Use a null indexer by default for Valkyrie

As long as Hyrax applications are using Wings, the index will be the one
provided by ActiveFedora. Using a null index by default allows us to start
"writing" data to the Valkyrie indexer throughout the codebase without the
overhead of running a separate index we'll never read from (see:
`Hyrax::Configuration#query_index_from_valkyrie`).

app/services/hyrax/solr_service.rb

@@ -119,9 +119,15 @@ def search_by_id(id, opts = {})
     private
 
       ##
-      # @api private
+      # @private
+      # Return the valkyrie solr index.
+      #
+      # Since this module depends closely on RSolr internals and makes use
+      # of `#connection`, it will always need to connect to a Solr index. Other
+      # valkyrie indexers used here would, at minimum, need to provide a
+      # functioning `rsolr` connection.
       def valkyrie_index
-        Hyrax.index_adapter
+        Valkyrie::IndexingAdapter.find(:solr_index)
       end
 
       ##

config/initializers/indexing_adapter_initializer.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 require 'valkyrie/indexing_adapter'
 require 'valkyrie/indexing/solr/indexing_adapter'
+require 'valkyrie/indexing/null_indexing_adapter'
 
 Rails.application.config.to_prepare do
   Valkyrie::IndexingAdapter.register(
@@ -9,4 +10,7 @@
     ),
     :solr_index
   )
+  Valkyrie::IndexingAdapter.register(
+    Valkyrie::Indexing::NullIndexingAdapter.new, :null_index
+  )
 end

lib/hyrax/configuration.rb

@@ -441,9 +441,9 @@ def iiif_metadata_fields
     attr_writer :iiif_metadata_fields
 
     ##
-    # @return [#save, #save_all, #delete, #wipe] an indexing adapter
+    # @return [#save, #save_all, #delete, #wipe!] an indexing adapter
     def index_adapter
-      @index_adapter ||= Valkyrie::IndexingAdapter.find(:solr_index)
+      @index_adapter ||= Valkyrie::IndexingAdapter.find(:null_index)
     end
 
     ##

lib/valkyrie/indexing/null_indexing_adapter.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+module Valkyrie
+  module Indexing
+    # A Valkyrie indexer that does nothing for all index requests. This is
+    # useful for applications using alternate/legacy (e.g. ActiveFedora)
+    # indexing strategies that don't want the overhead of running separate
+    # index requests.
+    class NullIndexingAdapter
+      def save(resource:)
+        :noop
+      end
+
+      def save_all(resources:)
+        :noop
+      end
+
+      def delete(resource:)
+        :noop
+      end
+
+      def wipe!
+        :noop
+      end
+    end
+  end
+end

spec/support/clean_solr.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 RSpec.configure do |config|
   config.before(:each, clean_index: true) do
-    client = Hyrax.index_adapter.connection
+    client = Valkyrie::IndexingAdapter.find(:solr_index).connection
     client.delete_by_query("*:*", params: { softCommit: true })
   end
 end