Support copying span contexts.

include/opentracing/span.h

@@ -26,6 +26,11 @@ class SpanContext {
   virtual void ForeachBaggageItem(
       std::function<bool(const std::string& key, const std::string& value)> f)
       const = 0;
+
+  // Clone creates a copy of SpanContext.
+  //
+  // Returns nullptr on failure.
+  virtual std::unique_ptr<SpanContext> Clone() const noexcept = 0;
 };
 
 struct LogRecord {

mocktracer/src/mock_span.cpp

@@ -117,7 +117,7 @@ void MockSpan::FinishWithOptions(const FinishSpanOptions& options) noexcept {
 
   data_.duration = finish_timestamp - start_steady_;
 
-  span_context_.SetData(data_.span_context);
+  span_context_.CopyData(data_.span_context);
 
   if (recorder_ != nullptr) {
     recorder_->RecordSpan(std::move(data_));

mocktracer/src/mock_span_context.cpp

@@ -21,12 +21,20 @@ void MockSpanContext::ForeachBaggageItem(
   }
 }
 
-void MockSpanContext::SetData(SpanContextData& data) {
+void MockSpanContext::CopyData(SpanContextData& data) const {
   data.trace_id = data_.trace_id;
   data.span_id = data_.span_id;
   std::lock_guard<std::mutex> lock_guard{baggage_mutex_};
   data.baggage = data_.baggage;
 }
+
+std::unique_ptr<SpanContext> MockSpanContext::Clone() const noexcept try {
+  auto result = std::unique_ptr<MockSpanContext>{new MockSpanContext{}};
+  CopyData(result->data_);
+  return std::unique_ptr<SpanContext>{result.release()};
+} catch (const std::exception& /*e*/) {
+  return nullptr;
+}
 }  // namespace mocktracer
 END_OPENTRACING_ABI_NAMESPACE
 }  // namespace opentracing

mocktracer/src/mock_span_context.h

@@ -33,7 +33,7 @@ class MockSpanContext : public SpanContext {
 
   uint64_t span_id() const noexcept { return data_.span_id; }
 
-  void SetData(SpanContextData& data);
+  void CopyData(SpanContextData& data) const;
 
   template <class Carrier>
   expected<void> Inject(const PropagationOptions& propagation_options,
@@ -49,6 +49,8 @@ class MockSpanContext : public SpanContext {
     return ExtractSpanContext(propagation_options, reader, data_);
   }
 
+  std::unique_ptr<SpanContext> Clone() const noexcept override;
+
  private:
   friend MockSpan;
 

src/noop.cpp

@@ -8,6 +8,10 @@ class NoopSpanContext : public SpanContext {
   void ForeachBaggageItem(
       std::function<bool(const std::string& key,
                          const std::string& value)> /*f*/) const override {}
+
+  std::unique_ptr<SpanContext> Clone() const noexcept override {
+    return std::unique_ptr<SpanContext>{new (std::nothrow) NoopSpanContext{}};
+  }
 };
 
 class NoopSpan : public Span {