fix: untraced only works with parent-based sampler

common/lib/opentelemetry/common/utilities.rb

@@ -10,6 +10,9 @@ module Common
     module Utilities
       extend self
 
+      UNTRACED_KEY = Context.create_key('untraced')
+      private_constant :UNTRACED_KEY
+
       STRING_PLACEHOLDER = ''.encode(::Encoding::UTF_8).freeze
 
       # Returns nil if timeout is nil, 0 if timeout has expired,
@@ -83,8 +86,17 @@ def truncate_attribute_value(value, limit)
         end
       end
 
+      # Disables tracing within the provided block.
       def untraced
-        OpenTelemetry::Trace.with_span(OpenTelemetry::Trace.non_recording_span(OpenTelemetry::Trace::SpanContext.new)) { yield } # rubocop:disable Style/ExplicitBlockArgument
+        Context.with_value(UNTRACED_KEY, true) do |ctx, _|
+          yield ctx
+        end
+      end
+
+      # Detects whether the current context has been set to disable tracing.
+      def untraced?(context = nil)
+        context ||= Context.current
+        !!context.value(UNTRACED_KEY)
       end
 
       # Returns a URL string with userinfo removed.

common/test/opentelemetry/common/utilities_test.rb

@@ -17,6 +17,17 @@ def shutdown(timeout: nil); end
 
   let(:common_utils) { OpenTelemetry::Common::Utilities }
 
+  describe '#untraced?' do
+    it 'returns true within an untraced block' do
+      assert_equal(true, common_utils.untraced { common_utils.untraced? })
+    end
+
+    it 'returns false outside an untraced block' do
+      common_utils.untraced {}
+      assert_equal(false, common_utils.untraced?)
+    end
+  end
+
   describe '#utf8_encode' do
     it 'happy path' do
       str = 'pristine ￢'.encode(Encoding::UTF_8)

sdk/lib/opentelemetry/sdk/trace/tracer.rb

@@ -28,9 +28,13 @@ def start_root_span(name, attributes: nil, links: nil, start_timestamp: nil, kin
         end
 
         def start_span(name, with_parent: nil, attributes: nil, links: nil, start_timestamp: nil, kind: nil)
+          with_parent ||= Context.current
+          if Common::Utilities.untraced?(with_parent)
+            return super(name, with_parent: with_parent, attributes: attributes, links: links, start_timestamp: start_timestamp, kind: kind)
+          end
+
           name ||= 'empty'
           kind ||= :internal
-          with_parent ||= Context.current
 
           @tracer_provider.internal_start_span(name, kind, attributes, links, start_timestamp, with_parent, @instrumentation_scope)
         end

sdk/test/opentelemetry/sdk/trace/tracer_test.rb

@@ -192,6 +192,13 @@
       _(span).wont_be :recording?
     end
 
+    it 'returns a no-op span within an untraced block' do
+      tracer_provider.sampler = Samplers::ALWAYS_ON
+      span = OpenTelemetry::Common::Utilities.untraced { tracer.start_span('op') }
+      _(span.context.trace_flags).wont_be :sampled?
+      _(span).wont_be :recording?
+    end
+
     it 'returns an unsampled span if sampler says record, but do not sample' do
       tracer_provider.sampler = record_sampler
       span = tracer.start_span('op', with_parent: context)