Extracting Qa.authority_for

Prior to this commit, we used different initialization logic for linked
data authorities and non-linked data.  Further, the initialization logic
was locked away inside of a controller.

With this commit, we expose a method that provides consistent
initialization logic; albeit with different returned classes that have
slightly different implementation nuances.  The application can then
rely on that and we can share initialization logic across controllers
and also expose a method that allows non-controller initialization to
begin to follow the same logic pathway.

Where are the tests?  The initializations are covered by controller
tests, so I have chosen not to write new ones.

There is further work to do because LinkedData and non-LinkedData there
are different signatures for `find` and `search`.  Ideally, we would
normalize the public facing implementation logic.  However, this commit
preserves backwards compatibility, simply introducing a new feature.

app/controllers/qa/linked_data_terms_controller.rb

@@ -158,8 +158,8 @@ def create_request_header_service
     end
 
     def init_authority
-      @authority = Qa::Authorities::LinkedData::GenericAuthority.new(vocab_param)
-    rescue Qa::InvalidLinkedDataAuthority => e
+      @authority = Qa.authority_for(vocab: params[:vocab], subauthority: params[:subauthority])
+    rescue Qa::InvalidAuthorityError, Qa::InvalidLinkedDataAuthority => e
       msg = e.message
       logger.warn msg
       render json: { errors: msg }, status: :bad_request

app/controllers/qa/terms_controller.rb

@@ -69,26 +69,14 @@ def check_vocab_param
   end
 
   def init_authority # rubocop:disable Metrics/MethodLength
-    begin
-      mod = authority_class.camelize.constantize
-    rescue NameError
-      msg = "Unable to initialize authority #{authority_class}"
-      logger.warn msg
-      render json: { errors: msg }, status: :bad_request
-      return
-    end
-    begin
-      @authority = if mod.is_a? Class
-                     mod.new
-                   else
-                     raise Qa::MissingSubAuthority, "No sub-authority provided" if params[:subauthority].blank?
-                     mod.subauthority_for(params[:subauthority])
-                   end
-    rescue Qa::InvalidSubAuthority, Qa::MissingSubAuthority => e
-      msg = e.message
-      logger.warn msg
-      render json: { errors: msg }, status: :bad_request
-    end
+    @authority = Qa.authority_for(vocab: params[:vocab],
+                                  subauthority: params[:subauthority],
+                                  # Included to preserve error message text
+                                  try_linked_data_config: false)
+  rescue Qa::InvalidAuthorityError, Qa::InvalidSubAuthority, Qa::MissingSubAuthority => e
+    msg = e.message
+    logger.warn msg
+    render json: { errors: msg }, status: :bad_request
   end
 
   def check_query_param

lib/qa.rb

@@ -30,6 +30,13 @@ def self.deprecation_warning(in_msg: nil, msg:)
     warn "[DEPRECATED] #{in_msg}#{msg}  It will be removed in the next major release."
   end
 
+  # Raised when the authority is not valid
+  class InvalidAuthorityError < RuntimeError
+    def initialize(authority_class)
+      super "Unable to initialize authority #{authority_class}"
+    end
+  end
+
   # Raised when the configuration directory for local authorities doesn't exist
   class ConfigDirectoryNotFound < StandardError; end
 
@@ -67,4 +74,39 @@ class MissingParameter < StandardError; end
 
   # Raised when data is returned but cannot be normalized
   class DataNormalizationError < StandardError; end
+
+  # @api public
+  # @since 5.11.0
+  #
+  # @param vocab [String]
+  # @param subauthority [String]
+  #
+  # @param try_linked_data_config [Boolean] when true attempt to check for a linked data authority;
+  #        this is included as an option to help preserve error messaging from the 5.10.0 branch.
+  #        Unless you want to mirror the error messages of `Qa::TermsController#init_authority` then
+  #        use the default value.
+  #
+  # @note :try_linked_data_config is included to preserve error message text; something which is
+  #       extensively tested in this gem.
+  #
+  # @return [#search, #find] an authority that will respond to #search and #find; and in some cases
+  #         #fetch.  This is provided as a means of normalizing how we initialize an authority.
+  #         And to provide a means to request an authority both within a controller request cycle as
+  #         well as outside of that cycle.
+  def self.authority_for(vocab:, subauthority: nil, try_linked_data_config: true)
+    authority_constant_name = "Qa::Authorities::#{vocab.to_s.camelcase}"
+    authority_constant = authority_constant_name.safe_constantize
+    if authority_constant.nil?
+      if try_linked_data_config
+        return Qa::Authorities::LinkedData::GenericAuthority.new(vocab.upcase.to_sym)
+      else
+        raise InvalidAuthorityError, authority_constant_name
+      end
+    end
+
+    return authority_constant.new if authority_constant.is_a?(Class)
+    return authority_constant.subauthority_for(subauthority) if subauthority.present?
+
+    raise Qa::MissingSubAuthority, "No sub-authority provided"
+  end
 end