Ensure any result that's nil result becomes useful

Add a middleware that should be placed at the bottom of the Invocation
stack. This middleware ensures that any time the invocation stack is
called, there is a return value. If the stack comes back nil, then a
message is put in its place implying an error. While this might not
technically be the case that the service errored, a lack of return value
should be considered an error and looked in to.

Author: Jon Yurek

lib/cc/service/invocation.rb

@@ -2,12 +2,14 @@
 require 'cc/service/invocation/with_retries'
 require 'cc/service/invocation/with_metrics'
 require 'cc/service/invocation/with_error_handling'
+require 'cc/service/invocation/with_return_values'
 
 class CC::Service::Invocation
   MIDDLEWARE = {
     retries: WithRetries,
     metrics: WithMetrics,
     error_handling: WithErrorHandling,
+    return_values: WithReturnValues,
   }
 
   attr_reader :result

lib/cc/service/invocation/with_return_values.rb

@@ -0,0 +1,18 @@
+class CC::Service::Invocation
+  class WithReturnValues
+    def initialize(invocation, message = nil)
+      @invocation = invocation
+      @message = message || "An internal error happened"
+    end
+
+    def call
+      result = @invocation.call
+      if result.nil?
+        { ok: false, message: @message }
+      else
+        result
+      end
+    end
+  end
+end
+

test/invocation_return_values_test.rb

@@ -0,0 +1,21 @@
+require File.expand_path('../helper', __FILE__)
+
+class InvocationReturnValuesTest < CC::Service::TestCase
+  def test_success_returns_upstream_result
+    handler = CC::Service::Invocation::WithReturnValues.new(
+      lambda { :return_value },
+      "error message"
+    )
+
+    assert_equal :return_value, handler.call
+  end
+
+  def test_empty_results_returns_hash
+    handler = CC::Service::Invocation::WithReturnValues.new(
+      lambda { nil },
+      "error message"
+    )
+
+    assert_equal( {ok: false, message: "error message"}, handler.call )
+  end
+end

test/invocation_test.rb

@@ -10,6 +10,28 @@ def test_success
     assert_equal :some_result, result
   end
 
+  def test_success_with_return_values
+    service = FakeService.new(:some_result)
+
+    result = CC::Service::Invocation.invoke(service) do |i|
+      i.with :return_values, "error"
+    end
+
+    assert_equal 1, service.receive_count
+    assert_equal :some_result, result
+  end
+
+  def test_failure_with_return_values
+    service = FakeService.new(nil)
+
+    result = CC::Service::Invocation.invoke(service) do |i|
+      i.with :return_values, "error"
+    end
+
+    assert_equal 1, service.receive_count
+    assert_equal( {ok: false, message: "error"}, result )
+  end
+
   def test_retries
     service = FakeService.new
     service.fake_error = RuntimeError.new