wip: fill in proc storage & expose both variables

* include/datadog/tracer.h
* include/datadog/tracer_signature.h
* src/datadog/tracer.cpp

Author: elsa

include/datadog/tracer.h

@@ -10,6 +10,8 @@
 // obtained from a `TracerConfig` via the `finalize_config` function.  See
 // `tracer_config.h`.
 
+#include <threads.h>
+
 #include <cstddef>
 #include <memory>
 
@@ -23,6 +25,9 @@
 #include "tracer_config.h"
 #include "tracer_signature.h"
 
+extern const void* elastic_apm_profiling_correlation_process_storage_v1;
+extern thread_local void* elastic_apm_profiling_correlation_tls_v1;
+
 namespace datadog {
 namespace tracing {
 
@@ -107,6 +112,7 @@ class Tracer {
   std::string config() const;
 
  private:
+  void correlate(const Span& span);
   void store_config();
 };
 

include/datadog/tracer_signature.h

@@ -19,7 +19,10 @@
 //    polling the Datadog Agent. See
 //    `RemoteConfigurationManager::process_response` in `remote_config.h`.
 
+#include <cstring>
+#include <memory>
 #include <string>
+#include <vector>
 
 #include "runtime_id.h"
 #include "string_view.h"
@@ -31,6 +34,15 @@
 namespace datadog {
 namespace tracing {
 
+namespace {
+void write_utf8_string(std::vector<uint8_t>& buffer, const std::string& str) {
+  uint32_t length = str.length();
+  buffer.insert(buffer.end(), reinterpret_cast<uint8_t*>(&length),
+                reinterpret_cast<uint8_t*>(&length) + sizeof(length));
+  buffer.insert(buffer.end(), str.begin(), str.end());
+}
+}  // namespace
+
 struct TracerSignature {
   RuntimeID runtime_id;
   std::string default_service;
@@ -47,6 +59,28 @@ struct TracerSignature {
         library_version(tracer_version),
         library_language("cpp"),
         library_language_version(DD_TRACE_STRINGIFY(__cplusplus), 6) {}
+
+  const std::unique_ptr<uint8_t*> generate_process_correlation_storage() {
+    std::vector<uint8_t> buffer;
+
+    // Currently, layout minor version is 2 to differ from Elastic's
+    // version which includes a socket path.
+    // Layout:
+    // https://github.com/elastic/apm/blob/149cd3e39a77a58002344270ed2ad35357bdd02d/specs/agents/universal-profiling-integration.md#process-storage-layout
+    uint16_t layout_minor_version = 2;
+    buffer.insert(buffer.end(),
+                  reinterpret_cast<uint8_t*>(&layout_minor_version),
+                  reinterpret_cast<uint8_t*>(&layout_minor_version) +
+                      sizeof(layout_minor_version));
+
+    write_utf8_string(buffer, default_service);
+    write_utf8_string(buffer, default_environment);
+    write_utf8_string(buffer, runtime_id.string());
+
+    uint8_t* res = new uint8_t[buffer.size()];
+    memcpy(res, buffer.data(), buffer.size());
+    return std::make_unique<uint8_t*>(res);
+  }
 };
 
 }  // namespace tracing

src/datadog/tracer.cpp

@@ -29,6 +29,9 @@
 #include "tracer_telemetry.h"
 #include "w3c_propagation.h"
 
+const void* elastic_apm_profiling_correlation_process_storage_v1 = nullptr;
+thread_local void* elastic_apm_profiling_correlation_tls_v1 = nullptr;
+
 namespace datadog {
 namespace tracing {
 
@@ -98,6 +101,11 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
     }
   }
 
+  // TODO: change the way this is done to handle programs that fork.
+  // TODO: add a flag to disable this by default.
+  elastic_apm_profiling_correlation_process_storage_v1 =
+      *signature_.generate_process_correlation_storage();
+
   if (config.log_on_startup) {
     logger_->log_startup([configuration = this->config()](std::ostream& log) {
       log << "DATADOG TRACER CONFIGURATION - " << configuration;
@@ -107,6 +115,13 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
   store_config();
 }
 
+void Tracer::correlate(const 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";
+}
+
 std::string Tracer::config() const {
   // clang-format off
   auto config = nlohmann::json::object({
@@ -198,6 +213,7 @@ Span Tracer::create_span(const SpanConfig& config) {
   Span span{span_data_ptr, segment,
             [generator = generator_]() { return generator->span_id(); },
             clock_};
+  correlate(span);
   return span;
 }
 
@@ -400,6 +416,7 @@ Expected<Span> Tracer::extract_span(const DictReader& reader,
   Span span{span_data_ptr, segment,
             [generator = generator_]() { return generator->span_id(); },
             clock_};
+  correlate(span);
   return span;
 }