feat: use custom labels to share trace & span ids

* include/datadog/tracer.h
* src/datadog/otel_identifiers.h
* src/datadog/span.cpp
* src/datadog/trace_segment.cpp
* src/datadog/tracer.cpp

Author: elsa

include/datadog/tracer.h

@@ -106,6 +106,9 @@ class Tracer {
   std::string config() const;
 
  private:
+#ifdef __linux__
+  void correlate(const Span& span);
+#endif
   void store_config();
 };
 

src/datadog/otel_identifiers.h

@@ -0,0 +1,12 @@
+#pragma once
+
+namespace datadog {
+namespace tracing {
+
+// Based on the OTel Trace API
+// https://opentelemetry.io/docs/specs/otel/trace/api
+#define OTEL_TRACE_ID_IDENTIFIER "TraceId"
+#define OTEL_SPAN_ID_IDENTIFIER "SpanId"
+
+}  // namespace tracing
+}  // namespace datadog

src/datadog/span.cpp

@@ -4,10 +4,12 @@
 #include <datadog/span_config.h>
 #include <datadog/string_view.h>
 #include <datadog/trace_segment.h>
+#include <datadog/tracer.h>
 
 #include <cassert>
 #include <string>
 
+#include "otel_identifiers.h"
 #include "span_data.h"
 #include "tags.h"
 
@@ -40,6 +42,19 @@ Span::~Span() {
     data_->duration = now - data_->start;
   }
 
+#ifdef __linux__
+  // When a span is finished, we must update the span_id to its parent's.
+  if (process_storage != nullptr && parent_id().has_value()) {
+    auto span_id = std::to_string(parent_id().value());
+    custom_labels_labelset_set(
+        custom_labels_current_set,
+        {sizeof(OTEL_SPAN_ID_IDENTIFIER),
+         reinterpret_cast<const unsigned char*>(OTEL_SPAN_ID_IDENTIFIER)},
+        {span_id.size(),
+         reinterpret_cast<const unsigned char*>(span_id.c_str())});
+  }
+#endif
+
   trace_segment_->span_finished();
 }
 

src/datadog/trace_segment.cpp

@@ -1,3 +1,9 @@
+#ifdef __linux__
+extern "C" {
+#include <customlabels/customlabels.h>
+}
+#endif
+
 #include <datadog/collector.h>
 #include <datadog/dict_reader.h>
 #include <datadog/dict_writer.h>
@@ -254,6 +260,15 @@ void TraceSegment::span_finished() {
     }
   }
 
+#ifdef __linux__
+  // When all spans are finished, so is the current trace.
+  if (process_storage != nullptr) {
+    auto saved_set = custom_labels_current_set;
+    custom_labels_current_set = nullptr;
+    custom_labels_labelset_free(saved_set);
+  }
+#endif
+
   telemetry::counter::increment(metrics::tracer::trace_segments_closed);
 }
 

src/datadog/tracer.cpp

@@ -21,6 +21,7 @@
 #include "hex.h"
 #include "json.hpp"
 #include "msgpack.h"
+#include "otel_identifiers.h"
 #include "platform_util.h"
 #include "random.h"
 #include "span_data.h"
@@ -108,6 +109,27 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
   store_config();
 }
 
+#ifdef __linux__
+void Tracer::correlate(const Span& span) {
+  custom_labels_current_set = custom_labels_labelset_new(2);
+  auto trace_id = span.trace_id().hex_padded();
+  custom_labels_labelset_set(
+      custom_labels_current_set,
+      {sizeof(OTEL_TRACE_ID_IDENTIFIER),
+       reinterpret_cast<const unsigned char*>(OTEL_TRACE_ID_IDENTIFIER)},
+      {trace_id.size(),
+       reinterpret_cast<const unsigned char*>(trace_id.c_str())});
+
+  auto span_id = std::to_string(span.id());
+  custom_labels_labelset_set(
+      custom_labels_current_set,
+      {sizeof(OTEL_SPAN_ID_IDENTIFIER),
+       reinterpret_cast<const unsigned char*>(OTEL_SPAN_ID_IDENTIFIER)},
+      {span_id.size(),
+       reinterpret_cast<const unsigned char*>(span_id.c_str())});
+}
+#endif
+
 std::string Tracer::config() const {
   // clang-format off
   auto config = nlohmann::json::object({
@@ -199,6 +221,11 @@ Span Tracer::create_span(const SpanConfig& config) {
   Span span{span_data_ptr, segment,
             [generator = generator_]() { return generator->span_id(); },
             clock_};
+
+#ifdef __linux__
+  if (correlate_full_host_profiles_) correlate(span);
+#endif
+
   return span;
 }
 
@@ -410,6 +437,11 @@ Expected<Span> Tracer::extract_span(const DictReader& reader,
   Span span{span_data_ptr, segment,
             [generator = generator_]() { return generator->span_id(); },
             clock_};
+
+#ifdef __linux__
+  if (correlate_full_host_profiles_) correlate(span);
+#endif
+
   return span;
 }