[GQL] BaseResolver: support #ready? in single and last

This adds support for the framework method `#ready?`, allowing resolvers
to use it and have derived single resolvers behave correctly.

app/graphql/resolvers/base_resolver.rb

@@ -3,27 +3,33 @@
 module Resolvers
   class BaseResolver < GraphQL::Schema::Resolver
     extend ::Gitlab::Utils::Override
+    include ::Gitlab::Utils::StrongMemoize
 
     def self.single
       @single ||= Class.new(self) do
+        def ready?(**args)
+          ready, early_return = super
+          [ready, select_result(early_return)]
+        end
+
         def resolve(**args)
-          super.first
+          select_result(super)
         end
 
         def single?
           true
         end
+
+        def select_result(results)
+          results&.first
+        end
       end
     end
 
     def self.last
-      @last ||= Class.new(self) do
-        def resolve(**args)
-          super.last
-        end
-
-        def single?
-          true
+      @last ||= Class.new(self.single) do
+        def select_result(results)
+          results&.last
         end
       end
     end

spec/graphql/resolvers/base_resolver_spec.rb

@@ -41,9 +41,35 @@ describe Resolvers::BaseResolver do
     end
   end
 
+  context 'when the resolver returns early' do
+    let(:resolver) do
+      Class.new(described_class) do
+        def ready?(**args)
+          [false, %w(early return)]
+        end
+
+        def resolve(**args)
+          raise 'Should not get here'
+        end
+      end
+    end
+
+    it 'runs correctly in our test framework' do
+      expect(resolve(resolver)).to contain_exactly('early', 'return')
+    end
+
+    it 'single selects the first early return value' do
+      expect(resolve(resolver.single)).to eq('early')
+    end
+
+    it 'last selects the last early return value' do
+      expect(resolve(resolver.last)).to eq('return')
+    end
+  end
+
   describe '.last' do
     it 'returns a subclass from the resolver' do
-      expect(last_resolver.last.superclass).to eq(last_resolver)
+      expect(last_resolver.last.ancestors).to include(last_resolver)
     end
 
     it 'returns the same subclass every time' do