Defer Active Storage service configuration until use

activestorage/app/models/active_storage/blob.rb

@@ -32,6 +32,7 @@ class ActiveStorage::Blob < ActiveRecord::Base
   has_secure_token :key, length: MINIMUM_TOKEN_LENGTH
   store :metadata, accessors: [ :analyzed, :identified ], coder: ActiveRecord::Coders::JSON
 
+  class_attribute :services, default: {}
   class_attribute :service
 
   has_many :attachments
@@ -49,8 +50,8 @@ class ActiveStorage::Blob < ActiveRecord::Base
   validates :service_name, presence: true
 
   validate do
-    if service_name_changed? && service_name
-      ActiveStorage::ServiceRegistry.fetch(service_name) do
+    if service_name_changed? && service_name.present?
+      services.fetch(service_name) do
         errors.add(:service_name, :invalid)
       end
     end
@@ -266,7 +267,7 @@ def purge_later
 
   # Returns an instance of service, which can be configured globally or per attachment
   def service
-    ActiveStorage::ServiceRegistry.fetch(service_name)
+    services.fetch(service_name)
   end
 
   private

activestorage/lib/active_storage.rb

@@ -38,7 +38,6 @@ module ActiveStorage
 
   autoload :Attached
   autoload :Service
-  autoload :ServiceRegistry
   autoload :Previewer
   autoload :Analyzer
 

activestorage/lib/active_storage/attached/model.rb

@@ -165,10 +165,10 @@ def purge_later
 
       private
         def validate_service_configuration(association_name, service)
-          return unless service
-
-          ServiceRegistry.fetch(service) do
-            raise ArgumentError, "Cannot configure service :#{service} for #{name}##{association_name}"
+          if service.present?
+            ActiveStorage::Blob.services.fetch(service) do
+              raise ArgumentError, "Cannot configure service :#{service} for #{name}##{association_name}"
+            end
           end
         end
     end

activestorage/lib/active_storage/engine.rb

@@ -14,6 +14,8 @@
 require "active_storage/analyzer/image_analyzer"
 require "active_storage/analyzer/video_analyzer"
 
+require "active_storage/service/registry"
+
 require "active_storage/reflection"
 
 module ActiveStorage
@@ -101,10 +103,25 @@ class Engine < Rails::Engine # :nodoc:
 
     initializer "active_storage.services" do
       ActiveSupport.on_load(:active_storage_blob) do
-        if config_choice = Rails.configuration.active_storage.service
-          ActiveStorage::Blob.service = ActiveStorage::ServiceRegistry.fetch(config_choice) do
-            raise ArgumentError, "Cannot load `Rails.application.config.active_storage.service`:\n#{config_choice}"
+        configs = Rails.configuration.active_storage.service_configurations ||=
+          begin
+            config_file = Pathname.new(Rails.root.join("config/storage.yml"))
+            raise("Couldn't find Active Storage configuration in #{config_file}") unless config_file.exist?
+
+            require "yaml"
+            require "erb"
+
+            YAML.load(ERB.new(config_file.read).result) || {}
+          rescue Psych::SyntaxError => e
+            raise "YAML syntax error occurred while parsing #{config_file}. " \
+                  "Please note that YAML must be consistently indented using spaces. Tabs are not allowed. " \
+                  "Error: #{e.message}"
           end
+
+        ActiveStorage::Blob.services = ActiveStorage::Service::Registry.new(configs)
+
+        if config_choice = Rails.configuration.active_storage.service
+          ActiveStorage::Blob.service = ActiveStorage::Blob.services.fetch(config_choice)
         end
       end
     end

activestorage/lib/active_storage/service/registry.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+module ActiveStorage
+  class Service::Registry #:nodoc:
+    def initialize(configurations)
+      @configurations = configurations.deep_symbolize_keys
+      @services = {}
+    end
+
+    def fetch(name)
+      services.fetch(name.to_sym) do |key|
+        if configurations.include?(key)
+          services[key] = configurator.build(key)
+        else
+          if block_given?
+            yield key
+          else
+            raise KeyError, "Missing configuration for the #{key} Active Storage service. " \
+              "Configurations available for the #{configurations.keys.to_sentence} services."
+          end
+        end
+      end
+    end
+
+    private
+      attr_reader :configurations, :services
+
+      def configurator
+        @configurator ||= ActiveStorage::Service::Configurator.new(configurations)
+      end
+  end
+end

activestorage/test/test_helper.rb

@@ -103,9 +103,8 @@ def fixture_file_upload(filename)
     end
 
     def with_service(service_name)
-      service = ActiveStorage::ServiceRegistry.fetch(service_name)
       previous_service = ActiveStorage::Blob.service
-      ActiveStorage::Blob.service =  service
+      ActiveStorage::Blob.service = ActiveStorage::Blob.services.fetch(service_name)
 
       yield
     ensure