder_non_docker: |-
+        The "docker" synced folder type can't be used because the  provider
+        in use is not Docker. This synced folder type only works with the
+        Docker provider. The provider this machine is using is: %{provider}

test/unit/plugins/providers/docker/synced_folder_spec.rb

@@ -0,0 +1,30 @@
+require_relative "../../../base"
+
+require Vagrant.source_root.join("plugins/providers/docker/synced_folder")
+
+describe VagrantPlugins::DockerProvider::SyncedFolder do
+  subject { described_class.new }
+
+  describe "#usable?" do
+    let(:machine) { double("machine") }
+
+    before do
+      machine.stub(provider_name: :docker)
+    end
+
+    it "is usable" do
+      expect(subject).to be_usable(machine)
+    end
+
+    it "is not usable if provider isn't docker" do
+      machine.stub(provider_name: :virtualbox)
+      expect(subject).to_not be_usable(machine)
+    end
+
+    it "raises an error if bad provider if specified" do
+      machine.stub(provider_name: :virtualbox)
+      expect { subject.usable?(machine, true) }.
+        to raise_error(VagrantPlugins::DockerProvider::Errors::SyncedFolderNonDocker)
+    end
+  end
+end