feat: write in tls on span creation

* BUILD.bazel
* include/datadog/tls_storage.h
* include/datadog/trace_segment.h
* include/datadog/tracer.h
* src/datadog/trace_segment.cpp
* src/datadog/tracer.cpp

Author: elsa

BUILD.bazel

@@ -109,6 +109,7 @@ cc_library(
       "include/datadog/span_matcher.h",
       "include/datadog/span_sampler_config.h",
       "include/datadog/string_view.h",
+      "include/datadog/tls_storage.h",
       "include/datadog/tracer.h",
       "include/datadog/tracer_config.h",
       "include/datadog/tracer_signature.h",

include/datadog/tls_storage.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <datadog/trace_id.h>
+
+#include <array>
+#include <cstdint>
+
+namespace datadog {
+namespace tracing {
+struct __attribute__((packed)) TLSStorage {
+  uint16_t layout_minor_version;
+  uint8_t valid;
+  uint8_t trace_present;
+  uint8_t trace_flags;
+  uint64_t trace_id_low;
+  uint64_t trace_id_high;
+  uint64_t span_id;
+  uint64_t transaction_id;
+};
+}  // namespace tracing
+}  // namespace datadog

include/datadog/trace_segment.h

@@ -102,6 +102,8 @@ class TraceSegment {
   const Optional<std::string>& origin() const;
   Optional<SamplingDecision> sampling_decision() const;
 
+  uint64_t local_root_id() const;
+
   Logger& logger() const;
 
   // Inject trace context for the specified `span` into the specified `writer`.

include/datadog/tracer.h

@@ -10,6 +10,7 @@
 // obtained from a `TracerConfig` via the `finalize_config` function.  See
 // `tracer_config.h`.
 
+#include <datadog/tls_storage.h>
 #include <threads.h>
 
 #include <cstddef>
@@ -26,7 +27,8 @@
 #include "tracer_signature.h"
 
 extern const void* elastic_apm_profiling_correlation_process_storage_v1;
-extern thread_local void* elastic_apm_profiling_correlation_tls_v1;
+extern thread_local struct datadog::tracing::TLSStorage*
+    elastic_apm_profiling_correlation_tls_v1;
 
 namespace datadog {
 namespace tracing {

src/datadog/trace_segment.cpp

@@ -142,6 +142,8 @@ Optional<SamplingDecision> TraceSegment::sampling_decision() const {
   return sampling_decision_;
 }
 
+uint64_t TraceSegment::local_root_id() const { return spans_.front()->span_id; }
+
 Logger& TraceSegment::logger() const { return *logger_; }
 
 void TraceSegment::register_span(std::unique_ptr<SpanData> span) {

src/datadog/tracer.cpp

@@ -12,6 +12,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <memory>
 
 #include "config_manager.h"
 #include "datadog_agent.h"
@@ -30,7 +31,8 @@
 #include "w3c_propagation.h"
 
 const void* elastic_apm_profiling_correlation_process_storage_v1 = nullptr;
-thread_local void* elastic_apm_profiling_correlation_tls_v1 = nullptr;
+thread_local struct datadog::tracing::TLSStorage*
+    elastic_apm_profiling_correlation_tls_v1 = nullptr;
 
 namespace datadog {
 namespace tracing {
@@ -115,11 +117,25 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
   store_config();
 }
 
-void Tracer::correlate(const Span&) {
+void Tracer::correlate(const Span& span) {
   // TODO: update this variablle with data
   // See Layout:
   // https://github.com/elastic/apm/blob/149cd3e39a77a58002344270ed2ad35357bdd02d/specs/agents/universal-profiling-integration.md#thread-local-storage-layout
-  elastic_apm_profiling_correlation_tls_v1 = (char*)"randomdata\n";
+  auto tls_storage_ptr = std::make_unique<struct TLSStorage>();
+  elastic_apm_profiling_correlation_tls_v1 = tls_storage_ptr.get();
+  struct TLSStorage* tls_data = elastic_apm_profiling_correlation_tls_v1;
+  tls_data->valid = 0;
+
+  tls_data->layout_minor_version = 1;
+  tls_data->trace_present = 1;  // We are in a span so no errors
+  tls_data->trace_flags = 0;    // IDK
+  auto trace_id = span.trace_id();
+  tls_data->trace_id_low = trace_id.low;
+  tls_data->trace_id_high = trace_id.high;
+  tls_data->span_id = span.id();
+  tls_data->transaction_id = span.trace_segment().local_root_id();
+
+  tls_data->valid = 1;
 }
 
 std::string Tracer::config() const {