support optimistic locking for ResourceForm

introduces a field and prepopulator for the (maybe confusingly named?) `version`
virtual attribute. `Hyrax::WorkForm` implements this by calling `model.etag`,
but we don't have that luxury here (since non-fedora backed valkyrie may not
have an etag at all). the logic added here is specific Wings adapter, pulling
out the etag for compatibility.

it's probably okay to keep this wings-specific indefinitely. other adapters
should support optimistic locking internally (indeed, so does Wings) so we can
phase out application-layer lock validation in favor of leaning on
Valkyrie/backend datastore features.

app/forms/hyrax/forms/resource_form.rb

@@ -16,6 +16,12 @@ def self.ResourceForm(work_class)
         self.model_class = work_class
 
         include Hyrax::FormFields(:core_metadata)
+
+        ##
+        # @return [String]
+        def self.inspect
+          "Hyrax::Forms::ResourceForm(#{model_class})"
+        end
       end
     end
 
@@ -36,6 +42,26 @@ class Permission < Hyrax::ChangeSet
         property :access, virtual: true, prepopulator: ->(_opts) { self.access = model.mode }
       end
 
+      ##
+      # @api private
+      #
+      # @note includes special handling for Wings, to support compatibility
+      #   with `etag`-driven, application-side lock checks. for non-wings adapters
+      #   we want to move away from application side lock validation and rely
+      #   on the adapter/database features instead.
+      LockKeyPopulator = lambda do |_options|
+        self.version =
+          case Hyrax.metadata_adapter
+          when Wings::Valkyrie::MetadataAdapter
+            model.persisted? ? Wings::ActiveFedoraConverter.convert(resource: model).etag : ''
+          else
+            Hyrax.logger.info 'trying to prepopulate a lock token for ' \
+                              "#{self.class.inspect}, but optimistic locking isn't " \
+                              "supported for the configured adapter: #{Hyrax.metadata_adapter.class}"
+            ''
+          end
+      end
+
       class_attribute :model_class
 
       delegate :depositor, :human_readable_type, to: :model
@@ -60,6 +86,16 @@ class Permission < Hyrax::ChangeSet
       property :member_ids, default: []
       property :member_of_collection_ids, default: []
 
+      # provide a lock token for optimistic locking; we name this `version` for
+      # backwards compatibility
+      #
+      # Hyrax handles lock token validation on the application side for legacy
+      # models and Wings so we provide a token even if optimistic locking on
+      # the model is disabled
+      #
+      # @see https://github.com/samvera/valkyrie/wiki/Optimistic-Locking
+      property :version, virtual: true, prepopulator: LockKeyPopulator
+
       # backs the child work search element;
       # @todo: look for a way for the view template not to depend on this
       property :find_child_work, default: nil, virtual: true

spec/forms/hyrax/forms/resource_form_spec.rb

@@ -242,6 +242,43 @@
     end
   end
 
+  describe '#version' do
+    context 'when using wings', valkyrie_adapter: :wings_adapter do
+      it 'prepopulates as empty before save' do
+        form.prepopulate!
+        expect(form.version).to eq ''
+      end
+
+      context 'with a saved work' do
+        let(:work) { FactoryBot.valkyrie_create(:hyrax_work) }
+
+        it 'prepopulates with the etag' do
+          af_object = Wings::ActiveFedoraConverter.convert(resource: work)
+
+          form.prepopulate!
+          expect(form.version).to eq af_object.etag
+        end
+      end
+    end
+
+    context 'when using a generic valkyrie adapter', valkyrie_adapter: :test_adapter do
+      it 'prepopulates as empty before save' do
+        form.prepopulate!
+        expect(form.version).to eq ''
+      end
+
+      context 'with a saved work' do
+        let(:work) { FactoryBot.valkyrie_create(:hyrax_work) }
+
+        it 'prepopulates empty' do
+          form.prepopulate!
+
+          expect(form.version).to eq ''
+        end
+      end
+    end
+  end
+
   describe '#visibility' do
     it 'can set visibility' do
       form.visibility = 'open'

spec/views/hyrax/base/_form.html.erb_spec.rb

@@ -7,6 +7,7 @@
     # mock the admin set options presenter to avoid hitting Solr
     allow(Hyrax::AdminSetOptionsPresenter).to receive(:new).and_return(options_presenter)
     allow(controller).to receive(:current_user).and_return(stub_model(User))
+    allow(view).to receive(:curation_concern).and_return(work)
     assign(:form, form)
     allow(controller).to receive(:action_name).and_return(controller_action)
     allow(controller).to receive(:repository).and_return(controller_class.new.repository)
@@ -39,12 +40,8 @@
     context 'with an existing object' do
       let(:work) { FactoryBot.valkyrie_create(:monograph) }
 
-      xit 'renders a form' do
-        # pending handling for #version/optimistic locking, and maybe other
-        # issues. a good way to make progress on ChangeSet forms is to enable
-        # this test, analyze the stacktrace, and try to patch.
+      it 'renders a form' do
         render
-
         expect(rendered).to have_selector("form[action='/concern/monographs/#{work.id}']")
       end
     end
@@ -65,7 +62,6 @@
       # TODO: stub_model is not stubbing new_record? correctly on ActiveFedora models.
       allow(work).to receive(:new_record?).and_return(true)
       allow(work).to receive(:member_ids).and_return([1, 2])
-      allow(view).to receive(:curation_concern).and_return(work)
       allow(controller).to receive(:controller_name).and_return('batch_uploads')
       allow(form).to receive(:permissions).and_return([])
       allow(form).to receive(:visibility).and_return('public')