Add rails error msg,stack,type to rack spans

lib/ddtrace/contrib/action_pack/action_controller/instrumentation.rb

@@ -66,6 +66,13 @@ def finish_processing(payload)
                 span.status = 1 if status.starts_with?('5')
               elsif Utils.exception_is_error?(exception)
                 span.set_error(exception)
+
+                # Some exception gets handled by Rails middleware before it can be set on Rack middleware
+                # The rack span is the root span of the request and should make sure it has the full exception
+                # set on it.
+                if env[:datadog_rack_request_span]
+                  env[:datadog_rack_request_span].set_error(exception)
+                end
               end
             ensure
               span.finish

test/contrib/rails/rack_middleware_test.rb

@@ -197,4 +197,27 @@ class FullStackTest < ActionDispatch::IntegrationTest
     assert_equal(request_span.get_tag('http.status_code'), '404')
     assert_equal(request_span.status, 0)
   end
+
+  test 'the rack span has all exception span tags set on rails ActionView Errors' do
+    get '/error_partial'
+
+    assert_response :error
+
+    # get spans
+    assert_operator(spans.length, :>=, 2, 'there should be at least 2 span')
+
+    rack_span = spans.first
+    controller_span = spans.last
+
+    # Rack span
+    assert_equal(rack_span.status, 1)
+    assert_equal(rack_span.get_tag('error.type'), 'ActionView::Template::Error')
+    refute_nil(rack_span.get_tag('error.stack'))
+    refute_nil(rack_span.get_tag('error.msg'))
+    refute_equal(rack_span.resource, controller_span.resource) # We expect the resource hasn't been overriden
+
+    # Controller span
+    assert_equal(controller_span.status, 1, 'span should be flagged as an error')
+    assert_equal(controller_span.get_tag('error.type'), 'ActionView::Template::Error')
+  end
 end